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The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine
international journal of hydrogen energy xxx (xxxx) xxx
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The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine Cheolwoong Park a,*, Yongrae Kim a, Young Choi a, Jeongwoo Lee a, Byeungjun Lim b a
Engine Research Team, Environmental System Research Division, Korea Institute of Machinery and Materials, Daejeon, 34103, Republic of Korea b Engine Component Technology Team, Aeropropulsion Division, Korea Aerospace Research Institute, Daejeon, 34133, Republic of Korea
highlights The maximum torque decreases due to the backfire as the hydrogen engine speed increases. The increase in the exhaust gas temperature affects the backfire with the increase in speed. The backfire occurs in the No. 4 cylinder under low-speed conditions due to a higher port temperature. The backfire starts in the cylinders located at the center of the engine under high-speed conditions.
article info
abstract
Article history:
The port-injection-type hydrogen engine is advantaged in that hydrogen gas is injected
Received 25 March 2019
into the intake pipe through a low-pressure fuel injector, and the mixing period with air is
Received in revised form
sufficient to produce uniform mixing, improving the thermal efficiency. A drawback is that
27 May 2019
the flame backfires in the intake manifold, reducing the engine output because the amount
Accepted 11 June 2019
of intake air is reduced, owing to the large volume of hydrogen. Here, the backfire mech-
Available online xxx
anism as a part of the development of full-load output capability is investigated, and a 2.4liter reciprocating gasoline engine is modified to a hydrogen engine with a hydrogen supply
Keywords:
system. To secure the stability and output performance of the hydrogen engine, the excess
Hydrogen engine
air ratio was controlled with a universal engine control unit.
Port fuel injection
The torque, excess air ratio, hydrogen fuel, and intake air flow rate changes in time
Engine speed
were compared under low- and high-engine speed conditions with a wide-open throttle.
Backfire
The excess air ratio depends on the change in the fuel amount when the throttle is
Excess air ratio
completely opened, and excess air ratio increase leads to fuel/air-mixture dilution by the
Cylinder-to-cylinder variations
surplus air in the cylinder. As the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase due to the occurrence of the backfire. The exhaust gas temperature also increases, except at an engine speed of 6000 rpm. Furthermore, the increase in exhaust gas temperature affects the backfire occurrence. At 2000 rpm, under low-speed and wide-open throttle conditions, backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high port temperature. In contrast, at 6000 rpm, under high-speed and wide-open throttle conditions, the backfire
* Corresponding author. E-mail address: [email protected] (C. Park). https://doi.org/10.1016/j.ijhydene.2019.06.058 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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starts at the No. 2 cylinder first because of a higher exhaust gas temperature, resulting in a lower excess air ratio in cylinders 2 and 3, located at the center of the engine. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The demand for a high-power mobile power source will increase significantly because the power source for highaltitude long-endurance (HALE) unmanned aerial vehicles (UAVs) and drone or mobile robots are very important unmanned technologies that serve as a countermeasure against aging, one of the future global megatrends. Although HALE UAV can be used with a charging and discharging storage type, such as solar cells and fuel cells, theoretically for several years, long-term research is required for commercialization because of its low technical maturity. On the other hand, gas turbines and reciprocating engines, which are propulsion methods of conventional aircraft, have the advantage of high technical maturity, although the time of flight is limited according to the amount of fuel. In addition, considering the fact that environmental issues are becoming a global agenda, the importance of using hydrogen as an energy source with no carbon emissions and high energy per unit weight is more emphasized [1,2]. Despite the investments in research and development by advanced countries of the HALE UAV, which is able to fly over a period of several days, the number of successful cases is very few, and there are few cases reported of practical size development. NASA has carried out research on the hydrogen fuel system design for HALE UAV, including a hydrogen reciprocating engine, storage, and supply system [3], and Boeing produced and tested a compact proto-type fuselage of the Phantom Eye using a hydrogen reciprocating engine by implementing the technologies gained through previous development experience and the storage of liquid hydrogen and supply technology applied to space launch vehicles [4]. The Global Observer propulsion system developed by Aerovironment drives the generator, with an internal combustion engine that uses hydrogen fuel and electricity generated by a generator as propeller drive, charge, and payload power [5]. The main task of the use of liquid hydrogen by the HALE UAVs is to maintain 7e10 days of flying time at an altitude of 20 km or more while installing mission equipment necessary for reconnaissance/surveillance and communication relaying [6]. Research and development of a reciprocating engine for an automobile using hydrogen fuel has been actively carried out, and although the technical maturity is high, it is difficult to predict the point of practical use because using high-pressure direct injection of hydrogen fuel or liquid injection technology is a long-term problem that must be solved [7e11]. Therefore, to complete the propulsion technology of HALE UAVs, the development of a hydrogen reciprocating engine, which experiences the problem of low output density due to the hydrogen volumetric density and early ignition, is required
even if hydrogen fuel is applied to a modified existing portinjection-type gasoline engine. The advantages of hydrogen fuel are a wide flammability limit (equivalence ratio F ¼ 0.1e7.2), increase in efficiency with a lean mixture (low-load region), low ignition energy, and lack of carbon monoxide or hydrocarbons generated [12]. Furthermore, since the octane number is high, the compression ratio can be increased. The basic characteristics of hydrogen combustion and the effect of different parameters were investigated and some controlling strategies were presented from previous studies [13e15]. The disadvantage is that this is difficult to control because the burning rate is high and the temperature of the adiabatic flame is about 100 C higher than that of gasoline, so nitrogen oxides can be generated [9,16]. The port-injection-type hydrogen engine has the advantage in that the hydrogen gas is injected into the intake pipe by using a low-pressure fuel injector, and the period of mixing with the air is sufficient that a uniform mixture is formed, and the thermal efficiency is improved. The drawback in that the flame backfires in the intake manifold, and the output of the engine is reduced because the amount of intake air is reduced due to the large volume of hydrogen [17,18]. It is known that the backfire, which was prematurely ignited by the hot spot, reverses back into the intake manifold during the valve overlap period. To suppress the backfire, methods such as reduction in combustion temperature by the use of the lean mixture, valve overlap control, and piston crevice volume cooling, as well as additional water injection have been applied [19e22]. In the case of dedicated hydrogen internal combustion engine, only a few experimental studies are available regarding the abnormal combustion and its cause analysis. In this study, to address the effect of flow motion characteristics and cylinder location, exhaust gas temperature and excess air ratio for each cylinder were measured and those are compared with 1D simulation results. The purpose of this study was to investigate the mechanism of backfire as a part of the development of fullload output capability to secure the stability and output performance of the hydrogen engine. The relationship between start of backfire and engine speed was also discussed and analyzed.
Experimental procedure Experimental setup Table 1 lists the specifications of the gasoline engine used in this study. A 2.4-liter gasoline engine with an intake port injection system was used while achieving an output power of at least 112 kW to realize the demand performance of a UAV.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Table 1 e Engine specifications. Engine type
Gasoline (base)/Hydrogen e Naturally aspirated
Number of cylinders Bore Stroke Displacement volume Compression ratio Maximum power output Maximum torque
4 88 mm 97 mm 2359 cc 10.5 131 kW at 6000 rpm 228.5 Nm at 4000 rpm
Natural gas injectors (NGI2, Bosch), which can operate at an injection pressure of approximately 0.8 MPa, are installed to supply hydrogen fuel instead of a gasoline injector. The engine control unit (ECU) (M800, Motec) was adopted to control combustion factors, such as the ignition timing, amount of air, and amount of fuel, to meet the experimental conditions. The main fuel, hydrogen, was stored in a reservoir composed of a series of hydrogen gas tanks with 10 MPa. Then, it was depressurized by a regulator and supplied to the engine through the hydrogen supply line. The air mass flow rate and hydrogen flow rate were measured using a mass flow meter with a thermal mass flow meter (S452-80, CSi-TEC) and a Coriolis-type mass flow meter (CMF025M, Micromotion), respectively. In addition, a wideband oxygen sensor (LSU 4.2, Bosch) and lambda meter (LA4, ETAS Co.) were installed in each exhaust port and junction to measure the air fuel ratio of each cylinder as well as overall. The engine speed and load were controlled using a 250-kW eddy current dynamometer (SE250HS, Dasan). Combustion pressure data were measured through an ignition-plug-type pressure sensor (6118BFD35, Kistler) and a combustion analyzer (DEWE-800-CA, DEWETRON Co.) to determine measures of combustion performance, such as combustion stability, represented by the coefficient of variation (COV) for the indicated mean effective pressure (IMEP) [23,24]. In addition, thermocouples and pressure sensors were installed to measure the temperature and pressure of the main engine parts, including the temperature and pressure of each intake/ exhaust gas port and manifold junction.
Experimental conditions A full-load swing was performed from 2000 rpm, which corresponds to low-speed output, to 6000 rpm, which is a rated output condition, at an interval of 1000 rpm for the full-load performance test. In addition to the main performance and characteristics such as output performance, thermal efficiency, and combustion stability, the cause of the change in the full-load performance characteristics with the change in the control factor was analyzed. The ignition timing was optimized according to the application of hydrogen fuel in each operating condition, so that it was operated under maximum brake torque (MBT) ignition timing conditions. The experimental data was acquired under stable operating conditions and an engine cooling water and engine oil temperature of 80 ± 2.5 C.
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Finally, to predict the effect of the amount of air introduced into each cylinder, we compared the air flow rate of each cylinder according to the change in the engine speed under backfire operating conditions, using 1D simulation software (WAVE, Ricardo Inc.). The engine model for 1D simulation is implemented by applying the measured engine geometry. The Wiebe model for spark ignition engine is applied to the hydrogen engine as well as the gasoline engine and the hydrogen combustion model (Crank angle degree at 50% of the heat from combustion has been released is 15 after top dead center) was established from the literature review. When the simulation was carried out under the stoichiometric air/fuel ratio conditions, the output and torque of the gasoline engine were 70e80%, and the fuel efficiency was improved to 40% due to the high energy density of hydrogen. The schematic diagram of 1D simulation model is shown in Fig. 1.
Results The experiment was performed to increase the fuel to the minimum excess air ratio condition in which the throttle was fully opened without backfire. The torque at this time was regarded as the maximum torque at the corresponding engine speed condition, and the maximum torque and the excess air ratio are shown in Fig. 2. As shown in the figure, as the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase, such that the backfire does not occur. In a general naturally aspirated spark ignition engine, the maximum torque is observed in the engine speed range of 3000e5000 rpm. The reason is that the amount of intake air mass plays an important role under stoichiometric mixture condition and the intake air flow losses occur due to less suction by slow piston movement at low engine speed and insufficient time to fill the cylinder completely at high engine speed, respectively. However, the supply of the hydrogen fuel is limited due to the occurrence of backfire and the amount of fuel input reduces compared to the required amount for the stoichiometric mixture condition as the engine speed increases. Consequently, the excess air increases with the increase in engine speed and the maximum torque is observed at 2000 rpm where the excess air ratio is 1.24 without any abnormal combustion, such as backfire. Fig. 3 shows changes in the thermal efficiency and combustion stability with increasing engine speed. Generally, in the case of a gasoline engine, the excess air ratio is proportional to the increase in the throttle opening degree, and when the throttle opening degree increases, the pumping loss decreases and, consequently, the thermal efficiency tends to increase. However, as in this study, the excess air ratio depends on the change in the fuel amount under the condition that the throttle is completely opened, and the increase in the excess air ratio leads to the dilution of the fuel/air mixture by the surplus air in the cylinder. As can be seen from the curve of the heat release rate shown in Fig. 4, an increase in the heat loss due to a decrease in the burning rate resulted in a reduction in the thermal efficiency. The increase of heat release rate is sharp and the main combustion duration is about 20 CAD for the engine speed of 2000 rpm. On the other hand, as the engine speed increases and consequently the
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 1 e Schematic diagram of 1D simulation model.
Fig. 2 e Torque and excess air ratio variations with engine speed changes under wide-open throttle conditions.
excess air ratio increases, the peak of the heat release rate gradually decreases and the combustion duration increases. The increase in exhaust loss, as the engine speed increases, can be another cause of the decrease in thermal efficiency. Although the combustion stability is drastically deteriorated as the excess air ratio increases in a gasoline engine, the use of hydrogen, whose lean flammability limit is very wide, results in the excess air ratio proportionally increasing as the engine speed increases, which does not affect the deterioration of the combustion stability. Consequently, the COV of IMEP does not exceed the value of 5% as the standard for an operable stable combustion.
Fig. 3 e Thermal efficiency and combustion stability variations with engine speed changes under wide-open throttle conditions.
As shown in Table 2, with the fuel injection system of the port injection system, which is at relatively low pressure, when the fuel injection duration increases to increase the torque as the engine speed increases, under high-speed operating conditions such as 6000 rpm, the fuel-injection duration reaches one cycle of the combustion stroke. Even when the fuel injection start timing is taken into consideration, it is difficult to say that the backfire is simply caused by valve overlap because the fuel injection period is overlapped with the overlap timing of the intake and exhaust valves. The exhaust gas temperature trends of Fig. 5 show that the
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 4 e Apparent heat release rate traces with engine speed changes under wide-open throttle conditions.
exhaust gas temperature increases under operating conditions, except at an engine speed of 6000 rpm, although the excess air ratio increases due to the backfire as the engine speed increases. This increase in the exhaust gas temperature affects the occurrence of backfire [21]. When the temperature of the inlet and outlet of the coolant water, shown in Fig. 5, are examined as a function of engine speed, the inlet-side coolant temperature, controlled by the coolant temperature controller flowing into the engine, is shown to be constant regardless of the engine speed. However, as the engine speed increases, the heat load increases as the flow rate increases. It is not easy to evaluate the heat loss from the heat release rate traces. However, when we compare the differences of the inlet and outlet temperature of coolant water, we can know that the
increased heat loss with the decreased in combustion speed due to the high excess air ratio affect the increase in the temperature differences. As described above, the combustion speed decreases, such that the heat loss increases and the cooling water temperature at the outlet side increases. The deterioration of thermal efficiency is the result of the increase in the heat and exhaust losses. Fig. 6 shows the torque, excess air ratio of each cylinder, hydrogen fuel, and intake air flow rate under backfire conditions at 2000 rpm, corresponding to low-speed, high-load operating conditions. Because the engine speed is constant with a wide-open throttle, the intake air flow rate is gradually decreased as the torque is increased, while the intake air flow rate is expected to be constant. This is because the volume of hydrogen as a gaseous fuel is larger than that of other hydrocarbon fuels among the mixture flowing into the intake manifold, as shown in the figure. Increasing the hydrogen fuel to increase the torque results in a decrease in the amount of air introduced by the increased hydrogen fuel. That is a typical cause of the drop in output in the hydrogen engine with the port injection system. As a result, the extent of the decrease in the excess air ratio significantly varies with the increase in the torque. Under constant excess air ratio conditions, while there is almost no change in the hydrogen fuel flow rate supplied on account of backfire, the flow rate of the intake air and the excess air ratio decreased sharply and then returned to the original level. Because of the occurrence of backfire, combustion occurred in the intake manifold instead of inside the combustion chamber, and the torque was reduced rapidly and then recovered. After approximately 15.8 s, after the changes in the torque and excess air ratio due to backfire were confirmed, the torque was decreased by reducing the intake air flow rate and hydrogen fuel flow rate to end the specified
Table 2 e Fuel injection duration and start of fuel injection timing with various engine speed. Engine speed Fuel injection duration [crank angle degree (CAD)] Start of fuel injection timing [CAD, after top dead center (ATDC)]
Fig. 5 e Exhaust gas temperature and inlet/outlet coolant temperature variations with engine speed changes under wide-open throttle conditions.
2000 rpm
3000 rpm
4000 rpm
5000 rpm
6000 rpm
152 327
230 220
311 198
481 28
684 174
Fig. 6 e Torque and excess air ratio for each cylinder, and hydrogen and air flow rate changes in time with the gradual increase in hydrogen fuel input at an engine speed of 2000 rpm.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 7 e Intake air temperature changes in time for each cylinder with the gradual increase in hydrogen fuel input at an engine speed of 2000 rpm.
Fig. 9 e Torque, excess air ratio for each cylinder, and hydrogen and air flow rate changes in time with the gradual increase in hydrogen fuel input at an engine speed of 6000 rpm.
test condition. The excess air ratio and the intake air flow rate varied greatly because the excess air ratio and ignition timing set by the mapping ECU were different for stable operation. It is noteworthy that backfire occurs in cylinder No. 4. As a result, as can be seen in Fig. 7, the intake air temperature of the No. 4 cylinder rapidly increases as the first action and that of the No. 2 cylinder follows while those of the other cylinders maintain constant values. The intake manifold of the plastic material is melted due to backfire occurring in the vicinity of the injector mounting of the No. 4 cylinder. When this phenomenon accumulates, a hole is generated, and the mixture of hydrogen and air is discharged to the outside, causing leakage. This backfire occurs in the No. 4 cylinder because of the fact that the mixture is heated by the relatively high temperature of the port, and thus, the possibility of backfire is higher than in the other cylinders. In the case of the series type-4 cylinder engine used in this experiment, as shown in Fig. 8, the water used to cool the combustion chamber is supplied from the inlet located at the lower side of the No. 1 cylinder, flowing upstream around the
combustion chamber liner, passing through the cooling water flow path merged at the head, and exiting from the head portion near the No. 4 cylinder. The cooling water that escaped in this way is cooled through the radiator and then supplied to the inlet at the lower portion of the No. 1 cylinder. Therefore, when the combustion heat of the engine is transmitted to the cooling water, the temperature of the intake port of the No. 4 cylinder near the exit portion becomes higher than that of the other cylinders, and when the temperature of the mixture is increased by the heat transfer, the reactivity of the mixture becomes higher, increasing the possibility of backfire. Fig. 9 shows the changes with time in the torque, excess air ratio, hydrogen fuel, and intake air flow rate under backfire conditions at 6000 rpm, which corresponds to the high-speed high-load operating conditions. As with the low-speed highload conditions, the flow rate of the supplied hydrogen gas increased, and the excess air ratio decreased. The torque also decreased sharply because of backfire at approximately 1.36 times the excess air ratio. Afterwards, the intake air flow rate and hydrogen fuel flow rate was decreased to reduce the
Fig. 8 e Schematic diagram of coolant flow in an engine cylinder block and head. Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 10 e Excess air ratio variations for each cylinder with engine speed changes under wide-open throttle conditions.
torque, and a slight increase in the intake air flow rate and excess air ratio was observed under different ECU mapping conditions. The difference from the 2000 rpm full-load operating conditions, the low-speed high-load operating conditions, is that the backfire starts at the No. 2 cylinder first, rather than at the No. 4 cylinder. The cause of this phenomenon can be found in the change in the excess air ratio and the exhaust gas temperature of each cylinder as the engine speed changes, as shown in Figs. 10 and 11. At the relatively lowspeed operating conditions of 2000 rpm and 3000 rpm, the exhaust temperature and excess air ratio are almost the same, regardless of the cylinder. However, as the engine speed increases, there is a variation between the cylinders. In particular, at engine speeds of 5000 rpm or more, the excess air ratios of No. 2 and 3 cylinders, located at the center of the engine, are lower than those of No. 1 and 4 cylinders, located at the edge of the engine, consequently resulting in a high exhaust gas temperature. The difference in the exhaust gas
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Fig. 12 e Total air flow rate of experiment and simulation, and air flow rate variations for each cylinder with engine speed changes under wide-open throttle conditions.
temperature is the greatest at 6000 rpm, showing a difference of about 30 , and the high exhaust gas temperature of No. 2 and 3 cylinders seem to be the cause of the backfire under high-speed operation conditions. To determine the cause of the difference in the excess air ratio per cylinder, 1D simulation software (WAVE, Ricardo Inc.) was used. As shown in Fig. 12, the intake air flow rate for each cylinder is calculated and compared with the engine speed change. At this time, a 1D model was constructed based on the 3D shape of the intake manifold, and the total air flow from the experiment was compared with the calculated value for the validation. The values were found to be nearly the same, as shown in the figure. As expected, the intake air flow rates of cylinders 2 and 3, located at the center of the engine, were lower than that of the other cylinders owing to a structural problem that resulted from the shape of the intake manifold and the intake port, which caused a difference in the excess air ratio.
Conclusions In the present study, combustion characteristics of the hydrogen engine for HALE UAV were investigated under various engine speeds and wide-open throttle operating conditions. To achieve stable combustion and maximize torque, the minimum excess air ratios vary and the ignition timings are optimized for the engine speed changes. The effect of cylinder-to-cylinder variations in the excess air ratio was assessed both experimentally and numerically to analyze the cause of the backfire trend in the hydrogen port fuel injection engine. The main results from the engine speed changes are summarized as follows.
Fig. 11 e Exhaust gas temperature variations for each cylinder with engine speed changes under wide-open throttle conditions.
1) As the engine speed increases, the maximum torque decreases because the excess air ratio that does not backfire continues to increase. The supply of the hydrogen fuel is limited due to the occurrence of backfire and the maximum torque is observed at 2000 rpm where the excess air ratio is 1.24 without backfire.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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2) The exhaust gas temperature increases under the operating conditions except under the condition of 6000 rpm and, consequently, the increase in the exhaust gas temperature affects the occurrence of backfire although the excess air ratio increases as the engine speed increases. 3) At 2000 rpm, under low-speed and wide-open throttle conditions, the backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high temperature of the port. 4) At 6000 rpm, under high-speed and wide-open throttle conditions, backfire starts in the No. 2 cylinder first because of the higher exhaust gas temperature, resulting in a lower excess air ratio in No. 2 and 3 cylinders, located at the center of the engine.
references
[1] Hudson MSL, Dubey PK, Pukazhselvan D, Pandey SK, Singh RK, Raghubanshi H, et al. Hydrogen energy in changing environmental scenario: Indian context. Int J Hydrogen Energy 2009;34:7358e67. [2] Eichlseder H, Wallner T, Freymann R, Ringler J. The potential of hydrogen internal combustion engines in a future mobility scenario. 2003. https://doi.org/10.4271/2003-01-2267. [3] Millis MG, Tornabene RT, Jurns JM, Guynn MD, Tomsik TM, VanOverbeke TJ. Hydrogen fuel system design trades for high-altitude long-endurance remotely-operated aircraft. NASA Tech Rep 2009. NASA/TM-2009-215521, E-16800. [4] Wilkinson RE, Benway RB. Liquid cooled turbocharged propulsion system for HALE application. 2015. https:// doi.org/10.1115/91-gt-399. [5] Warwick G. Hydrogen high-flier. Aviat Week Space Technol 2010;172:42e4. [6] Nickol CL, Guynn MD, Kohout LL, Ozoroski TA. High altitude long endurance uav analysis of alternatives and technology requirements development. 2007. [7] Kiesgen G, Klu¨ting M, Bock C, Fischer H. The new 12-cylinder hydrogen engine in the 7 series: the H2 ICE age has begun. SAE Technical Paper; 2006. [8] Tang X, Kabat DM, Natkin RJ, Stockhausen WF, Heffel J. Ford P2000 hydrogen engine dynamometer development. 2002. https://doi.org/10.4271/2002-01-0242. [9] Karim GA. Hydrogen as a spark ignition engine fuel. Int J Hydrogen Energy 2003;28:569e77.
[10] DeLuchi MA. Hydrogen vehicles: an evaluation of fuel storage, performance, safety, environmental impacts, and cost. Int J Hydrogen Energy 1989;14:81e130. https://doi.org/ 10.1016/0360-3199(89)90001-3. [11] Natkin RJ, Tang X, Boyer B, Oltmans B, Denlinger A, Heffel JW. Hydrogen IC engine boosting performance and NOx study. 2003. https://doi.org/10.4271/2003-01-0631. [12] Drell IL, Belles FE. Survey of hydrogen combustion properties. Supersedes NACA Res Memo 1957:E57D24. [13] Wang Z, Liu H, Reitz RD. Knocking combustion in sparkignition engines. Prog Energy Combust Sci 2017. https:// doi.org/10.1016/j.pecs.2017.03.004. [14] Luo Q he, Sun B gang. Inducing factors and frequency of combustion knock in hydrogen internal combustion engines. Int J Hydrogen Energy 2016. https://doi.org/10.1016/ j.ijhydene.2016.05.257. [15] Luo Q he, Sun B gang. Experiments on the effect of engine speed, load, equivalence ratio, spark timing and coolant temperature on the energy balance of a turbocharged hydrogen engine. Energy Convers Manag 2018. https:// doi.org/10.1016/j.enconman.2017.12.051. [16] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Prog Energy Combust Sci 2009;35:490e527. https:// doi.org/10.1016/j.pecs.2009.08.001. [17] Kukkonen CA, Shelef M. Hydrogen as an alternative automotive fuel: 1993 update. Warrendale, PA: SAE International; 1994. [18] Vanblarigan P. A hydrogen fuelled internal combustion engine designed for single speed/power operation. Int J Hydrogen Energy 1998;23:603e9. https://doi.org/10.1016/ S0360-3199(97)00100-6. [19] Caton JA. An investigation of cause of backfire and its control due to creviced volumes in hydrogen fueled engine. Trans ASME 2001;23:204e10. [20] Huynh TC, Kang JK, Noh KC, Lee JT, Caton J a. Controlling backfire for a hydrogen-fueled engine using external mixture injection. J Eng Gas Turbines Power 2008;130:062804. https:// doi.org/10.1115/1.2940353. [21] Duan J, Liu F, Sun B. Backfire control and power enhancement of a hydrogen internal combustion engine. Int J Hydrogen Energy 2014;39:4581e9. https://doi.org/10.1016/ j.ijhydene.2013.12.175. [22] Liu X, Liu F, Zhou L, Sun B, Schock HJ. Backfire prediction in a manifold injection hydrogen internal combustion engine. TMS07 Symp Mater Clean Power Syst 2008;33:3847e55. https://doi.org/10.1016/j.ijhydene.2008.04.051. [23] Stone R. Introduction to internal combustion engines. Basingstoke: Palgrave Macmillan; 2012. [24] Heywood JB. Internal combustion engine fundamentals. New York: McGraw-Hill; 1988.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine Cheolwoong Park a,*, Yongrae Kim a, Young Choi a, Jeongwoo Lee a, Byeungjun Lim b a
Engine Research Team, Environmental System Research Division, Korea Institute of Machinery and Materials, Daejeon, 34103, Republic of Korea b Engine Component Technology Team, Aeropropulsion Division, Korea Aerospace Research Institute, Daejeon, 34133, Republic of Korea
highlights The maximum torque decreases due to the backfire as the hydrogen engine speed increases. The increase in the exhaust gas temperature affects the backfire with the increase in speed. The backfire occurs in the No. 4 cylinder under low-speed conditions due to a higher port temperature. The backfire starts in the cylinders located at the center of the engine under high-speed conditions.
article info
abstract
Article history:
The port-injection-type hydrogen engine is advantaged in that hydrogen gas is injected
Received 25 March 2019
into the intake pipe through a low-pressure fuel injector, and the mixing period with air is
Received in revised form
sufficient to produce uniform mixing, improving the thermal efficiency. A drawback is that
27 May 2019
the flame backfires in the intake manifold, reducing the engine output because the amount
Accepted 11 June 2019
of intake air is reduced, owing to the large volume of hydrogen. Here, the backfire mech-
Available online xxx
anism as a part of the development of full-load output capability is investigated, and a 2.4liter reciprocating gasoline engine is modified to a hydrogen engine with a hydrogen supply
Keywords:
system. To secure the stability and output performance of the hydrogen engine, the excess
Hydrogen engine
air ratio was controlled with a universal engine control unit.
Port fuel injection
The torque, excess air ratio, hydrogen fuel, and intake air flow rate changes in time
Engine speed
were compared under low- and high-engine speed conditions with a wide-open throttle.
Backfire
The excess air ratio depends on the change in the fuel amount when the throttle is
Excess air ratio
completely opened, and excess air ratio increase leads to fuel/air-mixture dilution by the
Cylinder-to-cylinder variations
surplus air in the cylinder. As the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase due to the occurrence of the backfire. The exhaust gas temperature also increases, except at an engine speed of 6000 rpm. Furthermore, the increase in exhaust gas temperature affects the backfire occurrence. At 2000 rpm, under low-speed and wide-open throttle conditions, backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high port temperature. In contrast, at 6000 rpm, under high-speed and wide-open throttle conditions, the backfire
* Corresponding author. E-mail address: [email protected] (C. Park). https://doi.org/10.1016/j.ijhydene.2019.06.058 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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starts at the No. 2 cylinder first because of a higher exhaust gas temperature, resulting in a lower excess air ratio in cylinders 2 and 3, located at the center of the engine. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The demand for a high-power mobile power source will increase significantly because the power source for highaltitude long-endurance (HALE) unmanned aerial vehicles (UAVs) and drone or mobile robots are very important unmanned technologies that serve as a countermeasure against aging, one of the future global megatrends. Although HALE UAV can be used with a charging and discharging storage type, such as solar cells and fuel cells, theoretically for several years, long-term research is required for commercialization because of its low technical maturity. On the other hand, gas turbines and reciprocating engines, which are propulsion methods of conventional aircraft, have the advantage of high technical maturity, although the time of flight is limited according to the amount of fuel. In addition, considering the fact that environmental issues are becoming a global agenda, the importance of using hydrogen as an energy source with no carbon emissions and high energy per unit weight is more emphasized [1,2]. Despite the investments in research and development by advanced countries of the HALE UAV, which is able to fly over a period of several days, the number of successful cases is very few, and there are few cases reported of practical size development. NASA has carried out research on the hydrogen fuel system design for HALE UAV, including a hydrogen reciprocating engine, storage, and supply system [3], and Boeing produced and tested a compact proto-type fuselage of the Phantom Eye using a hydrogen reciprocating engine by implementing the technologies gained through previous development experience and the storage of liquid hydrogen and supply technology applied to space launch vehicles [4]. The Global Observer propulsion system developed by Aerovironment drives the generator, with an internal combustion engine that uses hydrogen fuel and electricity generated by a generator as propeller drive, charge, and payload power [5]. The main task of the use of liquid hydrogen by the HALE UAVs is to maintain 7e10 days of flying time at an altitude of 20 km or more while installing mission equipment necessary for reconnaissance/surveillance and communication relaying [6]. Research and development of a reciprocating engine for an automobile using hydrogen fuel has been actively carried out, and although the technical maturity is high, it is difficult to predict the point of practical use because using high-pressure direct injection of hydrogen fuel or liquid injection technology is a long-term problem that must be solved [7e11]. Therefore, to complete the propulsion technology of HALE UAVs, the development of a hydrogen reciprocating engine, which experiences the problem of low output density due to the hydrogen volumetric density and early ignition, is required
even if hydrogen fuel is applied to a modified existing portinjection-type gasoline engine. The advantages of hydrogen fuel are a wide flammability limit (equivalence ratio F ¼ 0.1e7.2), increase in efficiency with a lean mixture (low-load region), low ignition energy, and lack of carbon monoxide or hydrocarbons generated [12]. Furthermore, since the octane number is high, the compression ratio can be increased. The basic characteristics of hydrogen combustion and the effect of different parameters were investigated and some controlling strategies were presented from previous studies [13e15]. The disadvantage is that this is difficult to control because the burning rate is high and the temperature of the adiabatic flame is about 100 C higher than that of gasoline, so nitrogen oxides can be generated [9,16]. The port-injection-type hydrogen engine has the advantage in that the hydrogen gas is injected into the intake pipe by using a low-pressure fuel injector, and the period of mixing with the air is sufficient that a uniform mixture is formed, and the thermal efficiency is improved. The drawback in that the flame backfires in the intake manifold, and the output of the engine is reduced because the amount of intake air is reduced due to the large volume of hydrogen [17,18]. It is known that the backfire, which was prematurely ignited by the hot spot, reverses back into the intake manifold during the valve overlap period. To suppress the backfire, methods such as reduction in combustion temperature by the use of the lean mixture, valve overlap control, and piston crevice volume cooling, as well as additional water injection have been applied [19e22]. In the case of dedicated hydrogen internal combustion engine, only a few experimental studies are available regarding the abnormal combustion and its cause analysis. In this study, to address the effect of flow motion characteristics and cylinder location, exhaust gas temperature and excess air ratio for each cylinder were measured and those are compared with 1D simulation results. The purpose of this study was to investigate the mechanism of backfire as a part of the development of fullload output capability to secure the stability and output performance of the hydrogen engine. The relationship between start of backfire and engine speed was also discussed and analyzed.
Experimental procedure Experimental setup Table 1 lists the specifications of the gasoline engine used in this study. A 2.4-liter gasoline engine with an intake port injection system was used while achieving an output power of at least 112 kW to realize the demand performance of a UAV.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Table 1 e Engine specifications. Engine type
Gasoline (base)/Hydrogen e Naturally aspirated
Number of cylinders Bore Stroke Displacement volume Compression ratio Maximum power output Maximum torque
4 88 mm 97 mm 2359 cc 10.5 131 kW at 6000 rpm 228.5 Nm at 4000 rpm
Natural gas injectors (NGI2, Bosch), which can operate at an injection pressure of approximately 0.8 MPa, are installed to supply hydrogen fuel instead of a gasoline injector. The engine control unit (ECU) (M800, Motec) was adopted to control combustion factors, such as the ignition timing, amount of air, and amount of fuel, to meet the experimental conditions. The main fuel, hydrogen, was stored in a reservoir composed of a series of hydrogen gas tanks with 10 MPa. Then, it was depressurized by a regulator and supplied to the engine through the hydrogen supply line. The air mass flow rate and hydrogen flow rate were measured using a mass flow meter with a thermal mass flow meter (S452-80, CSi-TEC) and a Coriolis-type mass flow meter (CMF025M, Micromotion), respectively. In addition, a wideband oxygen sensor (LSU 4.2, Bosch) and lambda meter (LA4, ETAS Co.) were installed in each exhaust port and junction to measure the air fuel ratio of each cylinder as well as overall. The engine speed and load were controlled using a 250-kW eddy current dynamometer (SE250HS, Dasan). Combustion pressure data were measured through an ignition-plug-type pressure sensor (6118BFD35, Kistler) and a combustion analyzer (DEWE-800-CA, DEWETRON Co.) to determine measures of combustion performance, such as combustion stability, represented by the coefficient of variation (COV) for the indicated mean effective pressure (IMEP) [23,24]. In addition, thermocouples and pressure sensors were installed to measure the temperature and pressure of the main engine parts, including the temperature and pressure of each intake/ exhaust gas port and manifold junction.
Experimental conditions A full-load swing was performed from 2000 rpm, which corresponds to low-speed output, to 6000 rpm, which is a rated output condition, at an interval of 1000 rpm for the full-load performance test. In addition to the main performance and characteristics such as output performance, thermal efficiency, and combustion stability, the cause of the change in the full-load performance characteristics with the change in the control factor was analyzed. The ignition timing was optimized according to the application of hydrogen fuel in each operating condition, so that it was operated under maximum brake torque (MBT) ignition timing conditions. The experimental data was acquired under stable operating conditions and an engine cooling water and engine oil temperature of 80 ± 2.5 C.
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Finally, to predict the effect of the amount of air introduced into each cylinder, we compared the air flow rate of each cylinder according to the change in the engine speed under backfire operating conditions, using 1D simulation software (WAVE, Ricardo Inc.). The engine model for 1D simulation is implemented by applying the measured engine geometry. The Wiebe model for spark ignition engine is applied to the hydrogen engine as well as the gasoline engine and the hydrogen combustion model (Crank angle degree at 50% of the heat from combustion has been released is 15 after top dead center) was established from the literature review. When the simulation was carried out under the stoichiometric air/fuel ratio conditions, the output and torque of the gasoline engine were 70e80%, and the fuel efficiency was improved to 40% due to the high energy density of hydrogen. The schematic diagram of 1D simulation model is shown in Fig. 1.
Results The experiment was performed to increase the fuel to the minimum excess air ratio condition in which the throttle was fully opened without backfire. The torque at this time was regarded as the maximum torque at the corresponding engine speed condition, and the maximum torque and the excess air ratio are shown in Fig. 2. As shown in the figure, as the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase, such that the backfire does not occur. In a general naturally aspirated spark ignition engine, the maximum torque is observed in the engine speed range of 3000e5000 rpm. The reason is that the amount of intake air mass plays an important role under stoichiometric mixture condition and the intake air flow losses occur due to less suction by slow piston movement at low engine speed and insufficient time to fill the cylinder completely at high engine speed, respectively. However, the supply of the hydrogen fuel is limited due to the occurrence of backfire and the amount of fuel input reduces compared to the required amount for the stoichiometric mixture condition as the engine speed increases. Consequently, the excess air increases with the increase in engine speed and the maximum torque is observed at 2000 rpm where the excess air ratio is 1.24 without any abnormal combustion, such as backfire. Fig. 3 shows changes in the thermal efficiency and combustion stability with increasing engine speed. Generally, in the case of a gasoline engine, the excess air ratio is proportional to the increase in the throttle opening degree, and when the throttle opening degree increases, the pumping loss decreases and, consequently, the thermal efficiency tends to increase. However, as in this study, the excess air ratio depends on the change in the fuel amount under the condition that the throttle is completely opened, and the increase in the excess air ratio leads to the dilution of the fuel/air mixture by the surplus air in the cylinder. As can be seen from the curve of the heat release rate shown in Fig. 4, an increase in the heat loss due to a decrease in the burning rate resulted in a reduction in the thermal efficiency. The increase of heat release rate is sharp and the main combustion duration is about 20 CAD for the engine speed of 2000 rpm. On the other hand, as the engine speed increases and consequently the
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 1 e Schematic diagram of 1D simulation model.
Fig. 2 e Torque and excess air ratio variations with engine speed changes under wide-open throttle conditions.
excess air ratio increases, the peak of the heat release rate gradually decreases and the combustion duration increases. The increase in exhaust loss, as the engine speed increases, can be another cause of the decrease in thermal efficiency. Although the combustion stability is drastically deteriorated as the excess air ratio increases in a gasoline engine, the use of hydrogen, whose lean flammability limit is very wide, results in the excess air ratio proportionally increasing as the engine speed increases, which does not affect the deterioration of the combustion stability. Consequently, the COV of IMEP does not exceed the value of 5% as the standard for an operable stable combustion.
Fig. 3 e Thermal efficiency and combustion stability variations with engine speed changes under wide-open throttle conditions.
As shown in Table 2, with the fuel injection system of the port injection system, which is at relatively low pressure, when the fuel injection duration increases to increase the torque as the engine speed increases, under high-speed operating conditions such as 6000 rpm, the fuel-injection duration reaches one cycle of the combustion stroke. Even when the fuel injection start timing is taken into consideration, it is difficult to say that the backfire is simply caused by valve overlap because the fuel injection period is overlapped with the overlap timing of the intake and exhaust valves. The exhaust gas temperature trends of Fig. 5 show that the
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 4 e Apparent heat release rate traces with engine speed changes under wide-open throttle conditions.
exhaust gas temperature increases under operating conditions, except at an engine speed of 6000 rpm, although the excess air ratio increases due to the backfire as the engine speed increases. This increase in the exhaust gas temperature affects the occurrence of backfire [21]. When the temperature of the inlet and outlet of the coolant water, shown in Fig. 5, are examined as a function of engine speed, the inlet-side coolant temperature, controlled by the coolant temperature controller flowing into the engine, is shown to be constant regardless of the engine speed. However, as the engine speed increases, the heat load increases as the flow rate increases. It is not easy to evaluate the heat loss from the heat release rate traces. However, when we compare the differences of the inlet and outlet temperature of coolant water, we can know that the
increased heat loss with the decreased in combustion speed due to the high excess air ratio affect the increase in the temperature differences. As described above, the combustion speed decreases, such that the heat loss increases and the cooling water temperature at the outlet side increases. The deterioration of thermal efficiency is the result of the increase in the heat and exhaust losses. Fig. 6 shows the torque, excess air ratio of each cylinder, hydrogen fuel, and intake air flow rate under backfire conditions at 2000 rpm, corresponding to low-speed, high-load operating conditions. Because the engine speed is constant with a wide-open throttle, the intake air flow rate is gradually decreased as the torque is increased, while the intake air flow rate is expected to be constant. This is because the volume of hydrogen as a gaseous fuel is larger than that of other hydrocarbon fuels among the mixture flowing into the intake manifold, as shown in the figure. Increasing the hydrogen fuel to increase the torque results in a decrease in the amount of air introduced by the increased hydrogen fuel. That is a typical cause of the drop in output in the hydrogen engine with the port injection system. As a result, the extent of the decrease in the excess air ratio significantly varies with the increase in the torque. Under constant excess air ratio conditions, while there is almost no change in the hydrogen fuel flow rate supplied on account of backfire, the flow rate of the intake air and the excess air ratio decreased sharply and then returned to the original level. Because of the occurrence of backfire, combustion occurred in the intake manifold instead of inside the combustion chamber, and the torque was reduced rapidly and then recovered. After approximately 15.8 s, after the changes in the torque and excess air ratio due to backfire were confirmed, the torque was decreased by reducing the intake air flow rate and hydrogen fuel flow rate to end the specified
Table 2 e Fuel injection duration and start of fuel injection timing with various engine speed. Engine speed Fuel injection duration [crank angle degree (CAD)] Start of fuel injection timing [CAD, after top dead center (ATDC)]
Fig. 5 e Exhaust gas temperature and inlet/outlet coolant temperature variations with engine speed changes under wide-open throttle conditions.
2000 rpm
3000 rpm
4000 rpm
5000 rpm
6000 rpm
152 327
230 220
311 198
481 28
684 174
Fig. 6 e Torque and excess air ratio for each cylinder, and hydrogen and air flow rate changes in time with the gradual increase in hydrogen fuel input at an engine speed of 2000 rpm.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 7 e Intake air temperature changes in time for each cylinder with the gradual increase in hydrogen fuel input at an engine speed of 2000 rpm.
Fig. 9 e Torque, excess air ratio for each cylinder, and hydrogen and air flow rate changes in time with the gradual increase in hydrogen fuel input at an engine speed of 6000 rpm.
test condition. The excess air ratio and the intake air flow rate varied greatly because the excess air ratio and ignition timing set by the mapping ECU were different for stable operation. It is noteworthy that backfire occurs in cylinder No. 4. As a result, as can be seen in Fig. 7, the intake air temperature of the No. 4 cylinder rapidly increases as the first action and that of the No. 2 cylinder follows while those of the other cylinders maintain constant values. The intake manifold of the plastic material is melted due to backfire occurring in the vicinity of the injector mounting of the No. 4 cylinder. When this phenomenon accumulates, a hole is generated, and the mixture of hydrogen and air is discharged to the outside, causing leakage. This backfire occurs in the No. 4 cylinder because of the fact that the mixture is heated by the relatively high temperature of the port, and thus, the possibility of backfire is higher than in the other cylinders. In the case of the series type-4 cylinder engine used in this experiment, as shown in Fig. 8, the water used to cool the combustion chamber is supplied from the inlet located at the lower side of the No. 1 cylinder, flowing upstream around the
combustion chamber liner, passing through the cooling water flow path merged at the head, and exiting from the head portion near the No. 4 cylinder. The cooling water that escaped in this way is cooled through the radiator and then supplied to the inlet at the lower portion of the No. 1 cylinder. Therefore, when the combustion heat of the engine is transmitted to the cooling water, the temperature of the intake port of the No. 4 cylinder near the exit portion becomes higher than that of the other cylinders, and when the temperature of the mixture is increased by the heat transfer, the reactivity of the mixture becomes higher, increasing the possibility of backfire. Fig. 9 shows the changes with time in the torque, excess air ratio, hydrogen fuel, and intake air flow rate under backfire conditions at 6000 rpm, which corresponds to the high-speed high-load operating conditions. As with the low-speed highload conditions, the flow rate of the supplied hydrogen gas increased, and the excess air ratio decreased. The torque also decreased sharply because of backfire at approximately 1.36 times the excess air ratio. Afterwards, the intake air flow rate and hydrogen fuel flow rate was decreased to reduce the
Fig. 8 e Schematic diagram of coolant flow in an engine cylinder block and head. Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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Fig. 10 e Excess air ratio variations for each cylinder with engine speed changes under wide-open throttle conditions.
torque, and a slight increase in the intake air flow rate and excess air ratio was observed under different ECU mapping conditions. The difference from the 2000 rpm full-load operating conditions, the low-speed high-load operating conditions, is that the backfire starts at the No. 2 cylinder first, rather than at the No. 4 cylinder. The cause of this phenomenon can be found in the change in the excess air ratio and the exhaust gas temperature of each cylinder as the engine speed changes, as shown in Figs. 10 and 11. At the relatively lowspeed operating conditions of 2000 rpm and 3000 rpm, the exhaust temperature and excess air ratio are almost the same, regardless of the cylinder. However, as the engine speed increases, there is a variation between the cylinders. In particular, at engine speeds of 5000 rpm or more, the excess air ratios of No. 2 and 3 cylinders, located at the center of the engine, are lower than those of No. 1 and 4 cylinders, located at the edge of the engine, consequently resulting in a high exhaust gas temperature. The difference in the exhaust gas
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Fig. 12 e Total air flow rate of experiment and simulation, and air flow rate variations for each cylinder with engine speed changes under wide-open throttle conditions.
temperature is the greatest at 6000 rpm, showing a difference of about 30 , and the high exhaust gas temperature of No. 2 and 3 cylinders seem to be the cause of the backfire under high-speed operation conditions. To determine the cause of the difference in the excess air ratio per cylinder, 1D simulation software (WAVE, Ricardo Inc.) was used. As shown in Fig. 12, the intake air flow rate for each cylinder is calculated and compared with the engine speed change. At this time, a 1D model was constructed based on the 3D shape of the intake manifold, and the total air flow from the experiment was compared with the calculated value for the validation. The values were found to be nearly the same, as shown in the figure. As expected, the intake air flow rates of cylinders 2 and 3, located at the center of the engine, were lower than that of the other cylinders owing to a structural problem that resulted from the shape of the intake manifold and the intake port, which caused a difference in the excess air ratio.
Conclusions In the present study, combustion characteristics of the hydrogen engine for HALE UAV were investigated under various engine speeds and wide-open throttle operating conditions. To achieve stable combustion and maximize torque, the minimum excess air ratios vary and the ignition timings are optimized for the engine speed changes. The effect of cylinder-to-cylinder variations in the excess air ratio was assessed both experimentally and numerically to analyze the cause of the backfire trend in the hydrogen port fuel injection engine. The main results from the engine speed changes are summarized as follows.
Fig. 11 e Exhaust gas temperature variations for each cylinder with engine speed changes under wide-open throttle conditions.
1) As the engine speed increases, the maximum torque decreases because the excess air ratio that does not backfire continues to increase. The supply of the hydrogen fuel is limited due to the occurrence of backfire and the maximum torque is observed at 2000 rpm where the excess air ratio is 1.24 without backfire.
Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058
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2) The exhaust gas temperature increases under the operating conditions except under the condition of 6000 rpm and, consequently, the increase in the exhaust gas temperature affects the occurrence of backfire although the excess air ratio increases as the engine speed increases. 3) At 2000 rpm, under low-speed and wide-open throttle conditions, the backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high temperature of the port. 4) At 6000 rpm, under high-speed and wide-open throttle conditions, backfire starts in the No. 2 cylinder first because of the higher exhaust gas temperature, resulting in a lower excess air ratio in No. 2 and 3 cylinders, located at the center of the engine.
references
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Please cite this article as: Park C et al., The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.058