Feasibility of bioethanol and biobutanol as transportation fuel in spark-ignition engine: a review

Abstract

Fossil fuels depletion is a globally main issue because it is the primary transportation fuel. Furthermore, environmental issues including global warming and climate changes are problems that need to be dealt with immediately. Therefore, biofuels (bio-based fuels), i.e. bio-alcohols (bioethanol and biobutanol) which are produced from natural materials have emerged as promising transportation fuels because of their sustainability and environmental benefits which can reduce the dependency on crude oil reserves. Today, bioethanol is widely used as an option for transportation fuel or additives to gasoline in spark ignition (SI) engine due to its attractive properties of high octane number and reduced exhaust emissions. The next promising and competitive biofuel is biobutanol which has superior properties to be used in SI engine without engine modification. The aim of the present review is to highlight on the feasibility of the bioethanol and biobutanol as alternative transportation fuels in SI engine. The first section of this paper is an overview on bioethanol and biobutanol as gasoline alternatives. In the next section, comparative physicochemical properties of gasoline, bioethanol and biobutanol and their potential sources of production are presented. The effect of bioethanol and biobutanol with gasoline blends on engine performances, combustion analysis, exhaust emissions, engine durability and their effect on lubricating oil are discussed in the next section. The review study acknowledges that the bioethanol and biobutanol are capable of improving engine performances, combustion and also reducing exhaust emissions. However, the addition of alcohols in fuel blends leads to negative impacts on the engine durability and lubricating oil properties.

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M. N. A. Yusoff

Mohd Nur Ashraf bin Mohd Yusoff is currently pursuing his master’s degree in MSc (Eng) since 2015 in University of Malaya, Malaysia. He received his bachelor degree in Mechanical Engineering from the same university and graduated in 2014. Upon graduation, he started his research work as a Research Assistant in Center of Energy Sciences, University of Malaya. His research concentrates on the study on renewable energy sources for alternative transportation fuels.

N. W. M. Zulkifli

Dr Nurin Wahidah binti Mohd Zulkifli is working as a senior lecturer in the Department of Mechanical Engineering, University of Malaya since 2015 to date. Her research specialization is on Tribology. Dr Nurin obtained her undergraduate degree from University of Malaya (Malaysia). She continued her studies in MSc (Eng) & Doctoral Degree PhD in Monash University (Australia) and University of Malaya (Malaysia) respectively.

B. M. Masum

Md. Masum Billah is a Research Assistant, working on HIR project in the University of Malaya. Recently he was awarded Master of Engineering Science from University of Malaya, Malaysia. He received his bachelor degree with distinction in Mechanical Engineering at Chittagong University of Engineering and Technology, Bangladesh in 2010. Then, he started his industrial job as an assistant engineer at Akij Group of Industries, Bangladesh in November 2010. After one year, he started his research work in the Centre for Energy Sciences research group at the University of Malaya.

H. H. Masjuki

Professor Dr Masjuki Hj. Hassan obtained his Mechanical Engineering degree (BSc), at Leeds University, Leeds U.K. in 1977. He continued to pursue his MSc in Tribology and PhD from the same university and graduated in 1978 and 1982, respectively. Then, he was appointed as a lecturer in 1983 at the University of Malaya (UM). He is currently appointed as the Professor at Mechanical Engineering Department, UM. He is also one of the senate members of UM and secretary of Council of National Professors Engineering and Technology cluster. He is the founding President of Malaysian Tribology Society (MyTRIBOS) and the Director of the Centre for Energy Sciences.

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1. Introduction

The revolution of the industrialization sector and rising living standards are associated with huge energy consumption. Currently, the most significant growth of energy consumption is taking place in China with a rate of 5.5% per year.¹ The energies are typically consumed for transportation, manufacturing process, industrial facilities, lighting, etc. Meanwhile, transportation and industrial sectors are the main energy usages in Malaysia with 40.3% and 38.6%, respectively.² It is estimated that the total world energy consumption will be increased by 33.5% from 2010 to 2030.³ The existing amount of fossil fuel such as petroleum, coal and natural gas, which is the primary source of energy in the world is decreasing and it is assumed to essentially run out in the next 50 years. Fig. 1 shows approximately 80% of total usage is from fossil fuel.

Fig. 1 Fuels contribution to total world energy consumption.³

The burning of fossil fuel tends to cause environmental pollution as it releases greenhouse gases (GHGs), primarily CO₂ that aggravates global warming. The rise of global temperature caused by global warming could lead to extinction of millions of natural species and also bring harm to the ecosystem. It is shown that the emission of CO₂ has increased around 1.6 times in the last three decades.² Pollutants such as CO₂, NO_x, CO and SO₂ are emitted and they are extremely harmful for humans around the world. Moreover, acid rain, an air pollution has mostly occurred in industrial regions because of lack of control of fuel utilization. Governments are now seeking new policies to empower renewable energy to solve environmental issues. For example, the Kyoto Protocol (KP) 1997 mandates that countries involved in industrial activities must reduce pollutants by at least 5% below 1990 levels.²

The limited sources of non-renewable energy i.e. fossil fuels, which will not be able to sustain the needs for the next generation, leads us to study and discover new sustainable and renewable energy sources such as bio-energy. The bio-energy concept is focused on various renewable energies due to massive exploitation of fossil fuels since their discovery. Bio-energy is a renewable energy that uses natural resources for production of biofuels.⁴ The potential contribution of bio-energy in the future global energy supply ranges from 100 EJ per year to 400 EJ per year in 2050.⁵ Malaysia’s National Energy Policy of 1979 targets to have an efficient, safe, clean and environmentally friendly energy supply in the future.^6,7 Therefore, biofuel is a promising alternative to reduce the reliance on petroleum. An additional factor is less reliance on unstable geopolitics which indirectly improves national energy security.^8,9 This renewable energy source is produced from the natural materials (bio-based) as an alternative compared to conventional petroleum fuels. Therefore, biofuel creates stronger demands on various feedstocks while boosting agricultural economies and producers’ incomes.¹⁰ Biofuel production also encourages rural economies to become stronger. This is because agricultural crops and wastes are being used as the feedstock.¹¹ Therefore, the production of biofuel is a very promising potential energy supply to reduce biomass containing waste and reduce its disposal area.³ In addition, biofuel is available in different forms i.e. liquid or gas.¹² It consists of bioethanol, biodiesel, biogas, bio-methanol, bio-syngas (CO + H₂), bio-oil, etc.⁴ The production of global biofuel has increased three-fold from 2000 to 2007, from 4.8 billion gallons to 16.0 billion gallons. However, it still accounts for less than 3% of transportation fuel supply.¹³

The world has proposed the use of bioethanol and biodiesel as additive sources in liquid transportation fuel.^14,15 Currently, bioethanol is an alternative fuel for gasoline while biodiesel is an alternative fuel for diesel. They are able to reduce toxic emissions i.e. CO, HC, etc. and reduce smog pollution from the exhaust.^8,11,13,16 Nonetheless, conventional gasoline engines need major modifications to perform well with a higher concentration of bioethanol, i.e. pure ethanol (E100) can be used in flex fuel (FFV) engine only.⁸ Besides that, bioethanol is very difficult to handle and use relative to gasoline due to corrosive behaviour. The other option of liquid fuels that is able to replace conventional gasoline in transportation is biobutanol, which can be produced from the same feedstock as bioethanol, i.e. waste biomass or non-agricultural products. Biobutanol is a very competitive biofuel to be used for IC engines because it has many promising physicochemical properties that enhances engine performance. It is also a good potential fuel towards green energy consumption. Although the biotechnological production of biobutanol is much more complicated compared to bioethanol production, biobutanol has more advantages than bioethanol and gasoline. Nevertheless, the development on biofuel requires a large area to produce a huge amount of crops, and this aggravates serious environmental effects, i.e. soil corrosion, fertilizer run off, deforestation, eutrophication and salinity.⁸ In addition, biofuel production costs a lot of money and is ineffective to reduce CO₂ emission if compared to other options.¹¹

The aim of the present work is to review the literature regarding the feasibility of alcohols–gasoline fuel blends on spark ignition (SI) engine performances, combustion analysis and also exhaust emissions that are related to their physicochemical properties. The biofuels that are considered in the present work are; (i) bio-ethanol (ethanol) and (2) bio-butanol (butanol). These biofuels are proposed as potential biofuels regarding their production rate, availability and capability in engine performances and emissions. Numerous studies on ethanol and butanol addition on engine performances and combustion characteristics are discussed in detail. The analysis on engine performances of SI engine will be focused on engine torque, brake power, brake specific fuel consumption, brake thermal efficiency and also exhaust gas temperature. Meanwhile, the experimental findings on in-cylinder gas pressure and heat release rate of combustion will be further discussed in detail. Then, the analysis is followed by the effect on exhaust emissions i.e. CO₂, NO_x, HC and CO with addition of alcohol–gasoline fuel blends. The standard way is used by comparing the physicochemical properties between the alcohols fuel blend with the gasoline as a reference to analyse the effect on engine performance, combustion and exhaust emissions. Therefore, the obtained experimental findings can be explained clearly and effectively.

2. Bioethanol and biobutanol as transportation fuels

2.1 Bioethanol

Ethanol as a fuel is not a new concept as Samuel Morely had developed an engine that ran on ethanol in 1826. Besides that, Henry Ford’s Model T ran on ethanol in 1908. However, demands on ethanol declined after World War I and gasoline has dominated the market since the 1920s. In 1974, Solar Energy Research, Development, and Demonstration Act of 1974 promoted ethanol as a gasoline alternative due to the energy crisis. Ethanol is the most widely used biofuel in transportation due to rising oil price, tremendous risk of climate change, increasing on fuel vehicle demands, security of energy supply. Therefore, governments have authorized new policies to perform researches, develop and deploy more energy sources. In United States, the Energy Policy Act of 2005 shows the most significant steps by mandate the use of ethanol through the Renewable Fuel Standard (RFS).¹⁷ Besides that, the initiation of National Alcohol Fuel Programme (ProAlcool) in Brazil aims to increase the production of bioethanol in order to substitute the high cost and unsustainable petroleum-based products.¹⁴

Generally, ethanol can be divided into two types; bio-based ethanol (bioethanol) and synthetic ethanol. Bioethanol is produced from agricultural food and agricultural wastes i.e. corn, sugar cane, etc. fermenting sugars with yeast while synthetic ethanol is produced via catalytic hydration of ethylene, a petroleum by-product. Bioethanol and synthetic ethanol are the same product as they have the same chemical formulae, C₂H₅OH. Bioethanol or synthetic ethanol is a colourless liquid, transparent, neutral, volatile, flammable, miscible in both water and non-polar solvents and oxygenated liquid hydrocarbons, which has a pungent odour and sharp burning taste.^18,19

The analytical studies towards global ethanol production showed that most ethanol is produced by a fermentation process which contributes 97% while only 3% of ethanol is produced via catalytic hydration of ethylene.^20–22 The largest plants of synthetic ethanol in Germany and Scotland can produce about 4.4 million gallons per year. There are several multinational companies which produce synthetic ethanol i.e. Sasol (in Europe and South Africa), SADAF of the Saudi Arabia, Shell of the UK and Netherlands, BP of the UK and also Equistar (United States).^20,23 The production of synthetic ethanol is economically less attractive as compared to fermentation in USA due to the high cost of ethylene and abundance of raw materials of agricultural products as the feedstocks. Nevertheless, the production of synthetic ethanol is growing in Middle East countries, especially in Iran.

In the United States, more than 7.3 billion gallons of bioethanol were added to conventional gasoline in 2009 to achieve biofuel requirements.²⁴ Fig. 2 shows the graph of world bioethanol fuel production as it reached 4.5 billion gallons in year 2000, then rose up to 22.7 billion gallons in year 2012.²⁵ Bioethanol production is increasing steadily as nations are now looking to reduce oil imports, improve air quality and boost rural economies. In 2011, there are 31 countries in international level and 29 provinces which mandate the use of gasoline–bioethanol blends.²⁶

Fig. 2 Graph of world fuel ethanol production, 1975–2012.²⁵

Bioethanol–gasoline blends are often used in fuel injection engines of light duty vehicles as an alternative to gasoline or are used as fuel additives due to high octane number, faster flame speed, higher HoV and also broader flammability limits. These properties allow higher compression ratios and shorten combustion time, which give more advantages compared to pure gasoline.¹⁴ The burning of bioethanol in SI engine also reduces the emission of HC, CO, NO_x.^11,18 10 vol% ethanol in gasoline (E10) is commercialized by the automakers in United States as conventional vehicle fuel and it is widely known as gasohol by the public.²⁴ In addition, higher concentrations of fuel blends, i.e. 85 vol% ethanol in gasoline (E85) has been used for new FFVs. However, the 85 vol% ethanol blend (E85) cannot be used in a normal gasoline engine. FFVs were launched since 2003 in Brazil. As recorded, almost 90% of new cars which are sold today in Brazil have a flexible fuel engine and the gasoline sold contains 20–25% anhydrous ethanol and 100% hydrous ethanol (4–4.9% of water).^18,27,28 The addition of anhydrous ethanol fuel blend^29–31 and hydrous ethanol blends^32,33 on engine performances is widely studied by researchers. Meanwhile, the United States has nearly 8 million FFVs which can run 85 vol% ethanol blend (E85) on the road with various ranges of models such as sedans, pick-up trucks and minivans.³⁴ Table 1 below shows the usage of bioethanol–gasoline fuel blends in different countries.

Table 1 Bioethanol–gasoline fuel blends used in different countries^a8,35

Country	Bioethanol–gasoline fuel blend
a E2: 2 vol% ethanol–98 vol% gasoline; E3: 3 vol% ethanol–97 vol% gasoline; E4: 4 vol% ethanol–96 vol% gasoline; E5: 5 vol% ethanol–95 vol% gasoline; E7: 7 vol% ethanol–93 vol% gasoline; E7.5: 7.5 vol% ethanol–92.5 vol% gasoline; E7.8: 7.8 vol% ethanol–92.2 vol% gasoline; E8: 8 vol% ethanol–92 vol% gasoline; E8.5: 8.5 vol% ethanol–91.5 vol% gasoline; E10: 10 vol% ethanol–90 vol% gasoline; E15: 15 vol% ethanol–85 vol% gasoline; E20: 20 vol% ethanol–80 vol% gasoline; E25: 25 vol% ethanol–75 vol% gasoline; E75: 75 vol% ethanol–25 vol% gasoline; E85: 85 vol% ethanol–15 vol% gasoline; E100: 100 vol% ethanol or pure ethanol.
Angola	E10
Argentina	E5
Australia	E4: the blends are used in New South Wales
	E5: the blends are used in Queensland
Brazil	E25–E75: higher blends are used for flex fuel vehicles
	E100
Canada	E5: the blends are used nationally in the provinces of British Columbia, Alberta and Ontario
	E7.5: the blends are used in Saskatchewan province
	E8.5: the blends are used in Manitoba province
Colombia	E8
Costa Rica	E7
Ethiopia	E5
Guatemala	E5
India	E5
Indonesia	E3
Jamaica	E10
Malawi	E10
Malaysia	Not available
Mozambique	E10: the blends are used in 2012–2012
	E15: the blends are expected to be used in year 2016–2020
	E20: the blends are expected to be used in year 2021 onwards
Paraguay	E24
Peru	E7.8
Philippines	E10
South Africa	E10
South Korea	Not available
Sudan	E5
Thailand	E5
Turkey	E2
United States	E10 (gasohol): the blends used in Missouri, Montana, Florida, Hawaii, New Mexico, Oregon states
	E70–E85: blends varies with states
Uruguay	E5: the blends are expected to be used in 2015
Vietnam	E5
Zambia	E10

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The other alternative and more competitive biofuel for the use in SI engine is butanol. Butanol was discovered by Wirtz in 1852 as a regular constituent of fusel oil.³⁶ Louise Pasteur then clarified the synthesis of biobutanol at laboratory scale after 10 years in 1861.³⁷ The production of industrial acetone–butanol–ethanol (ABE) by fermentation of molasses and cereal grains using Clostridium acetobutylicum was achieved in 1912–1916 by the chemist Chaim Weizmann, University of Manchester UK.^38,39 The ABE fermentation continuously declined since 1950s, and butanol was produced via petrochemical process due to the lower price of petrochemicals and higher food demands of sugar and starchy grains.³⁶ Because of the high cost, low-yield and slow fermentations process of butanol, it could not compete on a commercial scale so it is only produced synthetically. However, there are many countries and big oil companies that looked at butanol again during the oil crisis in the 1970s. Further reasons are the rising price of petroleum oil and the increasing amounts of GHGs in the atmosphere.

Butanol (or butyl alcohol) is non-poisonous, less corrosive, less prone to water contamination, easily biodegradable and has higher energy content than ethanol, but it has similar energy content with gasoline.⁴⁰ Butanol exists with different isomers with respect to location of –OH and carbon chain structure. They all have the similar chemical properties but can be distinguished by their structures as listed in Table 2 shown below. The physical properties of the butanol isomers are different in octane number, boiling point, viscosity, etc., but the main applications are quite similar for certain usages.

Table 2 Molecular structure and main application of butanol isomers^36,40,41

Butanol isomers	Molecular structure	Main applications
1-Butanol (n-butanol)	CH3CH2CH2CH2OH	Solvent – for paints, resins, dyes, etc.
		Plasticizer – improve plastic process
		Chemical intermediate – for butyl esters or butyl ethers etc.
		Cosmetics – eye makeup, lipsticks
		Gasoline additive
sec-Butanol (2-butanol)	CH3CH(OH)CH2CH3	Solvents
		Chemical intermediate – for butanone, etc.
		Industrial cleaners – paint removers
		Perfumes or in artificial flavors
tert-Butanol	(CH3)3COH	Solvent
		Denaturant for ethanol
		Industrial cleaners – paint removers
		Gasoline additive – for octane booster and oxygenate
		Chemical intermediate – for MTBE, ETBE, TBHP, etc.
Isobutanol	(CH3)2CCH2OH	Solvent and additive – for paint
		Gasoline additive
		Industrial cleaners – paint removers
		Ink ingredient

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Biobutanol becomes an alternative to bioethanol and gasoline as transportation fuels in spark ignition engine due to its advantages in terms of physicochemical properties. Currently, bio-based n-butanol and isobutanol are considered as gasoline components to be blended in higher concentrations without any modification on a conventional gasoline engine.⁴² However, new automobiles FFVs that use 85 vol% ethanol blend (E85) cost a lot of money and are quite unaffordable for most buyers. By contrast, butanol fuel blends are able to replace conventional gasoline in existing cars without modifying the engine’s specifications. Szulczyk⁴³ explained that butanol can be blended with gasoline in any percentage up to 100 vol% of biobutanol in conventional SI engine. In addition, Ramey⁴⁴ successfully demonstrated by moving across America in 2005 and South Dakota in 2007 with pure butanol (Bu100) with unmodified SI engine. Besides that, biobutanol is a less corrosive fuel so it can be easily distributed via existing pipelines or distribution stations as compared to bioethanol. The lower amount of HoV and higher flash point of biobutanol compared to bioethanol is likely to indicate safer handling and usage. Moreover, biobutanol has double the amount of carbon of bioethanol and contains 25% more energy. This results on better fuel consumption of biobutanol relative to bioethanol.³⁶ Biobutanol also reduces exhaust emissions i.e. 95% of HC, 0.01% of CO and 37% of NO_x, relative to gasoline.⁴⁴

There are a few attempts to commercialize the biobutanol as an alternative fuel in transportation sectors by Ramey⁴⁴ with Department of Energy (DOE) from 1998 to 2003. However, it was not clarified as an alternative to gasoline by DOE or National Renewable Energy Laboratory. The International Clostridia Group has strived to acknowledge butanol fermentation for 25 years but this has been ignored by the producers as they are only presently concerned with bioethanol. In Europe, Fuel Quality Directive 2009/30/EC has allowed a maximum of 15 vol% of butanol in gasoline.⁴⁵

2.2 Potential feedstocks for bioethanol and biobutanol production

Several raw materials from biomass are still being studied to discover various available alternatives for biofuel production. On a fundamental basis, the various sources of the feedstocks can be classified into two main categories; (i) first-generation biofuel and (ii) second-generation biofuel.^18,46

The first generation of biofuel feedstock consists of sucrose (i.e. sugar cane, sugar beet, sweet sorghum) and starch rich crops (i.e. corn, milo, wheat, rice, cassava, potatoes, barley). The production of the first-generation biofuel is widely commercialized with approximately 50 billion litres produced annually.⁴⁷ Today, bioethanol from agricultural food crops i.e. corns are used commercially for the blend component in transportation fuel. However, the feedstock of the first generation appears unsustainable due to the increasing demands of biofuel production, and causes the rising of food prices and shortage of these edible materials.⁴⁷

As an alternative, the second generation of biofuels is recommended as an efficient fuel production. The second generation of the biofuel feedstock consists of non-food materials such as lignocellulosic biomass. The lignocellulosic materials consist of crop residues, corn stover, grasses, sawdust, woodchips, etc. It includes seaweed, pineapple leaf, banana peel, jatropha waste, oil palm frond, sugar cane bagasse and other major agro-residues. Cheese whey can be fermented to produce biobutanol only and cannot be utilised for bioethanol production.^43,48,49 The feedstocks are environmentally friendly and have the potential to give novel biofuels, as this feedstock is non-edible, being obtained from cheap raw materials and abundant plant waste biomass. In addition, the feedstocks can improve the energy balance of ethanol because of the reduced usage of fossil fuel energy to yield bioethanol. The cellulosic ethanol contributes to significant reduction of the life cycle of the GHG emissions i.e. CO₂ as these energy crops are nearly carbon neutral.⁴⁶ For example, switch grass can store more carbon in the soil than the agricultural food feedstocks, thus reducing the total GHGs emissions. The un-harvested roots of the grass creates soil organic carbon as the carbon negative.⁵⁰ Besides that, United State Department of Energy’s Centre for Transportation Research studied that the cellulosic ethanol offers highest reduction of GHG emissions compared to corns-derived bioethanol as illustrated in Fig. 3.⁵⁰ However, the cost for cellulosic ethanol production is not very effective due to technical barriers and the challenges that need to be solved before their potential could be realized. The comparison of the petroleum refinery products, and first- and second-generation biofuels production is illustrated in Fig. 4.

Fig. 3 Reduction of greenhouse gas (GHG) emissions from cellulosic bioethanol and corns-derived bioethanol blends.⁵⁰

Fig. 4 Comparison of petroleum refinery products, first- and second-generation biofuels production.⁴⁷

In addition, Asia is reported as the largest potential producer of biofuel from crop residues and waste crops due to higher biomass availability.⁵¹ Table 3 represents the biomass feedstocks and their potential ethanol yields.^46,47 United Kingdom (UK) has almost 148 000 hectares of sugar beet in year 2005 and produces nearly 1.25 million tonnes of sugar which means that it could yield around 6.5 million tonnes of bioethanol. There are also about 1.9 million hectares of wheat grown which produces almost 15 million tonnes of wheat grain and this could yield 4.3 million tonnes of bioethanol. However, the cost for raw materials is highly volatile, which can highly affect the total production cost. The price of the raw materials varies from different studies with the range of US$21–US$61 per metric ton dry matter.⁵² Besides that, the raw materials cost contributes 60–75% of the total bioethanol production cost.

Table 3 Biomass feedstocks and their potential ethanol yield^46,47

Feedstock	Potential ethanol yield, (litre per dry tonne of feedstock)
a Switchgrass alamo whole plant. Source: U.S. Department of Energy Biomass Program, theoretical ethanol yield calculator and biomass feedstock composition and property database.
Corn grain	470
Corn stover	428
Rice straw	416
Cotton gin trash	215
Forest thinnings	309
Hardwood sawdust	382
Bagasse	437
Mixed paper	440
Switchgrass^a	366

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Malaysia is one of the biggest producers of palm oil as it has become the most important commodity crop. This situation results in a great amount of waste production. Palm oil has been listed as the second most used oil in the world since 1985 just below soybean oil.⁶ Bioethanol and biobutanol are potentially produced from palm oil industrial wastes, i.e. palm empty fruit bunches (EFB), oil palm trunks, oil palm fronds and palm oil mill effluent (POME) as raw materials. Although Malaysia is one of the main producers of biodiesel, producers are encouraged to discover and commercialize bioethanol and biobutanol from palm oil waste due to their inexpensive lignocellulosic feedstock and renewable mass sources. In addition, Noomtim and Cheirsilp⁵³ studied the production of biobutanol from EFB that is hydrolysed by Clostridium acetobutylicum. Shukor et al.⁵⁴ also studied the production of butanol from palm kernel cake (PKC) via ABE fermentation by Clostridium saccharoperbutylacetonicum N1–4 using an empirical model. The PKC contains lignocellulose that is composed of 11.6% of cellulose and 61.5% of hemicellulose, including 3.7% of xylan and 57.8% of mannan.^54,55

Other than that, bananas are also one of the potential energy resources for bioethanol as it is the second largest produced fruit, contributing about 16.26% of the world’s total fruit production in 2007.⁵⁶ In 2001, the total planted area of bananas in Malaysia is around 33 704.2 hectares. Tock et al.⁵⁷ found that the maximum amount of potential power generation by banana biomass feedstock is 949.65 W, which is about 4.6% of Malaysia’s total available capacity, 20789 MW in 2007. Therefore, Malaysia is able to achieve this target if bananas are successfully used as an energy feedstock, as required in Fifth Fuel Policy (Eight Malaysia Plan 2001–2005) with the target of 5% of the total energy consumption. Banana has a huge of waste generated from its peels which otherwise may contribute detrimental effects to the environment. This is because commonly the banana peel is improperly disposed. The waste may produce hazardous gases to the environment such as hydrogen sulfide, ammonia, etc. during decomposition. Thus, bioethanol production from the agriculture waste from banana peel can overcome environmental issues. The banana peel produces higher LHV and can be considered as the best raw material to be utilized for fuel. Amylaceous and lignocellulosic materials which are found in the fruit and the organic residue are feedstocks that can be used to produce ethanol via hydrolysis, fermentation and distillation.⁵⁸

2.3 Production of bioethanol and biobutanol

Bioethanol is produced via different routes from several raw materials of feedstocks as shown in Table 4. The first-generation feedstocks i.e. sucrose-rich materials and starch-rich materials can be produced from alcoholic fermentation.^18,59 The starch rich crops consist of long chain polymers of glucose which need additional processing. The processing is by mixing and grinding with water to break down into simpler glucose before it is fermented by yeast into bioethanol, as shown in eqn (1) and (2). The corns-derived bioethanol production is obtained by dry milling and wet milling processes.⁴⁷ In the dry mill process, the starch from the corns is fermented into simple sugar before it is distilled into bioethanol. High value chemicals i.e. fragrances, flavouring agents, and food related products are removed during fermentation. Prior to bioethanol production, two economic valuable ethanol co-products are produced including distiller grains which is used for nutritious livestock feed and also carbon dioxide which is sold for industrial needs. There are three selective species of micro-organisms that can be used in fermentation for ethanol production; yeast (Saccharomyces species), bacteria (Zymomonas species) and mold (mycelium).⁴⁷ An abundance of research has been carried out to discover a supreme micro-organism to produce ethanol from different feedstocks. Practically, about 40–48% of glucose is converted into bioethanol, equivalent to 1000 kg of fermentable sugar producing 583 litre of pure ethanol (sp. gravity at 20 °C = 0.789).^47,60

Table 4 Bioethanol routes from different raw material feedstocks¹⁴

Raw material	Process
Wood	Acid hydrolysis and fermentation
Wood	Enzymatic hydrolysis and fermentation
Straw	Acid hydrolysis and fermentation
Straw	Enzymatic hydrolysis and fermentation
Wheat	Malting and fermentation
Sugar cane	Fermentation
Sugar beet	Fermentation
Corn grain	Fermentation
Corn stalk	Acid hydrolysis and fermentation
Sweet sorghum	Fermentation

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Lignocellulosic biomass consists of cellulose, hemicellulose and lignin as its main components. The biochemical production of lignocellulosic bioethanol has four major steps; (i) pre-treatment, (ii) hydrolysis-enzymatic and acid,⁴⁵ (iii) fermentation and (iv) distillation and evaporation as shown in Fig. 5. The production of lignocellulosic ethanol much more complicated as the pre-treatment process of hemicellulose is needed to increase the hydrolysis yield before it is hydrolysed and fermented into bioethanol. Hamelinck et al.⁶¹ reported that the hydrolysis with pre-treatment yields over 90% while the hydrolysis without pre-treatment yield less than 20%. The common method of dilute or concentrated acid hydrolysis is used to convert lignocellulose into fermentable sugars, and then the hydrolysate is fermented into bioethanol.

Fig. 5 Production process for bioethanol from lignocellulosic biomass.⁵²

Furthermore, bioethanol can be produced by thermochemical conversion.³ The biomass can be converted into bioethanol via two ways either thermochemically or in a biological process. Presently, the lignocellulosic materials are thermo-chemically gasified and the product of synthesis gas is fermented into bioethanol under specific conditions.^14,62

The estimated production cost of bioethanol from lignocellulosic feedstocks is discussed in previous studies^52,63–65 and in more advanced techno-economic evaluations.⁶⁶ The cost for enzymatic hydrolysis process is also a major contributor.⁵² The researches aim to improve the enzymatic hydrolysis with efficient enzymes and reducing enzyme production cost. In addition, the economical production of lignocellulosic bioethanol shows reliable estimated cost in laboratory scales, and is endorsed in pilot and demonstration plants. Developers such as Iogen Corps and Abengoa Bioenergy are currently operating with the demo-scale plants to yield lignocellulosic bioethanol.⁵² Table 5 shows the estimated cost of bioethanol production from different feedstocks.

Table 5 Estimation costs of bioethanol production from different feedstocks (exclusive of taxes)^14,67

	Year 2006	Long term about 2030
a Note range differs from row 1, for several factors such as refinery costs.b Excluding a few outliers above and below the range.
Price of oil, US$ per barrel	50–80
Corresponding pre-tax price of petroleum products, US cents per L	35–60^a
Corresponding price of petroleum products with taxes included	150–200 in EU^b
US cents per L (retail price)	About 80 in USA
Bioethanol from sugar cane	25–50	25–35
Bioethanol from corn	60–80	35–55
Bioethanol from beet	60–80	40–60
Bioethanol from wheat	70–95	45–65
Bioethanol from lignocellulose	80–110	25–65

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Similar to bioethanol, biobutanol can be produced from the same feedstocks, i.e. sugar crops, starch crops and lignocellulosic biomass. The biological production of biobutanol has been invented decades ago but the process is quite expensive compared to the petrochemical hydration process i.e. oxo-synthesis and aldol concentration. Therefore, almost all butanol is produced from petroleum and is known as petrobutanol. However, due to the depletion of fossil-fuel reserves and environmental issues, the interest on sustainable vehicle fuels, especially from non-edible materials, encourages the technological development in biobutanol fermentation.

Biobutanol can be produced via ABE fermentation. This alcoholic fermentation is called ABE fermentation as acetone, butanol and ethanol are the main products. The total concentration of solvents in ABE fermentation stock is 20 g L⁻¹ with butanol around 13 g L⁻¹ (ratio of butanol, acetone and ethanol is 6 : 3 : 1).^68–70 Previously, the cereal grains and sugar feedstocks were utilized for industrial scale by the ABE fermentation process. The study of butanol production by Qureshi and Blaschek⁷¹ pointed that the butanol can be produced at US$0.34 per kg based on the corn price at US$79.23 per ton with the ABE yield of 0.42 and the assumption of the co-products of CO₂ gas will be captured, compressed, and sold. However, the utilization of these food crops into biobutanol was condemned because of possible food shortages. Therefore, the researchers focus on the secondary generation biobutanol due to abundant cheaper raw materials as the feedstocks. The process to produce biobutanol from lignocellulosic feedstocks is illustrated in Fig. 6.^72,73

Fig. 6 Production process for biobutanol from lignocellulosic feedstocks.^72,73

ABE fermentation was the second largest industrial fermentation process after bioethanol by yeast fermentation.⁷⁰ This is basically because of the economic importance of acetone and butanol as petrochemical solvents. The substrates of the feedstocks can be fermented with different strains such as C. acetobutylicum, C. beijerinckii, C. sacharoperbutyl acetonicum and C. saccharobutylicum which are being used to utilize cellulolytic activities.⁷² ABE fermentation of biobutanol production can be done via different modes i.e. batch fermentation, fed-batch fermentation and continuous fermentation processes; free cell continuous fermentation, immobilized cells continuous fermentation and cells recycling and bleeding. Table 6 shows the comparison of the biomass feedstocks via different fermentation processes and their potential butanol yield/productivity. The challenges such as lower butanol titer and product inhibition are being resolved with strain improvement by mutation and genetic engineering. Other than that, it can be resolved with improvement of metabolic activities of the organism in acid producing and solvent producing pathways and also effective continuous fermentation process with promising recovery techniques i.e. gas stripping, distillation, liquid–liquid extraction, etc.^40,70 Presently, a number of developers i.e. Butamax Advanced Biofuels LLC, Swiss Butalco GmbH, American Gevo Inc., ButylFuel LLC and Advanced Biofuels LLC are developing their own fermentation process towards an economical synthesis of biobutanol.⁴⁵

Table 6 Comparison of biomass feedstocks and their potential biobutanol yield/productivity via different fermentation process^40,70

Feedstocks or substrate	Fermentation process	Strain used	Yield (g g^−1)/productivity (g L^−1 h^−1)	Maximum titer of ABE (g L^−1)	Ref.
Barley straw	Batch fermentation	C. beijerinkii P260	0.43/0.39	26.64	73
Wheat straw	Batch fermentation	C. beijerinkii P260	0.41/0.31	21.42	74
	Fed-batch fermentation	C. beijerinkii P260	—/0.36	16.59	75
Corn fibers	Batch fermentation	C. beijerinkii BA101	0.36–0.39/0.10	9.3	76
Corn stover & switchgrass (1 : 1)	Batch fermentation	C. beijerinkii P260	0.43/0.21	21.06	77
Switchgrass	Batch fermentation	C. beijerinkii P260	0.37/0.09	14.61	77
Sago starch	Free cell continuous fermentation	C. saccharobutylicum DSM 13864	0.29/0.85	9.1	78
Degermed corn	Free cell continuous fermentation	C. beijerinkii BA101	—/0.29–0.30	14.28	79
Whey permeate	Immobilized cells continues fermentation	C. acetobutylicum P262	3.5–3.6/0.36–1.10	8.6	80
Corn	Immobilized cells continues fermentation	C. acetobutylicum ATCC 55025	0.42/4.6	12.50 (butanol)	81 and 82
Sugar beet juice		C. beijerinkii CCM 6182	0.37/0.40	—	39

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3. Physicochemical properties of gasoline, bioethanol and biobutanol

The physicochemical properties indicate the quality of fuel to be combusted in SI engine.¹⁸ Table 7 summarizes the comparison of physical and chemical properties of gasoline, bioethanol and biobutanol.

Table 7 Physicochemical properties of gasoline, bioethanol and biobutanol

Property	Gasoline	Bioethanol	Biobutanol	Ref(s).
Chemical formula	∼C8H15.6	C2H5OH	C4H9OH	83
Molecular weight	100–105	46.07	74.12	84, 85
Oxygen content (wt%)	0	34.73	21.59	40, 43
Reid vapor pressure, RVP (kPa)	45–90	17	2.3	40
Research octane number, RON	95	106–130	94	40, 86
Motor octane number, MON	85	89–103	80	40
Molar mass (kg kmol^−1)	111.21	46.07	74.12	87, 88
Specific gravity at 20 °C	0.7392	0.7894	0.8097	87, 88
Boiling point (°C)	30–215	78.3	117.7	40
Flash point (°C)	−43	8	35	87
Auto ignition temperature (°C)	257	423	365	87
Stoichiometric air–fuel ratio	14.8	9	11.1	40
Adiabatic flame temperature (°C)	1970	1923	1960	87
Density (kg m^−3)	720–750	794	809	40
Low heating value, LHV (MJ kg^−1)	44.4	28.9	33.1	40
Heat of vaporization, HoV (MJ kg^−1)	0.32	0.92	0.71	40
Kinematic viscosity at 20 °C (mm^2 s^−1)	0.4–0.8	1.5	3.6	40

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3.1 Oxygen content

Oxygenated fuel i.e. alcohol fuel has higher oxygen content i.e. biobutanol has 21.59% oxygen and bioethanol has 34.73% oxygen, which promote higher complete combustion and lower exhaust emissions.^40,43

3.2 Octane number

Bioethanol has higher octane number compared to biobutanol and gasoline.⁴³ High-octane fuel prevents premature ignition that causes knocking which can damage the engine. The higher octane rating gives bioethanol advantages in improving the thermal efficiency. However, it emits 2–4 times higher level of acetaldehyde than gasoline, and hence is highly corrosive.¹⁶

3.3 Reid vapour pressure

Alcohol fuels, i.e. bioethanol and biobutanol have lower RVP as compared to gasoline, thus they bring problems when starting a cold engine especially during cold weather.^8,10,32 However, bioethanol is easier to evaporate relative to biobutanol. It means that it emits more volatile organic compound into the atmosphere as pollution especially during hot summer days. This volatile organic compound along with NO_x gases is converted by ultraviolet radiation into ground ozone pollution.⁴³ Thus, lower vapour pressure of biofuel brings both benefits and disadvantages to the performance.

3.4 Lower heating value

Carbon, C and hydrogen, H are liable to raise the heating value when the oxygen is declining during combustion. The energy content of biobutanol is approximately 82% of gasoline energy, while bioethanol has 65% of gasoline energy. Therefore, these biofuels have higher fuel consumption as compared to gasoline.

3.5 Density

The density of bioethanol and biobutanol are 794 kg m⁻³ and 809 kg m⁻³, respectively, which are higher than gasoline, which results in enhancing the volumetric fuel economy.

3.6 Boiling point

As the carbon chain length increases, the boiling point of alcohols increases. The boiling point of biobutanol and bioethanol are 117.7 and 78.3 °C, respectively. The boiling point of each alcohol influences their evaporative behaviour.

3.7 Heat of vaporization

HoV of bioethanol and biobutanol are quite higher than for gasoline, thus reducing the air–fuel mixture temperature during the intake stroke. Higher HoV improves knock resistance and achieves better volumetric efficiency of the engine. However, higher HoV of bioethanol and biobutanol leads to problems with engine start-up including when a running cold engine especially during cold weather due to the cooling effect of the air–fuel mixture at ambient temperature.^40,89 Besides that, higher latent HoV promotes higher emissions of organic gases.⁹⁰

3.8 Viscosity

Viscosity of biobutanol and bioethanol is higher than gasoline. These properties may adversely effect the fuel injection system due to higher flow resistance at lower temperature.⁴⁰

4. Effect of bio-based alcohols–gasoline fuel blends on SI engine

4.1 Engine performance

The performance of an engine can be defined as the maximum power or the maximum torque available at each speed within the engine operating ranges.⁹¹ Many studies have been carried out on SI engines vehicles using various alcohols–gasoline fuel blends to determine engine performances i.e. torque, brake power, brake specific fuel consumption (BSFC), brake thermal efficiency (BTE) and exhaust gas temperature (EGT). In this review, the effects of addition of ethanol and butanol in fuel blends on the SI engine performances are the main focus and will be discussed in detail.

4.1.1 Torque and brake power. By definition, torque, T is a turning force produced by the pressure from crankshaft of the piston. It depends on the length of the engine stroke, charge condition and average effective cylinder pressure.⁹² Meanwhile for brake power, BP is the power output produced by the engine without the power loss caused by the gear, transmission, friction, etc. This power output is called as brake power because the brake is used to slow down the shaft inside the dynamometer. The brake power can be expressed as in eqn (3);

Brake power, BP (kW) = 2πNT (3)

where N = engine speed (rpm), T = torque (N m⁻¹).

A large amount of literature acknowledges that the addition of ethanol in gasoline fuel blend increases the torque as compared with pure gasoline in SI engine. The oxygenated fuel such as ethanol in gasoline fuel blend produces a leaning mixture to increase the air–fuel equivalence ratio (λ) so that it promotes a better combustion and consequently produces a higher torque output.^31,93–96 This was revealed by the fundamental studies done by Hsieh et al.,⁹³ who determined the torque output yield by SI engine at 1000–4000 rpm of engine speed and full throttle condition is caused by leaning mixture created by ethanol in gasoline fuel blends. As is evident, 5 vol% up to 30 vol% ethanol in gasoline (E5–E30) gives higher torque output especially at higher speed of 4000 rpm and lower throttle valve opening at 20%. A typical result was reported by Wu et al.,⁹⁶ found that the lambda, λ could be reached under leaner condition as ethanol content is increased without changing the throttle opening and injection strategy. In support to this finding, the higher ethanol content gasoline fuel blend, i.e. 30 vol% of ethanol in gasoline (E30) gives highest torque at 4000 rpm of engine speed and 0.95 of λ. The same results were achieved by Masum et al.,⁹⁴ who investigated the torque performance of different volume denatured anhydrous ethanol (DAE)–gasoline blends; 10, 20, 30 and 50% by volume with gasoline. The higher oxygen content of DAE blend gives better complete combustion, thereby giving higher engine torque. Furthermore, the addition of ethanol promotes high brake power, caused by the faster flame speed. The brake power slightly increases by addition of DAE in gasoline fuel blends especially at high speed. However, it was observed that there is no significant change in brake power for the low engine speed with respective fuel changes.

The ethanol content in gasoline fuel blend increases the torque due to high latent heat of vaporization (HoV) of ethanol.^31,92,94,97 This provides lower temperature intake manifold and volumetric efficiency. As the HoV of ethanol is high, the charge temperature is lowered as the ethanol evaporates. A comprehensive study was done by Saridemir,⁹⁸ who discovered the torque and brake power increase as the ethanol content increases because ethanol has three times the evaporation rate of gasoline and better combustion performance.

In addition, the higher octane number of ethanol leads to higher torque of the engine as revealed by Masum et al.⁹² The experiment showed that the engine torque produced by 15 vol% ethanol in gasoline (E15) is the highest among the all blends of maximum octane number (MaxR), maximum petroleum displacement (MaxD), maximum heating value (MaxH), though the ethanol has lowest LHV. The enhanced octane number of 15 vol% ethanol blend (E15) improves the torque performance. Higher octane number leads to ignition delay that decelerates energy release rate and reduces the heat loss from the engine.⁹⁹

However, there is contradiction on the torque and brake power performances as Yüksel et al.¹⁰⁰ observed the reduction of engine torque and power output while using ethanol–gasoline blend at various engine speeds ranging from 1538 to 3845 rpm at different throttle opening position. The torque and power output of the engine decrease due to lower calorific values of ethanol–gasoline blend over pure unleaded gasoline (22.771 MJ kg⁻¹ for ethanol, 44.001 MJ kg⁻¹ for gasoline).^84,100

Limited studies were carried out on another potential gasoline alternative which is biobutanol on SI engine.¹⁰¹ The researchers concluded that the biobutanol promotes a standard level performance compared with the gasoline in SI engine and diesel in CI engine. This was examined by Xialong et al.¹⁰² who found that the torque of 30 vol% butanol in diesel is comparable to regular leaded gasoline at a low speed. However, the torque obviously drops at higher speed. The torque reduction at high speed was attributed to higher volumetric efficiency of biobutanol and much longer combustion delay due to its greater latent HoV. A similar case applies if the biobutanol is used as an alternative fuel to diesel in a single cylinder compression ignition (CI) engine as revealed by Al-Hasan and Al-Momany.¹⁰³ The brake power reduces with the respective isobutanol–diesel fuel blends because of the higher HoV of isobutanol. The combustion temperature decreases as the air–fuel mixture temperature at the beginning of the combustion stroke is lower.

4.1.2 Brake specific fuel consumption. Brake specific fuel consumption (BSFC) is simply a measure for the fuel efficiency of any reciprocating engine which indicates the usage of fuel during operation. In other words, BSFC is the ratio of the rate of fuel consumption to the brake power (g kW⁻¹ h⁻¹). Many manufacturers tried to determine a fixed engine operation with the least fuel consumption while still producing higher power. Obviously, lower amount of BSFC is desirable. The typical best value of brake specific fuel consumption for SI engine is about 75 × 10⁻⁶ g J⁻¹ = 270 g kW⁻¹ h⁻¹.⁹¹

The researchers revealed that the addition of ethanol in gasoline fuel blends gives negative feedback in terms of fuel consumption. The BSFC of the ethanol is increasing due to lower LHV of ethanol as compared to gasoline. As referred to Table 3, the LHV of the ethanol and gasoline are 28.9 and 44.4 MJ kg⁻¹, respectively. The experiments done by Koç et al.³¹ showed that the BSFC of 50 and 80 vol% ethanol blends (E50 and E80) are much higher than gasoline. The increment of BSFC is mainly caused by the percentage of ethanol in gasoline. The energy content of the ethanol is approximately 35% less than gasoline, thus more fuel is needed to produce the same amount of engine power.¹⁰⁴ In addition, Saridemir⁹⁸ showed the energy content of ethanol is approximately 25% less than gasoline. As the ethanol content in gasoline fuel blend increases, the energy content decreases. Therefore, more fuel is needed to produce the same power output at similar operating conditions. A similar effect of ethanol addition on BSFC has been found by other researchers.^87,105–107 Some researchers highlighted the higher density of alcohol may lead to increase in BSFC, but it is not explained in detail.^92,94,108 Furthermore, Dhaundiyal¹⁰⁹ reported BSFC increases with the increasing of volumetric percentage of ethanol due to bigger volumetric percentage of water and solubility with higher pressure, thereby enhancing the formation of azeotropes.

Interestingly, there is a contradiction revealed by Al-Hasan,²⁹ who found the BSFC of ethanol–gasoline blends were decreasing as the ethanol content increased up to 20 vol% ethanol blend (E20). The significant reduction of BSFC on the addition of ethanol as fuel additive to gasoline was stated to be caused by increased engine brake thermal efficiency behaviour.

The addition of biobutanol in gasoline fuel blends also promotes higher BSFC as compared with pure gasoline. The increase of BSFC from biobutanol fuel blend is caused by its lower calorific value. Biobutanol has greater LHV value than bioethanol but is comparable with gasoline. Varol et al.¹¹⁰ found the BSFC of 10 vol% of butanol, 10 vol% of ethanol and 10 vol% of methanol in gasoline (Bu10, E10 and M10) are higher than gasoline due to their lower energy content. Therefore, it is clearly observed that a greater amount of alcohol–gasoline fuel is required to achieve an equivalent energy relative to gasoline, and more fuel injection is required to maintain the torque. Similar results are reported by Pukalskas et al.¹¹¹ and Dernotte et al.⁸³ as the BSFC of biobutanol was higher with the increasing of biobutanol content in fuel blends, thereby more fuel is needed. The same case applies if different types of alcohols are used as reported by Masum et al.,¹⁰⁸ who concluded that the alcohols with higher carbon numbers, i.e. butanol has greater LHV because LHV increases with carbon number, thus less fuel blends are needed to yield the same engine power.

4.1.3 Brake thermal efficiency. Brake thermal efficiency is a measure of the efficiency or completeness of the engine to produce brake power from the thermal input over the fuel amount supplied. The BTE of the engine is calculated as following, eqn (4).¹¹²

Numerous studies have showed the effects of ethanol addition in spark ignition engine on brake thermal efficiency. The BTE can be improved with the oxygen content of the fuel and heat of vaporisation (HoV).¹¹³ In fact, alcohol–gasoline with low carbon number, i.e. methanol and ethanol has greater oxygen content than those blends with high carbon number, i.e. butanol and pentanol. Thus, the BTE value for low carbon number is greater than high carbon number of alcohols. The higher oxygen content of the fuel enhances the complete combustion; therefore the BTE is improved.⁹⁹ The ethanol–gasoline blend was proven to produce higher BTE than butanol–gasoline blend and pure gasoline as studied by Masum et al.¹⁰⁸

Interestingly, Yacoub et al.⁸⁷ reported that ethanol–gasoline blend gives higher thermal efficiency BTE relative to gasoline but with higher carbon alcohols, i.e. butanol–gasoline blend decreases the thermal efficiency relative to gasoline. Similar trend of BTE has been proven by Ansari et al.,¹¹⁴ who determined the BTE of ethanol fuel blends enhances the thermal efficiency due to better combustion efficiency. The BTE gradually increases at high brake power and decreases at low brake power with low percentages of ethanol. In contrast, Varol et al.¹¹⁰ concluded that the BTE for all alcohol–fuel blends are lower than for pure gasoline. It was recorded that the BTE of 10 vol% of methanol and 10 vol% of ethanol in gasoline (M10 and E10) are 4.5–6.8% lower and 10 vol% of butanol in gasoline (Bu10) is 2.8% lower than pure gasoline. In spite of that, all the fuel blends give the same BTE at lower speeds.

In fact, the fuel continues to vaporize in the compression stroke at high latent heat of vaporisation (HoV). During vaporization, the fuel absorbs the heat from the cylinder and the air–fuel mixture will be compressed easily hence improving BTE. The pressure and temperature decreases at the beginning of combustion as the ethanol content increases. This tends to increase the indicated work, i.e. increase the indicated efficiency. Al-Hasan²⁹ found that the thermal efficiency is improved with increasing of ethanol content in gasoline blends up to 20 vol% ethanol blend (E20) for all engine speeds. The typical finding was reached by Khieralla et al.¹¹⁵ who determined the highest thermal efficiency is achieved by 15 vol% ethanol blend (E15) compared to gasoline.

4.1.4 Exhaust gas temperature (EGT). Exhaust gas temperature (EGT) is a significant indicator of the cylinder temperature. EGT is also used to analyse the exhaust emission. In other words, EGT is a function of combustion temperature. The combustion temperature is also closely related to the heating value of the fuel (LHV). In addition, the formation of oxides of nitrogen, NO_x basically depends on combustion temperature.¹¹⁶ Therefore, the effect of alcohol content in fuel blends on EGT is fairly important due to advantageous properties of the alcohols. An alcohol such as ethanol which contains lower LHV yields lower combustion temperature that causes a reduction of EGT. Ansari and Verma¹¹⁴ had clearly observed reduction of EGT with increase of ethanol percentage. The heating value of ethanol is less than that of gasoline thereby reducing the combustion temperature and EGT.

Besides that, the reduction of EGT is caused by the oxygen content of alcohol in gasoline fuel blends. An oxygenated alcohol such as ethanol gives more advanced combustion hence reducing the exhaust temperature. Saridemir⁹⁸ observed the increase of oxygenated ratio in fuel blends, i.e. ethanol that reduced the EGT. Topgül et al.¹¹⁷ proposed that a greater amount of ethanol in fuel blend may reduce the exhaust temperature due to more efficient conversion process of heat to work. As evidence, 60 vol% of ethanol in gasoline (E60) showed lower exhaust temperature than pure gasoline. Moreover, higher latent heat of vaporisation (HoV) of ethanol than gasoline causes the reduction of exhaust temperature. More heat is absorbed by ethanol from the cylinder when it is vaporised. Therefore, the adiabatic flame temperature will decrease.¹¹⁸ The typical result was found by Elfasakhany¹¹⁹ who identified the improvement of EGT as the percentage of ethanol in fuel blend increases due to higher latent heat of vaporization of ethanol than gasoline.

The effect of EGT for addition of butanol in fuel blends shows an insignificant reduction in comparison to gasoline. More temperature drop takes place in the cylinder charge at the intake valve closure since butanol has higher HoV value than gasoline. Thus, it promotes reduction exhaust gas temperature at the end of the combustion. Singh et al.¹²⁰ reported that high concentration butanol–gasoline blends reduce the exhaust temperature relative to pure gasoline. Interestingly, a contradictory finding was revealed by Varol et al.¹¹⁰ as 10 vol% of butanol in gasoline (Bu10) gives higher EGT due to its high heating value and lower HoV that contributes to the high temperature of combustion. Table 8 summarizes the results of the effects of ethanol and butanol addition into gasoline blends on T, BP, BSFC, BTE and EGT from different researchers.

Table 8 Effects of ethanol and butanol addition into gasoline blends on T, BP, BSFC, BTE and EGT^a

Engine	Tested blend fuel	Operating condition	T	BP	BSFC	BTE	EGT	Ref
a MPI/MPFI = multiport fuel injection, DI = direct injection, MPEI = multipoint electronic injection, MFIE = multiport fuel injection engine, EFI = electronic fuel injection, C = cylinder, S = stroke, d = bore, CR = compression ratio, WC = water cooled, AC = air cooled, ↓ = decrease, ↑ = increase, E = ethanol, M = methanol, Bu = butanol, Pr = propanol, Pe = pentanol, DAE = denatured anhydrous ethanol, MaxH = maximum heating value optimum blend, MaxR = maximum research octane number optimum blend, MaxD = maximum petroleum displacement optimum blend.
Hydra, 1C, CR: 5 : 1–13 : 1, d = 80.26 mm, s = 88.9 mm	E0, E50, E85	Varying speed (1500–5000 rpm); CR: 10 : 1 and 11 : 1	E% ↑, T ↑	E%, BP ↑	E% ↑, BSFC ↑	—	—	31
		100% WOT throttle
New Sentra GA16DE, 1600 cc, d = 76.0 mm, s = 88.0 mm, CR = 9.5	E0, E5, E10, E20, E30	Varying speed (1000–4000 rpm)	E%, T ↑	—	Unchanged due to fuel injection strategy	—	—	93
		Varying throttle (0–100%)
GA6D, 4C, 1594 cc, CR = 10 : 1, MPI	DAE10, DAE20, DAE30, DAE50, gasoline	Varying speed (1000–4000 rpm)	DAE% ↑, T ↑	DAE% ↑, BP ↑	DAE% ↑, BSFC ↑	DAE% ↑, BTE ↑	—	94
		Throttle fixed at 50%
8V–4C inline SOHC, CR = 9.7, d = 71 mm, s = 83.6 mm, 1323 cc	E5, E10, E15, E20 (potato waste bioethanol)	Varying speed (1000–5000 rpm)	E%, T ↑	E%, BP ↑	E% ↑, BSFC ↑	E% ↑, BTE ↑	—	95
New Sentra GA16DE, 4C, 8V-DOHC, 1600 cc, d = 76 mm, s = 88 mm, CR: 9.5	E0, E5, E10, E20, E30	Varying speed (3000, 4000 rpm)	E% ↑, T ↑	—	—	—	—	96
		Varying throttle (0–100%)
		WOT
Proton Campro, 4C, MPEI, 1596 cc, d = 78 mm, s = 84 mm, CR = 10 : 1	Alcohol blends (MaxR, MaxH, MaxD, E15), gasoline	Varying speed (1000–6000 rpm)	T of alcohol blends > gasoline	—	BSFC of alcohol blends > gasoline	BTE of alcohol blends > gasoline	EGT of alcohol blends < gasoline	97
Proton Campro, 4C, 1596 cc, d = 78 mm, s = 84 mm, CR = 10 : 1	Alcohol blends (MaxR, MaxH, MaxD, E15) gasoline	Varying speed (1000–6000 rpm)	T of alcohol blends > gasoline	—	BSFC of alcohol blends > gasoline	—	—	92
		100% load condition
Gunt CT 100.20, 1C, AC, 4S, CR: 7 : 1, 400 cc, d = 87.3 mm, s = 66.7 mm	E0, E10, E20, E30, E40	Speed at 2500 and 3250 rpm	E% ↑, T ↑	E%, BP ↑	E% ↑, BSFC ↑	—	E% ↑, EGT ↓	98
		Full throttle 100%
		Engine oil temp 90 °C
Opel record L, WC, 4C, 1668 cc, d = 74 mm, s = 85 mm, CR = 8 : 1	E60, UG	Varying engine speed (idle to max speed)	E% ↑, T ↓	E% ↑, BP ↓	—	—	—	100
		Varying throttle 25–100%
1S, fuel injection, d = 56 mm, s = 49.5 mm, CR = 9.2	Bu30, gasoline	Cooling water temp. 80 °C	TBu30 is comparable to Tgasoline.	BPBu30 is comparable to BPgasoline.	Bu% ↑, BSFC ↑	—	—	102
		Oil temperature > 70 °C
		Varying speed 3000–8500 rpm
4C, 1596 cc, d = 78 mm, s = 84 mm, CR = 10 : 1, MPEFI	M20, E20, P20, Bu20	Varying speed (1000–6000 rpm)	T of alcohol blends > gasoline	—	BSFC of alcohol blends > gasoline	BTE of alcohol blends > gasoline	EGT of alcohol blends < gasoline	108
		At 100% load condition
SI engine, WC, CR = 8, 256.56 cc, d = 70 mm, s = 66.7 mm	E0, E10–E35	Varying engine speed 1000–1500 rpm	—	—	E% ↑, BSFC ↑	E% ↑, BTE ↓	—	109
	Bu0–Bu35					Bu% ↑, BTE ↓
Lombardini LM 250, 4S, 1C, 250 cc, CR = 6–10, AC & WC	E0, E25, E50, E75, E100	CR = 6/1, speed 2000 rpm	—	E% 50↑, BP ↑	E% ↑, BSFC ↑	—	—	106
		Full throttle		>E50, BP ↓
4S, 4C, WC, Eddy current dynamometer, CR = 9.2 : 1, d = 68.5 mm, s = 72 mm	E0, E5, E10, E15, E20	Varying speed (2100–5000 rpm)	—	—	E%↑, BSFC ↑	—	—	105
Toyota-Tercel-3A, 4S, 4C, 1452 cc, CR = 9 : 1	E0, E2.5–E25	Varying speed (1000–4000 rpm)	E% ↑ max 20%, T ↑	Unchanged	E% ↑ max 20%, BSFC↓	E% max 20%, BTE ↑	—	29
Ford, 4C, 4S, EFI, d = 89 mm, s = 95 mm, CR = 11.1	M10, E10, Bu10, gasoline	Varying speed (1000–4000 rpm)	—	—	BSFC of alcohol blends > gasoline	BTE of alcohol blends < gasoline	EGT of alcohol blends > gasoline	110
		Adjust throttle position to maintain the same brake T
Otto engine, 1392 cc, CR = 9.5, d = 73.5 mm, s = 82 mm	Bu0, Bu30, Bu50	Varying speed (2500–4000 rpm)	—	—	Bu% ↑, BSFC ↑	—	—	111
Honda D16Z6 engine, 4C, 16 valves, 1600 cc, CR = 9.6	Bu0, Bu20, Bu40, Bu60, Bu80	Speed at 2000 rpm	—	—	Bu% ↑, BSFC ↑	—	—	83
		BMEP of 262 kPa
		IMEP of 3.2 bars
LPGE (DATSU LT 200), 1C, 4S, AC, CR = 8.5	M100, E100, gasoline	Varying speed	T of alcohol blends > gasoline	—	BSFC of alcohol blends > gasoline	BTE of alcohol blends > gasoline	—	113
		Full throttle
Waukesha, 1C, d = 8.26 cm, s = 11.43 cm, 612 cc	Gasoline, M, E, Bu, Pe and Pr blends	Speed 1000 rpm	—	—	BSFC of all blends > gasoline, except E2.5	No. carbon of alcohol ↑, BTE ↓	—	87
		Wide open throttle
1C, 4S, enfield, AC, WC, d = 70 mm, s = 60 mm, CR = 10	E0, E20, E40, E60, E80, E100	Varying engine load (0, 440, 880, 1320, 1760, 2200 W)	—	—	E% ↑, BSFC ↑	BTE alcohol blends > gasoline	E% ↑, EGT ↓	114
		Constant speed 2800 rpm
Honda EMS3000 gasoline generator, 1C, 4S, AC, 272 cc, d = 76 mm, s = 95 mm	E0, E10, E15, E20, E25	Varying loads (0–100%)	E% ↑, T ↓	E% ↑, BP ↓	E% ↑, BSFC ↓	E% ↑, BTE ↓	—	115
Hydra, 1C, CR: 5 : 1–13 : 1, d = 80.26 mm, s = 88.9 mm	E0, E10, E20, E40, E60	Varying CR 8 : 1, 9 : 1, 10 : 1	E% ↑, T ↑	—	E% ↑, BSFC ↑	—	E% ↑, EGT ↓	117
		Constant speed 2000 rpm
		Full throttle
1C, 4S, AC, d = 65.1 mm, s = 44.4 mm, CR = 7, 600 cc	E0, E3, E7, E10	Varying speed (2600–3500 rpm)	E% ↑, T ↑	—	BSFC of blends > gasoline	—	E% ↑, EGT ↓	119
		Full throttle
Zen/Maruti Suzuki, 3C, 4S, WC, MPFI, 993 cc, d = 72 mm, s = 61 mm, CR = 8.8	Bu0, Bu5, Bu10, Bu20, Bu 50, Bu75	Varying speed (1500–4500 rpm)	—	—	Bu% ↑, BSFC ↑	Bu% ↑, BTE ↓	Bu% ↑, EGT ↓	120
		Varying torque (0–66 Nm)

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4.2 Combustion analysis

In a spark ignition (SI) engine, the combination of air and fuel flow past the valve into the combustion chamber and cylinder during intake stroke. Then the air–fuel mixture is compressed during the compression stroke. After that, the combustion is initiated by an electric discharge of the spark plug at the end of compression stroke under normal operating condition. The spark ignited flame moves steadily across the premixed air–fuel mixture until it reaches the combustion chamber walls before it is extinguished through the exhaust. Consequently, the combustion analysis is an important characteristic to be considered. Better combustion characteristics of the fuel will lead to higher torque and power. The characteristics of combustion, i.e. mass fraction burned, heat release rate and combustion duration were calculated from the in-cylinder pressure curve data.

4.2.1 In-cylinder pressure. Combustion analysis is the basic analysis for pressure and volume of the system. The in-cylinder pressure (ICP) versus crank angle is a significant characteristic of combustion analysis of an IC engine. The ICP varies with crank angle based on the results from the cylinder volume changes, combustion, heat transfer to the chamber walls, flow in and out of crevice regions and leakages.⁹¹ Numerous studies showed that the addition of ethanol gives variations on maximum in-cylinder pressure. Melo et al.,²⁸ showed that high ratio blends of hydrous ethanol–gasoline promotes maximum ICP due to the higher octane number of ethanol that promotes higher spark timing angle, especially for bigger load and high speed operating conditions. A similar result as reported by Balki et al.¹¹³ for two different alcohols; methanol and ethanol added separately into gasoline. Both alcohols have higher octane number and laminar flame speed than gasoline. Therefore, shorter period of time is taken for the combustion. In addition, higher latent heat of vaporisation of the alcohol fuels gives higher volumetric efficiency and BTE, thereby reflecting higher ICP. The combination of advanced combustion and higher laminar flame velocity of 0–100 vol% ethanol fuel blends cause faster combustion and reduce combustion initiation duration and also enhances the ICP (and therefore temperature).¹²¹ Moreover, a combination study of simulation and experimental analysis was conducted by Deng et al.¹²² with butanol–gasoline fuel blends who showed that the addition of oxygenated fuel such as butanol yields faster burning velocity so it gives higher peak of ICP for all engine speeds.

4.2.2 Heat release rate. Heat release rate (HRR) is the rate at which heat is generated during combustion. Heat release rate is calculated from the first law of thermodynamics during a cycle. The effect of alcohol addition in gasoline on HRR was studied by Siwale et al.,¹²³ who determined the increase in heat release rate with further spark timing. Faster burning rate in alcohols–gasoline blends is attributed to higher rate of heat release. The result obtained is similar as reported by Masum et al.⁹² as the HRR starts to increase earlier for all alcohol–gasoline fuel blends due to the faster flame speed of alcohols, and thereby the duration for the combustion is shortened. The combustion duration is decreasing as the ethanol ratio in the fuel blend increases. This is mainly because of the presence of oxygen within ethanol that contributes to faster flame speed as revealed by Turner et al.¹²¹ Alcohols such as ethanol enhance the combustion initiation and stability, thus increases the HRR.

The effect of biobutanol addition into gasoline fuel blend on the heat release analysis was reported by Deng et al.¹²⁴ as an efficient combustion process was achieved at the optimal operating parameters with increasing butanol fuel blend ratio. The oxygen content and leaner fuel–air mixture of biobutanol give more complete combustion and thus improve its combustion efficiency and HRR. Table 9 summarizes the results of the effects of ethanol and butanol addition into gasoline blends on ICP and HRR from different researchers.

Table 9 Effect of ethanol and butanol addition into gasoline blends on in cylinder pressure ICP and HRR^a

Engine	Tested blend fuel	Operating condition	ICP	HRR	Ref
a MPI/MPFI = multiport fuel injection, DI = direct injection, FFV = flex fuel vehicle, C = cylinder, S = stroke, d = bore, CR = compression ratio, WC = water cooled, AC = air cooled, ↓ = decrease, ↑ = increase, E = ethanol, M = methanol, Bu = butanol, MaxH = maximum heating value optimum blend, MaxR = maximum research octane number optimum blend, MaxD = maximum petroleum displacement optimum blend.
1.4L, Fiat FFV engine	E0, E30, E50, E80, E100	Varying speed (1500–4500 rpm)	E% ↑, ICP ↑	E% ↑, HRR ↑	28
		Varying torque (60, 105 Nm)
LPGE (DATSU LT 200), 1C, 4S, AC, CR = 8.5	M100, E100, gasoline	Varying speed	ICP of alcohol blends higher than gasoline	HRR of alcohol blends higher than gasoline	113
		Full throttle
1C, 4S SI engine, d = 56 mm, s = 49.5, 121.9 cc, CR: 9.2	Bu35, gasoline	Full load	Bu35 gives higher ICP than gasoline	—	122
		Varying speed (3000–8500 rpm)
Suzuki RS-416 1.6L, model T10M16A, d = 78 mm, s = 83 mm, CR = 11.1, 4C, 4V, MPI	Gasoline, M20, M70, M53Bu17	Lambda 1.1	Alcohols give higher ICP than gasoline	Alcohols give higher HRR	123
		Constant speed 2500 rpm
		Load 2.4–7.8 bars
Proton Campro, 4C, 1596 cc, d = 78 mm, s = 84 mm, CR = 10 : 1	Alcohol blends (MaxR, MaxH, MaxD, E15) gasoline	Varying speed (1000–6000 rpm)	ICP of alcohol blends higher than gasoline	HRR of alcohol blends higher than gasoline	92
		100% load condition
4V, 1C, d = 90 mm, s = 88.9 mm, CR = 11.5 : 1, DI	E0, E10, E20, E30, E50, E85, E100	Constant speed 1500 rpm	Ethanol have higher ICP than gasoline	E% ↑, HRR ↑	121
		Constant load 3.4 bar IMEP
4S, 1C, AC, 121.9 cc, d = 56 mm, s = 49.5 mm, CR = 9.2	Bu30, Bu35, Bu0	Full load	Bu% ↑, ICP ↑	Bu% ↑, HRR ↑	124
		Varying speed (3000 to 8500 rpm)

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4.3 Exhaust emission

Exhaust emission is an undesirable element i.e. flue gas that is emitted and discharged into the air as a result of fuel combustion in the internal combustion engine. Excessive release of the undesirable foreign substances into the air will aggravate the air quality, which can cause acid rain, health problems and also cause damage to the ecosystem. Caiazzo et al.¹²⁵ observed that road transportation contributes up to 53 000 premature deaths per year in the United States due to exhaust emissions. In the United Kingdom, the pollution experts from MIT, Massachusetts have observed almost 5000 premature deaths per year are caused by exhaust emission from vehicles, which is more than twice that from traffic accidents.¹²⁶

The combustion gases consist of non-toxic gases, i.e. nitrogen (N₂), water vapour (H₂O) and also carbon dioxide (CO₂) that contributes to global warming. Other toxic and very harmful gases such as carbon monoxide (CO) are discharged owing to incomplete combustion; hydrocarbons (HC) arise from unburned fuel, nitrogen oxides, NO_x result from high combustion temperatures, and ozone (O₃) and also particulate matters (PMs), i.e. soot, also are found. Fig. 7 depicts the proportion data of emissions yield by SI engine.¹²⁷ The amounts of these emissions also depend on the engine design including operating conditions.

Fig. 7 Pie chart of exhaust emissions in SI engine.¹²⁷

4.3.1 Carbon dioxide emission. In SI engine, the gases produced from the combustion of the fuel and air mixture are called exhaust gases. One of the largest amounts of exhaust gases is carbon dioxide (CO₂). With sufficient air, hydrocarbons will burn and generate heat to form CO₂ and water. CO₂ is a primary greenhouse gas emitted through human activities such as transportation. The CO₂ emission from motor vehicles is the main factor of anthropogenic influence that increases the CO₂ concentration in the atmosphere. The effect of excessive CO₂ in the atmosphere is that the earth’s temperature will be continuously rising, leading to climate changes and global warming. As a consequence, several efforts are being carried out by many agencies i.e. EPA and National Highway Traffic Safety Administration (NHTSA) of United States in order to produce a new generation of clean motor vehicles to reduce greenhouse gas emissions. This is achievable by improving the fuel usage for on-road vehicles and engines. An effective way to reduce CO₂ emission is by producing more energy from renewable sources, and in addition, implementing lower carbon content fuel in motor vehicles.

Studies on the effect of alcohols towards carbon dioxide have been conducted by many researchers. Research studies showed that the addition of ethanol in gasoline increases the amount of carbon dioxide emission. This is because the ethanol–gasoline fuel blends combust better than pure gasoline and the amount of non-complete combustion products, i.e. CO can be reduced.¹²⁸ As result, 10 vol% ethanol blend (E10) shows an increase of CO₂ of 5–10% because of improved combustion. A similar result was revealed by Yuksel and Yuksel¹⁰⁰ who identified the higher ethanol content in gasoline fuel blend, i.e. 60 vol% ethanol blend (E60) produces a greater amount of CO₂ emission due to improved combustion. In addition, the emission of CO₂ increases due to the increased oxygen content from the alcohols. Farkade and Pathre¹²⁹ identified the emission of CO₂ increased with the addition of methanol, ethanol and butanol in SI engine. This is perhaps because of the alcohol blends with higher oxygen content of 7.5 wt% produces more complete combustion of fuel, thereby increasing the amount of CO₂ emission.

Meanwhile, there are also several contradicting studies that revealed the reduction of CO₂ emission with the addition of alcohols in gasoline fuel blends. Kumar et al.¹⁰⁷ identified the ethanol–gasoline fuel blends reduces CO₂ concentration. Based on the study, 10 vol% ethanol blend (E10) can reduce the amount of CO₂ by 2.04 and 2.94% at 3000 and 4000 rpm, as compared to pure gasoline. Srinivasan and Saravanan¹³⁰ showed the addition of 2 vol% of isoheptanol as fuel additive in 60 vol% ethanol in gasoline (E60) reduces up to 7.7% by volume of CO₂ at 2800 rpm of engine speed due its complete combustion. Besides that, the addition of butanol in gasoline also reduces the amount of CO₂ emission by the engine. A comparison study was done by Singh et al.¹³¹ who studied two different alcohols fuel blends, 10 vol% of butanol and 10 vol% of ethanol in gasoline (Bu10 and E10). 10 vol% butanol blend (Bu10) was found to emit lower CO₂ than pure gasoline and 10 vol% ethanol blend (E10). The reduction of CO₂ is because of the faster flame speed but comparable calorific values of butanol to gasoline.

4.3.2 Nitrogen oxide emission. The oxides of nitrogen, NO_x consists of several compounds i.e. nitric oxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O), dinitrogen trioxide (N₂O₃), dinitrogen tetroxide (N₂O₄) and dinitrogen pentoxide (N₂O₅).^18,132 However, only NO and NO₂ are prominent while the other oxides are only formed in small quantities.¹³³ NO has no colour, odour or taste while NO₂ is a reddish-brown gas with a pungent, toxic, corrosive and irritating odour. NO_x is naturally formed and produced by human activities such as burning of fossil fuels. When the combustion of fuel is taking place inside the cylinder, the endothermic reaction of nitrogen and oxygen gases occurs at high temperature to produce NO_x. In other words, nitrogen and oxygen gases do not react at ambient temperatures. Additionally, NO_x is also an example of a GHG gas, which contributes to ozone depletion in the stratosphere. Nitrogen oxides lead to harmful effects such as acid deposition or acid rain. The mechanism of NO_x formation i.e. thermal, fuel, prompt NO_x and NO from nitrous oxide, N₂O were briefly studied by Masum et al.¹⁸

Substantial investigations have been carried out on SI engines to study the effect of ethanol and butanol in gasoline fuel blends on NO_x emission. Most studies concluded that the addition of ethanol and butanol with gasoline fuel blends reduces the amount of NO_x emission. The formation of NO_x is closely dependent on combustion temperature, oxygen concentration and also the residence time inside the combustion chamber.^97,134–136 The study done by Lin et al.¹³⁷ showed that the NO_x emission decreases as the ethanol content in the blended fuel increases. The reduction of NO_x emission is due to the lower combustion temperature since excess oxygen is present in ethanol. Based on the study, a significant reduction with the maximum of 86% of NO_x is reduced with addition of 6 vol% of ethanol in gasoline (E6). In addition, Zervas et al.¹³⁸ observed similar findings as the reduction of NO_x emission is caused by addition of oxygenated compounds such as ethanol.

Moreover, the amount of NO_x emission decreases due to higher latent HoV of ethanol as compared to pure gasoline. The temperature of ethanol blends decreases at the end of the intake stroke thereby the combustion temperature will also decrease. Lin et al.¹³⁷ showed that ethanol addition in gasoline fuel blends results in significant reduction of NO_x emission. This is because 9 vol% ethanol in gasoline (E9) reduces up to 77% of the mean average values of NO_x. The results are typically similar as reported by Liu et al.¹³⁹ who tested methanol–gasoline blend and Zervas et al.¹³⁸ who tested different fuel blends of eight hydrocarbons and four oxygenated compounds. Besides that, lower combustion temperature, caused by the higher latent heat and lower heating value of alcohol, reduces the amount of NO_x emission. The study towards two different alcohol fuel blends, ethanol and methanol by Canakci et al.¹³⁵ showed decreasing tendency on NO_x emission for 10 vol% of ethanol and 5 vol% of methanol in gasoline (E10 and M5), which give 15.5 and 9% maximum reduction of NO_x. Furthermore, the NO_x formation is caused by the increase of peak in-cylinder temperature.¹⁴⁰ The study of addition of ethanol in single and split injection strategies by Turner et al.¹²¹ showed lower amount of NO_x emission, attributed to the reduction of flame temperature. The addition of 30 and 85 vol% of ethanol in gasoline (E30 and E85) causes lower flame temperature which leads to lower exhaust temperature and thereby produces a lower amount of NO_x emission.

Nonetheless, there are some inconsistencies when the amount of NO_x emission is seen to increase with the addition of alcohols in gasoline fuel blends. A contradictory result was revealed by Schifter et al.¹⁴¹ as a small increase of NO_x emission is produced by the addition of more oxygenated compounds with the usage of 9 vol% of ethanol in gasoline (E9). Zervas and Tazerout¹⁴² identified an increase of NO_x emission is also caused by higher percentage of volumetric efficiency. The rise in volumetric efficiency causes lean operation that leads to more complete combustion or near stoichiometric. This case results from an increase of flame temperature, cylinder pressure and temperature.³⁰

Besides that, a significant increased amount of NO_x emission is revealed by Turner et al.¹²¹ with the usage of high ethanol content as pure ethanol (E100) produces higher NO_x that is attributed by higher in-cylinder pressure and temperature due to advanced combustion. Singh et al.¹³¹ indicated that the formation of NO_x is caused by the reaction between the nitrogen and oxygen under high pressure and temperature in the cylinder engine. Based on the study, 10 vol% of ethanol in gasoline (E10) shows an increase of NO_x formation because the ethanol leads to faster flame speed that gives quick combustion, thereby increasing the temperature of the chamber. However, 10 vol% of butanol in gasoline (Bu10) shows lower NO_x emission level compared to gasoline and it is slightly inferior to ethanol fuel blend due to comparable properties of butanol and gasoline in terms of density, laminar flame speed and also the flame temperature. A typical study was done by Gautam et al.⁸⁸ who observed the addition of different alcohols, methanol, ethanol, propanol, butanol and pentanol gives higher NO_x emissions with 12–16% increase compared to gasoline. This is explained by higher in-cylinder temperature upon alcohol addition.

4.3.3 Hydrocarbon emission. Hydrocarbons result from unburned mixture of the fuel molecules in the engine due to improper mixing and incomplete combustion. The formation of unburned HC leads to photochemical smog and ozone pollution.¹⁴³ There are approximately over 200 organic compounds that have been discovered in exhaust gas of SI engine. The typical HC composition in SI engine consists of paraffins, olefins and aromatics.⁹¹ The unburned hydrocarbon emissions (HC) from the SI engine is widely cited by Lavoie and Blumberg¹⁴⁴ since 1980. The production of HC emission from the SI engine is quite different with the CI engine. The HC emission is produced at the surface of sprays by the fuel over-leaned and also at the nozzle sac via fuel effusing.¹⁴⁵ There are four mechanisms of HC formation in SI engine which have been studied.

(1) The existence of propagating flame quenching layer at cold wall surfaces inside the combustion chamber.^145–151

(2) The unburned mixture of air–fuel trapped in the piston top land and ring crevices.^{144,145,150,152,153}

(3) Cyclic absorption or desorption processes of unburned fuel by lubricating cylinder oil films and deposits.^{144,145,148,154–156}

(4) Misfire and incomplete combustion of air/fuel mixture during engine cycles result in increase of HC emission formation.¹⁵⁰

Rigorous studies have been carried out by researchers to observe the effects of additional ethanol and butanol in gasoline fuel blends on HC emission. The progress study done by the researchers showed the significant reduction on HC emission with the addition of alcohols in fuel blends. The reduction of HC emission is caused by oxygen content in alcohol and leaning effect that enhances the combustion efficiency.³¹ A massive reduction of HC emission with the addition of ethanol is revealed by Singh et al.¹³¹ as higher oxygen content of 10 vol% ethanol in gasoline (E10) gives lower amount of HC emission compared to 10 vol% butanol in gasoline (Bu10) and gasoline. A similar study was conducted with higher ethanol content in gasoline fuel blends, where 60 vol% ethanol in gasoline (E60) emits higher reduction of HC emission level up to 16.45% ¹⁵⁷ at 5000 rpm and 31.45% at 2000 rpm.¹¹⁷ Faster flame speed of alcohol compared to gasoline coincidentally affects lower emissions of HC.¹⁵⁸ Therefore, it helps to improve complete combustion of alcohol fuel blends and thereby reduces the NO_x emission level.

In addition, the amount of HC emission slightly decreases with higher engine load. A study by Yasar¹⁵⁹ found that higher alcohol content in fuel blends, i.e. 50 vol% of methanol and 50 vol% butanol in gasoline (M50 and Bu50) reduce the amount of HC emission at higher load of 2400 W compared to low engine load of 500 W. This result is much more consistent than that of Taylor et al.¹⁶⁰ Other than that, HC emission is reduced at higher engine speeds compared to slower ones. The study done by Masum et al.⁹⁷ revealed that the addition of alcohols slightly reduces the HC emission level especially at high engine speed of 6000 rpm compared to a low speed of 1000 rpm. This is due to that the air–fuel mixture that homogenises at high engine speed tends to raise the in-cylinder temperature and enhances combustion efficiency.

Meanwhile, a small increase of HC emission has been reported by some researchers with the addition of ethanol. The formation of HC is affected by the large amount of cyclic variability that causes non-complete combustion which leads to increase of the HC emission. Ceviz and Yuksel¹⁶¹ found that the addition of ethanol with greater than 10 vol% in gasoline increases the formation of HC emission level. Also, the addition of 15 vol% and 20 vol% ethanol in gasoline (E15 and E20) induces a greater level of NO_x emission because of higher temperature at the intake manifold and low volumetric efficiency.

4.3.4 Carbon monoxide emission. Carbon monoxide is a toxic, odourless, tasteless and colourless gas. This pollutant is also formed during the burning of hydrocarbon fuels i.e. natural gas, petrol and diesel. Carbon monoxide, CO emission is one of the products from incomplete combustion of hydrocarbon fuels and also because of lack of air–fuel management.^91,162,163 Combustion is incomplete if there is insufficient amount of air in the air–fuel mixture. Another reason is because of time delay of the combustion cycle.¹⁶⁴

The production of CO emission is affected by the presence of oxygen in fuel blends to be run in the SI engine. Enhancing the leaning effect leads to low CO emission level.^135,165,166 Oxygenated fuel blends i.e. ethanol–gasoline reduces CO emission. He et al.¹⁶⁷ found that 10 and 30 vol% of ethanol in gasoline (E10 and E30) drastically lower the CO emission level. The presence of oxygen in ethanol effectively improves the combustion in rich mixtures. Besides that, Yasar¹⁵⁹ identified that methanol and butanol, that contain an oxygen ratio of 21.62 and 50 wt%, respectively, promote better complete combustion and thereby lower the CO emission level. In addition, Feng et al.¹⁶⁶ observed the CO emission is reduced with the addition of butanol i.e. 30 and 35 vol% of butanol in gasoline (Bu30 and Bu35) emit lower CO emission compared to gasoline, due to the oxygen content of butanol. Similar outcomes were reported by Rice et al.¹⁶⁸ and Gu et al.¹⁶⁹ as the CO emission is reduced with the addition of alcohol in gasoline. Besides that, the reduction of CO emission level is also caused by faster flame speed of ethanol that helps to contribute to complete combustion.^170,171 The higher LHV of the alcohol fuel blends accelerates the combustion that will lead to low CO emission. It is revealed by Masum et al.⁹⁷ that the MaxH (maximum heating value optimum fuel blend) reduces up to 12.4% CO emission compared to gasoline. Table 10 summarizes the results of the effects of ethanol and butanol addition into gasoline blends on exhaust emissions of CO₂, NO_x, HC, CO from different researchers.

Table 10 Effect of ethanol and butanol addition into gasoline blends on exhaust emissions of CO₂, NO_x, HC, and CO^a

Engine	Tested blend fuel	Operating condition	CO2	NOx	HC	CO	Ref.
a MPI/MPFI = multiport fuel injection, DI = direct injection, MPEI = multipoint electronic injection, EFI = electronic fuel injection, PFI = port fuel injection, C = cylinder, S = stroke, d = bore, CR = compression ratio, WC = water cooled, AC = air cooled, ↓ = decrease, ↑ = increase, E = ethanol, M = methanol, Bu = butanol, Pr = propanol, Pe = pentanol, MaxH = maximum heating value optimum blend, MaxR = maximum research octane number optimum blend, MaxD = maximum petroleum displacement optimum blend.
4E-FE DOHC 16V engine, 1332 cc, CR = 9.8, MPI fuel system	E0, E10	Full load	E10 > E0	—	E10 < E0 up to 5800 rpm	E10 < E0 (CO ↓ by 10–30%)	128
		Varying speed (1500–6500 rpm)	(CO2 ↑ by 5–10%)
Opel record L, WC, 4C, 1668 cc, d = 74 mm, s = 85 mm, CR = 8 : 1	E60, E0	Varying engine speed (idle to max speed)	E% ↑, CO2 (↑ 20%)	—	E% ↑, HC (↓ 80%)	E% ↑, CO (↓ 50%)	100
		Varying throttle 25–100%
Greaves MK-25, 1C, CR = 2.5 to 8, d = 70 mm, s = 66.7 mm, AC, WC	M5, M10, M15, E7, E14, E19, Bu12, Bu23, Bu35, gasoline	Constant speed 3000 rpm	M%, E%, Bu% ↑, CO2 ↑	—	M%, E%, Bu% ↑, HC ↓	M%, E%, Bu% ↑, CO ↓	129
		Load from 0 to full load
Make Maruti Wagon-R MPFI, 4C, 4S, s = 61 mm, d = 72 mm, 1100 cc, CR = 9.4	E5, E10, E15, E20	Varying speed (2100–5000 rpm)	E% ↑, CO2 ↓	—	E% ↑, HC ↑	—	107
		Varying load (no load and with load 5 kg)
3C, 4S, d = 86.5 mm, s = 72 mm, 796 cc, CR = 8.7, WC	E60 + 2.0, E50 + 1.0 isoheptanol additives	Varying speed (2000–2800 rpm)	E% ↑, CO2 ↓	E% ↑, NOx ↓	E% ↑, HC ↓	E% ↑, CO ↓	130
Birla Ecogen Genset, 4S, AC, 1C, d = 73 mm, s = 61 mm, 256 cc, CR = 5.1	E10, Bu10, gasoline	At constant speed 3000 rpm	CO2 from Bu10 < gasoline and E10	NO from E10 > gasoline and Bu10	HC from E10 < Bu10 < gasoline	CO from E10 < Bu10 < gasoline	131
Proton Campro, 4C, MPEI, 1596 cc, d = 78 mm, s = 84 mm, CR = 10 : 1	Alcohol blends, MaxR, MaxH, MaxD, E15, gasoline	Varying speed (1000–6000 rpm)	—	NOx of E15 > optimized blends > gasoline	HC of alcohol blends < gasoline	CO of alcohol blends < gasoline	97
Chassis dynamometer	E5, E10, M5, M10	Drive at speed 4 manually with gear ratio 1 : 1	M%, E% ↑, CO2 ↓ at 80 km h^−1 except M10 at 100 km h^−1	M%, E% ↑, NOx ↓ at 80 km h^−1 except M10 at 100 km h^−1	M%, E% ↑, HC ↓ at 80 km h^−1 and 100 km h^−1	M%, E% ↑, CO ↓ at 80 km h^−1	135
		4 diff Wheel power 5–20 kW
1.4i SI Engine Honda Civic, 4S, WC, MPI, 1398 cc, CR = 10.4, d = 75 mm, s = 79 mm		At 2 different speeds of 80 km h^−1 and 100 km h^−1
Honda GX 160, 1C SI engine, 4S, AC, d = 68 mm, s = 45 mm, CR = 8.5, 163 cc	E0, E3, E6, E9	Constant speed of 3600 rpm	—	E% ↑, NOx ↓	E% ↑, HC ↓	E% ↑, CO ↓	137
		Varying power 865, 1730 and 2595 W
1C, 4S SI engine, Jaguar guided DI, d = 90 mm, s = 88.9 mm, 565.6 cc, CR = 11.5	E0, E10, E20, E30, E50, E85, E100	Oil temp 85 ± 3 °C	—	For 1 injection; E% ↑, NOx ↓	E% ↑, HC ↓	For 1 injection; E% ↑, CO ↓	121
		Water temp 93 ± 3 °C
		Constant speed 1500 rpm				For split injection
		Constant load 3.4 bar IMEP				E85 and E100, CO ↑
3 different groups	Gasoline blends	Fuels were tested randomly for each vehicle except for sulfur content which tested from low to high sulfur	—	Small increase of NOx from oxygenated compounds L-MTBE and E	—	E% ↑, CO ↓ (3–6%) compared to MTBE	141
GT-1; 1989–1990 MY	L-MTBE, H-MTBE, E, H-AROM, H-OLEF, L-SULF, M-SULF, H-SULF, METRO, Rest country
GT-2; 12 1993–1998 MY
GT-3; 14 1999–2002 MY
Chassis dynamometer; Horiba ECDM-48 electric dynamometer
1C, 4S SI engine, WC, 763 cc, d = 90 mm, s = 120 mm	For experiment;	Constant speed at 1500 rpm	—	E% ↑, NO ↑	—	E% ↑, CO ↓	30
	E1.5 to E12	Varying CR = 7.75 and 8.25
	For numerical; blends up to E21	At full throttle condition
Waukesha, 1C, d = 8.26 cm, s = 11.43 cm, 611.7 cc	Gasoline, M, E, Bu, Pe and Pr blends	Speed 1000 rpm	CO2 of alcohol and gasoline are almost identical	NOx of alcohol > gasoline (12–16%)	HC of alcohol < gasoline	CO of alcohol and gasoline are almost identical	88
		Wide open throttle
Hydra, 1C, CR: 5 : 1–13 : 1, d = 80.26 mm, s = 88.9 mm	E0, E50, E85	Varying speed (1500–5000 rpm); CR: 10 : 1 and 11 : 1	—	E% ↑, NOx ↓	E% ↑, HC ↓	E% ↑, CO ↓	31
		100% WOT throttle
Hydra, 1C, CR: 5 : 1–13 : 1, d = 80.26 mm, s = 88.9 mm	E0, E10, E20, E40, E60	Varying speed (2000, 3000, 5000 rpm)	—	—	E% ↑, HC ↓	E% ↑, CO ↓	157
		At full throttle WOT
		Varying 6 CR (8 : 1–13 : 1)
Hydra, 1C, CR: 5 : 1–13 : 1, d = 80.26 mm, s = 88.9 mm	E0, E10, E20, E40, E60	Varying CR; 8 : 1, 9 : 1, 10 : 1	—	—	E% ↑, HC ↓	E% ↑, CO ↓	117
		Constant speed 2000 rpm
		At full throttle
Lombardini 1C SI engine, CR = 8.6, 349 cc, AC, d = 82 mm, s = 66 mm, 4S	M5, M15, M25, M35, M50, Bu5, Bu15, Bu25, Bu35, Bu50, gasoline	Constant speed 3000 rpm	M% ↑ up to 25%, CO2 ↑	M% ↑, NOx ↓	M% ↑and Bu% ↑, HC ↓	M% ↑and Bu% ↑, CO ↓	159
		Varying load 800, 1600 and 2400 W	CO2 of Bu% blend almost same to gasoline	NOx of Bu% blend almost same to gasoline
FIAT 4C, 4S, d = 86.4 mm, s = 67.4 mm, CR = 9.2, 1581 cc, WC	E0, E5, E10, E15, E20	Oil temp 50 °C + 5	E% ↑ up to 10%, CO2 ↑	—	E% ↑ up to 10%, HC ↓	E% ↑ up to 10%, CO ↓	161
		Constant speed 2000 rpm
Briggs and Stratton 1C 4S SI engine, d = 79.24 mm, s = 61.27 mm, 305 cc, CR = 8.1	Gasoline, Bu10, Bu15	Constant engine speed	—	NO ↑ for all Bu% in coated engine compared to uncoated head	Bu% ↑, HC ↓ for both coated and base engine	CO ↓ for all Bu% in coated engine compared with uncoated head	165
		Varying loads
		Coated and uncoated engine head
1C, 4S SI motorcycle engine, CR = 9.2	Gasoline, Bu30. Bu35	Varying speed 3000 to 8500 rpm, full load	Bu% ↑, CO2 ↑	Bu% ↑, NOx ↑	Bu% ↑, HC ↓	Bu% ↑, CO ↓	166
		Max brake torque
SI engine, EFI, d = 90.82 mm, s = 76.95 mm, CR = 8.2 mm	E0, E10, E30	Close-loop control at part engine loads; open loop control at full engine load	—	E% ↑, NOx ↓	E% ↑, HC ↓	E% ↑, CO ↓	167
HH368Q SI engine, 3C, CR = 9.4, PFI, d = 68.5 mm, s = 72 mm, 796 cc	Bu0, Bu10, Bu30, Bu40, Bu100	Varying loads		Bu% ↑, NOx ↑	Bu% ↑, HC ↓ except Bu100 > gasoline	Bu% ↑, CO ↓ except Bu100 > gasoline	169
		Constant speed 3000 rpm
		Fixed stoichiometric AFR
1C, SI motorcycle engine, AC, d = 56.5 mm, s = 49.5 mm, 124.1 cc, CR = 9.2	Gasoline, Bu35	Full load 3500–9000 rpm	Bu% ↑, CO2 ↑	E% ↑, NOx ↑	E% ↑, HC ↓	E% ↑, CO ↓	170
		Partial load 6500–8500 rpm

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4.3.5 Unregulated emissions. The unregulated emissions of aldehydes (HCO) i.e. formaldehyde, acetaldehyde, butyraldehyde, acrolein, propionaldehyde, methacrolein and benzaldehyde are released by alcohols and are commonly formed in the exhaust gases from vehicles. Such aldehyde emissions are harmful to human health.¹⁷² The study on the unregulated emissions showed that the formaldehyde and acetaldehyde emissions are richer in oxygen containing fuels than the gasoline.⁴⁵ The oxygenated fuel of ethanol fuel blend E85 shows highest emissions of acetaldehyde (98 mg km⁻¹) and the formaldehyde emissions of 7 mg km⁻¹. A similar result is also seen in other studies as the acetaldehyde emissions increase with increasing ethanol blend level due to oxidation.^167,173 The emissions of formaldehyde, acetaldehyde and acetone from ethanol containing fuel are 5.12 to 13.8 times higher than from neat gasoline.^93,174,175 However, the polynuclear aromatics from the burning of gasoline leads to worse environmental issues in spite of the aldehyde emission increase when ethanol is used as the fuel. Given this factor, higher composition of alcohol in the fuel blend gives a better air-quality than gasoline.¹⁶⁸

In a different study, Wallner et al.¹⁷⁶ reported that the formaldehyde and acetaldehyde emissions increased with n-butanol and isobutanol compared to ethanol fuel blends. The amount of formaldehyde emissions is higher for isobutanol fuel blends compared to ethanol fuel blends for most cases as illustrated in Fig. 8.¹⁷⁷ The amount of acetaldehyde emissions of both ethanol and isobutanol fuel blends increases as shown in Fig. 9.¹⁷⁷

Fig. 8 Graph of formaldehyde emissions for isobutanol and ethanol fuel blends.¹⁷⁷

Fig. 9 Graph of acetaldehyde emissions for isobutanol and ethanol fuel blends.¹⁷⁷

Formaldehyde emissions remain in a large amount during both cold start and hot start engine operation due to lack of catalyst to oxidize formaldehyde efficiently. However, the acetaldehyde emissions are reduced during hot-start operation (less than 0.3 mg km⁻¹).⁴⁵

Butyraldehyde emissions are generated by butanol fuel blends. Experiments done by Aakko-Saksa et al.⁴⁵ showed that the butyraldehyde emissions released by n-butanol are higher than isobutanol blend fuels (3–5 vs. (0.5–1 mg km⁻¹, respectively). The butyraldehyde emissions are almost zero during hot-start engine operation for all fuel blends.

4.4 Effect on engine durability

Before the alcohol fuels, e.g. ethanol were introduced in existing vehicles, a range of studies were carried out by researchers to investigate the effect of alcohol fuel blends on engine durability. A comprehensive study was done by Coordinating Research Council, Inc. (CRC)¹⁷⁸ of United States to investigate the effect of ethanol on engine durability for current, on-road and non-FFVs models in response to the US Energy Independence and Security Act, to mandate 36 billion gallons of renewable fuels for the upcoming year 2022. The study concluded that engines operating on 15 and 20 vol% of ethanol in gasoline (E15 and E20) will lead to failure. The presence of ethanol fuel causes three main engine wear mechanisms, i.e. abrasive wear, adhesive wear and corrosion. The reduction of the engine valve seat because of the wear mechanism causes poor sealing and high leakage, leading to loss of compression, cylinder misfires and also catalyst damage.¹⁷⁸ In addition, ethanol may degrade the metal, rubber and plastic parts of the fuel system because it is basically corrosive.¹⁷⁹ Therefore, anti-corrosion tanks, alcohol-tolerant rubber lines, seals and fuel pump diaphragms and plastic fuel system parts are needed to meet the challenge.

A post-test teardown analysis done by Hilbert¹⁸⁰ on internal parts of the Verado engine showed that the 15 vol% ethanol blend (E15) piston and connecting rod have higher amount of oil staining and carbon deposits than the pure gasoline (E0) engine parts as illustrated in Fig. 10 and 11. The visual inspection indicated that the 15 vol% ethanol blend (E15) engine experienced higher operating temperatures than pure gasoline. The wear pattern was observed on the 15 vol% ethanol blend (E15) exhaust cam lobe due to base circle contact as shown in Fig. 12.

Fig. 10 Piston carbon deposit comparison for cylinder 2.¹⁸⁰

Fig. 11 Connecting rod carbon deposit comparison for cylinder 2, E0 connecting rod (left) and E15 connecting rod (right).¹⁸⁰

Fig. 12 Exhaust cam lobes base circle details for cylinder 3, E0 cam lobe on Left, E15 cam lobe on Right.¹⁸⁰

However, the National Renewable Energy Laboratory (NREL) of United States refuted the CRC’s report and they stated that 15 and 20 vol% of ethanol in gasoline (E15 and E20) do not show the evidence of deterioration on engine durability.¹⁸¹ The review on 43 studies with the usage of 15 vol% ethanol blend (E15) by NREL showed that there is no abnormal deterioration in engine condition i.e. metal corrosion or elastomer swell. Besides that, the effect of the ethanol–diesel fuel blends in CI engine on engine durability have been studied and discussed. Archer Daniels Midland (ADM) of United States conducted a road test study on two trucks that use 15 vol% ethanol–diesel blend for over 400 000 km and no abnormal deterioration on engine condition was seen.^182,183 A similar result was obtained by Chicago Transit Authority (CTA) of United States, in that 15 buses running on 15 vol% ethanol–diesel blend for 434 500 km did not encounter any fuel related problems and abnormal maintenance issues.^182,183

4.5 Effect on lubricating oil

The lubricating oil is typically used for lubrication of an internal combustion engine. The role of the lubricating oil is to reduce the friction and cool the moving engine parts. The other function of lubricating oil is to prevent corrosion and wear, and also minimize deposit formation. The lubricating oil circulates around the internal part of the engine and its quality decreases with age. The usage of alcohol as an alternative fuel is believed to affect the properties of lubricating oil. Jaroonjitsathian et al.¹⁸⁴ investigated a four-stroke motorcycle engine running with 10 vol% ethanol blend (E10) for 100 h high speed cycle or 100 h composite cycle, and observed higher lubrication oil degradation. The effect of ethanol fuels on engine oil physicochemical properties was studied by Cousseau et al.¹⁸⁵ as the ethanol fuelled engine give a lower oxidation level and less total acid number (TAN) than gasoline fuelled engine.

The usage of higher ethanol blends, i.e. 85 vol% ethanol blend (E85) as the fuel in SI engine enhances the formation of deposits compared to neat gasoline.¹⁸⁶ The deposits are formed due to the higher vaporization of ethanol at high temperature, engine lubricating oil flow, blow-by gases (positive crankcase ventilation) and combustion gases (EGR).¹⁸⁷ Deposits such as gums are formed around the inlet valve, the injector tips of port injection engine and also combustion chamber.¹⁸⁶ Detergent additive is used to reduce the deposit formation. Besides that, the gum formation with associated rust or other particles inside the storage tanks can be dissolved and loosened by ethanol fuel.¹⁸⁶ Therefore, filters are needed to be changed more frequently as higher abundance of particulates plug the filters. In addition, an automatic emission spectroscopy (AES) analysis on lubricating oil showed a significant dissimilarity on the amount of wear particles between ethanol and gasoline fuelled engine.¹⁸⁵ The rubbing of moving metal of the engine from the wear surfaces forms some microscopic particles, which circulate in the oil and move against the engine parts, that will lead to wear.

Furthermore, ethanol fuel is also miscible with water, in contrast to neat gasoline.¹⁸⁸ Boons et al.¹⁸⁹ reported that the usage of high ethanol content, i.e. 85 vol% of ethanol in gasoline (E85) causes higher water level in the lubricating oil rather than the usage of neat gasoline in the engine. The presence of water in the lubricants may degrade the metal bearing fatigue life.¹⁹⁰ However, FFV engine fuelled with 85 vol% ethanol blend (E85) does not contribute the formation of valve train rust in lubricating oil.¹⁸⁹ Tomanik¹⁹¹ also reported that the engine parts that operate with the 85 vol% ethanol blend (E85) are less lubricated due to low lubricity of ethanol, and water-fuel dilution of lubricating oil during the cold start conditions, leads to bearing corrosion and piston ring spalling.

The usage of ethanol fuel blends in the engine causes tribological problems as the ethanol becomes contaminated with the lubricating oil. The effects on friction and film formation of the engine oil was studied by Costa and Spikes¹⁹² as it is related to the respective tribological problem. The viscosity for both based and formulated engine oil decreases with the addition of ethanol, and therefore reduce the elastohydrodynamic film thickness and the friction. The ethanol in base oil reduces the friction with formation of a boundary layer at low speed. Besides that, the ethanol in formulated oil reduces friction at higher speed as the viscosity of the lubricant is decreased. However, at lower speed the ethanol reduces the boundary layer thus increasing the friction.

5. Conclusion

The demands of energy consumption, depletion of fossil fuels energy sources and serious environmental issues are the main common problems under debate. Ethanol is used as a commercial alternative substitute for conventional gasoline due to its impressive properties of higher octane number. A new promising biofuel, biobutanol is also attractive to researchers due to its competitive properties compared to gasoline. The production of biobutanol is almost identical to that of bioethanol as they are from the same feedstocks, however biobutanol production is high cost, low-yield and takes a longer time, which leads to difficulty to compete commercially.

The research and development on bioethanol and biobutanol as gasoline substitutes is a highly active area. The review indicates that bioethanol and biobutanol are able to improve engine performances, combustion and also reduce exhaust emissions. The addition of alcohols blend gives higher T, BP, BSFC, BTE and lower EGT compared to gasoline, especially biobutanol that contains a higher carbon number than bioethanol, and thus improves the fuel properties i.e. RON, LHV, etc. Besides that, the higher flame speed of bioethanol and biobutanol gives higher ICP and HRR than gasoline. Bioethanol and biobutanol fuels are more environmental friendly than gasoline as the alcohol blends show reduced emission of CO and HC, notably bioethanol emits lowest CO amount. However, the alcohol blends emit more CO₂ and NO_x than gasoline due to higher oxygen concentration. The biobutanol fuels also give higher amount of unregulated emissions as they emit higher formaldehyde emissions than those of bioethanol and gasoline. The study also reviews that the addition of alcohols in the fuel blends leads to negative influences to the engine durability and lubricating oil. Ethanol causes problems to the engine such as corrosion on engine parts and contaminated lubricating oil that contributes engine failure.

By reference to this extensive data, the further study on bioethanol and biobutanol especially from different agricultural wastes on spark ignition engine performance, combustion, exhaust emissions, engine durability and lubricating oil is clearly desirable. Besides that, extensive study on the effect of alcohol fuels on engine durability and lubricating oil are essential, since alcohols are significant as gasoline alternatives due to their outstanding properties.

Nomenclature

SI	Spark ignition engine
CI	Compression ignition engine
IC	Internal combustion engine
GHG(s)	Greenhouse gases
RVP	Reid vapours pressure
RON	Research octane number
LHV	Lower heating value
HoV	Heat of vaporisation
T	Torque
BP	Brake power
BSFC	Brake specific fuel consumption
BTE	Brake thermal efficiency
EGT	Exhaust gas temperature
ICP	In-cylinder pressure
HRR	Heat release rate
CO2	Carbon dioxide
NOx	Nitrogen oxides
HC	Hydrocarbon
CO	Carbon monoxide
PM	Particulate materials
SO2	Sulfur dioxide
EGR	Exhaust gas recirculation
–OH	Hydroxyl
E	Ethanol, bioethanol
DAE	Denatured anhydrous ethanol
Bu	Butanol, biobutanol
M	Methanol
Pr	Propanol
Pe	Pentanol
E0/Bu0	Pure gasoline
E100	Pure ethanol or 100 vol% ethanol
Bu100	Pure butanol or 100 vol% butanol
MaxR	Maximum research octane number optimum blend
MaxH	Maximum heating value optimum blend
MaxD	Maximum petroleum displacement optimum blend
EFB	Palm empty fruit bunches
POME	Palm oil mill effluent
PKC	Palm kernel cake

Acknowledgements

The authors would like to thank University of Malaya for financial support through High Impact Research grant titled: Development of Alternative and Renewable Energy Career (DAREC); UM.C/HIR/MOHE/ENG/60 and also UM Research Grant titled: Optimization of High Quality Fuel for High Thermal Efficiency and Greenhouse Gas Reduction; FP032-2013A.

References

1. R. Saidur, A. E. Atabani and S. Mekhilef, Renewable Sustainable Energy Rev., 2011, 15, 2073–2086.
2. S. E. Hosseini and M. A. Wahid, Renewable Sustainable Energy Rev., 2013, 19, 454–462.
3. R. Saidur, E. A. Abdelaziz, A. Demirbas, M. S. Hossain and S. Mekhilef, Renewable Sustainable Energy Rev., 2011, 15, 2262–2289.
4. A. Gupta and J. P. Verma, Renewable Sustainable Energy Rev., 2015, 41, 550–567.
5. G. Berndes, M. Hoogwijk and R. van den Broek, Biomass Bioenergy, 2003, 25, 1–28.
6. M. H. M. Ashnani, A. Johari, H. Hashim and E. Hasani, Renewable Sustainable Energy Rev., 2014, 35, 244–257.
7. F. Sulaiman, N. Abdullah, H. Gerhauser and A. Shariff, Biomass Bioenergy, 2011, 35, 3775–3786.
8. P. Bajpai, Advances in Bioethanol, Springer, Patial, India, 2013.
9. M. Kumar and K. Gayen, in Biomass Conversion, ed. C. Baskar, S. Baskar and R. S. Dhillon, Springer, Berlin–Heidelberg, 2012, ch. 7, pp. 221–236,  DOI:10.1007/978-3-642-28418-2_7.
10. K. R. Szulczyk, B. A. McCarl and G. Cornforth, Renewable Sustainable Energy Rev., 2010, 14, 394–403.
11. D. Puppán, Periodica Polytechnica, Social and Management Sciences, 2002, 10, pp. 95–116.
12. A. Demirbas, Appl. Energy, 2009, 86, S108–S117.
13. W. Coyle, Amber Waves, 2007, 5, 24–29.
14. M. Balat and H. Balat, Appl. Energy, 2009, 86, 2273–2282.
15. M. F. Demirbas and M. Balat, Energy Convers. Manage., 2006, 47, 2371–2381.
16. T. V. Rasskazchikova, V. M. Kapustin and S. A. Karpov, Chem. Technol. Fuels Oils, 2004, 40, 203–210.
17. B. D. McGuire, Assessment of the bioenergy provisions in the 2008 farm bill, Association of Fish and Wildlife Agencies, 2012.
18. B. M. Masum, H. H. Masjuki, M. A. Kalam, I. M. Rizwanul Fattah, S. M. Palash and M. J. Abedin, Renewable Sustainable Energy Rev., 2013, 24, 209–222.
19. A. Ganguly, P. K. Chatterjee and A. Dey, Renewable Sustainable Energy Rev., 2012, 16, 966–972.
20. B. Roozbehani, M. Mirdrikvand, S. I. Moqadam and A. C. Roshan, Chem. Technol. Fuels Oils, 2013, 49, 115–124.
21. R. Nelson, M. A. Taylor, D. D. Davidson, L. M. Peters, US Pat., US2579601A, 1951.
22. Y. Maki, K. Sato, A. Isobe, N. Iwasa, S. Fujita, M. Shimokawabe and N. Takezawa, Appl. Catal., A, 1998, 170, 269–275.
23. Sugar Industry News, http://www.sugarinds.com/2011/08/worlds-top-20-ethanol-producing.html, accessed 4 September, 2015.
24. B. Booundy, S. W. Diegel, L. Wright and S. C. Davis, Biomass Energy Data Book, U.S. Department of Energy, 4th edn, 2011.
25. L. R. Brown, in Full planet, empty plates: The new geopolitics of food scarcity, ed. E. P. Institute, W. W. Norton & Company, New York, 2012.
26. J. L. Sawin, D. Barnes, E. Martinot, A. McCrone, J. Roussell, R. Sims and V. S. O’Brien, Renewables 2011 Global Status Report, REN21, France, 2011.
27. J. R. Tavares, M. S. Sthel, L. S. Campos, M. V. Rocha, G. R. Lima, M. G. da Silva and H. Vargas, Procedia Environ. Sci., 2011, 4, 51–60.
28. T. C. C. d. Melo, G. B. Machado, C. R. P. Belchior, M. J. Colaço, J. E. M. Barros, E. J. de Oliveira and D. G. de Oliveira, Fuel, 2012, 97, 796–804.
29. M. Al-Hasan, Energy Convers. Manage., 2003, 44, 1547–1561.
30. H. Bayraktar, Renewable Energy, 2005, 30, 1733–1747.
31. M. Koç, Y. Sekmen, T. Topgül and H. S. Yücesu, Renewable Energy, 2009, 34, 2101–2106.
32. R. C. Costa and J. R. Sodré, Fuel, 2010, 89, 287–293.
33. P. C. Vicentini and S. Kronberger, Rating the performance of Brazilian flex fuel Vehicles, SAE Technical Paper, 2005.
34. U. S. EPA, unpublished work.
35. J. L. Sawin, Renewables 2007 Global Status Report, Worldwatch Institute, Washington, 2008.
36. C. Jin, M. Yao, H. Liu, C.-f. F. Lee and J. Ji, Renewable Sustainable Energy Rev., 2011, 15, 4080–4106.
37. A. Ranjan and V. Moholkar, presented in part at the Proceeding of International Conference on Energy and Environment, 2009.
38. C. Weizmann, US Pat., US1315585, 1919.
39. O. Shapovalov and L. Ashkinazi, Russ. J. Appl. Chem., 2008, 81, 2232–2236.
40. P. Patakova, D. Maxa, M. Rychtera, M. Linhova, P. Fribert, Z. Muzikova, J. Lipovsky, L. Paulova, M. Pospisil, G. Sebor and K. Melzoch, Perspectives of Biobutanol Production and Use, 2011.
41. S. Szwaja and J. D. Naber, Fuel, 2010, 89, 1573–1582.
42. J. Niemistö, P. Saavalainen, R. Isomäki, T. Kolli, M. Huuhtanen and R. L. Keiski, Journal, 2013, 443–470.
43. K. R. Szulczyk, Int. J. Energy Environ., 2010, 1, 1–12.
44. D. E. Ramey, A. Eaglesham and R. Hardy, NABC Report, 2007, pp. 137–147.
45. P. Aakko-Saksa, P. Koponen, J. Kihlman, M. Reinikainen, E. Skyttä, L. Rantanen-Kolehmainen and A. Engman, Biogasoline options for conventional spark-ignition cars, VTT Technical Research Centre of Finland, Finland, 2011.
46. Ethanol Feedstocks, http://www.afdc.energy.gov/fuels/ethanol_feedstocks.html, accessed 1 October, 2015.
47. S. N. Naik, V. V. Goud, P. K. Rout and A. K. Dalai, Renewable Sustainable Energy Rev., 2010, 14, 578–597.
48. D. Ramey and S. T. Yang, Production of Butyric Acid and Butanol from Biomass Final Report, U.S. Department of Energy, Morgantown, WV, 2004.
49. D. T. Jones and D. R. Woods, Microbiol. Rev., 1986, 50, 484–524.
50. I. Ceres, Carbon dioxide/Net energy, Cares, Inc., California, United States, 2015.
51. S. Kim and B. E. Dale, Biomass Bioenergy, 2004, 26, 361–375.
52. B. Hahn-Hägerdal, M. Galbe, M. F. Gorwa-Grauslund, G. Lidén and G. Zacchi, Trends Biotechnol., 2006, 24, 549–556.
53. P. Noomtim and B. Cheirsilp, Energy Proc., 2011, 9, 140–146.
54. H. Shukor, N. K. N. Al-Shorgani, P. Abdeshahian, A. A. Hamid, N. Anuar, N. A. Rahman and M. S. Kalil, Bioresour. Technol., 2014, 170, 565–573.
55. L. G. A. Ong, S. Abd-Aziz, S. Noraini, M. I. A. Karim and M. A. Hassan, Appl. Biochem. Biotechnol., 2004, 118, 73–79.
56. D. Mohapatra, S. Mishra and N. Sutar, J. Sci. Ind. Res., 2010, 69, 323–329.
57. J. Y. Tock, C. L. Lai, K. T. Lee, K. T. Tan and S. Bhatia, Renewable Sustainable Energy Rev., 2010, 14, 798–805.
58. H. I. Velásquez-Arredondo, A. A. Ruiz-Colorado and S. de Oliveira junior, Energy, 2010, 35, 3081–3087.
59. R. Sims, M. Taylor, J. Saddler and W. Mabee, International Energy Agency, 2008, 16–20.
60. S. Lee, J. G. Speight and S.K. Loyalka, Handbook of alternative fuel technologies, CRC Press, 2014.
61. C. N. Hamelinck, G. van Hooijdonk and A. P. Faaij, Biomass Bioenergy, 2005, 28, 384–410.
62. A. Demirbaş, Energy Sources, 2005, 27, 327–337.
63. M. von Sivers and G. Zacchi, Bioresour. Technol., 1996, 56, 131–140.
64. L. R. Lynd, R. T. Elamder and C. E. Wyman, Appl. Biochem. Biotechnol., 1996, 57–58, 741–761.
65. D. S. D. Group, Feasibility study on an effective and sustainable bio-ethanol production program by Least Developed Countries as alternative to cane sugar export, Ministry of Agriculture, Nature and Food Quality (LNV), The Hague,The Netherlands, 2005.
66. R. Wooley, M. Ruth, J. Sheehan, K. Ibsen, H. Majdeski and A. Galvez, Lignocellulosic biomass to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and enzymatic hydrolysis current and futuristic scenarios, DTIC Document, 1999.
67. J. Pickett, D. Anderson, D. Bowles, T. Bridgwater, P. Jarvis, N. Mortimer, M. Poliakoff and J. Woods, Sustainable biofuels: prospects and challenges, The Royal Society, London, 2008.
68. T. Ezeji, N. Qureshi and H. Blaschek, Appl. Microbiol. Biotechnol., 2004, 63, 653–658.
69. A. van der Westhuizen, D. T. Jones and D. R. Woods, Appl. Environ. Microbiol., 1982, 44, 1277–1281.
70. M. Kumar and K. Gayen, Appl. Energy, 2011, 88, 1999–2012.
71. N. Qureshi and H. Blaschek, J. Ind. Microbiol. Biotechnol., 2001, 27, 292–297.
72. H. Liu, G. Wang, J. Zhang, in Liquid, Gaseous and Solid Biofuels – Conversion Techniques, ed. Z. Fang, Intech, 2013, pp. 175–198.
73. T. Ezeji and H. P. Blaschek, Bioresour. Technol., 2008, 99, 5232–5242.
74. N. Qureshi, B. C. Saha, B. Dien, R. E. Hector and M. A. Cotta, Biomass Bioenergy, 2010, 34, 559–565.
75. N. Qureshi, B. C. Saha, R. E. Hector, S. R. Hughes and M. A. Cotta, Biomass Bioenergy, 2008, 32, 168–175.
76. N. Qureshi, B. C. Saha and M. A. Cotta, Biomass Bioenergy, 2008, 32, 176–183.
77. N. Qureshi, T. C. Ezeji, J. Ebener, B. S. Dien, M. A. Cotta and H. P. Blaschek, Bioresour. Technol., 2008, 99, 5915–5922.
78. N. Qureshi, B. C. Saha, R. E. Hector, B. Dien, S. Hughes, S. Liu, L. Iten, M. J. Bowman, G. Sarath and M. A. Cotta, Biomass Bioenergy, 2010, 34, 566–571.
79. S. Liew, A. Arbakariya, M. Rosfarizan and A. Raha, Malays. J. Microbiol., 2006, 2, 42–50.
80. T. Ezeji, N. Qureshi and H. P. Blaschek, Process Biochem., 2007, 42, 34–39.
81. N. Qureshi and I. S. Maddox, J. Ferment. Bioeng., 1995, 80, 185–189.
82. W.-C. Huang, D. E. Ramey and S.-T. Yang, Appl. Biochem. Biotechnol., 2004, 115, 887–898.
83. J. Dernotte, C. Mounaim-Rousselle, F. Halter and P. Seers, Oil Gas Sci. Technol., 2009, 65, 345–351.
84. B. Yuksel, PhD thesis, Ataturk University, 1984.
85. D. American Petroleum Institute, Marketing and D. American Petroleum Institute. Refining, Alcohols and Ethers: A Technical Assessment of Their Application as Fuels and Fuel Components, American Petroleum Institute, 1988.
86. K. Owen, T. Coley, Automotive fuels reference book, Society of Automotive Engineers, Inc., USA, 2nd edn, 1995.
87. Y. Yacoub, R. Bata and M. Gautam, Proc. Inst. Mech. Eng., Part A, 1998, 212, 363–379.
88. M. Gautam, D. W. Martin and D. Carder, Proc. Inst. Mech. Eng., Part A, 2000, 214, 165–182.
89. U. Larsen, T. Johansen and J. Schramm, Ethanol as a Future Fuel for Road Transportation: Main report, DTU Mekanik, 2009.
90. F. Chiba, H. Ichinose, K. Morita, M. Yoshioka, Y. Noguchi and T. Tsukagoshi, High Concentration Ethanol Effect on SI Engine Emission, SAE Technical Paper, 2010.
91. J. B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, United States, 1988.
92. B. M. Masum, H. H. Masjuki, M. A. Kalam, S. M. Palash, M. A. Wakil and S. Imtenan, Energy Convers. Manage., 2014, 88, 382–390.
93. W.-D. Hsieh, R.-H. Chen, T.-L. Wu and T.-H. Lin, Atmos. Environ., 2002, 36, 403–410.
94. B. M. Masum, M. A. Kalam, H. H. Masjuki, S. M. A. Rahman and E. E. Daggig, RSC Adv., 2014, 4, 51220–51227.
95. G. Najafi, B. Ghobadian, T. Tavakoli, D. R. Buttsworth, T. F. Yusaf and M. Faizollahnejad, Appl. Energy, 2009, 86, 630–639.
96. C.-W. Wu, R.-H. Chen, J.-Y. Pu and T.-H. Lin, Atmos. Environ., 2004, 38, 7093–7100.
97. B. M. Masum, H. H. Masjuki, M. A. Kalam, S. M. Palash and M. Habibullah, J. Clean. Prod., 2015, 86, 230–237.
98. S. Saridemir, Energy Educ. Sci. Technol., Part A, 2012, 30, 727–736.
99. J. Campos-Fernandez, J. M. Arnal, J. Gomez, N. Lacalle and M. P. Dorado, Fuel, 2013, 107, 866–872.
100. F. Yüksel and B. Yüksel, Renewable Energy, 2004, 29, 1181–1191.
101. R. Bata, A. Elrod and T. Lewandowskia, SAE Technical Paper Series, 1989, 1, 890434,  DOI:10.4271/890434.
102. Y. Xialong, Y. Jing and L. Tieping, Int. Conf. Energy Environ. Technol., 2009, 402–405.
103. M. Al-Hasan and M. Al-Momany, Transport, 2008, 23, 306–310.
104. O. Can, I. Celikten and N. Usta, J. Eng. Sci., 2005, 11, 219–224.
105. J. Kumar, D. Trivedi, P. Mahar and R. Butola, IOSR Journal of Mechanical and Civil Engineering, 2013, 7, 71–78.
106. M. B. Celik, Appl. Therm. Eng., 2008, 28, 396–404.
107. J. Kumar, N. A. Ansari, V. Verma and S. Kumar, Am. J. Energy Res., 2013, 2(7), 191–201.
108. B. M. Masum, M. A. Kalam, H. H. Masjuki, S. M. Palash and I. M. R. Fattah, RSC Adv., 2014, 4, 27898–27904.
109. A. Dhaundiyal, Int. J. Appl. Sci. Eng. Res., 2014, 3, 129–152.
110. Y. Varol, C. Oner, H. F. Oztop and S. Altun, Energy Sources, Part A, 2014, 36, 938–948.
111. S. Pukalskas, Z. Bogdanovicius, E. Sendzikiene, V. Makareviciene and P. Janulis, Transport, 2009, 24, 301–307.
112. S. B. Shayan, S. Seyedpour, F. Ommi, S. Moosavy and M. Alizadeh, Int. J. Automot. Eng., 2011, 1, 219–227.
113. M. K. Balki, C. Sayin and M. Canakci, Fuel, 2014, 115, 901–906.
114. F. T. Ansari and A. P. Verma, Int. J. Eng., 2012, 1, 1–10.
115. A. F. Khieralla, A. B. A. Ibrahim, L. A. Keir, S. M. Lino and V. N. Joeshp, unpublished work.
116. I. M. Rizwanul Fattah, H. H. Masjuki, M. A. Kalam, M. Mofijur and M. J. Abedin, Energy Convers. Manage., 2014, 79, 265–272.
117. T. Topgül, H. S. Yücesu, C. Çinar and A. Koca, Renewable Energy, 2006, 31, 2534–2542.
118. S. Saridemir and T. Ergin, Energy Educ. Sci. Technol., Part A, 2012, 29, 1343–1354.
119. A. Elfasakhany, Int. J. Automot. Eng., 2014, 4, 609–620.
120. S. B. Singh, A. Dhar and A. K. Agarwal, Renewable Energy, 2015, 76, 706–716.
121. D. Turner, H. Xu, R. F. Cracknell, V. Natarajan and X. Chen, Fuel, 2011, 90, 1999–2006.
122. B. Deng, J. Yang, D. Zhang, R. Feng, J. Fu, J. Liu, K. Li and X. Liu, Appl. Energy, 2013, 108, 248–260.
123. L. Siwale, L. Kristóf, A. Bereczky, M. Mbarawa and A. Kolesnikov, Fuel Process. Technol., 2014, 118, 318–326.
124. B. Deng, J. Fu, D. Zhang, J. Yang, R. Feng, J. Liu, K. Li and X. Liu, Energy, 2013, 60, 230–241.
125. F. Caiazzo, A. Ashok, I. A. Waitz, S. H. L. Yim and S. R. H. Barrett, Atmos. Environ., 2013, 79, 198–208.
126. R. Pease, Traffic pollution kills 5000 a year in UK, says study, http://www.bbc.com/news/science-environment-17704116, accessed 19 March, 2015.
127. C. Soruşbay, unpublished work.
128. A. Pikunas, S. Pukalskas and J. Grabys, Journal of KONES, internal combustion engines, 2003, 10, 3–4.
129. H. S. Farkade and A. P. Panthre, Int. Conf. Adv. Nanomater. Emerging Eng. Technol., 2012, 2, 205–215.
130. C. A. Srinivasan and C. Saravanan, Int. J. Energy Environ., 2010, 1, 715–726.
131. E. Singh, M. K. Shukla, S. Pathak, V. Sood and N. Singh, Int. J. Eng. Res. Tech., 2014, 3, 993–999.
132. C. D. Cooper and F. C. Alley, Air pollution control a design approach, Waveland Press, Prospect Heights III, 1994.
133. F. Normann, K. Andersson, B. Leckner and F. Johnsson, Prog. Energy Combust. Sci., 2009, 35, 385–397.
134. A. Ozsezen, A. Turkcan, C. Sayin and M. Canakci, Energy, Explor. Exploit., 2011, 29, 525–541.
135. M. Canakci, A. N. Ozsezen, E. Alptekin and M. Eyidogan, Renewable Energy, 2013, 52, 111–117.
136. A. N. Ozsezen and M. Canakci, Energy, 2011, 36, 2747–2752.
137. W. Y. Lin, Y. Y. Chang and Y. R. Hsieh, J. Air Waste Manage. Assoc., 1995, 2010(60), 142–148.
138. E. Zervas, X. Montagne and J. Lahaye, Environ. Sci. Technol., 2003, 37, 3232–3238.
139. S. Liu, E. R. Cuty Clemente, T. Hu and Y. Wei, Appl. Therm. Eng., 2007, 27, 1904–1910.
140. I. Gravalos, D. Moshou, T. Gialamas, P. Xyradakis, D. Kateris and Z. Tsiropoulos, Renewable Energy, 2013, 50, 27–32.
141. I. Schifter, L. Diaz and E. Lopez-Salinas, J. Air Waste Manage. Assoc., 1995, 2005(55), 1289–1297.
142. E. Zervas and M. Tazerout, Atmos. Environ., 2000, 34, 3921–3929.
143. E. W. Kaiser, W. O. Siegl, Y. I. Henig, R. W. Anderson and F. H. Trinker, Environ. Sci. Technol., 1991, 25, 2005–2012.
144. G. A. Lavoie and P. N. Blumberg, Combust. Sci. Technol., 1980, 21, 225–258.
145. P. A. Lakshminarayanan and Y. V. Aghav, in Modelling Diesel Combustion, ed. F. F. Ling, Springer, 2010, ch. 11, pp. 147–166.
146. R. Blint and J. Bechtel, Hydrocarbon combustion near a cooled wall, SAE Technical Paper, 1982.
147. W. Daniel, presented in part at the Symposium (International) on Combustion, 1957.
148. G. Lavoie, J. LoRusso and A. Adamczyk, in Combustion Modeling in Reciprocating Engines, Plenum Press, 1980, p. 409.
149. J. LoRusso, E. Kaiser and G. Lavoie, Combust. Sci. Technol., 1981, 121–125,  DOI:10.1080/00102208108547511.
150. F. Alasfour, Energy Sources, 1999, 21, 379–394.
151. W. Daniel, Engine variable effects on exhaust hydrocarbon composition (a single-cylinder engine study with propane as the fuel), SAE Technical Paper, 1967.
152. J. Wentworth, Piston and ring variables affect exhaust hydrocarbon emissions, SAE Technical Paper, 1968.
153. W. W. Pulkrabek, Engineering fundamentals of the internal combustion engine, Prentice Hall Upper, Saddle River, NJ, 1997.
154. E. Kaiser, J. LoRusso, G. Lavoie and A. Adamczyk, Combust. Sci. Technol., 1982, 28, 69–73.
155. E. Kaiser, A. Adamczyk and G. Lavoie, presented in part at the Symposium (International) on Combustion, 1981.
156. G. Carrier, F. Fendell and P. Feldman, Combust. Sci. Technol., 1981, 25, 9–19.
157. H. S. Yücesu, T. Topgül, C. Çinar and M. Okur, Appl. Therm. Eng., 2006, 26, 2272–2278.
158. C. Sayin, Fuel, 2010, 89, 3410–3415.
159. A. Yasar, Metalurgija, 2010, 49, 335–338.
160. A. B. Taylor, D. P. Moran, A. J. Bell, N. G. Hodgson, I. S. Myburgh and J. J. Botha, Gasoline/alcoholblends: exhaust emissions, performance and burn-rate in a multi-valve production engine, SAE Technical Paper, 1996.
161. M. A. Ceviz and F. Yüksel, Appl. Therm. Eng., 2005, 25, 917–925.
162. M. Thompson, Carbon Monoxide, http://www.chm.bris.ac.uk/motm/co/coh.htm, accessed 25 April, 2015.
163. B. Wigg, R. Coverdill, C.-F. Lee and D. Kyritsis, Emissions characteristics of neat butanol fuel using a port fuel-injected, spark-ignition engine, SAE Technical Paper, 2011.
164. H. Bayindir, H. Yücesu and H. Aydin, Energy Sources, Part A, 2010, 33, 49–56.
165. N. Mittal, R. L. Athony, R. Bansal and C. Ramesh Kumar, Alexandria Engineering Journal, 2013, 52, 285–293.
166. R. Feng, J. Fu, J. Yang, Y. Wang, Y. Li, B. Deng, J. Liu and D. Zhang, Renewable Energy, 2015, 81, 113–122.
167. B.-Q. He, W. Jian-Xin, J.-M. Hao, X.-G. Yan and J.-H. Xiao, Atmos. Environ., 2003, 37, 949–957.
168. R. W. Rice, A. K. Sanyal, A. C. Elrod and R. M. Bata, J. Eng. Gas Turbines Power, 1991, 113, 377–381.
169. X. Gu, Z. Huang, J. Cai, J. Gong, X. Wu and C.-f. Lee, Fuel, 2012, 93, 611–617.
170. R. Feng, J. Yang, D. Zhang, B. Deng, J. Fu, J. Liu and X. Liu, Energy Convers. Manage., 2013, 74, 192–200.
171. M. Pechout, M. Mazac and M. Vojtisek-Lom, Effect of higher content n-butanol blends on combustion, exhaust emissions and catalyst performance of an unmodified SI vehicle engine, SAE Technical Paper, 2012.
172. P. Zarante, M. J. Da Silva, O. S. Valente and J. R. Sodré, Therm. Eng., 2010, 9, 35–39.
173. R. Magnusson, C. Nilsson and B. Andersson, Environ. Sci. Technol., 2002, 36, 1656–1664.
174. H.-R. Chao, T.-C. Lin, M.-R. Chao, F.-H. Chang, C.-I. Huang and C.-B. Chen, J. Hazard. Mater., 2000, 73, 39–54.
175. G. Rideout, M. Kirshenblatt and C. Prakash, Emissions from methanol, ethanol, and diesel powered urban transit buses, SAE Technical Paper, 1994.
176. T. Wallner and R. Frazee, Study of regulated and non-regulated emissions from combustion of gasoline, alcohol fuels and their blends in a DI-SI engine, SAE Technical Paper, 2010.
177. T. Wallner, N. Shidore and A. Ickes, presented in part at the 16th Directions in Engine Efficiency and Emissions Research (DEER) Conference, 2010.
178. I. Coordinating Research Council, Intermediate-level ethanol blends engine durability study, 2012.
179. F. Tester, Gas-caused (E10) Engine Damage and Performance Issues, http://www.fuel-testers.com/list_e10_engine_damage.html, accessed 15 September, 2015.
180. D. Hilbert, Contract, 2011, 303, 275–3000.
181. H. Jessen, E15 analysis shows no issues with engine durability, maintenance, http://ethanolproducer.com/articles/10365/e15-analysis-shows-no-issues-with-engine-durability-maintenance, accessed 15 September, 2015.
182. N. Marek and J. Evanoff, presented in part at the Proceedings of the air and waste management association 94th annual conference and exhibition, Orlando, FL, 2001.
183. A. C. Hansen, Q. Zhang and P. W. L. Lyne, Bioresour. Technol., 2005, 96, 277–285.
184. S. Jaroonjitsathian, N. Akarapanjavit, S. S. Sa-norh and S. Chanchaona, Investigation of 2-Wheeler Performance, Emissions, Driveability and Durability: Effect of Ethanol-Blended Gasoline, SAE Technical Paper, 2007.
185. T. Cousseau, J. Sebastian and A. Sinatora, Lyon, France, 2015.
186. C. Wyman, Handbook on bioethanol: production and utilization, CRC press, 1996.
187. Z. Stephian, S. Oleksiak, Influence of bioethanol fuels treatment for operational performance, ecological properties and GHG emissions of spark ignition engine, http://biotreth.eu/, accessed 24 September, 2015.
188. C. Argakiotis, R. Mishra, C. Stubbs and W. Weston, Renewable Energies and Power Quality Journal, 2014, 12, 1–6.
189. M. Boons, R. van den Bulk and T. King, The impact of E85 use on lubricant performance, SAE Technical Paper, 2008, Report 0148–7191.
190. R. E. Cantley, ASLE Trans., 1977, 20, 244–248.
191. E. Tomanik, Some tribological issues on flex-fuel engines, MAHLE Metal Leve SA, Sao Paulo, Brazil, 2012.
192. H. L. Costa and H. Spikes, Tribol. Trans., 2015, 58, 158–168.

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