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Investigation on the cold start characteristics of a hydrogen-enriched methanol engine
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation on the cold start characteristics of a hydrogen-enriched methanol engine Bo Zhang, Changwei Ji*, Shuofeng Wang, Yuchen Xiao College of Environmental and Energy Engineering and Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124, China
article info
abstract
Article history:
This paper experimentally investigated the effect of hydrogen addition on the cold start
Received 21 October 2013
performance of a methanol engine. The test was conducted on a modified four-cylinder
Received in revised form
gasoline engine. An electronically controlled hydrogen injection system was applied to
1 March 2014
realize the hydrogen port injection. The engine was started at an ambient temperature of
Accepted 2 April 2014
25 C with two hydrogen flow rates of 0 and 189 dm3/s, respectively. The results demon-
Available online xxx
strated that hydrogen addition availed elevating the peak engine speed and cylinder pressure during the cold start. Both flame development and propagation periods are
Keywords:
shortened after the hydrogen addition. When the hydrogen volume flow rate was raised
Hydrogen
from 0 to 189 dm3/s, HC, CO and total number of particulate emissions within 19 s from the
Methanol
onset of cold start were reduced by 68.7%, 75.2% and 72.4%, respectively. However, because
Cold start
of the enhanced in-cylinder temperature, NOx emissions were increased after the addition
Spark-ignition engine
of hydrogen.
Combustion
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Emissions
Introduction The application of clean alternative fuels is a feasible way for alleviating pressures on energy shortage and environmental pollution. Methanol is a renewable fuel that can be produced from kinds of methods and sources [1]. As methanol contains the oxygen atom and has a high H/C ratio, the combustion of methanol could exhaust lower carbon-related emissions than the pure gasoline engines [2]. Besides, the methanol has a high octane number which enables the engine to adopt larger compression ratios to improve the fuel economy [3]. According to the experimental results from Gong et al. [4], by optimizing the injectors, compression ratio and spark timing, the methanol engine could gain the highest thermal efficiency
larger than 30%. Also, the latent heat of methanol is roughly 1.5 times higher than that of gasoline [5]. This means that the evaporation of methanol helps cool the inlet charge and therefore benefits increasing the volumetric efficiency. However, the high latent heat and boiling temperature of methanol make its hard be fully evaporated at low temperatures, particularly at the cold start, idle and low load conditions. Thus, starting a methanol engine is relatively difficult at low ambient temperatures. Gong et al. [6,7] found that when the ambient temperature was lower than 16 C, the methanol engine cannot be successfully started without auxiliary start aids due to the insufficient combustible fueleair mixtures formation. Their tests also confirmed that, by adopting the inlet charge heating or adding small amounts of LPG and gasoline, the methanol engine could be started normally.
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). http://dx.doi.org/10.1016/j.ijhydene.2014.04.012 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Unfortunately, Li et al. [8] found that increasing the LPG content in the fueleair mixtures tended to cause the adversely raised formaldehyde emission. Besides, because of the sever fuel-film effect, the addition of gasoline tended to cause the increased HC emissions. According to previous investigations [9,10], more than 70% of HC emissions in the federal test procedure (FTP) are produced from the engine cold-start period. Thus, it is necessary to find a feasible way for ensuring a stable cold start of the methanol engine without increasing carbon-related emissions. Hydrogen is another promising fuel candidate for internal combustion engines [11,12]. Adding small amount of hydrogen to the fossil fuel engines is able to improve the engine thermal efficiency and reduce the toxic emissions [13,14]. Simio et al. [15] found that the combustion duration of heavy-duty natural gas engine was shortened after the hydrogen addition. Mariani et al. [16] studied the combustion characteristics of hydrogenblended natural gas engine and found that the addition of hydrogen in the fueleair mixtures resulted in the 16% reduction of CO2 emission. Huang and Wang et al. [17e19] investigated the effect of hydrogen addition on CNG combustion. The results confirmed that the addition of hydrogen availed increasing the burning velocity of natural gas due to the increased O, H and OH radicals in the flame. Ji and Wang et al. [20e22] found that the addition of hydrogen helped improve the fuel economy of gasoline engines. Meanwhile, the vehicle emissions level was improved from Euro-II of the original gasoline engine to Euro-IV of the hydrogen-enriched gasoline engine under the New European Driving Cycle (NEDC). They also tried to start an engine with hydrogenegasoline mixtures and found that the addition of hydrogen was effective on reducing HC and CO emissions during the cold start. There are also some reports showing the effect of hydrogen addition on performance of alcohol-fueled engines. Yousufuddin et al. [23] studied the performance of hydrogen-enriched ethanol engine. The results indicated that the addition of hydrogen benefited improving the engine thermal efficiency and shortening the combustion duration. Ji and Zhang et al. [24] found that the lean burn limit was extended and cyclic variation was reduced after the hydrogen addition for the methanol engine. However, although some articles have reported that the methanol engines are hard to be started without auxiliary system or LPG blending [6e8], there are still no investigations studying the effect of hydrogen addition on the cold start characteristics of a methanol engine. Compared with LPG, the hydrogen possesses much lower minimum ignition energy and wider flammability. Thus, the addition of hydrogen seems to be effective on improving the methanol engine performance at the cold start condition. In view of the above, this paper studied the performance of a hydrogen-enriched methanol engine at the cold start.
Experimental setup and procedure Experimental setup The test engine is a 1.6 L spark-ignition gasoline engine manufactured by Beijing Hyundai Motors. The methanol is introduced into the inlet plenum through the original gasoline
injectors. To ensure that there is no gasoline trapped in the fueling system, the fuel tank, rail and injectors are cleaned by the methanol before the test. Moreover, a hydrogen injection system with a hydrogen rail and four hydrogen injectors are fixed on the intake manifolds. Both injection timings and durations of hydrogen and methanol are governed by a selfdeveloped hybrid electronic control unit (HECU). The schematic diagram of the experimental system is given in Fig. 1. The cylinder pressure and crank angle position are detected through Kistler 6117BFD17 piezoelectric pressure transducer (measurement uncertainty: 0.3 bar) and Kistler 2613B optical encoder (measurement uncertainty: 0.1 CA, measurement sensitivity: 0.2 CA), respectively. A Dewe-800 combustion analyzer is adopted to analyze the pressure and crank angle data to obtain combustion parameters. A Horiba MEXA7100DEGR emissions analyzer is used to measure the untreated tailpipe emissions of CO, HC and NOx through regulated methods (measurement sensitivity: 1 ppm for HC and NOx emissions, 1% for CO emission; measurement uncertainty: <1%). The number of particulate emissions is measured by a DMS500 fast particulate spectrometer (measurement uncertainty: <10%). A D07-19BM thermal mass flow meter manufactured by Seven Star is applied to monitor the hydrogen flow rate (measurement uncertainties: <0.02 L/ min). A 20N060 thermal mass flow meter produced by Toceil is used to measure the air flow rate (measurement uncertainties:
Fig. 1 e The schematics of the experimental system 1. Hydrogen cylinder container 2. Hydrogen pressure adjusting valve 3. Hydrogen pressure meter 4. Hydrogen mass flow meter 5. Backfire arrestor 6. Hydrogen injector 7. Throttle 8. Air mass flow meter 9. Idle valve 10. OECU 11. HECU 12. Calibration computer 13. Methanol tank 14. Methanol mass flow meter 15. Methanol pump 16. Methanol injector 17. Ignition module 18. Pressure transducer with a spark plug 19. Optical encoder 20. Charge amplifier 21. A/D converter 22. Combustion analyzer 23. O2 sensor 24. A/F analyzer 25. Emissions sampling pipe 26. Horiba MEXA-7100DEGR emissions analyzer a. Signals from the OECU to the HECU b1. Calibration and control signals from the calibration computer to the HECU b2. Data signals from the HECU to the calibration computer.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
<0.1 L/min). To ensure the data accuracy, all measurement instruments are calibrated before the test.
Experimental procedure In all tests, the engine was started at the same ambient, coolant and lubricant temperatures of 25 C. The battery voltage was kept at 12 V. The engine main throttle was closed, so that the air flow rate into the engine was only governed by the idle bypass valve which was automatically adjusted through the engine original electric control unit (OECU). The engine was first started with the pure methanol. Then, the hydrogen flow rate was raised from 0 to 189 dm3/s through adjusting the hydrogen injection duration. For both starting strategies, the methanol injection duration was adjusted to be the minimum value while ensuring that the engine could be started successfully.
Results and discussion
3
start. When the engine is started successfully, the methanol attached on the surface of cold walls could be vaporized and joined the combustion again. Thus, it is reasonable that after 13 s from the onset of cold start, the fueleair mixture within the cylinder of methanol engine is actually richer than that of the hydrogen-enriched methanol engine. Thereby, after 13 s from the onset of cold start, the original methanol engine produces higher engine speed than the hydrogen-enriched methanol engine. Fig. 2 also demonstrated that the engine speeds of both methanol and hydrogen-enriched methanol engines are first increased and then decreased with the increase of time. This is because the OECU has to raise the opening of idle bypass valve within the first few cycles to enable more fueleair mixtures to be introduced into the cylinder. Therefore, the engine speeds of both methanol and hydrogen-enriched methanol engines are increased sharply first. Then, when the engine is started successfully, the OECU gradually reduces the opening of idle bypass valve so that the engine could be operated at the warming up condition. Thereby, the engine speed is decreased gradually in later time.
Engine speed Cylinder pressure Engine speed is a key symbol of the successful cold start. Fig. 2 shows the engine speed within 19 s from the onset of cold start. It can be found from Fig. 2 that the peak engine speed during the cold start rises from 2075 rpm of the pure methanol engine to 2211 rpm of the methanol engine enriched with 189 dm3/s hydrogen. This is because the flame speed of hydrogen is higher than that of methanol. Thus, the addition of hydrogen stimulates the combustion of fueleair mixture and therefore contributes to the raised peak engine speed during the cold start. Meanwhile, it is seen from Fig. 2 that the hydrogen-enriched methanol engine achieves higher engine speed within the first 13 s from the onset of cold start. But in later time, the 189 dm3/s hydrogen-enriched methanol engine produces lower engine speed than the pure methanol engine. The reason could be ascribed to the fact that, for the pure methanol engine, because of the severe fuel-film effect, the rich fueleair mixtures have to be adopted during the cold
Cylinder pressure is important on analyzing the engine combustion process. The variations of cylinder pressure with crank angle within the first five cycles are plotted in Fig. 3. It can be found from Fig. 3 that the peak cylinder pressure is obviously increased after the hydrogen addition. The maximum cylinder pressure of the first cycle is raised from 17.5 bar of the original engine to nearly 27.0 bar of the hydrogen-enriched methanol engine. This confirms the fact that the addition of hydrogen avails enhancing the cold start stability of the methanol engine. This is because the hydrogen has a much lower ignition energy and wider flammability than the methanol. Thus, the charge could be quickly ignited by the spark plug and combusted faster after the hydrogen addition. Fig. 3 also shows that, for a given hydrogen addition level, the peak cylinder pressure is gradually reduced. The reason could be attributed to the fact that when the engine is started,
Fig. 2 e Engine speed versus time within 19 s from the onset of cold start.
Fig. 3 e Cylinder pressure versus crank angle for the first five cycles from the cold start.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
the OECU gradually reduces the air flow rate to enable the engine to be run at warming up conditions. Thus, both the amount of combustible fueleair mixture and the cylinder pressure are dropped in later cycles and therefore results in the gradually decreased peak cylinder pressure after the cold start.
Flame development and propagation periods Flame development (CA0-10) and propagation (CA10-90) periods are important parameters for analyzing the combustion process. CA0-10 and CA10-90 are defined as the crank angle durations form spark discharge to 10% and from 10% to 90% heat release fraction. Figs. 4 and 5 display the variations of CA0-10 and CA10-90 with crank angle during the first 50 cycles from the onset of cold start. It is seen from Figs. 4 and 5 that both CA0-10 and CA10-90 are shortened with the hydrogen addition. This is because the hydrogen doesn’t need to be vaporized before forming the combustible fueleair mixtures. Thus, the addition of hydrogen helps reduce the negative effect of fuel film on combustion. Besides, the high diffusive speed of hydrogen may contribute to the improved homogeneity of fueleair mixtures. Therefore, the combustion of hydrogenemethanol mixtures is better than that of the pure methanol. On the other hand, as the ignition energy of hydrogen is lower than that of methanol, the charge could be ignited easier after the hydrogen addition. Moreover, since the hydrogen enrichment is beneficial for promoting the formation of O, H, and OH radicals in the laminar flame [17] and therefore accelerating the laminar flame propagation, the laminar flame speed of fueleair mixtures is effectively elevated after the hydrogen addition [25]. Furthermore, because of the high mass and thermal diffusivities of hydrogen, the laminar flame thickness is significantly reduced with hydrogen addition. This means that under the same turbulent intensity, the hydrogen-enriched flame tends to experience more wrinkling in comparison with the pure methanol flame [26], which also contributes to the heightened turbulent flame speed and consequently reduces the flame development and propagation periods.
Fig. 5 e CA10-90 versus crank angle for the first 50 cycles from the cold start.
It is found from Figs. 4 and 5 that both CA0-10 and CA10-90 of the methanol and hydrogen-enriched methanol engines are first prolonged and then shortened with the increase of cycle number. The possible reason could be ascribed to the fact that, to ensure a successful cold start, the rich mixture has to be adopted in the first few cycles. Therefore, because of the high fueleair mixtures flow rates and the adoption of rich mixtures, CA0-10 and CA10-90 are relatively short when the engine is just started. In later cycles, because of the reduced opening of idle bypass valve and gradually raised excess air ratio, the combustion duration is generally prolonged. However, when the cylinder is warmed up, the liquid methanol attached on the cold walls could be vaporized again. This may cause the increased equivalence ratio which could generally result in the shortened combustion duration. Also, as the engine is gradually warmed, the initial temperature of combustion is raised accordingly, which provides a better condition for the combustion [27]. Thus, for both methanol and hydrogen-enriched methanol engines, CA0-10 and CA10-90 are first prolonged and then shortened with the increase of cycle number.
Emissions
Fig. 4 e CA0-10 versus crank angle for the first 50 cycles from the cold start.
Fig. 6 illustrates the variations of HC emissions with time for two hydrogen addition levels. It is seen from Fig. 6 that when the hydrogen flow rate increases from 0 to 189 dm3/s, HC emissions within the first 19 s from the onset of cold start are averagely reduced by 68.7%. The reasons could be attributed to the fact that, because of the fuel-film effect, the pure methanol engine has to adopt rich mixtures to ensure the stable cold start. Thus, large amounts of unburnt fuels are directly expelled from the tail pipe without combustion, which causes the sharply increased HC emissions for the pure methanol engine. As hydrogen possesses a wide flammability, the methanol introduced into the cylinder is reduced after the hydrogen addition. Thereby, the addition of hydrogen avails reducing HC emissions caused by the incomplete combustion and misfire. Moreover, the short
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 6 e HC emissions versus time within 19 s from the onset of cold start.
5
Fig. 8 e NOx emissions versus time within 19 s from the onset of cold start.
quenching distance of hydrogen also helps reduce HC emissions caused by the quenching effect. It is found from Fig. 6 that HC emissions are gradually decreased after achieving its peak value since 3 s from the onset of cold start. This is because the gradually increased cylinder temperature provides better conditions for the fuel evaporation and combustion. Fig. 7 depicts the variation of CO emission with time within 19 s from the onset of cold start. Fig. 7 shows that when the hydrogen flow rate rises from 0 to 189 dm3/s, CO emission is averagely decreased by about 75.2% within 19 s from the onset of cold start. This is because the wide flammability of hydrogen ensures that the hydrogenemethanol mixtures to be completely burnt during the cold start. At the same time, the decreased methanol flow rate after the hydrogen addition also reduces the carbon atom in the fueleair mixtures, which could be ascribed as another reason for the dropped CO emission after the hydrogen enrichment.
Fig. 8 shows the variation of NOx emissions with time for the pure and hydrogen-enriched methanol engines during the first 19 s from the cold start. It is seen from Fig. 8 that NOx emissions from the hydrogen-enriched methanol engine are obviously higher than those from the pure methanol engine during the first few seconds. This is because the wide flammability and low ignition energy contributes to the complete combustion. Besides, the high flame temperature of hydrogen is also a reason for the increased NOx emissions of the hydrogen-enriched methanol engine. Fig. 9 plots the total particulate number ranging from 5 to 1000 nm versus time for the methanol and hydrogen-enriched methanol engines during the cold start. It is seen from Fig. 9 that the total particulate number is decreased by about 72.4% within the first 19 s from the onset of cold start. This is because the addition of hydrogen avails the combustion completeness of the fueleair mixtures. Moreover, the carbon content in the fuel is reduced after the hydrogen enrichment. This is beneficial for reducing the nucleation of particulate emissions.
Fig. 7 e CO emission versus time within 19 s from the onset of cold start.
Fig. 9 e Particulate number versus time within 19 s from the onset of cold start.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Conclusions This paper experimentally investigated the cold start performance of a hydrogen-enriched methanol engine. Because the high diffusion speed of hydrogen avails improving the charge homogeneity and the wide flammability of hydrogen benefits the complete combustion of fueleair mixtures, adding small amount of hydrogen contributes to the enhanced peak cylinder pressure and the peak engine speed of the methanol engine during cold start. This means that the hydrogen addition is capable of enhancing the cold start stability of methanol engines. Because of the enhanced combustion and short quenching distance of hydrogen, HC and CO emissions are reduced dramatically by hydrogen addition. Within the first 19 s from the onset of cold start, HC and CO emissions are averagely reduced by 68.7% and 75.2% when the hydrogen flow rate increases from 0 to 189 dm3/s. It is also confirmed that the addition of hydrogen avails reducing particulate emissions from the methanol engine during the cold start. However, because of the enhanced combustion, NOx emissions from the hydrogen-enriched methanol engine are obviously higher than those from the pure methanol engine. This suggests that proper methods should be taken to control NOx emissions from hydrogen-enriched methanol engines.
Acknowledgments This work was supported by National Program on Key Basic Research Project (973 Program) (Grant No. 2013CB228403), Key Program of Sci & Tech Project of Beijing Municipal Commission of Education (Grant No. KZ201210005002), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20111103110010) and Beijing Municipal Natural Science Foundation (Grant No. 3122006).
references
[1] Moreria JR, Goldmberg. The alcohol program. Energy Policy 1999;27:229e45. [2] Liu S, Eddy RCC, Hu T, Wei Y. Study of spark ignition engine fueled with methanol/gasoline fuel blends. Appl Therm Eng 2007;27:1904e10. [3] Celik MB, Ozdalyan B, Alkan F. The use of pure methanol as fuel at high compression ratio in a single cylinder gasoline engine. Fuel 2011;90:1591e8. [4] Gong C, Huang K, Jia J, Gao Q, Liu X. Improvement of fuel economy of a direct-injection spark-ignition methanol engine under light loads. Fuel 2011;90:1826e32. [5] Heywood JB. Internal combustion engine fundamentals. New York: McGraw-Hill; 1988. [6] Gong C, Deng B, Wang S, Su Y, Gao Q, Liu X. Combustion of a spark-ignition engine during cold start under cycle-by-cycle control. Energy Fuel 2008;22:2981e5. [7] Li J, Gong C, Liu B, Su Y, Dou H, Liu X. Combustion and hydrocarbon emissions from a spark-ignition engine fueled with gasoline and methanol during cold start. Energy Fuel 2009;23:4937e42.
[8] Li S, Gong C, Wang E, Yu X, Wang Z, Liu X. Emissions of formaldehyde and unburnt methanol form a spark-ignition methanol engine during cold start. Energy Fuel 2010;24:863e70. [9] Takeda K, Yaegashi T, Tai H. Mixture preparation and HC emissions of a 4-valve engine with port fuel injection during cold starting and warm-up. SAE Paper no. 950074; 1995. [10] Fischer H, Brereton GJ. Fuel injection strategies to minimize cold-start HC emission. SAE Paper no. 1997-02-24; 1997. [11] White CM, Steeper RR, Lutz AE. The hydrogen-fueled internal combustion engine: a technical review. Int J Hydrogen Energy 2006;31:1292e305. [12] Das LM. Hydrogen-oxygen reaction mechanism and its implication to hydrogen engine combustion. Int J Hydrogen Energy 1996;21:703e15. [13] Fennell D, Herreros J, Tsolakis A. Improving gasoline direct injection (GDI) engine efficiency and emissions with hydrogen from exhaust gas fuel reforming. Int J Hydrogen Energy. http://dx.doi.org/10.1016/j.ijhydene.2014.01.065; 2014. [14] Osama H, Ghazal. Performance and combustion characteristic of CI engine fueled with hydrogen enriched diesel. Int J Hydrogen Energy 2013;38:15469e76. [15] Simio LD, Gambino M, Iannaccone S. Experimental and numerical study of hydrogen addition in a natural gas heavy duty engine for a bus vehicle. Int J Hydrogen Energy 2013;38:6865e73. [16] Mariani A, Prati MV, Unich A, Morrone B. Combustion analysis of a spark ignition i.c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends. Int J Hydrogen Energy 2013;38:1616e23. [17] Wang J, Huang Z, Tang C, Miao H, Wang X. Numerical study of the effect of hydrogen addition on methaneeair mixtures combustion. Int J Hydrogen Energy 2009;34:1084e96. [18] Wang J, Huang Z, Tang C, Zheng J. Effect of hydrogen addition on early flame growth of lean burn natural gaseair mixtures. Int J Hydrogen Energy 2010;35:7246e52. [19] Huang B, Hu E, Huang Z, Zheng J, Liu B, Jiang D. Cycle-bycycle variations in a spark ignition engine fueled with natural gasehydrogen blends combined with EGR. Int J Hydrogen Energy 2009;34:8405e14. [20] Zhang M, Wang J, Xie Y, Jin W, Wei Z, Huang Z, et al. Flame front structure and burning velocity of turbulent premixed CH4/H2/air flames. Int J Hydrogen Energy 2013;38:11421e8. [21] Ji C, Wang S, Zhang B, Liu X. Emissions performance of a hybrid hydrogenegasoline engine-powered passenger car under the New European Driving Cycle. Fuel 2013;106:873e5. [22] Wang S, Ji C, Zhang B. Starting a spark-ignited engine with the gasolineehydrogen mixture. Int J Hydrogen Energy 2011;36:4461e8. [23] Yousufuddin S, Masood M. Effect of ignition timing and compression ratio on the performance of a hydrogeneethanol fuelled engine. Int J Hydrogen Energy 2009;34:6945e50. [24] Ji C, Zhang B, Wang S. Enhancing the performance of a sparkignition methanol engine with hydrogen addition. Int J Hydrogen Energy 2013;38:7490e8. [25] Ji C, Liu X, Wang S, Gao B, Yang J. Development and validation of a laminar flame speed correlation for the CFD simulation of hydrogen-enriched gasoline engines. Int J Hydrogen Energy 2013;38:1997e2006. [26] Ji C, Liu X, Gao B, Wang S, Yang J. Numerical investigation on the combustion process in a spark-ignited engine fueled with hydrogen-gasoline blends. Int J Hydrogen Energy 2013;38:11149e55. [27] Szwaja S, Naber JD. Dual nature of hydrogen combustion knock. Int J Hydrogen Energy 2013;38:12489e96.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation on the cold start characteristics of a hydrogen-enriched methanol engine Bo Zhang, Changwei Ji*, Shuofeng Wang, Yuchen Xiao College of Environmental and Energy Engineering and Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124, China
article info
abstract
Article history:
This paper experimentally investigated the effect of hydrogen addition on the cold start
Received 21 October 2013
performance of a methanol engine. The test was conducted on a modified four-cylinder
Received in revised form
gasoline engine. An electronically controlled hydrogen injection system was applied to
1 March 2014
realize the hydrogen port injection. The engine was started at an ambient temperature of
Accepted 2 April 2014
25 C with two hydrogen flow rates of 0 and 189 dm3/s, respectively. The results demon-
Available online xxx
strated that hydrogen addition availed elevating the peak engine speed and cylinder pressure during the cold start. Both flame development and propagation periods are
Keywords:
shortened after the hydrogen addition. When the hydrogen volume flow rate was raised
Hydrogen
from 0 to 189 dm3/s, HC, CO and total number of particulate emissions within 19 s from the
Methanol
onset of cold start were reduced by 68.7%, 75.2% and 72.4%, respectively. However, because
Cold start
of the enhanced in-cylinder temperature, NOx emissions were increased after the addition
Spark-ignition engine
of hydrogen.
Combustion
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Emissions
Introduction The application of clean alternative fuels is a feasible way for alleviating pressures on energy shortage and environmental pollution. Methanol is a renewable fuel that can be produced from kinds of methods and sources [1]. As methanol contains the oxygen atom and has a high H/C ratio, the combustion of methanol could exhaust lower carbon-related emissions than the pure gasoline engines [2]. Besides, the methanol has a high octane number which enables the engine to adopt larger compression ratios to improve the fuel economy [3]. According to the experimental results from Gong et al. [4], by optimizing the injectors, compression ratio and spark timing, the methanol engine could gain the highest thermal efficiency
larger than 30%. Also, the latent heat of methanol is roughly 1.5 times higher than that of gasoline [5]. This means that the evaporation of methanol helps cool the inlet charge and therefore benefits increasing the volumetric efficiency. However, the high latent heat and boiling temperature of methanol make its hard be fully evaporated at low temperatures, particularly at the cold start, idle and low load conditions. Thus, starting a methanol engine is relatively difficult at low ambient temperatures. Gong et al. [6,7] found that when the ambient temperature was lower than 16 C, the methanol engine cannot be successfully started without auxiliary start aids due to the insufficient combustible fueleair mixtures formation. Their tests also confirmed that, by adopting the inlet charge heating or adding small amounts of LPG and gasoline, the methanol engine could be started normally.
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). http://dx.doi.org/10.1016/j.ijhydene.2014.04.012 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Unfortunately, Li et al. [8] found that increasing the LPG content in the fueleair mixtures tended to cause the adversely raised formaldehyde emission. Besides, because of the sever fuel-film effect, the addition of gasoline tended to cause the increased HC emissions. According to previous investigations [9,10], more than 70% of HC emissions in the federal test procedure (FTP) are produced from the engine cold-start period. Thus, it is necessary to find a feasible way for ensuring a stable cold start of the methanol engine without increasing carbon-related emissions. Hydrogen is another promising fuel candidate for internal combustion engines [11,12]. Adding small amount of hydrogen to the fossil fuel engines is able to improve the engine thermal efficiency and reduce the toxic emissions [13,14]. Simio et al. [15] found that the combustion duration of heavy-duty natural gas engine was shortened after the hydrogen addition. Mariani et al. [16] studied the combustion characteristics of hydrogenblended natural gas engine and found that the addition of hydrogen in the fueleair mixtures resulted in the 16% reduction of CO2 emission. Huang and Wang et al. [17e19] investigated the effect of hydrogen addition on CNG combustion. The results confirmed that the addition of hydrogen availed increasing the burning velocity of natural gas due to the increased O, H and OH radicals in the flame. Ji and Wang et al. [20e22] found that the addition of hydrogen helped improve the fuel economy of gasoline engines. Meanwhile, the vehicle emissions level was improved from Euro-II of the original gasoline engine to Euro-IV of the hydrogen-enriched gasoline engine under the New European Driving Cycle (NEDC). They also tried to start an engine with hydrogenegasoline mixtures and found that the addition of hydrogen was effective on reducing HC and CO emissions during the cold start. There are also some reports showing the effect of hydrogen addition on performance of alcohol-fueled engines. Yousufuddin et al. [23] studied the performance of hydrogen-enriched ethanol engine. The results indicated that the addition of hydrogen benefited improving the engine thermal efficiency and shortening the combustion duration. Ji and Zhang et al. [24] found that the lean burn limit was extended and cyclic variation was reduced after the hydrogen addition for the methanol engine. However, although some articles have reported that the methanol engines are hard to be started without auxiliary system or LPG blending [6e8], there are still no investigations studying the effect of hydrogen addition on the cold start characteristics of a methanol engine. Compared with LPG, the hydrogen possesses much lower minimum ignition energy and wider flammability. Thus, the addition of hydrogen seems to be effective on improving the methanol engine performance at the cold start condition. In view of the above, this paper studied the performance of a hydrogen-enriched methanol engine at the cold start.
Experimental setup and procedure Experimental setup The test engine is a 1.6 L spark-ignition gasoline engine manufactured by Beijing Hyundai Motors. The methanol is introduced into the inlet plenum through the original gasoline
injectors. To ensure that there is no gasoline trapped in the fueling system, the fuel tank, rail and injectors are cleaned by the methanol before the test. Moreover, a hydrogen injection system with a hydrogen rail and four hydrogen injectors are fixed on the intake manifolds. Both injection timings and durations of hydrogen and methanol are governed by a selfdeveloped hybrid electronic control unit (HECU). The schematic diagram of the experimental system is given in Fig. 1. The cylinder pressure and crank angle position are detected through Kistler 6117BFD17 piezoelectric pressure transducer (measurement uncertainty: 0.3 bar) and Kistler 2613B optical encoder (measurement uncertainty: 0.1 CA, measurement sensitivity: 0.2 CA), respectively. A Dewe-800 combustion analyzer is adopted to analyze the pressure and crank angle data to obtain combustion parameters. A Horiba MEXA7100DEGR emissions analyzer is used to measure the untreated tailpipe emissions of CO, HC and NOx through regulated methods (measurement sensitivity: 1 ppm for HC and NOx emissions, 1% for CO emission; measurement uncertainty: <1%). The number of particulate emissions is measured by a DMS500 fast particulate spectrometer (measurement uncertainty: <10%). A D07-19BM thermal mass flow meter manufactured by Seven Star is applied to monitor the hydrogen flow rate (measurement uncertainties: <0.02 L/ min). A 20N060 thermal mass flow meter produced by Toceil is used to measure the air flow rate (measurement uncertainties:
Fig. 1 e The schematics of the experimental system 1. Hydrogen cylinder container 2. Hydrogen pressure adjusting valve 3. Hydrogen pressure meter 4. Hydrogen mass flow meter 5. Backfire arrestor 6. Hydrogen injector 7. Throttle 8. Air mass flow meter 9. Idle valve 10. OECU 11. HECU 12. Calibration computer 13. Methanol tank 14. Methanol mass flow meter 15. Methanol pump 16. Methanol injector 17. Ignition module 18. Pressure transducer with a spark plug 19. Optical encoder 20. Charge amplifier 21. A/D converter 22. Combustion analyzer 23. O2 sensor 24. A/F analyzer 25. Emissions sampling pipe 26. Horiba MEXA-7100DEGR emissions analyzer a. Signals from the OECU to the HECU b1. Calibration and control signals from the calibration computer to the HECU b2. Data signals from the HECU to the calibration computer.
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<0.1 L/min). To ensure the data accuracy, all measurement instruments are calibrated before the test.
Experimental procedure In all tests, the engine was started at the same ambient, coolant and lubricant temperatures of 25 C. The battery voltage was kept at 12 V. The engine main throttle was closed, so that the air flow rate into the engine was only governed by the idle bypass valve which was automatically adjusted through the engine original electric control unit (OECU). The engine was first started with the pure methanol. Then, the hydrogen flow rate was raised from 0 to 189 dm3/s through adjusting the hydrogen injection duration. For both starting strategies, the methanol injection duration was adjusted to be the minimum value while ensuring that the engine could be started successfully.
Results and discussion
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start. When the engine is started successfully, the methanol attached on the surface of cold walls could be vaporized and joined the combustion again. Thus, it is reasonable that after 13 s from the onset of cold start, the fueleair mixture within the cylinder of methanol engine is actually richer than that of the hydrogen-enriched methanol engine. Thereby, after 13 s from the onset of cold start, the original methanol engine produces higher engine speed than the hydrogen-enriched methanol engine. Fig. 2 also demonstrated that the engine speeds of both methanol and hydrogen-enriched methanol engines are first increased and then decreased with the increase of time. This is because the OECU has to raise the opening of idle bypass valve within the first few cycles to enable more fueleair mixtures to be introduced into the cylinder. Therefore, the engine speeds of both methanol and hydrogen-enriched methanol engines are increased sharply first. Then, when the engine is started successfully, the OECU gradually reduces the opening of idle bypass valve so that the engine could be operated at the warming up condition. Thereby, the engine speed is decreased gradually in later time.
Engine speed Cylinder pressure Engine speed is a key symbol of the successful cold start. Fig. 2 shows the engine speed within 19 s from the onset of cold start. It can be found from Fig. 2 that the peak engine speed during the cold start rises from 2075 rpm of the pure methanol engine to 2211 rpm of the methanol engine enriched with 189 dm3/s hydrogen. This is because the flame speed of hydrogen is higher than that of methanol. Thus, the addition of hydrogen stimulates the combustion of fueleair mixture and therefore contributes to the raised peak engine speed during the cold start. Meanwhile, it is seen from Fig. 2 that the hydrogen-enriched methanol engine achieves higher engine speed within the first 13 s from the onset of cold start. But in later time, the 189 dm3/s hydrogen-enriched methanol engine produces lower engine speed than the pure methanol engine. The reason could be ascribed to the fact that, for the pure methanol engine, because of the severe fuel-film effect, the rich fueleair mixtures have to be adopted during the cold
Cylinder pressure is important on analyzing the engine combustion process. The variations of cylinder pressure with crank angle within the first five cycles are plotted in Fig. 3. It can be found from Fig. 3 that the peak cylinder pressure is obviously increased after the hydrogen addition. The maximum cylinder pressure of the first cycle is raised from 17.5 bar of the original engine to nearly 27.0 bar of the hydrogen-enriched methanol engine. This confirms the fact that the addition of hydrogen avails enhancing the cold start stability of the methanol engine. This is because the hydrogen has a much lower ignition energy and wider flammability than the methanol. Thus, the charge could be quickly ignited by the spark plug and combusted faster after the hydrogen addition. Fig. 3 also shows that, for a given hydrogen addition level, the peak cylinder pressure is gradually reduced. The reason could be attributed to the fact that when the engine is started,
Fig. 2 e Engine speed versus time within 19 s from the onset of cold start.
Fig. 3 e Cylinder pressure versus crank angle for the first five cycles from the cold start.
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the OECU gradually reduces the air flow rate to enable the engine to be run at warming up conditions. Thus, both the amount of combustible fueleair mixture and the cylinder pressure are dropped in later cycles and therefore results in the gradually decreased peak cylinder pressure after the cold start.
Flame development and propagation periods Flame development (CA0-10) and propagation (CA10-90) periods are important parameters for analyzing the combustion process. CA0-10 and CA10-90 are defined as the crank angle durations form spark discharge to 10% and from 10% to 90% heat release fraction. Figs. 4 and 5 display the variations of CA0-10 and CA10-90 with crank angle during the first 50 cycles from the onset of cold start. It is seen from Figs. 4 and 5 that both CA0-10 and CA10-90 are shortened with the hydrogen addition. This is because the hydrogen doesn’t need to be vaporized before forming the combustible fueleair mixtures. Thus, the addition of hydrogen helps reduce the negative effect of fuel film on combustion. Besides, the high diffusive speed of hydrogen may contribute to the improved homogeneity of fueleair mixtures. Therefore, the combustion of hydrogenemethanol mixtures is better than that of the pure methanol. On the other hand, as the ignition energy of hydrogen is lower than that of methanol, the charge could be ignited easier after the hydrogen addition. Moreover, since the hydrogen enrichment is beneficial for promoting the formation of O, H, and OH radicals in the laminar flame [17] and therefore accelerating the laminar flame propagation, the laminar flame speed of fueleair mixtures is effectively elevated after the hydrogen addition [25]. Furthermore, because of the high mass and thermal diffusivities of hydrogen, the laminar flame thickness is significantly reduced with hydrogen addition. This means that under the same turbulent intensity, the hydrogen-enriched flame tends to experience more wrinkling in comparison with the pure methanol flame [26], which also contributes to the heightened turbulent flame speed and consequently reduces the flame development and propagation periods.
Fig. 5 e CA10-90 versus crank angle for the first 50 cycles from the cold start.
It is found from Figs. 4 and 5 that both CA0-10 and CA10-90 of the methanol and hydrogen-enriched methanol engines are first prolonged and then shortened with the increase of cycle number. The possible reason could be ascribed to the fact that, to ensure a successful cold start, the rich mixture has to be adopted in the first few cycles. Therefore, because of the high fueleair mixtures flow rates and the adoption of rich mixtures, CA0-10 and CA10-90 are relatively short when the engine is just started. In later cycles, because of the reduced opening of idle bypass valve and gradually raised excess air ratio, the combustion duration is generally prolonged. However, when the cylinder is warmed up, the liquid methanol attached on the cold walls could be vaporized again. This may cause the increased equivalence ratio which could generally result in the shortened combustion duration. Also, as the engine is gradually warmed, the initial temperature of combustion is raised accordingly, which provides a better condition for the combustion [27]. Thus, for both methanol and hydrogen-enriched methanol engines, CA0-10 and CA10-90 are first prolonged and then shortened with the increase of cycle number.
Emissions
Fig. 4 e CA0-10 versus crank angle for the first 50 cycles from the cold start.
Fig. 6 illustrates the variations of HC emissions with time for two hydrogen addition levels. It is seen from Fig. 6 that when the hydrogen flow rate increases from 0 to 189 dm3/s, HC emissions within the first 19 s from the onset of cold start are averagely reduced by 68.7%. The reasons could be attributed to the fact that, because of the fuel-film effect, the pure methanol engine has to adopt rich mixtures to ensure the stable cold start. Thus, large amounts of unburnt fuels are directly expelled from the tail pipe without combustion, which causes the sharply increased HC emissions for the pure methanol engine. As hydrogen possesses a wide flammability, the methanol introduced into the cylinder is reduced after the hydrogen addition. Thereby, the addition of hydrogen avails reducing HC emissions caused by the incomplete combustion and misfire. Moreover, the short
Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012
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Fig. 6 e HC emissions versus time within 19 s from the onset of cold start.
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Fig. 8 e NOx emissions versus time within 19 s from the onset of cold start.
quenching distance of hydrogen also helps reduce HC emissions caused by the quenching effect. It is found from Fig. 6 that HC emissions are gradually decreased after achieving its peak value since 3 s from the onset of cold start. This is because the gradually increased cylinder temperature provides better conditions for the fuel evaporation and combustion. Fig. 7 depicts the variation of CO emission with time within 19 s from the onset of cold start. Fig. 7 shows that when the hydrogen flow rate rises from 0 to 189 dm3/s, CO emission is averagely decreased by about 75.2% within 19 s from the onset of cold start. This is because the wide flammability of hydrogen ensures that the hydrogenemethanol mixtures to be completely burnt during the cold start. At the same time, the decreased methanol flow rate after the hydrogen addition also reduces the carbon atom in the fueleair mixtures, which could be ascribed as another reason for the dropped CO emission after the hydrogen enrichment.
Fig. 8 shows the variation of NOx emissions with time for the pure and hydrogen-enriched methanol engines during the first 19 s from the cold start. It is seen from Fig. 8 that NOx emissions from the hydrogen-enriched methanol engine are obviously higher than those from the pure methanol engine during the first few seconds. This is because the wide flammability and low ignition energy contributes to the complete combustion. Besides, the high flame temperature of hydrogen is also a reason for the increased NOx emissions of the hydrogen-enriched methanol engine. Fig. 9 plots the total particulate number ranging from 5 to 1000 nm versus time for the methanol and hydrogen-enriched methanol engines during the cold start. It is seen from Fig. 9 that the total particulate number is decreased by about 72.4% within the first 19 s from the onset of cold start. This is because the addition of hydrogen avails the combustion completeness of the fueleair mixtures. Moreover, the carbon content in the fuel is reduced after the hydrogen enrichment. This is beneficial for reducing the nucleation of particulate emissions.
Fig. 7 e CO emission versus time within 19 s from the onset of cold start.
Fig. 9 e Particulate number versus time within 19 s from the onset of cold start.
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Conclusions This paper experimentally investigated the cold start performance of a hydrogen-enriched methanol engine. Because the high diffusion speed of hydrogen avails improving the charge homogeneity and the wide flammability of hydrogen benefits the complete combustion of fueleair mixtures, adding small amount of hydrogen contributes to the enhanced peak cylinder pressure and the peak engine speed of the methanol engine during cold start. This means that the hydrogen addition is capable of enhancing the cold start stability of methanol engines. Because of the enhanced combustion and short quenching distance of hydrogen, HC and CO emissions are reduced dramatically by hydrogen addition. Within the first 19 s from the onset of cold start, HC and CO emissions are averagely reduced by 68.7% and 75.2% when the hydrogen flow rate increases from 0 to 189 dm3/s. It is also confirmed that the addition of hydrogen avails reducing particulate emissions from the methanol engine during the cold start. However, because of the enhanced combustion, NOx emissions from the hydrogen-enriched methanol engine are obviously higher than those from the pure methanol engine. This suggests that proper methods should be taken to control NOx emissions from hydrogen-enriched methanol engines.
Acknowledgments This work was supported by National Program on Key Basic Research Project (973 Program) (Grant No. 2013CB228403), Key Program of Sci & Tech Project of Beijing Municipal Commission of Education (Grant No. KZ201210005002), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20111103110010) and Beijing Municipal Natural Science Foundation (Grant No. 3122006).
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Please cite this article in press as: Zhang B, et al., Investigation on the cold start characteristics of a hydrogen-enriched methanol engine, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.012