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Starting a spark-ignited engine with the gasoline–hydrogen mixture
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Starting a spark-ignited engine with the gasolineehydrogen mixture Shuofeng Wang, Changwei Ji*, Bo Zhang College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
article info
abstract
Article history:
Because of the increased fuel-film effect and dropped combustion temperature, spark-
Received 10 November 2010
ignited (SI) gasoline engines always expel large amounts of HC and CO emissions during
Received in revised form
the cold start period. This paper experimentally investigated the effect of hydrogen addi-
3 January 2011
tion on improving the cold start performance of a gasoline engine. The test was carried out
Accepted 5 January 2011
on a 1.6-L, four-cylinder, SI engine equipped with an electronically controlled hydrogen injection system. A hybrid electronic control unit (HECU) was applied to control the opening and closing of hydrogen and gasoline injectors. Under the same environmental
Keywords:
condition, the engine was started with the pure gasoline and gasolineehydrogen mixture,
Hydrogen
respectively. After the addition of hydrogen, gasoline injection duration was adjusted to
Gasoline
ensure the engine to be started successfully. All cold start experiments were performed at
Combustion
the same ambient, coolant and oil temperatures of 17 C. The test results showed that
Emissions
cylinder and indicated mean effective pressures in the first cycle were effectively improved
Cold start
with the increase of hydrogen addition fraction. Engine speed in the first 20 start cycles
SI engines
increased with hydrogen blending ratio. However, in later cycles, engine speed varied only a little with and without hydrogen addition due to the adoption of close loop control on engine speed. Because of the low ignition energy and high flame speed of hydrogen, both flame development and propagation durations were shortened after hydrogen addition. HC and CO emissions were dropped markedly after hydrogen addition due to the enhanced combustion process. When the hydrogen flow rate increased from 0 to 2.5 and 4.3 L/min, the instantaneous peak HC emissions were sharply reduced from 57083 to 17850 and 15738 ppm, respectively. NOx emissions were increased in the first 5 s and then reduced later after hydrogen addition. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The limited fossil fuel reserves and increased air pollution have pushed studies on improving the thermal efficiency and emissions performance of internal combustion (IC) engines. Because of the decreased combustion temperature and reduced charge homogeneity, spark-ignited (SI) engines always expel large amounts of HC and CO emissions at cold start. According
to previous studies [1], HC emissions during the cold start period account for about 70e90% of the total HC emissions in the federal test procedure (FTP). Meanwhile, because the low temperature stimulates the formation of fuel film, more fuel should be injected in the initial cycles to ensure the engine starting successfully. However, large amounts of the unburnt and incompletely combusted fuels are exhausted from the tail pipe as HC and CO emissions due to the fuel-rich combustion at
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.020
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cold start [2]. Moreover, as the temperature at engine exhaust manifolds is also lower than the light-off temperature of the three-way catalytic convert (TWC) when the engine is just started, the TWC is not able to convert HC, CO and NOx emissions into H2O, CO2 and N2 [3]. Engine cold start researches have been highlighted in recent years. Quader et al. [4] found that when the ambient temperature was 22 C, 57% of the injected gasoline could be vaporized to support the combustion. But when the temperature further dropped to 0 C, only 30% of the fuel was vaporized to form the combustible mixture. Gong et al. [5e7] carried out a series of studies focusing on the cold start characteristic of the methanol engine. He found that the engine could not be started without auxiliary start aids when the ambient air temperature was below 16 C, due to the high latent heat and poor evaporation of methanol. When the engine was preheated by a glow plug, the amount of methanol required for a successful start decreased by about 30%, and the peak cylinder pressure in the first firing cycle increased by about 180%. Li et al. [8] investigated the effects of ambient temperature, excess air ratio, battery voltage and spark timing on the cold start characteristic of an LPG engine. The test results showed that the lower the battery voltage was, the higher the HC emissions were. At an excess air ratio of 0.74, when the battery voltage increased from 10.6 to 12.8 V, HC emissions were reduced by about 27%. Comparatively, spark timing had little effect on the engine cold start HC emissions. As the new European test procedure exacts the measurement of the engine cold start emissions, improving the performance during cold start is crucial for SI engines to meet more and more stringent emissions standards. Therefore, some researchers have focused on investigations related with the engine cold start performance [9e11]. Compared with the traditional fossil fuels, hydrogen is seen as the most promising fuel that can be applied in IC engines. Because hydrogen has many excellent combustion and physicochemical properties, the hydrogen-fueled engine could gain better combustion and emissions performance than the gasoline engine [12e17]. As a gaseous fuel, the hydrogen-fueled SI engine is not affected by the fuel-film effect. Thus, less fuel is required during the cold start for the hydrogen engine than that for the gasoline engine. Besides, the low ignition energy and wide flammability of hydrogen help the fuel be ignited easily and combusted completely at low temperatures and lean conditions. However, because of the low volume energy density and high flame temperature of hydrogen, the pure hydrogen-fueled engine always encounters the reduced power output and increased NOx emissions [18,19]. Besides, the limited hydrogen infrastructure and difficulties in hydrogen on-board storage also block the wide commercialization of the pure hydrogen-fueled engine. Comparatively, adding a small amount of hydrogen to the fossil fuel-powered engine avails improving its economic and emissions performance [20e23]. Moreover, the decreased hydrogen consumption of the hydrogen-blended engines also alleviates concerns on hydrogen refilling and storage [24]. Huang et al. [25e27] experimentally investigated the effect of hydrogen addition on the performance of a CNG engine. The test results demonstrated that engine thermal efficiency was enhanced and cyclic variation was reduced after hydrogen blending. He also conducted an experiment on a constant
volume combustion vessel to investigate the effect of hydrogen addition on early flame state of the natural gaseair mixture [28]. The experimental results showed that the fuel burning velocity was effectively increased with the increase of hydrogen blending level, due to the raised O and OH radicals after hydrogen addition. Besides, the flame stability at lean conditions was enhanced with hydrogen addition. Akansu et al. [29,30] ran an SI engine fueled with hydrogen-natural gas blends at various speeds and excess air ratios. The test results demonstrated that CO emission was reduced with the increase of engine speed and excess air ratio. The addition of hydrogen was beneficial for increasing engine thermal efficiency and cylinder pressure, especially at lean conditions. Moreover, HC and CO emissions were also decreased for the hydrogen-blended natural gas engine. Ji et al. [31e34] also carried out a series of experiments to investigate the effect of hydrogen addition on improving gasoline and ethanol engines performance at idle and normal operating conditions. The experimental results verified that hydrogen addition was effective on improving engine thermal efficiency, reducing cyclic variation and dropping HC and CO emissions. Besides, because of the dropped combustion temperature resulting from the improved engine indicated thermal efficiency and decreased fuel energy flow rate, NOx emissions were reduced with the increase of hydrogen addition fraction at idle. However, although many papers have investigated the effect of hydrogen addition on combustion and emissions performance of the gasoline, CNG and ethanol engines under idle and normal operating conditions, there is not any public report on starting a gasoline engine with hydrogen addition. Since the unique properties of hydrogen tend to help SI engines gain the improved cold start performance, there is a strong motivation to experimentally study the effect of hydrogen addition on combustion and emissions performance of a gasoline engine during cold start. In this paper, a modified four-cylinder gasoline engine equipped with a hydrogen port-injection system and a hybrid electronic control unit (HECU) was used to investigate the cold start performance of an SI engine fueled with hydrogen-gasoline blends. The engine was started at the same ambient, coolant and oil temperatures of 17 C. Two hydrogen flow rates of 2.5 and 4.3 L/min were adopted in the experiment.
2.
Experimental setup and procedure
2.1.
Experimental setup
The test engine is a four-cylinder SI engine produced by Beijing Hyundai Motors, which has a displacement of 1.6 L, a rated power output of 82.32 kW at 6000 rpm and a rated torque output of 143.28 N m at 4500 rpm. To realize the hydrogen port injection, a hydrogen injection system is developed including a hydrogen rail and four hydrogen injectors mounted on the engine intake manifolds while the original gasoline injection system is kept unchanged. A HECU which could communicate with the engine original electronic control unit (OECU) to acquire sensor signals and a calibration computer to get the desired control strategy is applied to control the opening and closing of hydrogen and gasoline
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injectors. With this system, the amount of hydrogen addition and excess air ratio of the fueleair mixture can be adjusted online. The hydrogen used in this test is produced from water electrolysis, which is stored at 16 MPa in the stainless steel cylinders outside the engine lab. When the test begins, hydrogen is supplied to the hydrogen rail and injectors at 0.3 MPa after two steps of pressure reduction. A flame arrestor is placed in the hydrogen supply pipe to prevent the backfire of hydrogen. A DMF-1-1-A Coriolis fuel mass flow meter is adopted to measure the instantaneous gasoline flow rate (measurement uncertainty: <0.17 g/min, response time: 0.5 s). The air and hydrogen mass flow rates are monitored by a 20N060 and a D07-19BM thermal mass flowmeters produced by Toceil and Seven Star, respectively (measurement uncertainties: <0.1 L/ min for air, <0.02 L/min for hydrogen). A Kistler 2613B optical encoder is connected to the crank shaft to collect crank angle signals which can be used to determine crank position and calculate the instantaneous engine speed (crank angle resolution: 0.2 CA, measurement deviation <0.01 CA). A Kistler 6117BCD17 cylinder pressure transducer with a spark plug is used to acquire the cylinder pressure and enforce the ignition of the fourth cylinder (measurement uncertainty: <0.3 bar). The crank angle and cylinder pressure signals are recorded and treated by a Dewetron combustion analyzer to extract some indepth combustion information. The exhaust emissions of NOx, HC and CO are measured by a Horiba MEXA-7100D emissions analyzer and the measurement sensitivities are 1 ppm for NOx and HC emissions and 0.001% for CO emission. NOx are measured by the chemiluminescent method, HC emissions are determined by the hydrogen flame ionization detection method, and CO is detected by the nondispersive infrared method. The measurement uncertainties are less than 1% of the measured values for HC, CO and NOx emissions. The detailed description of the experimental system can be found in Ref. [34].
2.2.
Experimental procedure
All experiments were carried out at an ambient temperature of 17 C. The coolant and lubricant oil temperatures were also adjusted to 17 C before each experiment. For all testing points, the main throttle was kept closed, and the battery voltage was 11.9 V. The OECU was applied to control the idle bypass valve opening and spark timing automatically. The HECU was adopted to control the injection timings and durations of hydrogen and gasoline via the commands from a calibration computer and engine sensor signals acquired from the OECU. At the same environmental condition, the engine was firstly started with the pure gasoline, and then started with the gasoline-hydrogen mixture at hydrogen volume flow rates of 2.5 and 4.3 L/min, respectively. Because the formation and evaporation of fuel film was unstable, the open loop control of the charge excess air ratio was applied at engine cold start. At the open loop control mode, gasoline injection duration after hydrogen addition was adjusted to ensure a successful engine start. For each testing point, the in-cylinder pressures for over 100 consecutive cycles were collected and analyzed through the Dewe-CA combustion analysis software to obtain the
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profiles of in-cylinder pressure versus crank angle, and then curves of CA0e10 and CA10e90, etc against cycle number were deduced.
3.
Results and discussion
3.1. Cylinder pressure and indicated mean effective pressure Fig. 1 displays the variation of cylinder pressure (P) with crank angle in the first five cycles for three hydrogen flow rates at cold start. It can be found from Fig. 1 that the engine peak cylinder pressure in each cycle is effectively enhanced with the increase of hydrogen addition level. In the first cycle, the maximum cylinder pressure increases by about 51.1% and 69.2% when hydrogen flow rate rises from 0 to 2.5 and 4.3 L/min, respectively. This is because the low engine temperature at cold start leads to the worsened fuel-film effect which means only a limited amount of the injected gasoline could be vaporized for joining the combustion at cold start. Thus, the increased mixture excess air ratio and deteriorated mixture homogeneity meet together in the initial cycles, which consequently result in the prolonged combustion duration and reduced cylinder pressure. As a gaseous fuel, the injected hydrogen can be introduced into the cylinder quickly without forming fuel film. Besides, the high diffusion speed of hydrogen also avails stimulating the charge flow and improving the mixture homogeneity. As the enhanced mixture homogeneity benefits the fast and complete combustion of the hydrogen-gasoline mixture, the peak cylinder pressure in the first cycle is generally increased with hydrogen addition. Fig. 1 also shows that, for a given hydrogen flow rate, the peak cylinder pressure is gradually reduced cycle by cycle in the first five cycles. The reason can be attributed to the fact that the OECU generally adopts the increased opening of the idle bypass valve to ensure the engine starting successfully. Because the increased idle bypass valve opening permits more air and fuel to be inducted into the cylinder, the cylinder pressure gains the highest value
Fig. 1 e Cylinder pressure versus crank angle at three hydrogen flow rates.
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in the first cycle. Once the engine is started, since the increased engine speed results in the raised throttling loss, the maximum cylinder pressure is reduced after the first cycle as more work is spent for overcoming the pumping loss. Meanwhile, when the engine speed reaches the target value, the OECU would slightly reduce the opening of idle bypass valve, and the decreased idle bypass valve opening also causes the increased pumping loss and dropped fuel energy flow rate. Thus, at a specified hydrogen addition fraction, the peak cylinder pressure is dropped cycle by cycle. Indicated mean effective pressure (Imep) indicates engine working capability. Fig. 2 shows the variation of indicated mean effective pressure with cycle number for the first 100 cycles under three hydrogen flow rates at cold start. As it is shown in Fig. 2, for all hydrogen addition levels, Imep firstly increases and then decreases with the increase of cycle number. This is because at cold start, the engine speed should be firstly raised to be higher than the target starting speed to ensure a successful start, and then decreases to a stable idle speed. So, Imep increases at the initial cycles and then drops to a relatively stable value. At the same time, Fig. 2 shows that Imep rises with the increase of hydrogen flow rate, especially in the initial cycles. The possible explanation could be that the high flame speed of hydrogen contributes to the shortened combustion duration after hydrogen addition (see Figs. 4 and 5) which helps reduce the cooling and exhaust losses and benefits improving engine thermal efficiency. Thus, Imep increases with hydrogen addition. However, for the pure gasoline engine, when the engine is started, because the cold working condition intensifies the fuel-film effect which affects adversely the atomization and vaporization of gasoline and its mixing with air, the reduced fuel energy flowing into the cylinder is attained inevitably, which leads to the reduced Imep in the initial cycles. As the high diffusion speed of hydrogen avails improving charge homogeneity and complete combustion, Imep is enhanced with the increase of hydrogen
Fig. 2 e Imep versus cycle number at three hydrogen flow rates.
flow rate. Fig. 2 also demonstrates that the variation of Imep with cycle number is getting flat with the increase of hydrogen flow rate. This is because the low ignition energy and fast flame speed of hydrogen contribute to the reduced engine cyclic variation. Moreover, the flame structure of hydrogen is also more stable than that of gasoline [35]. Thus, the reduced cyclic variation after hydrogen addition finally leads to the smoother start-up process with the gasoline-hydrogen mixture than the pure gasoline.
3.2.
Engine speed
Engine speed is a key factor for judging cold start performance. Fig. 3 depicts the variation of engine speed (N ) with cycle number for three hydrogen flow rates at cold start. It can be found from Fig. 3 that, because of the shortened combustion duration and improved indicated mean effective pressure after hydrogen addition, the peak engine speed is increased from 2017 rpm of the pure gasoline start to 2048 and 2144 rpm of the hydrogen-blended engine start at hydrogen flow rates of 2.5 and 4.3 L/min, respectively. Meanwhile, because the high diffusion and combustion speeds of hydrogen decrease engine combustion duration, the relevant cycle for reaching the peak engine speed is also advanced after hydrogen addition, which varies from the 10th cycle of the pure gasoline start to the 8th and 6th cycles of the hydrogen-blended engine start at hydrogen flow rates of 2.5 and 4.3 L/min, respectively. However, when the cycle number exceeds 20, engine speeds for all hydrogen flow rates show a similar value due to the adoption of close loop control on engine idle speed.
3.3.
Combustion
Flame development (CA0e10) and propagation (CA10e90) periods reflect the combustion process and quality. CA0e10 and CA10e90 are defined as the crank angle durations from spark discharge to 10% and from 10% to 90% heat release of the total fuel, respectively. Figs. 4 and 5 display the variations of CA0e10 and CA10e90 with cycle number for three
Fig. 3 e Engine speed versus cycle number at three hydrogen flow rates.
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Fig. 4 e CA0e10 versus cycle number at three hydrogen flow rates.
hydrogen flow rates at cold start. Because of the high oscillation in combustion during cold start, to clearly show the trend of flame development and propagation periods with cycle number, the lines in Figs. 4 and 5 are curve fitted from the measured CA0e10 and CA10e90. It can be found from Figs. 4 and 5 that both CA0e10 and CA10e90 are effectively shortened after hydrogen blending. When hydrogen flow rate rises from 0 to 4.3 L/min, CA0e10 and CA10e90 are shortened by about 41.8% and 26.5% in the first firing cycle, respectively. This can be ascribed to the fact that as the ignition energy of hydrogen is only 1/10 to that of gasoline, the hydrogen-gasoline mixture can be ignited much more easily than the pure gasoline, so hydrogen addition causes the decreased CA0e10. Besides, the increased O and OH radicals after hydrogen addition also contribute to the fast and complete combustion of the fueleair mixture which further results in the shortened
Fig. 5 e CA10e90 versus cycle number at three hydrogen flow rates.
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CA0e10 [28,36]. Because of the high burning velocity of hydrogen, the flame of the hydrogen-gasoline mixture can be propagated more quickly than that of gasoline. Meanwhile, the improved mixture homogeneity and reduced fuel-film effect after hydrogen addition benefit the complete and quick combustion of the hydrogenegasoline mixture. Therefore, CA10e90 is also reduced obviously with the increase of hydrogen flow rate. Figs. 4 and 5 also demonstrate that, for a given hydrogen blending level, CA0e10 is gradually increased to a stable value, but CA10e90 is first prolonged and then slightly shortened with the increase of cycle number within the first 100 cycles from the initiation of the cold start. The possible reason can be attributed to the fact that, at cold start, the OECU raises the idle bypass valve opening to let more fuel-air mixture be introduced into the cylinders so that the engine could be started successfully. As the raised idle bypass valve opening leads to the enhanced charge motion and combustion, both CA0e10 and CA10e90 are shorter in initial cycles from the onset of the cold start than those in later cycles. Because the engine is started at a relatively low ambient temperature of 17 C, the injected gasoline can be easily adhered to some cold surfaces, such as piston, cylinder wall and intake and exhaust valves. However, when the mixture is ignited, the raised cylinder temperature helps these fuels be vaporized again to support the combustion. Thus, different from CA0e10, CA10e90 is slightly shortened with the increase of cycle number in later cycles.
3.4.
Emissions
Because of the low combustion temperature and the adoption of rich mixture for counteracting the fuel-film effect, HC and CO emissions are inevitably raised at cold start for gasoline engines. The effects of hydrogen addition on engine toxic emissions of HC, CO and NOx before a three-way catalytic converter at cold start condition are discussed in this section. Fig. 6 shows the variation of HC emissions with time for three hydrogen flow rates at cold start. As it is shown in Fig. 6, for all hydrogen flow rates, HC emissions first increase quickly
Fig. 6 e HC versus time at three hydrogen flow rates.
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and then decrease to a stable value with the increase of time. This is because the rich mixture is used to ensure the engine start successfully in the initial cycles. Thus, the unburned fuel is expelled from the cylinder as HC emissions. When the engine is gradually warmed, since the stoichiometric fueleair mixture is used for the engine warming up, the fuel can be combusted more completely in later cycles than in the initial cycles. It is pleasing to see from Fig. 6 that when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, the peak instantaneous HC emissions are sharply reduced from 57083 to 17850 and 15738 ppm, respectively. The proper explanation could be that, for the same excess air ratio, the reduced gasoline flow rate is accomplished after the addition of hydrogen, causing the decreased source of HC emissions. Meanwhile, the short quenching distance of hydrogen also permits the flame of the hydrogenegasoline mixture to be propagated closer to the cylinder wall and crevices than that of the pure gasoline, and therefore reduces the HC emissions caused by the crevice effect. Moreover, the wide flammability of hydrogen also permits the hydrogen-gasoline mixture to be ignited and combusted more stably in the initial cycles where the actual combustion air-to-fuel ratio is relatively high due to the intensified fuel-film effect. As the flame stability at early combustion stage is enhanced after hydrogen addition, less HC emissions are exhausted from the engine with the hydrogenegasoline blends start than with the pure gasoline start. Fig. 7 plots the variation of CO emission with time for three hydrogen flow rates at cold start. It is seen from Fig. 7 that, at a given hydrogen addition level, because of the incomplete combustion in the initial cycles and the adoption of stoichiometric airefuel mixture in later cycles, CO also first rises and then drops with the increase of time. Fig. 7 demonstrates that the addition of hydrogen is effective on reducing engine CO emission during the cold start process, and when hydrogen flow rate increases from 0 to 4.3 L/min, the maximum instantaneous CO emission during the first 50 s is reduced by about 32.6%. This can be attributed to the fact that CO is mainly caused by the incomplete combustion of gasoline. As the high flame speed of hydrogen avails reducing the
Fig. 8 e NOx versus time at three hydrogen flow rates.
incomplete combustion, CO is effectively reduced after hydrogen addition. Meanwhile, since the high diffusion speed of hydrogen also contributes to the improved mixture homogeneity and the reduced fuel-rich area, the addition of hydrogen benefits reducing CO emission from gasoline engines at cold start. Fig. 8 displays the variation of NOx emissions with time for three hydrogen flow rates at cold start. As it is shown in Fig. 8, because of the shortened combustion duration and increased Imep after hydrogen addition in initial cycles, the maximum instantaneous NOx emissions are increased with the increase of hydrogen flow rate. The peak NOx emissions rise by about 61.1% and 93.7% when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, respectively. However, it is satisfying to find from Fig. 8 that, when the engine runs under the stable idle condition, NOx emissions from the engine started with the hydrogen-gasoline mixture are lower than those from the original engine, and this trend is consistent with that in our previous studies performed at idle and stoichiometric conditions [31]. The average NOx emissions within 50 s after the onset of the cold start drop from 116.4 to 105.8 and 99.6 ppm when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, respectively. The proper interpretation can be that the shortened combustion duration and enhanced combustion after hydrogen blending result in the reduced engine cooling and exhaust losses and improved thermal efficiency. Thereby, for a specified idle speed, the increased thermal efficiency means less fuel is required to drive the engine. As the reduced fuel energy flow rate causes the dropped peak cylinder temperature, NOx emissions are smoothly decreased with the increase of hydrogen flow rate after 10 s from the onset of the cold start.
4.
Fig. 7 e CO versus time at three hydrogen flow rates.
Conclusions
This paper introduced an experimental investigation which was performed on a gasoline-fueled SI engine equipped with an electronically controlled hydrogen port-injection system and a HECU to explore the combustion and emissions
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characteristics of an SI engine which is cold started with the gasoline-hydrogen mixture. The main conclusions are listed as follows: 1. The maximum cylinder pressure and indicated mean effective pressure in the first cycle are increased obviously after hydrogen addition. In the first cycle, the peak cylinder pressure increases by about 51.1% and 69.2% when hydrogen flow rate rises from 0 to 2.5 and 4.3 L/min, respectively. 2. Engine speed in the first 20 cycles increases with the increase of hydrogen flow rate. When the cycle number exceeds 20, engine speed is roughly the same before and after hydrogen addition, due to the adoption of close loop control on engine speed. 3. Because of the low ignition energy and high flame speed of hydrogen, flame development and propagation periods are effectively shortened for the engine started with hydrogenegasoline blends. 4. As the high diffusion speed of hydrogen avails improving mixture homogeneity and contributes to the fast and complete combustion of the hydrogen-gasoline mixture, HC and CO emissions are decreased markedly with the increase of hydrogen flow rate. When hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, the maximum instantaneous HC emissions are sharply reduced from 57083 to 17850 and 15738 ppm, respectively. 5. NOx emissions in the first 5 s after cold start are increased with the increase of hydrogen flow rate, due to the enhanced combustion and raised cylinder temperature after hydrogen blending. However, NOx emissions with the hydrogen-gasoline mixture start are lower than those with the pure gasoline start after 10 s from the onset of the cold start.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 50976005). The authors also appreciate all students in the group for their help with the experiment.
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[31] Ji C, Wang S. Combustion and emissions performance of a hybrid hydrogenegasoline engine at idle and lean conditions. Int J Hydrogen Energy 2010;35:346e55. [32] Wang S, Ji C, Zhang B. Effect of hydrogen addition on combustion and emissions performance of a spark-ignited ethanol engine at idle and stoichiometric conditions. Int J Hydrogen Energy 2010;35:9205e13. [33] Wang S, Ji S, Zhang M, Zhang B. Reducing the idle speed of a spark-ignited gasoline engine with hydrogen addition. Int J Hydrogen Energy 2010;35:10580e8. [34] Ji C, Wang S, Zhang B. Combustion and emissions characteristics of a hybrid hydrogenegasoline engine under various loads and lean conditions. Int J Hydrogen Energy 2010;35:5714e22. [35] Ozdor N, Dulger M, Sher E. Cyclic variability in spark ignition engines e a literature survey. SAE Paper No. 1994;940987. [36] Ahsan RC, Surbramanyam RG. Laser induced fluorescence measurements of radical concentrations in hydrogenehydrocarbon hybrid gas fuel flames. Int J Hydrogen Energy 2000;25:1119e27.
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Starting a spark-ignited engine with the gasolineehydrogen mixture Shuofeng Wang, Changwei Ji*, Bo Zhang College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
article info
abstract
Article history:
Because of the increased fuel-film effect and dropped combustion temperature, spark-
Received 10 November 2010
ignited (SI) gasoline engines always expel large amounts of HC and CO emissions during
Received in revised form
the cold start period. This paper experimentally investigated the effect of hydrogen addi-
3 January 2011
tion on improving the cold start performance of a gasoline engine. The test was carried out
Accepted 5 January 2011
on a 1.6-L, four-cylinder, SI engine equipped with an electronically controlled hydrogen injection system. A hybrid electronic control unit (HECU) was applied to control the opening and closing of hydrogen and gasoline injectors. Under the same environmental
Keywords:
condition, the engine was started with the pure gasoline and gasolineehydrogen mixture,
Hydrogen
respectively. After the addition of hydrogen, gasoline injection duration was adjusted to
Gasoline
ensure the engine to be started successfully. All cold start experiments were performed at
Combustion
the same ambient, coolant and oil temperatures of 17 C. The test results showed that
Emissions
cylinder and indicated mean effective pressures in the first cycle were effectively improved
Cold start
with the increase of hydrogen addition fraction. Engine speed in the first 20 start cycles
SI engines
increased with hydrogen blending ratio. However, in later cycles, engine speed varied only a little with and without hydrogen addition due to the adoption of close loop control on engine speed. Because of the low ignition energy and high flame speed of hydrogen, both flame development and propagation durations were shortened after hydrogen addition. HC and CO emissions were dropped markedly after hydrogen addition due to the enhanced combustion process. When the hydrogen flow rate increased from 0 to 2.5 and 4.3 L/min, the instantaneous peak HC emissions were sharply reduced from 57083 to 17850 and 15738 ppm, respectively. NOx emissions were increased in the first 5 s and then reduced later after hydrogen addition. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The limited fossil fuel reserves and increased air pollution have pushed studies on improving the thermal efficiency and emissions performance of internal combustion (IC) engines. Because of the decreased combustion temperature and reduced charge homogeneity, spark-ignited (SI) engines always expel large amounts of HC and CO emissions at cold start. According
to previous studies [1], HC emissions during the cold start period account for about 70e90% of the total HC emissions in the federal test procedure (FTP). Meanwhile, because the low temperature stimulates the formation of fuel film, more fuel should be injected in the initial cycles to ensure the engine starting successfully. However, large amounts of the unburnt and incompletely combusted fuels are exhausted from the tail pipe as HC and CO emissions due to the fuel-rich combustion at
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.020
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cold start [2]. Moreover, as the temperature at engine exhaust manifolds is also lower than the light-off temperature of the three-way catalytic convert (TWC) when the engine is just started, the TWC is not able to convert HC, CO and NOx emissions into H2O, CO2 and N2 [3]. Engine cold start researches have been highlighted in recent years. Quader et al. [4] found that when the ambient temperature was 22 C, 57% of the injected gasoline could be vaporized to support the combustion. But when the temperature further dropped to 0 C, only 30% of the fuel was vaporized to form the combustible mixture. Gong et al. [5e7] carried out a series of studies focusing on the cold start characteristic of the methanol engine. He found that the engine could not be started without auxiliary start aids when the ambient air temperature was below 16 C, due to the high latent heat and poor evaporation of methanol. When the engine was preheated by a glow plug, the amount of methanol required for a successful start decreased by about 30%, and the peak cylinder pressure in the first firing cycle increased by about 180%. Li et al. [8] investigated the effects of ambient temperature, excess air ratio, battery voltage and spark timing on the cold start characteristic of an LPG engine. The test results showed that the lower the battery voltage was, the higher the HC emissions were. At an excess air ratio of 0.74, when the battery voltage increased from 10.6 to 12.8 V, HC emissions were reduced by about 27%. Comparatively, spark timing had little effect on the engine cold start HC emissions. As the new European test procedure exacts the measurement of the engine cold start emissions, improving the performance during cold start is crucial for SI engines to meet more and more stringent emissions standards. Therefore, some researchers have focused on investigations related with the engine cold start performance [9e11]. Compared with the traditional fossil fuels, hydrogen is seen as the most promising fuel that can be applied in IC engines. Because hydrogen has many excellent combustion and physicochemical properties, the hydrogen-fueled engine could gain better combustion and emissions performance than the gasoline engine [12e17]. As a gaseous fuel, the hydrogen-fueled SI engine is not affected by the fuel-film effect. Thus, less fuel is required during the cold start for the hydrogen engine than that for the gasoline engine. Besides, the low ignition energy and wide flammability of hydrogen help the fuel be ignited easily and combusted completely at low temperatures and lean conditions. However, because of the low volume energy density and high flame temperature of hydrogen, the pure hydrogen-fueled engine always encounters the reduced power output and increased NOx emissions [18,19]. Besides, the limited hydrogen infrastructure and difficulties in hydrogen on-board storage also block the wide commercialization of the pure hydrogen-fueled engine. Comparatively, adding a small amount of hydrogen to the fossil fuel-powered engine avails improving its economic and emissions performance [20e23]. Moreover, the decreased hydrogen consumption of the hydrogen-blended engines also alleviates concerns on hydrogen refilling and storage [24]. Huang et al. [25e27] experimentally investigated the effect of hydrogen addition on the performance of a CNG engine. The test results demonstrated that engine thermal efficiency was enhanced and cyclic variation was reduced after hydrogen blending. He also conducted an experiment on a constant
volume combustion vessel to investigate the effect of hydrogen addition on early flame state of the natural gaseair mixture [28]. The experimental results showed that the fuel burning velocity was effectively increased with the increase of hydrogen blending level, due to the raised O and OH radicals after hydrogen addition. Besides, the flame stability at lean conditions was enhanced with hydrogen addition. Akansu et al. [29,30] ran an SI engine fueled with hydrogen-natural gas blends at various speeds and excess air ratios. The test results demonstrated that CO emission was reduced with the increase of engine speed and excess air ratio. The addition of hydrogen was beneficial for increasing engine thermal efficiency and cylinder pressure, especially at lean conditions. Moreover, HC and CO emissions were also decreased for the hydrogen-blended natural gas engine. Ji et al. [31e34] also carried out a series of experiments to investigate the effect of hydrogen addition on improving gasoline and ethanol engines performance at idle and normal operating conditions. The experimental results verified that hydrogen addition was effective on improving engine thermal efficiency, reducing cyclic variation and dropping HC and CO emissions. Besides, because of the dropped combustion temperature resulting from the improved engine indicated thermal efficiency and decreased fuel energy flow rate, NOx emissions were reduced with the increase of hydrogen addition fraction at idle. However, although many papers have investigated the effect of hydrogen addition on combustion and emissions performance of the gasoline, CNG and ethanol engines under idle and normal operating conditions, there is not any public report on starting a gasoline engine with hydrogen addition. Since the unique properties of hydrogen tend to help SI engines gain the improved cold start performance, there is a strong motivation to experimentally study the effect of hydrogen addition on combustion and emissions performance of a gasoline engine during cold start. In this paper, a modified four-cylinder gasoline engine equipped with a hydrogen port-injection system and a hybrid electronic control unit (HECU) was used to investigate the cold start performance of an SI engine fueled with hydrogen-gasoline blends. The engine was started at the same ambient, coolant and oil temperatures of 17 C. Two hydrogen flow rates of 2.5 and 4.3 L/min were adopted in the experiment.
2.
Experimental setup and procedure
2.1.
Experimental setup
The test engine is a four-cylinder SI engine produced by Beijing Hyundai Motors, which has a displacement of 1.6 L, a rated power output of 82.32 kW at 6000 rpm and a rated torque output of 143.28 N m at 4500 rpm. To realize the hydrogen port injection, a hydrogen injection system is developed including a hydrogen rail and four hydrogen injectors mounted on the engine intake manifolds while the original gasoline injection system is kept unchanged. A HECU which could communicate with the engine original electronic control unit (OECU) to acquire sensor signals and a calibration computer to get the desired control strategy is applied to control the opening and closing of hydrogen and gasoline
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injectors. With this system, the amount of hydrogen addition and excess air ratio of the fueleair mixture can be adjusted online. The hydrogen used in this test is produced from water electrolysis, which is stored at 16 MPa in the stainless steel cylinders outside the engine lab. When the test begins, hydrogen is supplied to the hydrogen rail and injectors at 0.3 MPa after two steps of pressure reduction. A flame arrestor is placed in the hydrogen supply pipe to prevent the backfire of hydrogen. A DMF-1-1-A Coriolis fuel mass flow meter is adopted to measure the instantaneous gasoline flow rate (measurement uncertainty: <0.17 g/min, response time: 0.5 s). The air and hydrogen mass flow rates are monitored by a 20N060 and a D07-19BM thermal mass flowmeters produced by Toceil and Seven Star, respectively (measurement uncertainties: <0.1 L/ min for air, <0.02 L/min for hydrogen). A Kistler 2613B optical encoder is connected to the crank shaft to collect crank angle signals which can be used to determine crank position and calculate the instantaneous engine speed (crank angle resolution: 0.2 CA, measurement deviation <0.01 CA). A Kistler 6117BCD17 cylinder pressure transducer with a spark plug is used to acquire the cylinder pressure and enforce the ignition of the fourth cylinder (measurement uncertainty: <0.3 bar). The crank angle and cylinder pressure signals are recorded and treated by a Dewetron combustion analyzer to extract some indepth combustion information. The exhaust emissions of NOx, HC and CO are measured by a Horiba MEXA-7100D emissions analyzer and the measurement sensitivities are 1 ppm for NOx and HC emissions and 0.001% for CO emission. NOx are measured by the chemiluminescent method, HC emissions are determined by the hydrogen flame ionization detection method, and CO is detected by the nondispersive infrared method. The measurement uncertainties are less than 1% of the measured values for HC, CO and NOx emissions. The detailed description of the experimental system can be found in Ref. [34].
2.2.
Experimental procedure
All experiments were carried out at an ambient temperature of 17 C. The coolant and lubricant oil temperatures were also adjusted to 17 C before each experiment. For all testing points, the main throttle was kept closed, and the battery voltage was 11.9 V. The OECU was applied to control the idle bypass valve opening and spark timing automatically. The HECU was adopted to control the injection timings and durations of hydrogen and gasoline via the commands from a calibration computer and engine sensor signals acquired from the OECU. At the same environmental condition, the engine was firstly started with the pure gasoline, and then started with the gasoline-hydrogen mixture at hydrogen volume flow rates of 2.5 and 4.3 L/min, respectively. Because the formation and evaporation of fuel film was unstable, the open loop control of the charge excess air ratio was applied at engine cold start. At the open loop control mode, gasoline injection duration after hydrogen addition was adjusted to ensure a successful engine start. For each testing point, the in-cylinder pressures for over 100 consecutive cycles were collected and analyzed through the Dewe-CA combustion analysis software to obtain the
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profiles of in-cylinder pressure versus crank angle, and then curves of CA0e10 and CA10e90, etc against cycle number were deduced.
3.
Results and discussion
3.1. Cylinder pressure and indicated mean effective pressure Fig. 1 displays the variation of cylinder pressure (P) with crank angle in the first five cycles for three hydrogen flow rates at cold start. It can be found from Fig. 1 that the engine peak cylinder pressure in each cycle is effectively enhanced with the increase of hydrogen addition level. In the first cycle, the maximum cylinder pressure increases by about 51.1% and 69.2% when hydrogen flow rate rises from 0 to 2.5 and 4.3 L/min, respectively. This is because the low engine temperature at cold start leads to the worsened fuel-film effect which means only a limited amount of the injected gasoline could be vaporized for joining the combustion at cold start. Thus, the increased mixture excess air ratio and deteriorated mixture homogeneity meet together in the initial cycles, which consequently result in the prolonged combustion duration and reduced cylinder pressure. As a gaseous fuel, the injected hydrogen can be introduced into the cylinder quickly without forming fuel film. Besides, the high diffusion speed of hydrogen also avails stimulating the charge flow and improving the mixture homogeneity. As the enhanced mixture homogeneity benefits the fast and complete combustion of the hydrogen-gasoline mixture, the peak cylinder pressure in the first cycle is generally increased with hydrogen addition. Fig. 1 also shows that, for a given hydrogen flow rate, the peak cylinder pressure is gradually reduced cycle by cycle in the first five cycles. The reason can be attributed to the fact that the OECU generally adopts the increased opening of the idle bypass valve to ensure the engine starting successfully. Because the increased idle bypass valve opening permits more air and fuel to be inducted into the cylinder, the cylinder pressure gains the highest value
Fig. 1 e Cylinder pressure versus crank angle at three hydrogen flow rates.
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in the first cycle. Once the engine is started, since the increased engine speed results in the raised throttling loss, the maximum cylinder pressure is reduced after the first cycle as more work is spent for overcoming the pumping loss. Meanwhile, when the engine speed reaches the target value, the OECU would slightly reduce the opening of idle bypass valve, and the decreased idle bypass valve opening also causes the increased pumping loss and dropped fuel energy flow rate. Thus, at a specified hydrogen addition fraction, the peak cylinder pressure is dropped cycle by cycle. Indicated mean effective pressure (Imep) indicates engine working capability. Fig. 2 shows the variation of indicated mean effective pressure with cycle number for the first 100 cycles under three hydrogen flow rates at cold start. As it is shown in Fig. 2, for all hydrogen addition levels, Imep firstly increases and then decreases with the increase of cycle number. This is because at cold start, the engine speed should be firstly raised to be higher than the target starting speed to ensure a successful start, and then decreases to a stable idle speed. So, Imep increases at the initial cycles and then drops to a relatively stable value. At the same time, Fig. 2 shows that Imep rises with the increase of hydrogen flow rate, especially in the initial cycles. The possible explanation could be that the high flame speed of hydrogen contributes to the shortened combustion duration after hydrogen addition (see Figs. 4 and 5) which helps reduce the cooling and exhaust losses and benefits improving engine thermal efficiency. Thus, Imep increases with hydrogen addition. However, for the pure gasoline engine, when the engine is started, because the cold working condition intensifies the fuel-film effect which affects adversely the atomization and vaporization of gasoline and its mixing with air, the reduced fuel energy flowing into the cylinder is attained inevitably, which leads to the reduced Imep in the initial cycles. As the high diffusion speed of hydrogen avails improving charge homogeneity and complete combustion, Imep is enhanced with the increase of hydrogen
Fig. 2 e Imep versus cycle number at three hydrogen flow rates.
flow rate. Fig. 2 also demonstrates that the variation of Imep with cycle number is getting flat with the increase of hydrogen flow rate. This is because the low ignition energy and fast flame speed of hydrogen contribute to the reduced engine cyclic variation. Moreover, the flame structure of hydrogen is also more stable than that of gasoline [35]. Thus, the reduced cyclic variation after hydrogen addition finally leads to the smoother start-up process with the gasoline-hydrogen mixture than the pure gasoline.
3.2.
Engine speed
Engine speed is a key factor for judging cold start performance. Fig. 3 depicts the variation of engine speed (N ) with cycle number for three hydrogen flow rates at cold start. It can be found from Fig. 3 that, because of the shortened combustion duration and improved indicated mean effective pressure after hydrogen addition, the peak engine speed is increased from 2017 rpm of the pure gasoline start to 2048 and 2144 rpm of the hydrogen-blended engine start at hydrogen flow rates of 2.5 and 4.3 L/min, respectively. Meanwhile, because the high diffusion and combustion speeds of hydrogen decrease engine combustion duration, the relevant cycle for reaching the peak engine speed is also advanced after hydrogen addition, which varies from the 10th cycle of the pure gasoline start to the 8th and 6th cycles of the hydrogen-blended engine start at hydrogen flow rates of 2.5 and 4.3 L/min, respectively. However, when the cycle number exceeds 20, engine speeds for all hydrogen flow rates show a similar value due to the adoption of close loop control on engine idle speed.
3.3.
Combustion
Flame development (CA0e10) and propagation (CA10e90) periods reflect the combustion process and quality. CA0e10 and CA10e90 are defined as the crank angle durations from spark discharge to 10% and from 10% to 90% heat release of the total fuel, respectively. Figs. 4 and 5 display the variations of CA0e10 and CA10e90 with cycle number for three
Fig. 3 e Engine speed versus cycle number at three hydrogen flow rates.
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Fig. 4 e CA0e10 versus cycle number at three hydrogen flow rates.
hydrogen flow rates at cold start. Because of the high oscillation in combustion during cold start, to clearly show the trend of flame development and propagation periods with cycle number, the lines in Figs. 4 and 5 are curve fitted from the measured CA0e10 and CA10e90. It can be found from Figs. 4 and 5 that both CA0e10 and CA10e90 are effectively shortened after hydrogen blending. When hydrogen flow rate rises from 0 to 4.3 L/min, CA0e10 and CA10e90 are shortened by about 41.8% and 26.5% in the first firing cycle, respectively. This can be ascribed to the fact that as the ignition energy of hydrogen is only 1/10 to that of gasoline, the hydrogen-gasoline mixture can be ignited much more easily than the pure gasoline, so hydrogen addition causes the decreased CA0e10. Besides, the increased O and OH radicals after hydrogen addition also contribute to the fast and complete combustion of the fueleair mixture which further results in the shortened
Fig. 5 e CA10e90 versus cycle number at three hydrogen flow rates.
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CA0e10 [28,36]. Because of the high burning velocity of hydrogen, the flame of the hydrogen-gasoline mixture can be propagated more quickly than that of gasoline. Meanwhile, the improved mixture homogeneity and reduced fuel-film effect after hydrogen addition benefit the complete and quick combustion of the hydrogenegasoline mixture. Therefore, CA10e90 is also reduced obviously with the increase of hydrogen flow rate. Figs. 4 and 5 also demonstrate that, for a given hydrogen blending level, CA0e10 is gradually increased to a stable value, but CA10e90 is first prolonged and then slightly shortened with the increase of cycle number within the first 100 cycles from the initiation of the cold start. The possible reason can be attributed to the fact that, at cold start, the OECU raises the idle bypass valve opening to let more fuel-air mixture be introduced into the cylinders so that the engine could be started successfully. As the raised idle bypass valve opening leads to the enhanced charge motion and combustion, both CA0e10 and CA10e90 are shorter in initial cycles from the onset of the cold start than those in later cycles. Because the engine is started at a relatively low ambient temperature of 17 C, the injected gasoline can be easily adhered to some cold surfaces, such as piston, cylinder wall and intake and exhaust valves. However, when the mixture is ignited, the raised cylinder temperature helps these fuels be vaporized again to support the combustion. Thus, different from CA0e10, CA10e90 is slightly shortened with the increase of cycle number in later cycles.
3.4.
Emissions
Because of the low combustion temperature and the adoption of rich mixture for counteracting the fuel-film effect, HC and CO emissions are inevitably raised at cold start for gasoline engines. The effects of hydrogen addition on engine toxic emissions of HC, CO and NOx before a three-way catalytic converter at cold start condition are discussed in this section. Fig. 6 shows the variation of HC emissions with time for three hydrogen flow rates at cold start. As it is shown in Fig. 6, for all hydrogen flow rates, HC emissions first increase quickly
Fig. 6 e HC versus time at three hydrogen flow rates.
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and then decrease to a stable value with the increase of time. This is because the rich mixture is used to ensure the engine start successfully in the initial cycles. Thus, the unburned fuel is expelled from the cylinder as HC emissions. When the engine is gradually warmed, since the stoichiometric fueleair mixture is used for the engine warming up, the fuel can be combusted more completely in later cycles than in the initial cycles. It is pleasing to see from Fig. 6 that when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, the peak instantaneous HC emissions are sharply reduced from 57083 to 17850 and 15738 ppm, respectively. The proper explanation could be that, for the same excess air ratio, the reduced gasoline flow rate is accomplished after the addition of hydrogen, causing the decreased source of HC emissions. Meanwhile, the short quenching distance of hydrogen also permits the flame of the hydrogenegasoline mixture to be propagated closer to the cylinder wall and crevices than that of the pure gasoline, and therefore reduces the HC emissions caused by the crevice effect. Moreover, the wide flammability of hydrogen also permits the hydrogen-gasoline mixture to be ignited and combusted more stably in the initial cycles where the actual combustion air-to-fuel ratio is relatively high due to the intensified fuel-film effect. As the flame stability at early combustion stage is enhanced after hydrogen addition, less HC emissions are exhausted from the engine with the hydrogenegasoline blends start than with the pure gasoline start. Fig. 7 plots the variation of CO emission with time for three hydrogen flow rates at cold start. It is seen from Fig. 7 that, at a given hydrogen addition level, because of the incomplete combustion in the initial cycles and the adoption of stoichiometric airefuel mixture in later cycles, CO also first rises and then drops with the increase of time. Fig. 7 demonstrates that the addition of hydrogen is effective on reducing engine CO emission during the cold start process, and when hydrogen flow rate increases from 0 to 4.3 L/min, the maximum instantaneous CO emission during the first 50 s is reduced by about 32.6%. This can be attributed to the fact that CO is mainly caused by the incomplete combustion of gasoline. As the high flame speed of hydrogen avails reducing the
Fig. 8 e NOx versus time at three hydrogen flow rates.
incomplete combustion, CO is effectively reduced after hydrogen addition. Meanwhile, since the high diffusion speed of hydrogen also contributes to the improved mixture homogeneity and the reduced fuel-rich area, the addition of hydrogen benefits reducing CO emission from gasoline engines at cold start. Fig. 8 displays the variation of NOx emissions with time for three hydrogen flow rates at cold start. As it is shown in Fig. 8, because of the shortened combustion duration and increased Imep after hydrogen addition in initial cycles, the maximum instantaneous NOx emissions are increased with the increase of hydrogen flow rate. The peak NOx emissions rise by about 61.1% and 93.7% when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, respectively. However, it is satisfying to find from Fig. 8 that, when the engine runs under the stable idle condition, NOx emissions from the engine started with the hydrogen-gasoline mixture are lower than those from the original engine, and this trend is consistent with that in our previous studies performed at idle and stoichiometric conditions [31]. The average NOx emissions within 50 s after the onset of the cold start drop from 116.4 to 105.8 and 99.6 ppm when hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, respectively. The proper interpretation can be that the shortened combustion duration and enhanced combustion after hydrogen blending result in the reduced engine cooling and exhaust losses and improved thermal efficiency. Thereby, for a specified idle speed, the increased thermal efficiency means less fuel is required to drive the engine. As the reduced fuel energy flow rate causes the dropped peak cylinder temperature, NOx emissions are smoothly decreased with the increase of hydrogen flow rate after 10 s from the onset of the cold start.
4.
Fig. 7 e CO versus time at three hydrogen flow rates.
Conclusions
This paper introduced an experimental investigation which was performed on a gasoline-fueled SI engine equipped with an electronically controlled hydrogen port-injection system and a HECU to explore the combustion and emissions
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characteristics of an SI engine which is cold started with the gasoline-hydrogen mixture. The main conclusions are listed as follows: 1. The maximum cylinder pressure and indicated mean effective pressure in the first cycle are increased obviously after hydrogen addition. In the first cycle, the peak cylinder pressure increases by about 51.1% and 69.2% when hydrogen flow rate rises from 0 to 2.5 and 4.3 L/min, respectively. 2. Engine speed in the first 20 cycles increases with the increase of hydrogen flow rate. When the cycle number exceeds 20, engine speed is roughly the same before and after hydrogen addition, due to the adoption of close loop control on engine speed. 3. Because of the low ignition energy and high flame speed of hydrogen, flame development and propagation periods are effectively shortened for the engine started with hydrogenegasoline blends. 4. As the high diffusion speed of hydrogen avails improving mixture homogeneity and contributes to the fast and complete combustion of the hydrogen-gasoline mixture, HC and CO emissions are decreased markedly with the increase of hydrogen flow rate. When hydrogen flow rate increases from 0 to 2.5 and 4.3 L/min, the maximum instantaneous HC emissions are sharply reduced from 57083 to 17850 and 15738 ppm, respectively. 5. NOx emissions in the first 5 s after cold start are increased with the increase of hydrogen flow rate, due to the enhanced combustion and raised cylinder temperature after hydrogen blending. However, NOx emissions with the hydrogen-gasoline mixture start are lower than those with the pure gasoline start after 10 s from the onset of the cold start.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 50976005). The authors also appreciate all students in the group for their help with the experiment.
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