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Comparison of the performance of a spark-ignited gasoline engine blended with hydrogen and hydrogen–oxygen mixtures
Energy 36 (2011) 5832e5837
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Energy journal homepage: www.elsevier.com/locate/energy
Comparison of the performance of a spark-ignited gasoline engine blended with hydrogen and hydrogeneoxygen mixtures Shuofeng Wang, Changwei Ji*, Jian Zhang, Bo Zhang College of Environmental and Energy Engineering, Beijing University of Technology, #100 Pingleyuan, Chaoyang District, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2011 Received in revised form 22 August 2011 Accepted 26 August 2011 Available online 17 September 2011
This paper compared the effects of hydrogen and hydrogeneoxygen blends (hydroxygen) additions on the performance of a gasoline engine at 1400 rpm and a manifolds absolute pressure of 61.5 kPa. The tests were carried out on a 1.6 L gasoline engine equipped with a hydrogen and oxygen injection system. A hybrid electronic control unit was applied to adjust the hydrogen and hydroxygen volume fractions in the intake increasing from 0% to about 3% and keep the hydrogen-to-oxygen mole ratio at 2:1 in hydroxygen tests. For each testing condition, the gasoline flow rate was adjusted to maintain the mixture global excess air ratio at 1.00. The test results confirmed that engine fuel energy flow rate was decreased after hydrogen addition but increased with hydroxygen blending. When hydrogen or hydroxygen volume fraction in the intake was lower than 2%, the hydroxygen-blended gasoline engine produced a higher thermal efficiency than the hydrogen-blended gasoline engine. Both the additions of hydrogen and hydroxygen help reduce flame development and propagation periods of the gasoline engine. HC emissions were reduced whereas NOx emissions were raised with the increase of hydrogen and hydroxygen addition levels. CO was slightly increased after hydrogen blending, but reduced with hydroxygen addition. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Hydroxygen Gasoline Combustion Emissions SI engines
1. Introduction Concerning the fossil fuel depletion, developing new alternative fuels has become a pressing issue nowadays. Moreover, the adversely increased environmental pollution also requires controlling the harmful emissions during the combustion process [1]. Hydrogen is a green and renewable fuel, which combustion produces no carbon related emissions, such as CO, HC and CO2. Generally, hydrogen can be produced from wind, solar and other renewable energies [2e5]. Besides, hydrogen has many excellent combustion and physicochemical properties that avail enhancing the engine performance. Thus, hydrogen has been proved to be one of the most promising alternative fuels for vehicle engines [6e9]. Sopena et al. [10] compared the combustion and emissions characteristics of the hydrogen and gasoline engines. It is found that thermal efficiency of the hydrogen engine was obviously higher than that of the gasoline engine. Moreover, since hydrogen possesses a wide flammability, NOx emissions from the hydrogen engine can be reduced by adopting lean combustion. However, because of the low volume energy density of hydrogen, the torque
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.08.042
output of the hydrogen engine is obviously lower than that of the gasoline engine. Although the application of hydrogen direct injection could heighten the power output of a hydrogen engine, the poor lubricity of hydrogen brings new challenges to the lifespan of the high-pressure hydrogen injector [11,12]. Comparatively, since hydrogen has many good combustion and physicochemical properties, fueling an engine with the blends of hydrogen and other conventional fuels could also enable the engine to gain the improved thermal efficiency and emissions performance [13e15]. Because of the high flame and diffusion speeds of hydrogen, the hydrogen-blended engines could gain the shortened flame development and propagation durations which contribute to the improved engine thermal efficiency. Moreover, the wide flammability of hydrogen also enables the hydrogen engines to run at high excess air ratios where the dropped combustion temperature and decreased cooling loss can be achieved [16]. Huang et al. [17,18] found that the lean burn limit of the natural gas engine was extended, and HC emissions were decreased with the increase of hydrogen addition fraction. Ma et al. [19e21] investigated the cyclic variation characteristic of a hydrogen-enriched CNG engine. The experimental results confirmed that the coefficient of variation in indicated mean effective pressure was distinctly reduced with the increase of hydrogen blending fraction. Meanwhile, the engine lean burn limit was also extended after hydrogen addition. Ji et al.
S. Wang et al. / Energy 36 (2011) 5832e5837
[22e26] investigated the performance of a hydrogen-blended gasoline engine under various operating conditions. The test results confirmed that the engine cyclic variation was eased after hydrogen addition. Besides, the engine thermal efficiency was continuously increased with the increase of hydrogen blending ratio. Because of the low hydrogen consumption rate when the hydrogen is used as a fuel additive for SI engines, the onboard hydrogen generator can be adopted to provide hydrogen for the hydrogen-blended engines, which alleviates the concerns on hydrogen refilling and onboard storage [27e29]. Meanwhile, except for hydrogen, oxygen is also produced in the water electrolysis process, which is regarded as a combustion promoter that contributes to the fast and complete combustion of the fueleair mixtures. Thus, the hydrogeneoxygen blends (hydroxygen) producing by the water electrolysis hydrogen generator seem to be capable of further improving the performance of the gasoline engines. Uykur et al. [30] found that the effect of adding 10% hydrogen and 5% oxygen on improving the flame speed of the methane-air mixture was equivalent to that of adding 20% hydrogen only. Duger et al. [27] investigated the performance of a gasoline engine blended with hydrogen and oxygen produced from an onboard water electrolysis hydrogen generator. It was found that the engine fuel economy was markedly enhanced at city-driving conditions after blending the hydroxygen. However, as previous studies were mainly performed with premixed hydrogeneoxygen-air mixtures [27,30], the hydroxygen volume fraction in the total intake gas was hard to be changed. But for the modern SI engines, the hydroxygen blending fraction should be varied to enable the engine to gain the best performance at different operating conditions. Moreover, few publications compared the performance of a gasoline engine blended with hydrogen and hydroxygen. As the hydrogen generator produces hydrogen and oxygen simultaneously, it is of necessity to compare the effects of hydrogen and hydroxygen additions on the performance of gasoline engines, so that engine engineers could decide when the hydrogen and oxygen blends or just hydrogen should be added to the gasoline engine. Thereby, there is a strong motivation to quantitatively investigate and compare the effects of hydrogen and hydroxygen additions on combustion and emissions characteristics of a gasoline engine. Furthermore, the stoichiometric hydrogeneoxygen blends can be easily ignited, due to the wide flammability and low ignition energy of hydrogen. Thus, the premixed hydroxygen used in previous studies [27,30] tends to cause severe misfire in the hydroxygen supplying pipe at high engine loads. To ensure the safety, it is of necessity to develop new hydroxygen supplying strategies. Therefore, in this paper, a new hydrogen and oxygen injection system was added to the modified gasoline engine, which enables the hydrogen and oxygen to be introduced into the intake manifolds separately. Besides, a new hybrid electronic control unit was adopted to govern the hydrogen and hydroxygen volume fractions in the intake and the global excess air ratio of the hydroxygenegasolineeair mixtures, which provides a feasible way to feed the engine with the suitable fuel at different operating conditions. 2. Experimental setup and procedure 2.1. Experimental setup Fig. 1 shows the schematic diagram of the experimental system. The test engine is a 1.6 L, SI engine manufactured by Beijing Hyundai Motors. The engine has a rated power of 82.32 kW at 6000 rpm and a rated torque of 143.28 Nm at 4500 rpm. As the standard hydroxygen has a hydrogen-to-oxygen mole ratio of 2:1,
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Fig. 1. Schematics of the experimental system. 1. Oxygen bottle; 2. Oxygen pressure adjuster; 3. Oxygen pressure indicator; 4. Oxygen flow meter; 5. Hydrogen cylinder assembly; 6. Hydrogen pressure adjuster; 7. Hydrogen pressure indicator; 8. Hydrogen flow meter; 9. Air flow meter; 10. Throttle; 11. Idle bypass valve; 12. Oxygen injector; 13. Original electronic control unit (OECU); 14. Hybrid electronic control unit (HECU); 15. Calibration computer; 16. Fuel tank; 17. Fuel mass flow meter; 18. Fuel pump; 19. Ignition module; 20. Gasoline injector; 21. Flame arrestor; 22. Hydrogen injector; 23. Pressure transducer with spark plug; 24. Oxygen sensor; 25. A/F analyzer; 26. Sampling pipe; 27. Emissions analyzer; 28. Combustion analyzer; 29. A/D converter; 30. Charge amplifier; 31. Optical encoder; 32. Crankshaft a. Signals from the OECU to HECU; b1. Signals from the calibration PC to HECU; b2. Signals from the HECU to calibration PC.
the premixed hydrogeneoxygen blends can be easily ignited and consequently causes severe backfire in the hydroxygen rail. Thus, new hydroxygen supplying strategies are required to enable the hydrogen and oxygen to be introduced into the intake manifolds separately. To avoid backfire, four hydrogen injectors connecting with a hydrogen rail are mounted on the intake manifolds underneath the original gasoline injectors, and an oxygen injector is fixed at the intake plenum. In the modified engine, as the hydrogen and oxygen were introduced into the engine separately, no backfire was found during the experiment. The opening and closing of hydrogen, oxygen and gasoline injectors are controlled by a self-developed hybrid electronic control unit (HECU) according to commands from a calibration computer and sensor signals from the engine original electronic control unit (OECU). Thus, the hydroxygen or hydrogen volume fraction in the total intake and the global excess air ratio of the hydrogenegasolineeair mixtures can be adjusted freely by the HECU which provides a feasible way for feeding the engine with the suitable fuel or fuel blends under different engine operating conditions. A GW160 eddy-current dynamometer is connected with the crankshaft to control the engine speed and load (measurement deviations: 1 rpm in speed, 0.28 Nm in torque). An FC2210 fuel mass flow meter is adopted to determine the mass flow rate of gasoline (measurement uncertainty: <0.33 g/min). The air, oxygen and hydrogen mass flow rates are monitored by a 20N060, a D07-19B and a D07-19BM thermal mass flow meters, respectively (measurement uncertainties: <0.1 L/min for air, <0.02 L/min for oxygen and hydrogen). A Kistler 6117BFD17 spark plug-integrated pressure transducer is used to measure the cylinder pressure and enforce the ignition of the fourth cylinder (measurement uncertainty: <0.3 bar). A Kistler 2613B optical encoder is utilized to measure the crank angle position (crank angle resolution: 0.2 CA, measurement deviation <0.01 CA). A Dewetron combustion analyzer is applied to record the cylinder pressures for 200 consecutive cycles and calculate the heat release rate, etc. NOx, HC and CO emissions are measured by a Horiba MEXA-7100D EGR emissions analyzer. The measurement sensitivities are 1 ppm for
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NOx, CO and HC emissions. NOx are measured by the chemiluminescent method, HC emissions are determined by the hydrogen flame ionization detection method, and CO emission is detected by the nondispersive infrared method. The measurement uncertainties are less than 1% of the measured values for all emissions. 2.2. Experimental procedure All experiments were conducted at the constant coolant and lubricant oil temperatures of 90 1 C and 95 1 C, respectively. The engine was run at a typical city driving speed of 1400 rpm [31] and an intake manifolds absolute pressure (MAP) of 61.5 kPa. The spark timing for the maximum brake torque (MBT) was applied to all testing points. The engine was first run with the pure gasoline. Then, the hydrogen and hydroxygen volume fractions in the intake were gradually increased from 0% to 3%, respectively. Besides, to simulate the case of hydrogen and oxygen produced by a water electrolysis hydrogen generator, the hydrogen-to-oxygen mole ratio was fixed at 2:1 in the hydroxygen tests, which is so-called the standard hydroxygen (2H2/O2). For a specified hydrogen or hydroxygen blending ratio, the gasoline flow rate was adjusted to keep the global excess air ratio of the fueleair mixtures at 1.00. The hydrogen or hydroxygen volume fraction in the intake (am), global excess air ratios of the hydrogenegasolineeair (lH2 ) and the standard hydroxygenegasolineeair (lH2 þO2 ) mixtures and fuel energy flow rate (Ef) are defined as:
am ¼ Vm =ðVm þ Vair Þ
(1)
lH2 ¼ ðVair rair Þ= VH2 rH2 AFH2 þ mgas AFgas l2H2 þO2 ¼ ðVair rair Þ= mgas AFgas
Ef ¼ VH2 rH2 LHVH2 þ mgas LHVgas
(2) (3) (4)
In Eqs. (1)e(4), Vm and Vair are the measured volumetric flow rates of the hydrogen or standard hydroxygen and air at normal conditions (L/min), respectively; mgas represents the measured gasoline mass flow rate (g/min); rH2 and rair symbolize hydrogen and air densities at normal conditions (g/L); AFH2 and AFgas are the stoichiometric air-to-fuel ratios of hydrogen and gasoline; LHVH2 and LHVgas are the lower heating values of hydrogen and gasoline (kJ/g), respectively.
Fig. 2. hi versus am with hydrogen and standard hydroxygen additions.
the standard hydroxygen-blended gasoline engine could gain higher thermal efficiency than the pure hydrogen-enriched gasoline engine when am is smaller than 2%. Fig. 3 shows the variations of fuel energy flow rate (Ef) with am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. As it is shown in Fig. 3, Ef is raised with the increase of the standard hydroxygen addition fraction whereas decreased with the increase of hydrogen blending fraction. When am rises from 0% to 2.8%, Ef is decreased by 5.74% for the hydrogen-blended gasoline engine, but increased by 3.14% for the standard hydroxygen-blended gasoline engine. This is because the lower heating value of the standard hydroxygen is higher than that of the stoichiometric air-gasoline-hydrogen mixtures. Thus, the addition of the standard hydroxygen contributes to the increased Ef. However, because the volume energy density of hydrogen is lower than that of gasoline, Ef is decreased with the increase of hydrogen addition level. Since both the engine indicted thermal efficiency and fuel energy flow rate are raised after the standard hydroxygen blending, it can be deduced that the addition of the standard hydroxygen leads to the enhanced engine torque output.
3. Results and discussion 3.1. Indicated thermal efficiency and fuel energy flow rate The engine indicated thermal efficiency is crucial for evaluating the engine fuel economy. Fig. 2 depicts indicated thermal efficiency (hi) versus am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 2 demonstrates that, since the addition of hydrogen helps enhance the fast and complete combustion of the fueleair mixture, hi is improved after the additions of hydrogen and the standard hydroxygen. It is also seen from Fig. 2 that, when am is smaller than 2%, the engine blended with the standard hydroxygen produces a higher indicated thermal efficiency than that blended with the pure hydrogen. Under the testing conditions, hi achieves the peak value of 35.7% at the standard hydroxygen volume fraction in the intake of 0.75%. The possible explanation could be ascribed to the fact that, as the addition of hydroxygen increases the oxygen fraction in the intake, the fuel-rich area in the cylinder can be slightly reduced after the hydroxygen blending. Since the decreased fuelrich area helps the complete combustion of the fueleair mixtures,
Fig. 3. Ef versus am with hydrogen and standard hydroxygen additions.
S. Wang et al. / Energy 36 (2011) 5832e5837
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3.2. Combustion analysis Flame development and propagation durations directly reflect the engine combustion quality. Figs. 4 and 5 plot the variations of flame development (CA0e10) and propagation (CA10e90) periods with am. CA0e10 and CA10e90 are defined as the crank angle durations from the spark discharge to 10% and from 10% to 90% heat release of the total fuel, respectively. It can be seen from Fig. 4 that, since the ignition energy of hydrogen is only 1/10 of that of gasoline and the addition of hydrogen stimulates the formation of O and OH radicals [32], the hydrogen and hydroxygen-blended gasoline engines could gain a shorter CA0e10 than the original engine. Moreover, as hydrogen possesses a high flame speed, CA10e90 is also decreased with the increase of hydrogen and hydroxygen addition fractions. When am is smaller than 2.5%, the standard hydroxygen-blended gasoline engine produces a shorter CA0e10 and CA10e90 than the hydrogen-blended engine. But when am is further increased, both CA0e10 and CA10e90 of the hydroxygenblended engine are slightly longer than those of the hydrogenblended engine. This could be attributed to the fact that, as the increased oxygen concentration helps reduce the lean-oxygen area and therefore avails the fast and complete combustion of the fueleair mixtures, the standard hydroxygen-blended gasoline engine could gain shorter CA0e10 and CA10e90 than the hydrogen-enriched gasoline engine at low blending levels. However, for a specified am, the hydrogen fraction in the hydroxygen-blended engine is lower than that in the hydrogenblended engine. Thus, CA0e10 and CA10e90 of the engine blended with the standard hydroxygen are longer than those of the hydrogen-blended gasoline engine when am is further increased.
Fig. 5. CA10e90 versus am with hydrogen and standard hydroxygen additions.
Because of the incomplete combustion of gasoline and the high combustion temperature in cylinders, SI engines always exhaust large amounts of HC, CO and NOx emissions at city driving conditions. This section compares the effects of hydrogen and the standard hydroxygen additions on reducing the toxic emissions from the gasoline engine before a three-way catalytic convertor. Fig. 6 shows the variations of HC emissions with am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. It can be found from Fig. 6 that, since hydrogen has a short quenching distance which benefits
decreasing HC emissions caused by the crevice effect, the additions of hydrogen and hydroxygen lead to the reduced HC emissions. Besides, the formation of HC emissions is closely related with the chemical equilibrium process. As the raised cylinder temperature after hydrogen or hydroxygen addition helps ease the formation of HC emissions during the combustion process [33], HC emissions are effectively reduced after hydrogen and hydroxygen additions. It is also found from Fig. 6 that, when am exceeds 1.5%, the effect of hydrogen addition on decreasing HC emissions is more pronounced than that of the standard hydroxygen addition. This is because at low mixture blending levels, the addition of the standard hydroxygen results in the increased oxygen concentration which benefits enhancing the complete combustion. But for a given mixture addition fraction, the hydrogen content in the standard hydroxygenegasolineeair mixtures is obviously lower than that in the hydrogenegasolineeair mixture. Therefore, at high mixture blending ratios, as the short quenching distance of hydrogen and decreased C-atom gradually become key factors for reducing HC emissions, the hydrogen-enriched gasoline engine could expel less
Fig. 4. CA0e10 versus am with hydrogen and standard hydroxygen additions.
Fig. 6. HC versus am with hydrogen and standard hydroxygen additions.
3.3. Emissions
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Fig. 8 depicts NOx emissions versus am for the hydrogen and the standard hydroxygen-blended gasoline engines at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 8 demonstrates that NOx emissions are raised after hydrogen and hydroxygen additions. This is because hydrogen possesses a high adiabatic flame temperature and the additions of hydrogen and hydroxygen lead to the shortened combustion duration, the cylinder temperature tends to be increased after hydrogen and hydroxygen additions. As the increased combustion temperature may shift the thermodynamic equilibrium to stimulate the formation of NOx emissions [1], NOx are increased after hydrogen and hydroxygen additions. Moreover, as the hydroxygen provides more oxygen which could stimulate the formation of NOx, for a specified am, NOx emissions from the standard hydroxygen-blended engine are higher than those from the hydrogen-blended gasoline engine. 4. Conclusions
Fig. 7. CO versus am with hydrogen and standard hydroxygen additions.
amounts of HC emissions than the standard hydroxygen-blended gasoline engine when am is larger than 1.5%. Fig. 7 displays CO emission versus am for the hydrogen and the standard hydroxygen-blended gasoline engines at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 7 shows that CO emission increases with the increase of hydrogen volume fraction in the intake whereas decreases with the increase of the standard hydroxygen addition fraction. The possible reason can be ascribed to the fact that, since hydrogen has a wide flammability and a high flame speed, the added hydrogen tends to be ignited and combusted prior to the gasoline and therefore causes some lean-oxygen areas in the cylinder. Thus, CO is increased after hydrogen addition at the stoichiometric condition. Comparatively, the oxygen content in the hydroxygen could further oxidize CO into CO2 which is not the case for the pure hydrogen addition. Thereby, the addition of hydroxygen is more effective on reducing CO emission than the pure hydrogen addition. When the hydroxygen volume fraction in the intake rises from 0% to 2.8%, CO is reduced by 21.86% for the standard hydroxygen-blended gasoline engine.
This paper investigated and compared the effects of hydrogen and standard hydroxygen additions on improving the performance of a gasoline engine at 1400 rpm and a MAP of 61.5 kPa under stoichiometric conditions. The main conclusions are summarized as follows: At low blending fractions, the standard hydroxygen-blended gasoline engine produces a higher thermal efficiency and shorter flame development and propagation durations than the hydrogenenriched gasoline engine. But at high blending fractions, the addition of pure hydrogen tends to help the engine gain a higher thermal efficiency than the standard hydroxygen addition. Since the volume energy density of the standard hydroxygen is higher than that of the hydrogenegasolineeair mixtures, the increased fuel energy flow rate is gained after the hydroxygen addition, which symbolizes that the standard hydroxygen-blended gasoline engine could gain a higher torque output than the original and hydrogenenriched gasoline engines. The standard hydroxygen-blended gasoline engine produces lower CO emission than the original and hydrogen-enriched gasoline engines. At low blending fractions, the addition of hydroxygen is more effective on reducing HC emissions. However, because of the raised combustion temperature, both the additions of hydrogen and hydroxygen lead to the increased NOx emissions. Due to the further increased oxygen concentration in the cylinder, the hydroxygen-blended gasoline engine expels larger amounts of NOx than the hydrogen-enriched engine. Thus, proper techniques are required to control NOx emissions from the standard hydroxygen-blended gasoline engines. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 50976005). References
Fig. 8. NOx versus am with hydrogen and standard hydroxygen additions.
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Contents lists available at SciVerse ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Comparison of the performance of a spark-ignited gasoline engine blended with hydrogen and hydrogeneoxygen mixtures Shuofeng Wang, Changwei Ji*, Jian Zhang, Bo Zhang College of Environmental and Energy Engineering, Beijing University of Technology, #100 Pingleyuan, Chaoyang District, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2011 Received in revised form 22 August 2011 Accepted 26 August 2011 Available online 17 September 2011
This paper compared the effects of hydrogen and hydrogeneoxygen blends (hydroxygen) additions on the performance of a gasoline engine at 1400 rpm and a manifolds absolute pressure of 61.5 kPa. The tests were carried out on a 1.6 L gasoline engine equipped with a hydrogen and oxygen injection system. A hybrid electronic control unit was applied to adjust the hydrogen and hydroxygen volume fractions in the intake increasing from 0% to about 3% and keep the hydrogen-to-oxygen mole ratio at 2:1 in hydroxygen tests. For each testing condition, the gasoline flow rate was adjusted to maintain the mixture global excess air ratio at 1.00. The test results confirmed that engine fuel energy flow rate was decreased after hydrogen addition but increased with hydroxygen blending. When hydrogen or hydroxygen volume fraction in the intake was lower than 2%, the hydroxygen-blended gasoline engine produced a higher thermal efficiency than the hydrogen-blended gasoline engine. Both the additions of hydrogen and hydroxygen help reduce flame development and propagation periods of the gasoline engine. HC emissions were reduced whereas NOx emissions were raised with the increase of hydrogen and hydroxygen addition levels. CO was slightly increased after hydrogen blending, but reduced with hydroxygen addition. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Hydroxygen Gasoline Combustion Emissions SI engines
1. Introduction Concerning the fossil fuel depletion, developing new alternative fuels has become a pressing issue nowadays. Moreover, the adversely increased environmental pollution also requires controlling the harmful emissions during the combustion process [1]. Hydrogen is a green and renewable fuel, which combustion produces no carbon related emissions, such as CO, HC and CO2. Generally, hydrogen can be produced from wind, solar and other renewable energies [2e5]. Besides, hydrogen has many excellent combustion and physicochemical properties that avail enhancing the engine performance. Thus, hydrogen has been proved to be one of the most promising alternative fuels for vehicle engines [6e9]. Sopena et al. [10] compared the combustion and emissions characteristics of the hydrogen and gasoline engines. It is found that thermal efficiency of the hydrogen engine was obviously higher than that of the gasoline engine. Moreover, since hydrogen possesses a wide flammability, NOx emissions from the hydrogen engine can be reduced by adopting lean combustion. However, because of the low volume energy density of hydrogen, the torque
* Corresponding author. Tel./fax: þ86 1067392126. E-mail address: [email protected] (C. Ji). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.08.042
output of the hydrogen engine is obviously lower than that of the gasoline engine. Although the application of hydrogen direct injection could heighten the power output of a hydrogen engine, the poor lubricity of hydrogen brings new challenges to the lifespan of the high-pressure hydrogen injector [11,12]. Comparatively, since hydrogen has many good combustion and physicochemical properties, fueling an engine with the blends of hydrogen and other conventional fuels could also enable the engine to gain the improved thermal efficiency and emissions performance [13e15]. Because of the high flame and diffusion speeds of hydrogen, the hydrogen-blended engines could gain the shortened flame development and propagation durations which contribute to the improved engine thermal efficiency. Moreover, the wide flammability of hydrogen also enables the hydrogen engines to run at high excess air ratios where the dropped combustion temperature and decreased cooling loss can be achieved [16]. Huang et al. [17,18] found that the lean burn limit of the natural gas engine was extended, and HC emissions were decreased with the increase of hydrogen addition fraction. Ma et al. [19e21] investigated the cyclic variation characteristic of a hydrogen-enriched CNG engine. The experimental results confirmed that the coefficient of variation in indicated mean effective pressure was distinctly reduced with the increase of hydrogen blending fraction. Meanwhile, the engine lean burn limit was also extended after hydrogen addition. Ji et al.
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[22e26] investigated the performance of a hydrogen-blended gasoline engine under various operating conditions. The test results confirmed that the engine cyclic variation was eased after hydrogen addition. Besides, the engine thermal efficiency was continuously increased with the increase of hydrogen blending ratio. Because of the low hydrogen consumption rate when the hydrogen is used as a fuel additive for SI engines, the onboard hydrogen generator can be adopted to provide hydrogen for the hydrogen-blended engines, which alleviates the concerns on hydrogen refilling and onboard storage [27e29]. Meanwhile, except for hydrogen, oxygen is also produced in the water electrolysis process, which is regarded as a combustion promoter that contributes to the fast and complete combustion of the fueleair mixtures. Thus, the hydrogeneoxygen blends (hydroxygen) producing by the water electrolysis hydrogen generator seem to be capable of further improving the performance of the gasoline engines. Uykur et al. [30] found that the effect of adding 10% hydrogen and 5% oxygen on improving the flame speed of the methane-air mixture was equivalent to that of adding 20% hydrogen only. Duger et al. [27] investigated the performance of a gasoline engine blended with hydrogen and oxygen produced from an onboard water electrolysis hydrogen generator. It was found that the engine fuel economy was markedly enhanced at city-driving conditions after blending the hydroxygen. However, as previous studies were mainly performed with premixed hydrogeneoxygen-air mixtures [27,30], the hydroxygen volume fraction in the total intake gas was hard to be changed. But for the modern SI engines, the hydroxygen blending fraction should be varied to enable the engine to gain the best performance at different operating conditions. Moreover, few publications compared the performance of a gasoline engine blended with hydrogen and hydroxygen. As the hydrogen generator produces hydrogen and oxygen simultaneously, it is of necessity to compare the effects of hydrogen and hydroxygen additions on the performance of gasoline engines, so that engine engineers could decide when the hydrogen and oxygen blends or just hydrogen should be added to the gasoline engine. Thereby, there is a strong motivation to quantitatively investigate and compare the effects of hydrogen and hydroxygen additions on combustion and emissions characteristics of a gasoline engine. Furthermore, the stoichiometric hydrogeneoxygen blends can be easily ignited, due to the wide flammability and low ignition energy of hydrogen. Thus, the premixed hydroxygen used in previous studies [27,30] tends to cause severe misfire in the hydroxygen supplying pipe at high engine loads. To ensure the safety, it is of necessity to develop new hydroxygen supplying strategies. Therefore, in this paper, a new hydrogen and oxygen injection system was added to the modified gasoline engine, which enables the hydrogen and oxygen to be introduced into the intake manifolds separately. Besides, a new hybrid electronic control unit was adopted to govern the hydrogen and hydroxygen volume fractions in the intake and the global excess air ratio of the hydroxygenegasolineeair mixtures, which provides a feasible way to feed the engine with the suitable fuel at different operating conditions. 2. Experimental setup and procedure 2.1. Experimental setup Fig. 1 shows the schematic diagram of the experimental system. The test engine is a 1.6 L, SI engine manufactured by Beijing Hyundai Motors. The engine has a rated power of 82.32 kW at 6000 rpm and a rated torque of 143.28 Nm at 4500 rpm. As the standard hydroxygen has a hydrogen-to-oxygen mole ratio of 2:1,
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Fig. 1. Schematics of the experimental system. 1. Oxygen bottle; 2. Oxygen pressure adjuster; 3. Oxygen pressure indicator; 4. Oxygen flow meter; 5. Hydrogen cylinder assembly; 6. Hydrogen pressure adjuster; 7. Hydrogen pressure indicator; 8. Hydrogen flow meter; 9. Air flow meter; 10. Throttle; 11. Idle bypass valve; 12. Oxygen injector; 13. Original electronic control unit (OECU); 14. Hybrid electronic control unit (HECU); 15. Calibration computer; 16. Fuel tank; 17. Fuel mass flow meter; 18. Fuel pump; 19. Ignition module; 20. Gasoline injector; 21. Flame arrestor; 22. Hydrogen injector; 23. Pressure transducer with spark plug; 24. Oxygen sensor; 25. A/F analyzer; 26. Sampling pipe; 27. Emissions analyzer; 28. Combustion analyzer; 29. A/D converter; 30. Charge amplifier; 31. Optical encoder; 32. Crankshaft a. Signals from the OECU to HECU; b1. Signals from the calibration PC to HECU; b2. Signals from the HECU to calibration PC.
the premixed hydrogeneoxygen blends can be easily ignited and consequently causes severe backfire in the hydroxygen rail. Thus, new hydroxygen supplying strategies are required to enable the hydrogen and oxygen to be introduced into the intake manifolds separately. To avoid backfire, four hydrogen injectors connecting with a hydrogen rail are mounted on the intake manifolds underneath the original gasoline injectors, and an oxygen injector is fixed at the intake plenum. In the modified engine, as the hydrogen and oxygen were introduced into the engine separately, no backfire was found during the experiment. The opening and closing of hydrogen, oxygen and gasoline injectors are controlled by a self-developed hybrid electronic control unit (HECU) according to commands from a calibration computer and sensor signals from the engine original electronic control unit (OECU). Thus, the hydroxygen or hydrogen volume fraction in the total intake and the global excess air ratio of the hydrogenegasolineeair mixtures can be adjusted freely by the HECU which provides a feasible way for feeding the engine with the suitable fuel or fuel blends under different engine operating conditions. A GW160 eddy-current dynamometer is connected with the crankshaft to control the engine speed and load (measurement deviations: 1 rpm in speed, 0.28 Nm in torque). An FC2210 fuel mass flow meter is adopted to determine the mass flow rate of gasoline (measurement uncertainty: <0.33 g/min). The air, oxygen and hydrogen mass flow rates are monitored by a 20N060, a D07-19B and a D07-19BM thermal mass flow meters, respectively (measurement uncertainties: <0.1 L/min for air, <0.02 L/min for oxygen and hydrogen). A Kistler 6117BFD17 spark plug-integrated pressure transducer is used to measure the cylinder pressure and enforce the ignition of the fourth cylinder (measurement uncertainty: <0.3 bar). A Kistler 2613B optical encoder is utilized to measure the crank angle position (crank angle resolution: 0.2 CA, measurement deviation <0.01 CA). A Dewetron combustion analyzer is applied to record the cylinder pressures for 200 consecutive cycles and calculate the heat release rate, etc. NOx, HC and CO emissions are measured by a Horiba MEXA-7100D EGR emissions analyzer. The measurement sensitivities are 1 ppm for
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NOx, CO and HC emissions. NOx are measured by the chemiluminescent method, HC emissions are determined by the hydrogen flame ionization detection method, and CO emission is detected by the nondispersive infrared method. The measurement uncertainties are less than 1% of the measured values for all emissions. 2.2. Experimental procedure All experiments were conducted at the constant coolant and lubricant oil temperatures of 90 1 C and 95 1 C, respectively. The engine was run at a typical city driving speed of 1400 rpm [31] and an intake manifolds absolute pressure (MAP) of 61.5 kPa. The spark timing for the maximum brake torque (MBT) was applied to all testing points. The engine was first run with the pure gasoline. Then, the hydrogen and hydroxygen volume fractions in the intake were gradually increased from 0% to 3%, respectively. Besides, to simulate the case of hydrogen and oxygen produced by a water electrolysis hydrogen generator, the hydrogen-to-oxygen mole ratio was fixed at 2:1 in the hydroxygen tests, which is so-called the standard hydroxygen (2H2/O2). For a specified hydrogen or hydroxygen blending ratio, the gasoline flow rate was adjusted to keep the global excess air ratio of the fueleair mixtures at 1.00. The hydrogen or hydroxygen volume fraction in the intake (am), global excess air ratios of the hydrogenegasolineeair (lH2 ) and the standard hydroxygenegasolineeair (lH2 þO2 ) mixtures and fuel energy flow rate (Ef) are defined as:
am ¼ Vm =ðVm þ Vair Þ
(1)
lH2 ¼ ðVair rair Þ= VH2 rH2 AFH2 þ mgas AFgas l2H2 þO2 ¼ ðVair rair Þ= mgas AFgas
Ef ¼ VH2 rH2 LHVH2 þ mgas LHVgas
(2) (3) (4)
In Eqs. (1)e(4), Vm and Vair are the measured volumetric flow rates of the hydrogen or standard hydroxygen and air at normal conditions (L/min), respectively; mgas represents the measured gasoline mass flow rate (g/min); rH2 and rair symbolize hydrogen and air densities at normal conditions (g/L); AFH2 and AFgas are the stoichiometric air-to-fuel ratios of hydrogen and gasoline; LHVH2 and LHVgas are the lower heating values of hydrogen and gasoline (kJ/g), respectively.
Fig. 2. hi versus am with hydrogen and standard hydroxygen additions.
the standard hydroxygen-blended gasoline engine could gain higher thermal efficiency than the pure hydrogen-enriched gasoline engine when am is smaller than 2%. Fig. 3 shows the variations of fuel energy flow rate (Ef) with am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. As it is shown in Fig. 3, Ef is raised with the increase of the standard hydroxygen addition fraction whereas decreased with the increase of hydrogen blending fraction. When am rises from 0% to 2.8%, Ef is decreased by 5.74% for the hydrogen-blended gasoline engine, but increased by 3.14% for the standard hydroxygen-blended gasoline engine. This is because the lower heating value of the standard hydroxygen is higher than that of the stoichiometric air-gasoline-hydrogen mixtures. Thus, the addition of the standard hydroxygen contributes to the increased Ef. However, because the volume energy density of hydrogen is lower than that of gasoline, Ef is decreased with the increase of hydrogen addition level. Since both the engine indicted thermal efficiency and fuel energy flow rate are raised after the standard hydroxygen blending, it can be deduced that the addition of the standard hydroxygen leads to the enhanced engine torque output.
3. Results and discussion 3.1. Indicated thermal efficiency and fuel energy flow rate The engine indicated thermal efficiency is crucial for evaluating the engine fuel economy. Fig. 2 depicts indicated thermal efficiency (hi) versus am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 2 demonstrates that, since the addition of hydrogen helps enhance the fast and complete combustion of the fueleair mixture, hi is improved after the additions of hydrogen and the standard hydroxygen. It is also seen from Fig. 2 that, when am is smaller than 2%, the engine blended with the standard hydroxygen produces a higher indicated thermal efficiency than that blended with the pure hydrogen. Under the testing conditions, hi achieves the peak value of 35.7% at the standard hydroxygen volume fraction in the intake of 0.75%. The possible explanation could be ascribed to the fact that, as the addition of hydroxygen increases the oxygen fraction in the intake, the fuel-rich area in the cylinder can be slightly reduced after the hydroxygen blending. Since the decreased fuelrich area helps the complete combustion of the fueleair mixtures,
Fig. 3. Ef versus am with hydrogen and standard hydroxygen additions.
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3.2. Combustion analysis Flame development and propagation durations directly reflect the engine combustion quality. Figs. 4 and 5 plot the variations of flame development (CA0e10) and propagation (CA10e90) periods with am. CA0e10 and CA10e90 are defined as the crank angle durations from the spark discharge to 10% and from 10% to 90% heat release of the total fuel, respectively. It can be seen from Fig. 4 that, since the ignition energy of hydrogen is only 1/10 of that of gasoline and the addition of hydrogen stimulates the formation of O and OH radicals [32], the hydrogen and hydroxygen-blended gasoline engines could gain a shorter CA0e10 than the original engine. Moreover, as hydrogen possesses a high flame speed, CA10e90 is also decreased with the increase of hydrogen and hydroxygen addition fractions. When am is smaller than 2.5%, the standard hydroxygen-blended gasoline engine produces a shorter CA0e10 and CA10e90 than the hydrogen-blended engine. But when am is further increased, both CA0e10 and CA10e90 of the hydroxygenblended engine are slightly longer than those of the hydrogenblended engine. This could be attributed to the fact that, as the increased oxygen concentration helps reduce the lean-oxygen area and therefore avails the fast and complete combustion of the fueleair mixtures, the standard hydroxygen-blended gasoline engine could gain shorter CA0e10 and CA10e90 than the hydrogen-enriched gasoline engine at low blending levels. However, for a specified am, the hydrogen fraction in the hydroxygen-blended engine is lower than that in the hydrogenblended engine. Thus, CA0e10 and CA10e90 of the engine blended with the standard hydroxygen are longer than those of the hydrogen-blended gasoline engine when am is further increased.
Fig. 5. CA10e90 versus am with hydrogen and standard hydroxygen additions.
Because of the incomplete combustion of gasoline and the high combustion temperature in cylinders, SI engines always exhaust large amounts of HC, CO and NOx emissions at city driving conditions. This section compares the effects of hydrogen and the standard hydroxygen additions on reducing the toxic emissions from the gasoline engine before a three-way catalytic convertor. Fig. 6 shows the variations of HC emissions with am at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. It can be found from Fig. 6 that, since hydrogen has a short quenching distance which benefits
decreasing HC emissions caused by the crevice effect, the additions of hydrogen and hydroxygen lead to the reduced HC emissions. Besides, the formation of HC emissions is closely related with the chemical equilibrium process. As the raised cylinder temperature after hydrogen or hydroxygen addition helps ease the formation of HC emissions during the combustion process [33], HC emissions are effectively reduced after hydrogen and hydroxygen additions. It is also found from Fig. 6 that, when am exceeds 1.5%, the effect of hydrogen addition on decreasing HC emissions is more pronounced than that of the standard hydroxygen addition. This is because at low mixture blending levels, the addition of the standard hydroxygen results in the increased oxygen concentration which benefits enhancing the complete combustion. But for a given mixture addition fraction, the hydrogen content in the standard hydroxygenegasolineeair mixtures is obviously lower than that in the hydrogenegasolineeair mixture. Therefore, at high mixture blending ratios, as the short quenching distance of hydrogen and decreased C-atom gradually become key factors for reducing HC emissions, the hydrogen-enriched gasoline engine could expel less
Fig. 4. CA0e10 versus am with hydrogen and standard hydroxygen additions.
Fig. 6. HC versus am with hydrogen and standard hydroxygen additions.
3.3. Emissions
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Fig. 8 depicts NOx emissions versus am for the hydrogen and the standard hydroxygen-blended gasoline engines at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 8 demonstrates that NOx emissions are raised after hydrogen and hydroxygen additions. This is because hydrogen possesses a high adiabatic flame temperature and the additions of hydrogen and hydroxygen lead to the shortened combustion duration, the cylinder temperature tends to be increased after hydrogen and hydroxygen additions. As the increased combustion temperature may shift the thermodynamic equilibrium to stimulate the formation of NOx emissions [1], NOx are increased after hydrogen and hydroxygen additions. Moreover, as the hydroxygen provides more oxygen which could stimulate the formation of NOx, for a specified am, NOx emissions from the standard hydroxygen-blended engine are higher than those from the hydrogen-blended gasoline engine. 4. Conclusions
Fig. 7. CO versus am with hydrogen and standard hydroxygen additions.
amounts of HC emissions than the standard hydroxygen-blended gasoline engine when am is larger than 1.5%. Fig. 7 displays CO emission versus am for the hydrogen and the standard hydroxygen-blended gasoline engines at 1400 rpm, a MAP of 61.5 kPa and MBT spark timing. Fig. 7 shows that CO emission increases with the increase of hydrogen volume fraction in the intake whereas decreases with the increase of the standard hydroxygen addition fraction. The possible reason can be ascribed to the fact that, since hydrogen has a wide flammability and a high flame speed, the added hydrogen tends to be ignited and combusted prior to the gasoline and therefore causes some lean-oxygen areas in the cylinder. Thus, CO is increased after hydrogen addition at the stoichiometric condition. Comparatively, the oxygen content in the hydroxygen could further oxidize CO into CO2 which is not the case for the pure hydrogen addition. Thereby, the addition of hydroxygen is more effective on reducing CO emission than the pure hydrogen addition. When the hydroxygen volume fraction in the intake rises from 0% to 2.8%, CO is reduced by 21.86% for the standard hydroxygen-blended gasoline engine.
This paper investigated and compared the effects of hydrogen and standard hydroxygen additions on improving the performance of a gasoline engine at 1400 rpm and a MAP of 61.5 kPa under stoichiometric conditions. The main conclusions are summarized as follows: At low blending fractions, the standard hydroxygen-blended gasoline engine produces a higher thermal efficiency and shorter flame development and propagation durations than the hydrogenenriched gasoline engine. But at high blending fractions, the addition of pure hydrogen tends to help the engine gain a higher thermal efficiency than the standard hydroxygen addition. Since the volume energy density of the standard hydroxygen is higher than that of the hydrogenegasolineeair mixtures, the increased fuel energy flow rate is gained after the hydroxygen addition, which symbolizes that the standard hydroxygen-blended gasoline engine could gain a higher torque output than the original and hydrogenenriched gasoline engines. The standard hydroxygen-blended gasoline engine produces lower CO emission than the original and hydrogen-enriched gasoline engines. At low blending fractions, the addition of hydroxygen is more effective on reducing HC emissions. However, because of the raised combustion temperature, both the additions of hydrogen and hydroxygen lead to the increased NOx emissions. Due to the further increased oxygen concentration in the cylinder, the hydroxygen-blended gasoline engine expels larger amounts of NOx than the hydrogen-enriched engine. Thus, proper techniques are required to control NOx emissions from the standard hydroxygen-blended gasoline engines. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 50976005). References
Fig. 8. NOx versus am with hydrogen and standard hydroxygen additions.
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