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Effect of combustion duration on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel
International Journal of Hydrogen Energy 25 (2000) 581±589
Eect of combustion duration on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel Jehad A.A. Yamin a,*, H.N. Gupta a, B.B. Bansal a, O.N. Srivastava b a
Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi, 221 005, India b Department of Physics, Banaras Hindu University, Varanasi, India
Abstract The deteriorating quality of air due to exhaust emissions and the increasing number of motor vehicles in the world, is sending alarming waves throughout the world to try to do something to cut o or signi®cantly reduce these emissions in order to save our planet. Scientists have found that hydrogen presents the best and unprecedented solution to this problem, for its superior combustion qualities and availability. This paper discusses analytically one aspect of combustion i.e. combustion duration and how it is aected by an engine's operating parameters like compression ratio, equivalence ratio, spark plug location, spark timing and engine speed, and how it aects an engine's performance parameters like brake speci®c fuel consumption, brake mean eective pressure, thermal eciency, as well as emission characteristics. # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction During 1994, the global motor vehicle population including passenger cars, trucks, buses, motorcycles and three-wheeled vehicles exceeded 700 million for the ®rst time in history. While most of these vehicles remain concentrated in the highly industrialized countries of the OECD, an increasing number of urbanized areas in developing countries contain large numbers of vehicles. Cities such as Delhi, Mexico, Bangkok and Seoul are certainly among those experiencing the most congested roads in the world. While these vehicles have brought many advantages in
* Corresponding author. Tel.: +91-542-316-434; fax: +91542-316-428. E-mail address: [email protected] (J.A.A. Yamin).
increasing the mobility and ¯exibility of millions of people, creating more jobs and enhancing many aspects of the quality of life, these bene®ts have been partially oset by excess pollution which adversely aect the quality of life. Motor vehicles emit large quantities of carbon monoxide, hydrocarbons, nitrogen oxides and such toxic substances like ®ne particles. Each one of these can cause adverse eects on human health and environment. Because of this rapid growing rate in vehicle population in the world and the high emission rates from these vehicles, serious air pollution problems have become an increasingly common phenomenon in modern life. Initially, these problems were limited to city centers; but, today lakes, streams and even remote forests are also experiencing signi®cant degradation. As more and more evidence of man-made impact on the upper at-
0360-3199/00/$20.00 # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 3 1 - 2
582
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
mosphere accumulate, concerns are increasing that motor vehicles are contributing to global changes which could alter the climate of the planet. This alarming fact has led researchers to try to ®nd an alternative to the presently used fuels, which could be pollution-free, environmentally friendly, economical and compatible with today's automobiles. Among the suggested alternatives, gaseous fuels have shown excellent performance over others like alcohol. Among the gaseous fuels suggested were: CNG (Methane CH4), LPG (mainly Propane C3H8) and Hydrogen (H2). However, since, theoretically, burning hydrogen produces only water (H2O) with no CO, CO2, HC or ®ne particles, and since it can be produced without causing any ecological disorder, hydrogen as a future fuel has been drawing greater attention. In the present paper, hydrogen as a fuel has been considered. Hydrogen has the following advantages over gasoline: 1. Reduced deposits due to more homogeneous mixture formation. 2. Reduced engine oil dilution and increased oil life. 3. Reduced engine wear, hence increased engine life. 4. Higher compression ratios can be used which may solve the problem of reduced power output due to reduction in volumetric eciency. 5. Elimination of emissions of CO and HC. 6. Increased fuel economy due to possible operation at leaner mixtures. However, the introduction of electronic injectors and the availability of lead-free gasoline have eroded these advantages in eciency and emissions previously enjoyed by hydrogen fueled engines. Thus, future use of hydrogen in automobiles area depends upon further improvements in utilization of hydrogen in I. C. Engines. This paper throws light on one of the factors that has a greater eect on engine performance i.e. combustion duration. An analytical model was developed, tested and veri®ed against the experimental data of several engines and used to study the eect of various operating parameters on combustion duration as well as the eect of combustion duration on the engine performance and emission parameters, in order to get to a better understanding of the interaction between these parameters which will help the engine designers while designing for hydrogen.
2. Availability and suitability of hydrogen as an S. I. Engine fuel For any fuel to be considered as an alternative fuel, it has to ful®l certain criteria. The basic criteria for selecting any alternative is:
Table 1 Properties of hydrogen Characteristic
Hydrogen
Chemical formula Relative molecular mass (kg/kmol) Density at 158C (kg/l) Stoichiometric A/F (kg/kg) Flame speed (m/s) Flammability limits in air (% vol): Upper Lower Lower calori®c value (kJ/kg)
H2 2.015 0.0898 34.3 127 74.5 4.1 119.657
1. Availability: the fuel has to be in abundant supply or, preferably, derived from renewable sources. 2. High speci®c energy content. 3. Easy transportation and storage. 4. Minimum environmental pollution and resource depletion. 5. Good safety and handling properties. Hydrogen has provided its superiority over gasoline in most of these criteria. It can be noticed from Table 1 that with hydrogen, the engine tends to operate at leaner mixtures, making engine operation more economical. With a higher calori®c value, lower density and lower boiling point, hydrogen used in vapor form engine operation and life is signi®cantly improved with respect to gasoline. From the viewpoint of engine performance parameters, operation with hydrogen reduces the BSFC. However, because of the loss of volumetric eciency, mainly due to high inlet temperature, engines tend to produce (20%) lesser power than that
Table 2 Engine design and operating conditions Engine speed
Variable
Cylinder bore Stroke Connecting rod length Displacement volume Compression ratio Intake valve Diameter Opens Closes Exhaust valve Diameter Opens Closes Ignition timing
76.2 mm 111.125 mm 233.35 mm 506 cm3 Variable 35.0 mm 98 BTDC 368 ABDC 30.05 mm 428 BTDC 78 ABDC Variable
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
583
Fig. 1. Comparison between experimental and calculated results.
of gasoline. Some of the properties of the fuel used are shown in Table 1.
3. Brief description of the model and validation The simulation program called `SimHydrogEn' is based on the theory developed by researchers [1±7] and is an extension of the work of Gupta et al. [3]. This has been largely modi®ed to cover a wide range
of engines and the gas exchange process and turbulent combustion model included. The engine modeled and used for the validation of the program is the E6/T Ricardo Variable Compression Engine. Brief technical data are shown in Table 2. The results of the mathematical model were then veri®ed against the experimental data of the engine supplied by the manufacturer, as shown in Fig. 1. The ®gure shows that the results predicted by the mathematical model are very close (within 3%) of the ex-
Fig. 2. Eect of engine speed and equivalence ratio on combustion duration at MBT and WOT.
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J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 3. Eect of compression ratio and spark plug location on combustion duration at MBT and WOT.
perimental results. This veri®es that the model developed can be used to a great degree of accuracy.
4. Results and discussion Discussion on the results has been presented in three parts: 1. Eect of operating conditions on the combustion duration. 2. Eect of combustion duration on performance parameters. 3. Eect of combustion duration on emission characteristics.
4.1. Eect of operating conditions on the combustion duration 4.1.1. Eect of engine speed on combustion duration Fig. 2 shows that the combustion duration (in ms) decreases as the engine speed (in rpm) increases. This is a clear eect of turbulence. As the engine speed increases, the turbulence inside the cylinder increases, leading to a better heat transfer between the burned and unburned zones. 4.1.2. Eect of equivalence ratio on the combustion duration Referring to Fig. 2, it can be observed that operating at lean or rich mixtures tends to increase the combustion duration. This eect is more predominant at higher speeds. This is because of the lesser thermal energy liberated from the leaner mixtures which
increases the ignition delay and slows the ¯ame propagation. The ¯ame temperature is low at lean and rich mixtures. Further, the incomplete combustion due to oxygen de®ciency at rich mixtures also has an adverse eect over the ¯ame speed. Also from this ®gure it can be seen that the combustion duration is minimum at equivalence ratios (hereafter denoted by `l') nearly equal to 1.0 for all engine speeds. 4.1.3. Eect of compression ratio on combustion duration Fig. 3 shows that the combustion duration decreases as the compression ratio increases. This is because of the increase in the end-of-compression temperature and pressure and decrease in the fraction residual gases. This creates a favorable condition for the reduction of ignition lag and increase in the ¯ame speed. In the present analysis, a compression ratio equal to 9.0 has been chosen for further analysis because most of the present day automobiles (S. I. Engines) have compression ratios in this range. 4.1.4. Eect of spark plug location on combustion duration Before proceeding with the discussion it would be appropriate to de®ne the term XSP used in the program. XSP is a non-dimensional parameter referring to the ratio of the distance between the spark plug location from the nearest wall to the cylinder diameter. Fig. 3 shows the eect of the compression ratio and spark plug location on the combustion duration. The graphs have been plotted for l=1.0 and engine speed=2500 rpm. Referring to Fig. 3, it can be seen that as we move the spark from the peripheral position (i.e. XSP=0.08)
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
585
Fig. 4. (a) Eect of spark plug location on BMEP at dierent engine speeds at MBP and WOT; (b) eect of spark plug location on BSFC at dierent engine speeds at MBT and WOT; (c) eect of spark plug location on oxides of nitrogen at dierent engine speeds at MBT and WOT.
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J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 5. Eect of Spark advance on combustion duration.
to the center (i.e. XSP=0.5) the combustion duration decreases. This is because of the decrease in the distance traveled by the ¯ame. The spark location has a greater eect in suppressing detonation. For selecting the best location for the present study an analysis on the eect of XSP on some engine performance and emission parameters was made. Referring to Fig. 4 (a± c), it can be observed that there is no signi®cant change in the power, economy parameters and emission characteristics between XSP=0.08 and XSP=0.5. Taking the possibility of detonation into account, location XSP=0.29 has been selected for further analysis. 4.1.5. Eect of spark advance on combustion duration As is clear from Fig. 5, retarding or advancing the
spark timing beyond MBT leads to increasing the combustion duration, reaching its minimum at MBT. This timing is selected to eliminate the eect of spark timing. 4.2. Eect of combustion duration on engine's performance parameters After studying the eect of operating parameters on the combustion duration, the eect of combustion duration on engine performance has been examined. Before proceeding further, let us discuss some aspects of the combustion process. Theoretically, the rate of combustion should be such that combustion duration is minimum with the high
Fig. 6. Eect of combustion duration on peak unburned temperature.
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
587
Fig. 7. Eect of combustion duration on BMEP.
rate of pressure rise. Pressure should be maximum just at TDC to produce greater force acting through a long period of the power stroke. This means however, that the product will have enough time to lose some of its heat to the coolant, resulting in poor performance added to the roughness of the engine operation. Therefore, in practice, engines are so designed that only 50% of the pressure rise is completed by the TDC, resulting in peak pressure and temperature occurring at 10±158 after TDC which reduces the heat loss and make the engine operation smooth. Therefore, for best results, the combustion has to be completed within 158 after TDC. Figs. 6±7 show that an increase in the combustion duration causes the peak temperature and the brake mean eective pressure to decrease. This is because of
the increased heat losses which is shown in Fig. 8. On the other hand, because increasing combustion duration increases the lean mis®re limit, there is a decrease in the BSFC to certain limit as shown in Fig. 9, and improvement of the thermal eciency as shown in Fig. 10. Therefore, it has become clear that any attempt to increase the combustion duration either by reducing the compression ratio, locating the spark near the periphery or operating at leaner mixtures, is going to improve the engine's economy, but is going to reduce the power output. Further, examination of the ®gures leads to a conclusion that for better performance in terms of power and economy, the combustion duration has to be between 4±6 ms which means that the engine should run on a mixture slightly leaner than stoichiometric (l=0.9±1.0).
Fig. 8. Eect of combustion duration on percentage heat loss.
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J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 9. Eect of combustion duration on BSFC.
Fig. 10. Eect of combustion duration on brake thermal eciency.
Fig. 11. Eect of combustion duration on nitric oxides level.
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
4.3. Eect of combustion duration on emission characteristics From the emission point of view, it may be seen from Fig. 11 that NOX emissions are lower when combustion duration is high. This is because it allows more time for the combustion to complete. Further, at higher combustion duration (Fig. 6), the peak temperature is low and therefore the formation of NOX is reduced. On the other hand, decreasing the combustion duration beyond a certain limit, reduces the concentration of NOX because of the lesser time of exposure of products of combustion to cylinder's peak temperature. If the mixture is made progressively rich, the combustion duration decreases. 5. Conclusions 1. Engine operating parameters have to be carefully chosen by the designer, taking into account their eect on the combustion duration. 2. Any attempt to control emissions by operating the engine at leaner mixtures has to take into account the eect on other variables like B.M.E.P. and B.S.F.C. 3. Combustion duration has a signi®cant eect on both performance and emission characteristics of the engine and has to be carefully designed to achieve the best engine performance characteristics.
589
Acknowledgements The authors are deeply grateful to the hydrogen energy group, Department of Mechanical Engineering, for their continuous support and encouragement while carrying out this work.
References [1] Heywood JB. Internal combustion engine fundamentals. Singapore: McGraw±Hill Book Company, 1989. [2] Benson RS. In: The thermodynamics and gas dynamics of internal combustion engines, vol. 1. Oxford: Clarendon Press, 1982. [3] Gupta HN, Bansal BB, Mohan R. Computer simulation of power cycle for spark-ignition engine. IE(I)-JournalMC 1995;76 November. [4] Gupta HN Theoretical and experimental investigation of multi-cylinder spark ignition engine with air injection and catalytic reactor. Ph.D. Thesis, Faculty of Technology, University of Manchester 1977. [5] Arkhangelsky V. Motor vehicle engines. Moscow: Mir Publishers, 1971. [6] Beretta GP, Rashidi M, Keck JC. Turbulent ¯ame propagation and combustion in spark ignition engines. Combustion and Flame 1983;52:217±45. [7] Blizard NC, Keck JC. Experimental and theoretical investigation of turbulent burning model for internal combustion engine. SAE 740191 1974.
Eect of combustion duration on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel Jehad A.A. Yamin a,*, H.N. Gupta a, B.B. Bansal a, O.N. Srivastava b a
Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi, 221 005, India b Department of Physics, Banaras Hindu University, Varanasi, India
Abstract The deteriorating quality of air due to exhaust emissions and the increasing number of motor vehicles in the world, is sending alarming waves throughout the world to try to do something to cut o or signi®cantly reduce these emissions in order to save our planet. Scientists have found that hydrogen presents the best and unprecedented solution to this problem, for its superior combustion qualities and availability. This paper discusses analytically one aspect of combustion i.e. combustion duration and how it is aected by an engine's operating parameters like compression ratio, equivalence ratio, spark plug location, spark timing and engine speed, and how it aects an engine's performance parameters like brake speci®c fuel consumption, brake mean eective pressure, thermal eciency, as well as emission characteristics. # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction During 1994, the global motor vehicle population including passenger cars, trucks, buses, motorcycles and three-wheeled vehicles exceeded 700 million for the ®rst time in history. While most of these vehicles remain concentrated in the highly industrialized countries of the OECD, an increasing number of urbanized areas in developing countries contain large numbers of vehicles. Cities such as Delhi, Mexico, Bangkok and Seoul are certainly among those experiencing the most congested roads in the world. While these vehicles have brought many advantages in
* Corresponding author. Tel.: +91-542-316-434; fax: +91542-316-428. E-mail address: [email protected] (J.A.A. Yamin).
increasing the mobility and ¯exibility of millions of people, creating more jobs and enhancing many aspects of the quality of life, these bene®ts have been partially oset by excess pollution which adversely aect the quality of life. Motor vehicles emit large quantities of carbon monoxide, hydrocarbons, nitrogen oxides and such toxic substances like ®ne particles. Each one of these can cause adverse eects on human health and environment. Because of this rapid growing rate in vehicle population in the world and the high emission rates from these vehicles, serious air pollution problems have become an increasingly common phenomenon in modern life. Initially, these problems were limited to city centers; but, today lakes, streams and even remote forests are also experiencing signi®cant degradation. As more and more evidence of man-made impact on the upper at-
0360-3199/00/$20.00 # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 3 1 - 2
582
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
mosphere accumulate, concerns are increasing that motor vehicles are contributing to global changes which could alter the climate of the planet. This alarming fact has led researchers to try to ®nd an alternative to the presently used fuels, which could be pollution-free, environmentally friendly, economical and compatible with today's automobiles. Among the suggested alternatives, gaseous fuels have shown excellent performance over others like alcohol. Among the gaseous fuels suggested were: CNG (Methane CH4), LPG (mainly Propane C3H8) and Hydrogen (H2). However, since, theoretically, burning hydrogen produces only water (H2O) with no CO, CO2, HC or ®ne particles, and since it can be produced without causing any ecological disorder, hydrogen as a future fuel has been drawing greater attention. In the present paper, hydrogen as a fuel has been considered. Hydrogen has the following advantages over gasoline: 1. Reduced deposits due to more homogeneous mixture formation. 2. Reduced engine oil dilution and increased oil life. 3. Reduced engine wear, hence increased engine life. 4. Higher compression ratios can be used which may solve the problem of reduced power output due to reduction in volumetric eciency. 5. Elimination of emissions of CO and HC. 6. Increased fuel economy due to possible operation at leaner mixtures. However, the introduction of electronic injectors and the availability of lead-free gasoline have eroded these advantages in eciency and emissions previously enjoyed by hydrogen fueled engines. Thus, future use of hydrogen in automobiles area depends upon further improvements in utilization of hydrogen in I. C. Engines. This paper throws light on one of the factors that has a greater eect on engine performance i.e. combustion duration. An analytical model was developed, tested and veri®ed against the experimental data of several engines and used to study the eect of various operating parameters on combustion duration as well as the eect of combustion duration on the engine performance and emission parameters, in order to get to a better understanding of the interaction between these parameters which will help the engine designers while designing for hydrogen.
2. Availability and suitability of hydrogen as an S. I. Engine fuel For any fuel to be considered as an alternative fuel, it has to ful®l certain criteria. The basic criteria for selecting any alternative is:
Table 1 Properties of hydrogen Characteristic
Hydrogen
Chemical formula Relative molecular mass (kg/kmol) Density at 158C (kg/l) Stoichiometric A/F (kg/kg) Flame speed (m/s) Flammability limits in air (% vol): Upper Lower Lower calori®c value (kJ/kg)
H2 2.015 0.0898 34.3 127 74.5 4.1 119.657
1. Availability: the fuel has to be in abundant supply or, preferably, derived from renewable sources. 2. High speci®c energy content. 3. Easy transportation and storage. 4. Minimum environmental pollution and resource depletion. 5. Good safety and handling properties. Hydrogen has provided its superiority over gasoline in most of these criteria. It can be noticed from Table 1 that with hydrogen, the engine tends to operate at leaner mixtures, making engine operation more economical. With a higher calori®c value, lower density and lower boiling point, hydrogen used in vapor form engine operation and life is signi®cantly improved with respect to gasoline. From the viewpoint of engine performance parameters, operation with hydrogen reduces the BSFC. However, because of the loss of volumetric eciency, mainly due to high inlet temperature, engines tend to produce (20%) lesser power than that
Table 2 Engine design and operating conditions Engine speed
Variable
Cylinder bore Stroke Connecting rod length Displacement volume Compression ratio Intake valve Diameter Opens Closes Exhaust valve Diameter Opens Closes Ignition timing
76.2 mm 111.125 mm 233.35 mm 506 cm3 Variable 35.0 mm 98 BTDC 368 ABDC 30.05 mm 428 BTDC 78 ABDC Variable
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
583
Fig. 1. Comparison between experimental and calculated results.
of gasoline. Some of the properties of the fuel used are shown in Table 1.
3. Brief description of the model and validation The simulation program called `SimHydrogEn' is based on the theory developed by researchers [1±7] and is an extension of the work of Gupta et al. [3]. This has been largely modi®ed to cover a wide range
of engines and the gas exchange process and turbulent combustion model included. The engine modeled and used for the validation of the program is the E6/T Ricardo Variable Compression Engine. Brief technical data are shown in Table 2. The results of the mathematical model were then veri®ed against the experimental data of the engine supplied by the manufacturer, as shown in Fig. 1. The ®gure shows that the results predicted by the mathematical model are very close (within 3%) of the ex-
Fig. 2. Eect of engine speed and equivalence ratio on combustion duration at MBT and WOT.
584
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 3. Eect of compression ratio and spark plug location on combustion duration at MBT and WOT.
perimental results. This veri®es that the model developed can be used to a great degree of accuracy.
4. Results and discussion Discussion on the results has been presented in three parts: 1. Eect of operating conditions on the combustion duration. 2. Eect of combustion duration on performance parameters. 3. Eect of combustion duration on emission characteristics.
4.1. Eect of operating conditions on the combustion duration 4.1.1. Eect of engine speed on combustion duration Fig. 2 shows that the combustion duration (in ms) decreases as the engine speed (in rpm) increases. This is a clear eect of turbulence. As the engine speed increases, the turbulence inside the cylinder increases, leading to a better heat transfer between the burned and unburned zones. 4.1.2. Eect of equivalence ratio on the combustion duration Referring to Fig. 2, it can be observed that operating at lean or rich mixtures tends to increase the combustion duration. This eect is more predominant at higher speeds. This is because of the lesser thermal energy liberated from the leaner mixtures which
increases the ignition delay and slows the ¯ame propagation. The ¯ame temperature is low at lean and rich mixtures. Further, the incomplete combustion due to oxygen de®ciency at rich mixtures also has an adverse eect over the ¯ame speed. Also from this ®gure it can be seen that the combustion duration is minimum at equivalence ratios (hereafter denoted by `l') nearly equal to 1.0 for all engine speeds. 4.1.3. Eect of compression ratio on combustion duration Fig. 3 shows that the combustion duration decreases as the compression ratio increases. This is because of the increase in the end-of-compression temperature and pressure and decrease in the fraction residual gases. This creates a favorable condition for the reduction of ignition lag and increase in the ¯ame speed. In the present analysis, a compression ratio equal to 9.0 has been chosen for further analysis because most of the present day automobiles (S. I. Engines) have compression ratios in this range. 4.1.4. Eect of spark plug location on combustion duration Before proceeding with the discussion it would be appropriate to de®ne the term XSP used in the program. XSP is a non-dimensional parameter referring to the ratio of the distance between the spark plug location from the nearest wall to the cylinder diameter. Fig. 3 shows the eect of the compression ratio and spark plug location on the combustion duration. The graphs have been plotted for l=1.0 and engine speed=2500 rpm. Referring to Fig. 3, it can be seen that as we move the spark from the peripheral position (i.e. XSP=0.08)
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
585
Fig. 4. (a) Eect of spark plug location on BMEP at dierent engine speeds at MBP and WOT; (b) eect of spark plug location on BSFC at dierent engine speeds at MBT and WOT; (c) eect of spark plug location on oxides of nitrogen at dierent engine speeds at MBT and WOT.
586
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 5. Eect of Spark advance on combustion duration.
to the center (i.e. XSP=0.5) the combustion duration decreases. This is because of the decrease in the distance traveled by the ¯ame. The spark location has a greater eect in suppressing detonation. For selecting the best location for the present study an analysis on the eect of XSP on some engine performance and emission parameters was made. Referring to Fig. 4 (a± c), it can be observed that there is no signi®cant change in the power, economy parameters and emission characteristics between XSP=0.08 and XSP=0.5. Taking the possibility of detonation into account, location XSP=0.29 has been selected for further analysis. 4.1.5. Eect of spark advance on combustion duration As is clear from Fig. 5, retarding or advancing the
spark timing beyond MBT leads to increasing the combustion duration, reaching its minimum at MBT. This timing is selected to eliminate the eect of spark timing. 4.2. Eect of combustion duration on engine's performance parameters After studying the eect of operating parameters on the combustion duration, the eect of combustion duration on engine performance has been examined. Before proceeding further, let us discuss some aspects of the combustion process. Theoretically, the rate of combustion should be such that combustion duration is minimum with the high
Fig. 6. Eect of combustion duration on peak unburned temperature.
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
587
Fig. 7. Eect of combustion duration on BMEP.
rate of pressure rise. Pressure should be maximum just at TDC to produce greater force acting through a long period of the power stroke. This means however, that the product will have enough time to lose some of its heat to the coolant, resulting in poor performance added to the roughness of the engine operation. Therefore, in practice, engines are so designed that only 50% of the pressure rise is completed by the TDC, resulting in peak pressure and temperature occurring at 10±158 after TDC which reduces the heat loss and make the engine operation smooth. Therefore, for best results, the combustion has to be completed within 158 after TDC. Figs. 6±7 show that an increase in the combustion duration causes the peak temperature and the brake mean eective pressure to decrease. This is because of
the increased heat losses which is shown in Fig. 8. On the other hand, because increasing combustion duration increases the lean mis®re limit, there is a decrease in the BSFC to certain limit as shown in Fig. 9, and improvement of the thermal eciency as shown in Fig. 10. Therefore, it has become clear that any attempt to increase the combustion duration either by reducing the compression ratio, locating the spark near the periphery or operating at leaner mixtures, is going to improve the engine's economy, but is going to reduce the power output. Further, examination of the ®gures leads to a conclusion that for better performance in terms of power and economy, the combustion duration has to be between 4±6 ms which means that the engine should run on a mixture slightly leaner than stoichiometric (l=0.9±1.0).
Fig. 8. Eect of combustion duration on percentage heat loss.
588
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
Fig. 9. Eect of combustion duration on BSFC.
Fig. 10. Eect of combustion duration on brake thermal eciency.
Fig. 11. Eect of combustion duration on nitric oxides level.
J.A.A. Yamin et al. / International Journal of Hydrogen Energy 25 (2000) 581±589
4.3. Eect of combustion duration on emission characteristics From the emission point of view, it may be seen from Fig. 11 that NOX emissions are lower when combustion duration is high. This is because it allows more time for the combustion to complete. Further, at higher combustion duration (Fig. 6), the peak temperature is low and therefore the formation of NOX is reduced. On the other hand, decreasing the combustion duration beyond a certain limit, reduces the concentration of NOX because of the lesser time of exposure of products of combustion to cylinder's peak temperature. If the mixture is made progressively rich, the combustion duration decreases. 5. Conclusions 1. Engine operating parameters have to be carefully chosen by the designer, taking into account their eect on the combustion duration. 2. Any attempt to control emissions by operating the engine at leaner mixtures has to take into account the eect on other variables like B.M.E.P. and B.S.F.C. 3. Combustion duration has a signi®cant eect on both performance and emission characteristics of the engine and has to be carefully designed to achieve the best engine performance characteristics.
589
Acknowledgements The authors are deeply grateful to the hydrogen energy group, Department of Mechanical Engineering, for their continuous support and encouragement while carrying out this work.
References [1] Heywood JB. Internal combustion engine fundamentals. Singapore: McGraw±Hill Book Company, 1989. [2] Benson RS. In: The thermodynamics and gas dynamics of internal combustion engines, vol. 1. Oxford: Clarendon Press, 1982. [3] Gupta HN, Bansal BB, Mohan R. Computer simulation of power cycle for spark-ignition engine. IE(I)-JournalMC 1995;76 November. [4] Gupta HN Theoretical and experimental investigation of multi-cylinder spark ignition engine with air injection and catalytic reactor. Ph.D. Thesis, Faculty of Technology, University of Manchester 1977. [5] Arkhangelsky V. Motor vehicle engines. Moscow: Mir Publishers, 1971. [6] Beretta GP, Rashidi M, Keck JC. Turbulent ¯ame propagation and combustion in spark ignition engines. Combustion and Flame 1983;52:217±45. [7] Blizard NC, Keck JC. Experimental and theoretical investigation of turbulent burning model for internal combustion engine. SAE 740191 1974.