Influence of oxygen enriched hydrogen gas as a combustion catalyst in a DI diesel engine operating with varying injection time of a diesel fuel

Abstract

Automobiles emit many pollutants. These pollutants become a big threat to our environment. This study examines the influence of oxygen enriched hydrogen (OEH) gas as a combustion catalyst in a DI diesel engine operating with varying injection times of a diesel fuel. For this study, OEH gas was produced by the process of electro-chemical dissociation of water. The OEH gas at 4.6 litres per minute (lpm) was aspirated into an engine cylinder along with intake air at varied injection times of a diesel fuel. Three injection times were selected. The first was the standard injection time of 23° BTDC (before top dead centre) recommended by the engine manufacturer, the second one was the retarded injection time of 19° BTDC and the third one was the advanced injection time of 27° BTDC. When the OEH gas was inducted at 100% rated load of the engine at the standard injection time, the brake thermal efficiency increased by 16.45% and the emissions of oxides of nitrogen (NO_X) increased by 16.9%. All other engine-out emissions, such as carbon monoxide (CO), unburned hydrocarbon (UBHC) and smoke, were reduced by 15.38%, 19.7% and 28.57%, respectively, compared to diesel combustion. At the advanced injection time, the brake thermal efficiency and the NO_X emission were increased by 19.03% and 21.42%, respectively. Other engine-out emissions, such as CO, UBHC and smoke, were reduced by 11.53%, 22.72% and 30.95%, respectively. However, at the retarded injection time, the brake thermal efficiency increased by 12.21% and engine-out emissions, such as CO, UBHC, NO_X and smoke, were reduced by 7.69%, 12.12%, 9.04% and 19.04%, respectively. From the data, it is evident that the diesel engine can be operated efficiently using the OEH gas as a combustion catalyst with the optimized injection timing of a diesel fuel.

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1. Introduction

Day by day, our environment is getting degraded because of various pollutants emitted from various sources. To reduce the same, numerous methods have been researched. One of the methods is using alternative fuels in automotives. Alternative fuels are comparatively clean fuels.¹ Hydrogen is emerging as one of the favorite alternative fuels. It also acts as an energy carrier. The properties of hydrogen that make it an eligible automotive fuel are low ignition energy, low density, wide flammability limit, high diffusivity, and high flame speed.² Due to its wider flammability limit, it can efficiently burn lean mixtures, resulting in fewer amounts of exhaust emissions. Currently, hydrogen is produced by the methods of steam reforming and partial oxidation of hydrocarbons.³ However, when either relatively small hydrogen quantities or high purity hydrogen are required, processes such as water electrolysis, ammonia decomposition and methanol reforming are used.⁴ Hydrogen can be generated by splitting water. Various techniques used to split water are electrolysis, plasmolysis, magnetolysis, thermal approach, use of light, bio-catalytic decomposition, and radiolysis.⁵

When hydrogen is supplemented into the combustion process, the in-cylinder pressure and the thermal efficiency are increased.⁶ This substantial improvement in combustion is due to the fast and clean burning characteristics of hydrogen in comparison to conventional fuels.⁷ Increasing the flame speed of hydrogen reduces the engine-out emissions.⁸

Numerous studies have previously reported on hydrogen combustion in diesel engines in different conditions. The combustion process of a diesel engine can be enhanced by supplementing a small amount of hydrogen to the diesel fuel.⁷

In 1820, Cecil⁹ studied the design and construction of a hydrogen engine and how hydrogen energy could be utilized effectively. Most likely, this is the most primitive development made in hydrogen-fueled engines. The properties of hydrogen are listed in Table 1. Some researchers tried to use hydrogen as a sole fuel in the diesel engine. The self-ignition temperature of hydrogen is 858 K.¹⁰ It is impractical to ignite hydrogen just by the heat of compression. Thus, it needs assistance to start its combustion. Wong¹¹ used a ceramic glow plug as an ignition starter.

Table 1 Important properties of hydrogen¹⁰

Properties of hydrogen
Limits of flammability in air	4–75% vol
Minimum energy for ignition	0.02 mJ
Auto-ignition temperature	858 K
Quenching gap in NTP air	0.064 cm
Burning velocity in NTP air	265–325 cm s^−1
Diffusion coefficient in NTP air	0.61 cm^2 s^−1
Heat of combustion (LCV)	119.93 MJ kg^−1

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The dual-fuel engine operating with hydrogen consumes less fuel than a pure diesel operating engine, resulting in a lower level of smoke emission.¹² This method of operation enables the full realization of a higher brake thermal efficiency. However, it creates the problem of storage of hydrogen. One of the feasible solutions to this problem is to generate hydrogen on-board. One of the processes that assist the on-board generation of hydrogen is the electro-chemical dissociation of water. The addition of oxygen to the air–fuel mixture considerably enhances brake thermal efficiency and significantly decreases emissions from the exhaust.⁷ Santilli,¹³ in his analysis, used various measurement techniques to find the compositions of a mixture of hydrogen and oxygen gas produced via an electrolyzer. This mixture was appreciably different from other known gas mixtures. Bari and Esmaeil¹⁴ performed their experimental work in a four-cylinder, direct injection, water-cooled diesel engine. A H₂–O₂ mixture produced by water electrolysis was inducted into the cylinder at various load conditions of the engine. Their result showed that at 19 kW of load, the brake thermal efficiency increased from 32% to 34.6%, whereas HC emission decreased from 187 ppm to 85 ppm. A smaller amount of carbon dioxide (CO₂) of 2.06 ppm was also observed. However, NO_X emission increased from 220 ppm to 280 ppm. Yilmaz et al.¹⁵ investigated the effect of hydroxy gas on the emission and performance characteristics of a four cylinder, four stroke compression ignition engine. They produced the hydroxy gas by the process of electrolysis. The result of their investigations showed that the addition of hydroxy gas to the engine without any modification resulted in an increase of engine torque output by an average of 19.1%, reducing CO emissions by an average of 13.5%, HC emissions by an average of 5% and SFC by an average of 14%. Birtas et al.¹⁶ carried out a test on a naturally aspirated direct injection tractor diesel engine with four cylinders in-line having a total capacity of 3759 cm³. Hydrogen rich gas (HRG) produced by water electrolysis process was aspirated along with air stream inducted into the engine cylinder. The result showed that by adding HRG, smoke was reduced by 30%, while NO_X concentrations were increased up to 14% compared to pure diesel operation.

Injection timing plays an important role in reducing the engine-out emissions. A number of studies by several researchers indicate its significance. Shioji and Mohammadi¹⁷ carried out an investigation on diesel engine performance and emission characteristics when LCG (low calorific gases) and LCG with a small portion of hydrogen were inducted into the inlet manifold of a four-stroke single cylinder naturally aspirated direct-injection diesel engine with varied injection times of diesel fuel. They varied the injection timing of diesel fuel in the range of 7.5–15° BTDC at the engine load of 0.6 MPa. The results indicated that advancing injection timing improved the thermal efficiency. This trend was similar for both fuels. This advancement in injection timing also improved smoke, THC and CO emissions but NO_X emission worsened with the thermal efficiency remaining the same as diesel fuel operation.

From the vast literature on hydrogen usage in diesel engines, it was noted that vital work had not been carried out in optimizing the usage of OEH gas in a DI diesel engine. Thus, with an aim to fill this gap, the author has published numerous research papers in this area.^18–20 To expand this study, the effect of OEH gas addition on the combustion of diesel under variable injection timing of diesel fuel was analyzed.

2. Present experimental method

The present method provides a practical solution for the onboard production of hydrogen. It avoids storing hydrogen in heavy pressurized storage tanks. In the current process, the hydrogen was produced with oxygen at the desired rate by the electro-chemical dissociation of water. An electrolyzer dissociated the aqueous electrolytic solution consisting of water and electrolyte into a new mixture of gas. This gas mixture consisted of single atoms of H and O, also called as atomic hydrogen and oxygen, and dual molecules of H₂, O₂ and H₂O.¹³ This mixture was named as OEH gas. The generated gas was aspirated into the engine cylinder along with intake air at varied injection timings of diesel fuel. Three injection times were selected. The first was the standard injection time of 23° BTDC recommended by the engine manufacturer, the second one was the retarded injection time of 19° BTDC and the third one was the advanced injection time of 27° BTDC. The injection times were varied by modifying the shim thickness at the link point between the pump and the engine.²¹ In this experiment, petroleum diesel combustion with a standard injection time of 23° BTDC was taken as a base line to compare the efficiency of the test engine at different injection times of the diesel fuel operating under the influence of OEH gas with a flow rate of 4.6 lpm at different load ranges of the test engine. All the experimental data were collected after the engine reached the steady state.

3. Test engine setup

The present experimental investigation was conducted in a single cylinder, water-cooled, four stroke, DI diesel engine made by Kirloskar, providing a rated power of 5.9 kW at a speed of 1800 rpm and having a compression ratio of 17.5 : 1. The detailed engine specifications are given in Table 2. The experiments were conducted at a constant speed of 1800 rpm with variable load. The load ranged from no load condition to full load condition (0–100% rated load of the engine in steps of 25%). The operating parameters such as injection time of diesel fuel and injection pressure of the diesel fuel recommended by the manufacturer are 23° BTDC and 200 bar injection pressure, respectively. The governor of the engine was used to control the engine speed. An eddy current dynamometer was coupled to the engine for its loading. The flow of OEH gas was controlled by a digital mass flow controller made by Aalborg. The in-cylinder pressure of the engine was measured with a Kistler air-cooled piezoelectric pressure transducer. K type thermocouples were used to measure the temperatures of cooling water, inlet air, and exhaust gas. A Crypton 290 EN2 five gas analyzer was used to measure exhaust gas emissions such as CO, CO₂, UBHC, NO_X and excess oxygen. An AVL smoke meter was used to measure the smoke in terms of Hartridge Smoke Units (HSU). The schematic arrangement of the experiment is shown in Fig. 1.

Table 2 Engine specifications

Specifications of test engine
Make and model	Kirloskar, SV1
General	4-Stroke/vertical
Type	Compression ignition
Number of cylinder	One
Bore	87.5 mm
Stroke	110 mm
Cubic capacity	661 cm^3
Clearance volume	37.8 cm^3
Compression ratio	17.5 : 1
Rated output	5.9 kW
Rated speed	1800 rpm
Combustion chamber	Hemispherical open
Type of cooling	Water-cooled

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Fig. 1 Schematic arrangement of experimental setup.

4. Experimental procedure

When DC power of 12 V was supplied, the potential difference between the anode electrodes and the cathode electrodes, along with the aqueous electrolyte solution of NaOH present in the electrolyzer, generated OEH gas by the process of the electro-chemical dissociation of water. The generated gas was then passed through a drier, a flashback arrestor and a flame trap before mixing it with inlet air. The drier was used to remove the moisture content present in the gas. The flashback arrestor and flame trap were used to suppress the flame in case a backfire from the engine occurred.

5. Experimental uncertainty

In the present experimental investigation, many physical quantities were measured using various instruments. All the instruments were calibrated prior to their use. Uncertainties for the present experimental work are detailed in Table 3. The uncertainties for basic measurements, such as temperature, speed, and time, were taken as the value of the least count of relevant instruments. The possible error that may occur in measuring quantities of CO, CO₂, UBHC, NO_X, O₂, and smoke was taken from manuals supplied by the manufacturers of instruments. The uncertainty for derived quantities was computed on the basis of Holman's method.²² This was based on the work of Kline and McClintock.²³

Table 3 Experimental uncertainties

Variable	Uncertainty
Speed	±1 rpm
Temperature	±1°
Time	±0.1 s
Pressure	±0.6164%
Brake power	±0.9434%
Fuel flow	±0.7319%
NOX	±10 ppm
CO	±0.01%
CO2	±0.03%
UBHC	±1 ppm
Smoke	±1 HSU

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6. Results and discussion

6.1 Brake thermal efficiency (BTE)

Brake thermal efficiency is the real indication of the efficiency of an engine. It is defined as the degree with which the chemical energy available in the fuel is converted into useful work. A graphical representation of the effect of OEH gas on the brake thermal efficiency at different rated load conditions of a test engine at different injection timings of diesel fuel is shown in Fig. 2. When OEH gas is used as a combustion catalyst in diesel combustion, the rate of increase in brake thermal efficiency is higher than for base line operation. The experimental results showed that under the influence of OEH gas at 100% rated load, the brake thermal efficiency increased by 16.45%, 12.21%, and 19.03% for the standard injection timing of 23° BTDC, retarded injection timing of 19° BTDC and advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This increase in brake thermal efficiency is due to the higher heat content of the hydrogen present in the gas mixture, its high flame velocity, and the presence of atomic hydrogen and oxygen in the gas.¹³ The atomic hydrogen and oxygen present in the gas are highly energetic and more reactive than their dual molecule counterparts.²⁰ This intrinsic characteristic enables fissure of the heavier hydrocarbon molecules of diesel fuel and initiates a chain reaction, which results in high-efficiency combustion when the petroleum diesel initiates the ignition.²⁰ When the test engine was operated at the retarded injection time of 19° BTDC with OEH gas, it resulted in a 3.63% decrease in brake thermal efficiency compared to that at the standard injection time operation with OEH gas, while a decrease of 5.73% in brake thermal efficiency was observed as compared to the advanced injection time operation of 27° BTDC with the OEH gas. During the retarded injection time operation, a part of the combustion took place during the expansion stroke. This was also confirmed by the in-cylinder pressure curve at this injection time. At 27° BTDC, the maximum brake thermal efficiency was obtained compared to other injection timings. This result confirms the result obtained by Mohammadi et al.²⁴ Brake thermal efficiency increased by 2.22% at the advanced injection time of 27° BTDC compared to the standard injection time of 23° BTDC.

Fig. 2 Variation of BTE with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.2 Brake specific energy consumption (BSEC)

Fig. 3 represents the variation of BSEC with BP when OEH gas of 4.6 lpm was added in the diesel combustion process at different injection timings of the diesel fuel. The experimental results showed that the BSEC increased when the injection time was retarded, whereas it decreased when the injection time was advanced. Under the influence of the OEH gas at 100% rated load, the BSEC decreased by 14.12%, 10.88%, and 15.99% for the standard injection timing of 23° BTDC, the retarded injection timing of 19° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. Dulger and Ozcelik²⁵ reported that using on-board hydrogen, the fuel consumption and the engine-out emissions could be reduced, and they attributed this decrease in BSEC to the efficient catalytic nature of hydrogen gas. This resulted in the formation of a uniform fuel–air mixture and extraction of more energy from the diesel fuel. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in a 3.77% increase in BSEC compared to the standard injection time operation, and a 6.08% increase in BSEC as compared to the advanced injection time operation of 27° BTDC. This might be due to a shorter ignition delay period,²⁴ which resulted in low efficiency combustion. At 27° BTDC, the minimum BSEC was obtained compared to other injection timings. The BSEC was decreased by 2.22% at the advanced injection time of 27° BTDC compared to the standard injection time of 23° BTDC. This might be due to the participation of a more homogeneous mixture of fuel and air in the combustion process, which resulted in an improved combustion.

Fig. 3 Variation of BSEC with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.3 Carbon monoxide emission (CO)

Fig. 4 depicts the comparison of CO emission for petroleum diesel and diesel with the OEH gas of 4.6 lpm at different injection timings of diesel fuel. When the test engine was operated at the retarded injection time of 19° BTDC at the rated load of the test engine, it resulted in increases of 9.09% and 4.34% in CO emission compared to 23° BTDC and 27° BTDC, respectively. This might be due to the inefficient mixing of fuel and air because some fuel particles in fuel-rich zones might never react with oxygen. The CO emission decreased by 7.69%, 15.38%, and 11.53% at 19° BTDC, 23° BTDC, and 27° BTDC, respectively, compared to the base line operation. This might be due to the high diffusion property of hydrogen and its high flame velocity, resulting in intense combustion. This result confirms the result obtained by Bari and Esmaeil.¹⁴ At 27° BTDC, CO emission increased by 4.54% as compared to that for 23° BTDC operation.

Fig. 4 Variation of CO with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.4 Carbon dioxide emission (CO₂)

Fig. 5 displays the comparison of CO₂ emission when the OEH gas of 4.6 lpm was supplemented in the diesel combustion process at different injection timings of diesel fuel. Advancing the injection time of the diesel fuel increased the CO₂ emission, whereas retarding the injection time reduced the CO₂ emission. Under the influence of OEH gas at full rated load of the engine, CO₂ emission increased by 12.12% and 9.09% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to the spontaneous combustion of OEH gas when its ignition was initiated by a pilot diesel fuel. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in decreases of 5.4% and 2.77% in CO₂ emission compared to that when 23° BTDC and 27° BTDC were applied, respectively. This might be due to the improper conversion of CO to CO₂ due to decreases in combustion temperatures, resulting in less intense combustion. At 23° BTDC, the maximum CO₂ emission was emitted from the engine compared to other injection timings. The CO₂ emission decreased by 2.7% at 27° BTDC compared to that at 23° BTDC. This might be due to the dissociation of CO₂ into CO and excess oxygen. The increase in CO emission at 100% load also justified this.

Fig. 5 Variation of CO₂ with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.5 Unburned hydrocarbon emission (UBHC)

Fig. 6 shows the comparison of UBHC emission when the OEH gas of 4.6 lpm was added in the diesel combustion process at different injection timings of diesel fuel. The advancement of injection time decreases the UBHC emission, whereas retarding the injection amplifies the same. Under the influence of the OEH gas at 100% rated load of the engine, UBHC emission decreased by 19.7% and 22.72% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to enhanced H/C ratio in the overall fuel mixture. This result confirms the result obtained by Shioji and Mohammadi.¹⁷ At the retarded injection timing of 19° BTDC, UBHC emission decreased by 12.12% compared to the base line operation. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in increases of 9.43% and 13.72% in UBHC emission compared to 23° BTDC and 27° BTDC, respectively. This might be due to the low homogeneity of the combustible mixture formed during the ignition delay period. At 27° BTDC, the minimum UBHC emission was exhausted from the engine compared to other injection timings. The UBHC emission decreased by 3.77% at 27° BTDC compared to 23° BTDC. This might be due to the formation of a proper mixture with enough oxygen to burn all the fuel particles.

Fig. 6 Variation of UBHC with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.6 Oxides of nitrogen emission (NO_X)

Fig. 7 represents the comparison of NO_X emission when the OEH gas of 4.6 lpm was added into the diesel combustion process at different injection timings of diesel fuel. The advancement of injection time enhanced NO_X emission, whereas retarding the injection helped to reduce the same. Under the influence of OEH gas at 100% rated load, NO_X emission increased by 16.9% and 21.42% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to an enhanced pre-mixed burning phase²⁶ as a result of the instantaneous combustion of OEH gas when it was ignited by the pilot diesel fuel. The heat release rate curve, shown in Fig. 11, also confirmed this. This result confirms the result obtained by Tomita et al.,²⁶ who investigated the effect of hydrogen injection in a single cylinder, four-stroke diesel engine by varying the injection timing of light oil from 60° BTDC to 5° ATDC. Their results showed that when the injection timing of the light oil was near 25° BTDC, the NO_X emission was increased and exhibited the maximum value. In the present experiment, at the retarded injection timing of 19° BTDC, NO_X emission was decreased by 9.04% compared to the base line operation. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in decreases of 22.19% and 25.09% in NO_X emission compared to 23° BTDC and 27° BTDC, respectively. This might be due to a low temperature atmosphere prevailing in the combustion chamber because less time was available to form a homogeneous mixture during the ignition delay period, which resulted in a drop in the combustion temperature.²⁷ At 27° BTDC, the maximum NO_X emission occurred in the engine compared to other injection timings. NO_X emission increased by 3.86% at 27° BTDC compared to 23° BTDC. This might be due to an increase in the ignition delay period.²⁶ When the start of fuel injection timing was earlier, the initial air temperature and pressure was lower. This caused the ignition delay period to increase, which in-turn increased the premixed burning phase, the cylinder gas temperature and the NO_X emissions.²⁸

Fig. 7 Variation of NO_X with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.7 Smoke emission

Fig. 8 displays the comparison of smoke emission when the OEH gas of 4.6 lpm was added in the diesel combustion process at different injection timings of the diesel fuel and petroleum diesel combustion at the standard injection timing. The experimental results showed that smoke emission was increased when the injection time was retarded and it decreased when the injection time was advanced. Under the influence of the OEH gas at 100% rated load of the engine, the smoke emission decreased by 28.57%, 19.04%, and 30.95% for the standard injection timing of 23° BTDC, the retarded injection timing of 19° BTDC, and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This result confirms the result obtained by Birtas et al.¹⁶ When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in an increase of 13.33% in smoke emission compared to the standard injection time operation, whereas an increase of 17.24% in smoke emission compared to the advanced injection time operation of 27° BTDC. When the injection time of diesel fuel was retarded, the regions of the better air–fuel mixing were decreased. This in-turn decreased the pre-mixed combustion phase²⁴ and the heat release rate. At 27° BTDC, the minimum smoke emission was obtained compared to other injection timings. The smoke emission decreased by 3.33% at the advanced injection time of 27° BTDC compared to the standard injection time of 23° BTDC. When the diesel fuel was injected at the advanced injection time, the fuel had sufficient time to mix with air molecules.²⁶ This resulted in the formation of a more homogeneous mixture of fuel and air.²⁶ When this mixture was ignited, the combustion resulted in less smoke emission compared to other injection timed operations.

Fig. 8 Variation of smoke emission with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.8 Excess oxygen emission

Fig. 9 depicts the comparison of excess oxygen emission for petroleum diesel and diesel with the OEH of 4.6 lpm at different injection timings of diesel fuel. The experimental results showed that the excess oxygen emission increased when the injection timing was retarded and decreased when the injection timing was advanced. When the test engine was operated at the retarded injection time of 19° BTDC at the rated load of the test engine, it resulted in increases of 1.19% and 6% in excess oxygen emission compared to 23° BTDC and 27° BTDC, respectively. This might be due to the existence of more fuel-rich zones at this injection timed operation. The excess oxygen emission decreased by 7.78%, 8.87%, and 13.01% at 19° BTDC, 23° BTDC, and 27° BTDC, respectively, compared to the base line operation. This might be due to the high diffusion co-efficient of the hydrogen present in the gas mixture and its low activation energy, resulting in efficient combustion.²⁹ This confirms the result obtained by Avadhanula et al.³⁰ in their investigation on hydrogen fueled engines. At 27° BTDC, the excess oxygen emitted from the engine was lower as compared to other injection timed operations. The excess oxygen available at the exhaust of the engine at the advanced injection time of 27° BTDC decreased by 4.54% compared to that of 23° BTDC operation. This decrease might be due to the occurrence of more molecular collisions during the combustion at this injection timed operation than at other injection timed operations.

Fig. 9 Variation of excess oxygen emission with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.9 Exhaust gas temperature (EGT)

Fig. 10 illustrates the comparison of EGT of petroleum diesel combustion when the OEH gas of 4.6 lpm was added in the diesel combustion process at different injection timings of the diesel fuel. Advancing the injection time decreased the EGT, whereas retarding the injection time augmented the same. Under the influence of the OEH gas at the maximum load of the test engine, EGT increased by 6.41% and 4.61% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to an enhanced premixed burning phase resulting from the spontaneous combustion of OEH gas, which increased the average cylinder temperature. At the retarded injection timing of 19° BTDC, EGT increased by 8.71% compared to the base line operation. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in increases of 2.16% and 3.92% in the EGT compared to 23° BTDC and 27° BTDC injection timed operations. This might be due to the improper expansion of combustion gases because little time was available for expansion. This result confirms the result obtained by Fathi et al.²⁷ in their CFD analysis of hydrogen fueled engines. At 27° BTDC, the minimum EGT was exhausted from the engine compared to other injection timings. The EGT decreased by 1.68% at 27° BTDC compared to that at 23° BTDC. The peak pressure was achieved at around a 362 degree crank angle for 27° BTDC combustion. This facilitated a more complete expansion of combustion gases when compared to other injection timed operations. The heat release curve at this injection time also confirmed this statement.

Fig. 10 Variation of EGT with BP for different injection timings of diesel fuel with OEH gas of 4.6 lpm.

6.10 Heat release rate (HRR)

Fig. 11 compares heat release rate with crank angle when OEH gas of 4.6 lpm was inducted into the diesel combustion process at different injection timings of diesel fuel at the rated load of the engine. The advancement of injection time augmented the heat release rate, whereas retarding the injection time helped to decrease the same. Under the influence of the OEH gas of 4.6 lpm at 100% rated load of the engine, the heat release rate increased by 13.75% and 16.25% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to more constant volume combustion. This led to an enhanced premixed combustion phase. This result confirms the result obtained by Tomita et al.²⁶ At the retarded injection timing of 19° BTDC, the heat release rate decreased by 7.5% compared to the base line operation. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in decreases of 18.68% and 20.43% in heat release rate compared to 23° BTDC and 27° BTDC operations, respectively. When the engine was operated at the retarded injection time of diesel fuel, the fuel was introduced into the cylinder at a comparatively higher pressure and temperature environment. Owing to this, the ignition delay period and the pre-mixed combustion phase were reduced.²⁴ At 27° BTDC, the maximum heat release rate was obtained compared to the other injection timings. The heat release rate increased by 2.19% at 27° BTDC compared to that at 23° BTDC. This might be due to the elevated flame temperature and a less heterogeneous fuel–air mixture at this injection timed operation.

Fig. 11 Variation of HRR with CA for different injection timings of diesel fuel with OEH gas of 4.6 lpm at the rated load.

6.11 In-cylinder pressure

Fig. 12 compares in-cylinder pressure with crank angle when the OEH gas of 4.6 lpm was inducted in the diesel combustion process at different injection timings of diesel fuel at rated load of the engine. Advancing the injection time of diesel fuel amplified the peak in-cylinder pressure, whereas retarding the injection time helped to decrease the same. When the OEH gas of 4.6 lpm was introduced to the diesel combustion at 100% rated load, the peak in-cylinder pressure increased by 5.71% and 10.72% for the standard injection timing of 23° BTDC and the advanced injection timing of 27° BTDC, respectively, compared to the base line operation. This might be due to an enhanced pre-mixed burning phase. When the pre-mixed burning was enhanced, flame propagation through the hydrogen–air mixture led to rapid heat release rates, increased peak cylinder pressure and temperature, and improved brake thermal efficiency.³¹ At the retarded injection timing of 19° BTDC, the peak in-cylinder pressure was decreased by 2.85% compared to the base line operation. When the test engine was operated at the retarded injection time of 19° BTDC, it resulted in 8.1% and 12.25% decreases in peak in-cylinder pressures compared to 23° BTDC and 27° BTDC, respectively. At 27° BTDC, the maximum peak in-cylinder pressure was obtained compared to the other injection timings. The peak in-cylinder pressure increased by 4.72% at 27° BTDC compared to that at 23° BTDC. The peak in-cylinder pressure primarily depends on mixing rate, temperature and availability of oxidants such as OH and oxygen radicals. At the injection time of 27° BTDC, all these facts were more pronounced and resulted in a more homogeneous mixture and efficient combustion.

Fig. 12 Variation of in-cylinder pressure with CA for different injection timings of diesel fuel with OEH gas of 4.6 lpm at the rated load.

7. Conclusion

The results of the present study distinctly show that the performance of a DI diesel engine can be enhanced and all engine-out emissions can be reduced using the OEH gas as a combustion catalyst in diesel combustion with change in the injection timing of a diesel fuel. Some of the important conclusions drawn from the present experimental study are presented in this section. The optimized injection timing of the diesel fuel is 19° BTDC when 4.6 lpm of the OEH gas is used as a combustion catalyst in diesel combustion. When the OEH gas of 4.6 lpm was inducted at 100% rated load of the engine with diesel injection timings of 23° BTDC, 27° BTDC, and 19° BTDC, the brake thermal efficiency increased by 16.45%, 19.03% and 12.21%, respectively, compared to the base line operation. This might be due to the high catalytic nature of the OEH gas, which enhances the overall combustion phenomena. The NO_X emission increased by 16.9% and 21.42% for the injection timings of 23° BTDC and 27° BTDC respectively. However, when the injection timing was retarded to 19° BTDC, it was reduced by 9.04%. The smoke emissions were reduced by 28.57%, 30.95%, and 19.04% for the injection timings of 23° BTDC, 27° BTDC, and 19° BTDC, respectively. UBHC emissions were reduced by 19.7%, 22.72%, and 12.12% for the injection timings of 23° BTDC, 27° BTDC, and 19° BTDC, respectively. The use of the OEH gas as a combustion catalyst in DI diesel engine combustion results in improvements in performance and reduction in emissions, which can be observed from the abovementioned results. In particular, the retarded injection timing of 19° BTDC gives a significant improvement in performance as well as emission characteristics of a test engine compared to the base line operation, including NO_X emission, which was not achieved by other injection timed operations. Based on the overall analysis, it is concluded that the DI diesel engine can be operated comfortably and efficiently using the OEH gas as a combustion catalyst with the retarded injection timing of a diesel fuel.

Abbreviations

BP	Brake power
BSEC	Brake specific energy consumption
BTDC	Before top dead centre
BTE	Brake thermal efficiency
CA	Crank angle
CI	Compression ignition
CO	Carbon monoxide
CO2	Carbon dioxide
DI	Direct injection
EGT	Exhaust gas temperature
H2–O2	Hydrogen–oxygen
HRG	Hydrogen rich gas
HRR	Heat release rate
HSU	Hartridge smoke unit
LCG	Low calorific gases
lpm	Litres per minute
NaOH	Sodium hydroxide
NOX	Oxides of nitrogen
OEH	Oxygen enriched hydrogen
UBHC	Unburned hydrocarbon

Acknowledgements

The author would like to thank Professor Dr K. Annamalai of the Department of Automobile Engineering, Anna University, Chennai and Professor Dr A. R. Pradeepkumar of the Department of Mechanical Engineering, Dhanalakshmi College of Engineering, Chennai for their immense help to complete this study successfully.

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