Development of a large-sized direct injection hydrogen engine for a stationary power generator

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Development of a large-sized direct injection hydrogen engine for a stationary power generator Taku Tsujimura*, Yasumasa Suzuki Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Japan

article info

abstract

Article history:

A number of studies on hydrogen engines have targeted small-sized engines for passenger

Received 6 July 2018

vehicles. By contrast, the present study focuses on a large-sized engine for a stationary

Received in revised form

power generator. The objective of this study is to simultaneously achieve low NOx emis-

30 August 2018

sion without aftertreatment, and high thermal efficiency and torque. Experimental anal-

Accepted 25 September 2018

ysis has been conducted on a single-cylinder test engine equipped with a gas injector for

Available online xxx

direct hydrogen injection. The injection strategy adopted in this study aims generating inhomogeneity of hydrogen mixtures within the engine cylinder by setting the injection

Keywords:

pressure at a relatively low level while injecting hydrogen through small orifices. High

Hydrogen engine

levels of EGR and increased intake boost pressures are also adopted to reduce NOx emis-

Stationary power generator

sion and enhance torque. The results showed that extreme levels of EGR and air-fuel in-

Low NOx

homogeneity can suppress NOx emission and the occurrence of abnormal combustion

High power

with little negative impact on the efficiency of hydrogen combustion. The maximum IMEP achieved under these conditions is 1.46 MPa (135 Nm@1000 rpm) with engine-out NOx emission of less than 150 ppm (ISNOx < 0.55 g/kW) for an intake boost pressure of 175 kPa and EGR rate of around 50%. To achieve further improvement of the IMEP and thermal efficiency, the Atkinson/Miller cycle was attempted by increasing the expansion ratio and retarding the intake valve closing time of the engine. The test engine used in this study finally achieved an IMEP of 1.64 MPa (150 Nm@1000 rpm) with less than 100 ppm of NOx emission (ISNOx < 0.36 g/kWh) and more than 50% of ITE. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction It is prominent that environmental issues and global warming have become more crucial over the last decades. The Paris agreement was adopted in December 2015 through consensus decision to mitigate global warming [1]. Keeping a global temperature rise below 2  C in the 21st century, which is the aim of the agreement, strongly requires the

decarbonization not only of the transportation sector but also any other sectors by switching over from conventional carbon-based fuels to renewable sources like solar, wind, and geothermal energy. Hydrogen can be produced from water and turns back to water after being combusted or electrochemically reacted for power generation purposes. The role of hydrogen in a low-carbon economy becomes critical and is necessary to utilize it efficiently and cleanly for preventing global warming [2,3].

* Corresponding author. Hydrogen Energy Carrier Team, Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-2-9, Machiikedai, Koriyama, 963-0298, Fukushima, Japan. E-mail address: [email protected] (T. Tsujimura). https://doi.org/10.1016/j.ijhydene.2018.09.178 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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Nomenclatures AC alternate current BMEP break mean effective pressure BTE break thermal efficiency CA crank angle CA10 crank angle of 10%cumulative heat release CA50 crank angle of 50%cumulative heat release CA90 crank angle of 90%cumulative heat release CA10-90 crake angle duration between CA10 and CA90 COV_IMEP coefficient of variation of the IMEP CO carbon monoxide carbon dioxide CO2 deg. degree EGR exhaust gas recirculation LP-EGR low pressure EGR EZEV equivalent zero emission vehicle FCV fule cell vehicle FT-IR Fourier transform infrared spectroscopy g/kWh gram per kilowatt hour HC hydrocarbon H.R.R. apparent heat release rate Hz Hertz hydrogen H2 water H2O ICE internal combustion engine IMEP indicated mean effective pressure ITE indicated thermal efficiency K Kelvin m/s meter per second NL normal litter NL/min normal litter per minute Nm Newton meter NOx nitrogen oxides ISNOx indicated specific emission of NOx oxygen O2 Pa pascal kPa kilo pascal MPa mega pascal ppm parts per million rpm rotation per minute RH relative humidity TDC top dead center aTDC after top dead center vol.% volume percent 4 equivalence ratio duration of injection event Dtinj intake gas pressure pin injection presuure pinj ignition timing tign

The development of hydrogen fuel cell vehicles (FCV) has remarkably progressed over the last years. Toyota Motor Corporation. Has already launched the first commercial FCV back in 2015 [4]. However, the price of an FCV is still much higher than conventional vehicles, and a fuel cell essentially has a disadvantage in cost because the power output of a fuel

cell system is proportional to the number of cells stacked in the system. Meanwhile, the development of an internal combustion engine (ICE) fueled with hydrogen has a long history of many years. Escher et al. reported the progress in hydrogen engine back in 1976 [5], and since then an enormous number of technical papers have been published. One of the reasons why hydrogen is attractive as a fuel for ICE is its combustion characteristics. As compared with conventional fuels, hydrogen is significantly more flammable, and its flammable range is much wider from 4 to 75 vol% [6]. The required minimum ignition energy is one-tenth of gasoline fuel, and the laminar flame speed is 2.9 m/s which is over seven times faster than the laminar flame speed of conventional fuels. Thus, such unique combustion characteristics of hydrogen make it a suitable fuel for the internal combustion engine. In addition, the auto-ignition temperature of hydrogen is about 850 K that is 100 K higher than gasoline. This allows engine operation at higher compression ratios, and increased efficiency. According to the review paper of Escher et al., in which they investigated 50 research studies and demonstrations on hydrogen-fueled engines [5], hydrogen is expected to realize a theoretical engine cycle with a higher polytropic index. Then, a hydrogen engine is supposed to be able to operate with wide open throttling referred to as quality governing, while quantity governing at a moreor-less fixed fuel/air condition near stoichiometric conditions as operated in ordinary gasoline engines. This can ensure that the hydrogen engine can achieve higher indicated thermal efficiency (ITE) than the gasoline engine. The comprehensive overview with 281 references done by Verhelst et al. in 2009 [7] describes hydrogen combustion fundamentals, engine control strategies, and computer simulation models. Ford Motor Company, BMW, and Mazda have been developing hydrogen-fueled vehicles, and they successfully demonstrated the dedicated hydrogen vehicles to show superior performance on emissions and fuel economy [8e10]. Ford Motor Company had built and tested the first production viable vehicle P2000 with a hydrogen-fueled ICE that could run unthrottled at lean hydrogen mixtures [8,11]. BMW had tested a specially designed engine with external and internal mixture formation systems to investigate the effects of various injection strategies on hydrogen engine performance [9,12]. The test results showed that the combination of external and internal mixture formation systems allows the hydrogen engine to run quite efficiently under part-load and lean conditions. Even at full-load operation, external mixture formation as well as with direct injection (DI) could be implemented with stoichiometric mixtures. According to the BMW's operation strategy, lean mixture operation was used only for low engine loads, and stoichiometric mixture operation was adopted for higher engine loads with the use of an aftertreatment catalyst. The engine achieved 1.8 MPa of IMEP (indicated mean effective pressure) at an engine speed of 4000 rpm which was higher than the one of the base gasoline engine [9]. The research group of Argonne National Laboratory evaluated several mixture formation strategies of direct hydrogen injection to reduce NOx emission while achieving peak engine efficiencies. The group implemented engine experiments over a speed range from 1000 to 3000 rpm and a load range from 0.17 to 1.43 MPa of BMEP (brake mean

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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effective pressure) [13,14]. The results showed that the engine achieved BTE (brake thermal efficiency) above 35% at over about 80% of the tested range. The corresponding NOx map was dominated by less than 0.1 g/kWh of NOx. At 2000 rpm and 1.35 MPa of BMEP, NOx emission measured was 0.87 g/ kWh. However, NOx emission was increased at higher speeds and loads which means that further optimization of the mixture formation is required. The life cycle assessment performed by Mazda presents that a hydrogen-fueled ICE vehicle has the potential to decrease CO2 emission by 57% compared to a conventional gasoline-fueled vehicle throughout its life cycle [15]. Further research and development are necessary to improve the life cycle efficiency of hydrogen's potential as a future energy carrier and reduce its cost [16]. As mentioned above, the hydrogen ICE has been fascinating researchers and engineers due to the hydrogen's potential as a clean, efficient, and sustainable fuel for an ICE. In other words, most of the research studies have been performed so far are related to hydrogen engines for passenger vehicles with a swept volume per cylinder of 500 cm3 or lower. Recently, the importance of utilizing hydrogen as a fuel for stationary power generators is becoming more significant than ever due to the hydrogen's potential as a flexible energy carrier. However, there are only a limited number of projects aiming to develop a large-sized hydrogen engine. Olavson et al. developed a hydrogen-fueled engine, that is one the largest in the literature, with a swept volume of 6.96 L (1.74 L/ cylinder), based on the Caterpillar 3004, a four-cylinder diesel engine. They developed the hydrogen engine as an alternative for powering underground mining machinery in 1984 [17]. The hydrogen engine with a specially designed external mixture formation system successfully satisfied power and efficiency that was comparable to the conventional naturally-aspirated diesel engine. However, the peak BMEP achieved was just over 700 kPa since abnormal combustion like backfiring as well as demand for NOx emission below 0.7 g/kWh limited the BMEP of the hydrogen engine. The group of National Traffic Safety and Environment Laboratory and the Tokyo City University were involved in a national project targeting to develop a large-sized (medium-duty) truck with a multi-cylinder DI spark-ignition hydrogen engine system [18]. The engine developed in the project was based on a four-cylinder diesel engine with a swept volume of 4.73L (1.18 L/cylinder). The group adopted a combustion control strategy which could balance low NOx emission (0.7 g/kWh), IMEP of 0.85 MPa, and ITE of 41% under limited engine operating conditions. The torque of the hydrogen engine was about 20% lower than the baseline diesel engine. The deficient torque of a hydrogen engine can be improved by intake boosting if abnormal combustion like pre-ignition and knocking does not occur. Verhelst et al. attempted to increase the power output by equipping their test engine with an EGR (Exhaust Gas Recirculation) system and a supercharging set-up [19,20]. They found through the comparison between lean burn operation without aftertreatment and stoichiometric operation with EGR and aftertreatment, that lean operation with supercharging is a more useful way to obtain better efficiency and lower NOx emission. As is obvious, the lean operation needs higher supercharging pressure to keep the equivalence ratio

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low enough for avoiding abnormal combustion and unacceptable levels of NOx emission. Otherwise, lean engine operation would essentially result in deficient torque or power than what would be obtained by a stoichiometric gasoline engine operation. On the other hand, when supercharged stoichiometric mixtures with EGR are chosen, 30% higher power output compared to gasoline can be obtained with sacrificed fuel economy for the de-NOx catalyst. As mentioned above, a similar strategy adopted by the BMW group realized the world highest IMEP of the hydrogen engine [9]. A research group from the Indian Institute of Technology has been recently developing stoichiometric or over stoichiometric mixture strategies with cooled EGR, turbocharging, and deNOx catalytic aftertreatment to achieve higher torque while preventing the occurrence of abnormal combustion and high NOx generation [21]. In case of over stoichiometric conditions, unburnt hydrogen was used as a NOx reductant, and a peak torque of 180 Nm at 3600 rpm was achieved with over 800 ppm of NOx (BMEP was around 0.9 MPa). The group is currently performing further studies with different catalysts and support materials to achieve effective NOx reduction. According to the technical papers on hydrogen engines reviewed above, sufficient reduction of engine-out NOx emission at high engine loads has not been succeeded yet. Considering that most of the research and development is targeting small-sized engines for passenger vehicles, there is currently lack of technical knowledge and achievement on larger size hydrogen engines. The present study focuses on a large-sized engine for stationary power generation application. The objective of this study is to simultaneously achieve sufficiently low engine-out NOx emission without any aftertreatment, and high thermal efficiency and torque. In this study, engine experiments have been conducted on a converted single-cylinder diesel engine with a swept volume of 1.3 L/cylinder. The baseline engine was modified by changing the piston design, and the addition of a spark plug and a gas injector for direct hydrogen injection. A combination of extremely high levels of EGR and intake boosting as well as hydrogen mixture formation strategies have been applied to achieve NOx emission lower than 200 ppm without an aftertreatment system. The desired IMEP is higher than 1.35 MPa (140 Nm) at 1000 rpm which is the level obtained by the baseline diesel engine. Unlike the injection strategies previously studied for the DI hydrogen engines, the injection strategy proposed in this study attempts to generate inhomogeneity of hydrogen mixtures within the engine cylinder by setting the injection pressure at relatively low levels while injecting hydrogen through small orifices. In the last part of this paper, a special designed camshaft and a thiner engine head gasket have been applied to reproduce an Atkinson/ Miller cycle operation, aiming to obtain further torque enhancement with low NOx emission and high thermal efficiency.

Experimental set up The experiments of this study were conducted on a singlecylinder engine based on a 4-stroke diesel engine. The diesel fuel injector was replaced by a prototype gas injector

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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(HOERBIGER, HPI GEN5), driven by an electromagnetic valve. Two additional holes on the engine head were drilled to install a cold-rated spark plug and a piezo-type pressure transducer (Kistler, 6056 A). Table 1 shows the specifications of the singlecylinder test engine used in the experiments. The test engine is coupled to an AC dynamometer (HORIBA, DYNAS3 LI145), and all the engine tests were conducted at steady-state conditions. The swept volume of the engine is 1.3 L/cylinder which is over two times larger than those tested in previous research studies on hydrogen engines. The shape of the fire deck is flat, not pent roof since the base engine is a diesel engine. Meanwhile, the geometry of the piston cavity was redesigned as the original cavity geometry was reentrant shape optimally designed for diesel spray combustion. The newly designed cavity looks like a shallow dish with a reduced surface area by 26.5% compared to the original one, while its volume is only 0.4% smaller than the original one. In order to reduce the geometric compression ratio from 17.5 to 12.9, the engine head gasket was replaced with a thicker one. The camshaft and valve timings for the intake and exhaust are the same as those for the original engine except for the case attempting the Atkinson/Miller cycle described in the last part of this paper. All the engine experiments were conducted in unthrottled operation at a constant speed of 1000 rpm aiming to develop a large-sized engine for a stationary power generator that runs much slower than a small-sized engine for passenger vehicles. Slowing engine speed might bring unsavory influences like frequent occurrence of knocking on the test engine. Temperatures of the cooling water and lubricant oil are kept constant around 80 deg. C by the stand-alone cooling system for all the experiments. Fig. 1 shows the cross-section diagram of the test engine and the enlarged nozzle tip used in this study. The gas injector is installed at the center of the engine head, and the nozzle tip has 16 holes of 0.5 mm diameter. Regarding hydrogen injection strategies, Verhelst et al. mentioned in their review paper that a proper design of the mixture formation process is crucial for achieving high engine

efficiencies and meeting stringent emissions targets [7]. Nevertheless, they pointed out that direct hydrogen injection is still at a research stage. In general, a high-pressure gas injector consists of a solenoid valve and a spring that lifts up and down a needle valve to control the intermittent flow rate. Some parts without lubricant frequently slide and contact along with some injection events. Therefore, it is concerned that the sealing portions are damaged and allow some gas to leak out of the injector. It is also concerned that the sliding parts with significantly narrow clearance are stuck with contaminations. In fact, the prototype gas injector used in this study occasionally suffered from severe leakage at the leading edge of the injector. It is consequently certain that the development of a highly reliable gas injector is necessary for realizing a DI hydrogen engine to come into practical use. In addition, reducing the required gas pressure is important not only for lowering the technical barriers of the gas injector but also reducing the number of durable materials, energy consumed for gas compression, and system cost. Because of the reasons mentioned above, the pressure of the hydrogen gas was kept constant at about 5 MPa for most of the experiments. The hydrogen injection timings and periods were varied to meet the required amount of fuel injection, and to vary the uniformity of hydrogen mixtures. Fig. 2 presents a schematic diagram of the engine test apparatus. The test apparatus consists of the single-cylinder engine, a data acquisition unit, an intake gas supply system including a supercharger, and exhaust gas analyzers. The temperature and humidity of the fresh air can be controlled at 25 deg. C and 50 %RH respectively. The flow rate of the fresh air was measured using a laminar flow meter (Tsukasa Sokken, LFE-100B). In the experiments, EGR was simulated by diluting fresh air with nitrogen gas to avoid any damage of the supercharger's compressor due to condensation of water vapor existing in the exhaust gas of the hydrogen engine. As shown in Fig. 2, nitrogen gas was introduced upstream of the supercharger so that this system could work like a lowpressure EGR (LP-EGR) system. Differently from a high-

Table 1 e Specifications of the single cylinder test engine and the hydrogen injector. Engine type No. of cylinder Bore x Stroke Displacement Geometric compression ratio Number of valves Effective ratios of compression/expansion Maximum in-cylinder pressure Engine speed Intake gas boosting Exhaust gas aftertreatment Spark plug type Ignition timing H2 injection Gas injector Injection pressure Nozzle hole diameter No. of nozzle hole Injection timing Injection period

e

Water-cooled four-stroke

e mm cm3 e e e MPa rpm e e e deg.CA aTDC e MPa mm e deg.CA aTDC deg.CA

1 115  125 1298 12.9 4 Compression: 11.0 Expansion:11.8 13 1000 Supercharger Without Cold-rated plug 37 to 1.6 Prototype 3 and 5 0.5 16 138 to 68 22 to 77

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 5

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Fig. 1 e Cross-section diagram of the single-cylinder test engine and. an enlarged illustration of the injector nozzle tip.

pressure EGR system, the LP-EGR system doesn't need to raise exhaust gas pressure. Therefore, it is easy to represent extreme levels of EGR without sacrifice of engine efficiency even when high intake boost pressure is adopted. It was confirmed that some experiments with actual EGR reproduced similar results to the simulated EGR. Hydrogen gas in high-pressure cylinders was depressurized at a given pressure and supplied to the gas injector installed on the engine's head. A thermal mass flow meter was used to measure the hydrogen flow rate. The accuracy of the hydrogen flow rate measured during engine experiments will be discussed in the next section. A combustion analyzer (ONO SOKKI, DS3000) with a multichannel data acquisition unit saved and processed all the measured data on pressures, temperatures, flow rates, crank signals, etc. It is common practice in steady-state engine experiments, a number of engine cycles to be repeated until steady-state operation under a given condition is achieved. Once the engine stabilizes, the in-cylinder pressure signals for 100 engine cycles are averaged to calculate the indicated work and apparent heat release rate, as well as the in-cylinder average temperature, heat balance, and other parameters. Similarly, the engine experiments conducted in this study involve engine operation for a number of cycles under a given condition to establish steady-state operation of hydrogen combustion. However, in this study, the in-cylinder pressure signals for only 10 engine cycles were averaged to record precise combustion behaviors of hydrogen such as prognostications of pre-ignition and knocking. Therefore, the temporal profiles of in-cylinder pressure and rate of heat release shown in the latter sections look wavy and fluctuated.

Exhaust gas was continuously sampled into two lines, one for an exhaust gas analyzer (HORIBA, MEXA-ONE) to analyze CO2, CO, HC, NOx, and O2, and the other for an FT-IR gas analyzer (BEST SOKKI, BOB-2000FT) to analyze 27 constituents. The FTIR gas analyzer was coupled to a hydrogen sensor for measuring the rates of unburnt hydrogen in the exhaust gas. In addition, the hydrogen sensor was used to compensate hydrogen's interference with the measured constituents because hydrogen can influence infra-red absorbance of other constituents by Lorentz effect. Moreover, the FT-IR gas analyzer can also compensate interferences of H2O and O2.

Results and discussions Accuracy of measured hydrogen flow rate It is significantly important to precisely measure the hydrogen flow rate during engine experiments for evaluating the engine performance, especially the fuel economy. In the engine experiments conducted in this study, hydrogen pressure was set to 5 MPa by depressurizing the pressure of the hydrogen cylinders. The hydrogen flow rate is measured with a thermal mass flow meter (TOKYO KEISO, TF-6340). An accuracy of the thermal mass flow meter was double-checked with a pressure difference flow meter (ALICAT, M-500SLPM-D). Injection pressure and durations were set to the similar conditions to the engine experiments. Injection frequency was set to 8.3 Hz that is equivalent to an engine speed of 1000 rpm. It was found that the thermal mass flow meter undermeasures by 6% than the pressure difference flow meter. This discrepancy is likely

Fig. 2 e Schematic diagram of the engine test apparatus. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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to be caused by that as gas density flowing through the capillary tube in the thermal mass flow meter was lower than the one in the main tube. This discrepancy will be compensated when hydrogen flow rate is measured in engine tests.

Combustion behavior of the hydrogen-fueled engine Effect of timings of direct hydrogen injection According to several research results on hydrogen engines with external mixture formation systems, lean combustion without intake throttling is an adequate way to use hydrogen simply, cleanly, and efficiently as fuel for ICE due to its superior combustion characteristics. It is clarified that the leaner the hydrogen mixture becomes, the more NOx emission can be suppressed, even below 100 ppm (f z 0.5), and 10 ppm (f z 0.38) [11,12]. Lean combustion strategy has also been adopted to develop a hydrogen engine for a series hybrid vehicle system [22,23]. All engine operations were taking place at wide open throttle minimizing pumping losses and maximizing efficiency even when the engine was operating in onoff mode. This hybrid vehicle's hydrogen engine achieved NOx emission of 10e20 ppm for f of 0.4 meeting the Equivalent Zero Emission Vehicle (EZEV) level by implementing a homogeneous mixing of hydrogen and air. Meanwhile, the engine used in this study has an internal mixture formation system, and incomplete mixing is supposed to influence the combustion performance. Therefore, the effect of inhomogeneity of hydrogen distribution in the engine cylinder on engine performance has been investigated. As described above, unlike previous research on hydrogen engines using internal mixture formation with high-pressure (over 10 MPa) hydrogen, this study aims to slow mixture

formation by implementing a lower hydrogen pressure. For investigating the effect of inhomogeneous mixture formation on engine performance, the injection timings tinj were varied from 138 to 68 deg. CAaTDC. The injection pressure pinj and amount of the injected hydrogen were constant at 5 MPa and 109 NL/min. respectively which is equivalent to f of 0.46. Because of the low-pressure hydrogen injection through small orifices, the injection event required a significantly long injection period Dtinj of 49.2 deg. CA (8.2 ms), therefore, remarkably short time was given for the mixture formation process. Fig. 3 shows the effects of tinj on the engine performance for a constant spark timing tign of 8 deg. CAaTDC, an intake air pressure pin of 100 kPa, and engine speed of 1000 rpm. The CA10, 50, and 90 values stand for the crank angle position where 10, 50, and 90% of the cumulative heat is released. Except for the cases that tinj were 138 and 128 deg. CAaTDC, there was a slight effect of tinj on the CA50, while the time duration between CA10 and CA90 (CA10-90) was not remarkably affected. This can be confirmed by Fig. 4 showing the instantaneous in-cylinder pressure and rate of heat release for different injection timings. tinj did not influence the onsets of heat release, and the onsets of heat release did not delay from the onsets of tign. In this experiment, tinj was widely varied, but non-significant effect on combustion performance was noticed when the injection timing was set after 120 deg. CAaTDC. It was reasonable that the similar combustion phases and durations resulted in similar thermal efficiencies and IMEPs regardless of tinj, as shown in Fig. 3. It is concluded that the inhomogeneous mixture formation represented in this study gives relatively robust combustion of the hydrogen gas. On the other hand, when tinj was set before 120 deg. CAaTDC, the variation of tinj influenced the

Fig. 3 e Effect of injection timing on engine performance.

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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CAaTDC. Therefore, it is assumed that inhomogeneous mixtures existed within the engine cylinder at tign because the DI strategy was given much shorter mixing periods than external mixture formation. It is also presumed that the low pinj with slow mixing strategy adopted in this study most likely generated inhomogeneity and hydrogen-rich mixtures within the engine cylinder, which as a result caused high NOx emission formation. The next section aims to clarify the effects of inhomogeneity of mixtures on the engine performance.

Effects of inhomogeneity of mixtures on engine performance

Fig. 4 e Histories of in-cylinder pressure and rate of heat release with various injection timings.

combustion phase and NOx emission. It is presumed that incylinder flow was remaining even after the time when the intake valve closed and that the in-cylinder flow encouraged mixing of hydrogen injected before 120 deg. CAaTDC. However, in this study, NOx emission was around 500 ppm that is much higher than the previous research study using external mixture formation [11], even when tinj was set to 138 deg.

The engine experiments were conducted with two different injection pressures pinj to investigate the effects of inhomogeneity of mixtures distributing in the engine cylinder on engine performance. Similar to the previous section, an intake air pressure pin and engine speed were set to 100 kPa, and 1000 rpm, respectively. The total amount of hydrogen injected was 96 NL/min. (equivalent f is 0.4) which is the same for the two conditions, and pinj, injection duration Dtinj, and injection timing tinj are different. It was aimed that in the case of higher pinj, earlier tinj and shorter Dtinj were applied to promote the formation of a relatively homogeneous mixture. By contrast, for the case with the lower pinj, a later tinj and longer Dtinj were implemented to develop a relatively heterogeneous mixture. The spark ignition timing tign was swung to obtain the best efficiency and emissions. The experimental conditions and results are shown in Fig. 5. The temporal histories of the in-cylinder pressure and rate of heat release are shown in Fig. 6 for the cases of representative tign. The ITE in the case with the lower pinj and slower mixing was insensitive

Fig. 5 e Effect of inhomogeneous mixtures on engine performance. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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Fig. 6 e Histories of in-cylinder pressure and rate of heat release with various injection strategies.

to tign, while ITE in the case with the higher-pressure injection declined as tign was retarded. It should be noted that similar IMEP or ITE were obtained for both injection strategies by optimizing tign, while in terms of NOx emission, there are remarkable differences between the injection strategies. The higher pinj and earlier tinj were set, the lower NOx emissions became regardless of tign. It could be assumed that the heterogeneity of mixtures varied by the injection strategy has a significant effect on NOx emission. This assumption should be clarified by further research about the effects of injection strategy on engine performance, and this will be performed as future work.

Effects of hydrogen amount (equivalence ratio) on engine performance The combustion characteristics of unthrottled lean operation without EGR were investigated in this study. The equivalence ratio f of overall hydrogen/air mixtures were changed from 0.23 to 0.45 by varying only the injection duration Dtinj. The injection pressure pinj and timing tinj were set at 5 MP and 88 deg. CAaTDC, respectively. Fig. 7 presents the engine performance under three different f conditions for various spark ignition timings tign. As a matter of course, IMEP was dependent on f, and the optimum tign for maximum torque was advanced as f became smaller because CA50 was delayed

Fig. 7 e Effect of equivalence ratio on engine performance. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 5

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Fig. 8 e Histories of in-cylinder pressure, temperature, and rate of heat release with equivalence ratio.

along with the decrease in f. It is clear that the smaller f became, the longer the combustion duration between CA10 to CA90 became. The combustion behaviors with given f are shown in Fig. 8. Even when f was extremely low at 0.23, the onset of combustion did not remarkably delay from the onset of the ignition. As illustrated in CA50 and CA10-90 charts of

Fig. 7, the rate of heat release became sharp, and the combustion duration was shortened along with an increase in f. Also, the overall f is strongly related to the averaged incylinder temperatures and NOx emissions. It is clear that there is a trade-off relationship between f and NOx emission. As found in Ref. [11], extremely lean operations emitted

Fig. 9 e Effect of intake boosting on engine performance. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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increase in f regardless of pin. It is considered that the increased in-cylinder temperature and f resulted in higher NOx emission. In analogy with what's shown in Figs. 7 and 8, Fig. 10 shows that f had a great influence on combustion characteristics. It is clearly seen that the higher f became, the steeper initial rate of heat release became with shorter combustion duration. The peak of heat release increased with an increase in f. Unlike the non-boosting conditions, pre-ignition frequently occurred when tign was advanced for the cases with boosted intake pressure. In general, intake boosting can play an important role in increasing the power output of an engine. However, particularly in the case of a large-sized engine running at low engine speeds, it is likely that pre-ignition limits operative engine loads of the hydrogen engine, while a small-sized engine realized increasing the power output by means of supercharging [9,19,20].

Effects of EGR rate on the combustion performance of hydrogen engine

Fig. 10 e Histories of in-cylinder pressure and rate of heat release with changing intake boost pressures and an amount of hydrogen injected.

unburnt hydrogen and resulted in bad thermal efficiency. For lean operation, the necessity for tign advancement to obtain a high ITE led to a steep increase of NOx emission. As overall f was kept at around 0.3, a significantly low NOx and increased ITE could be achieved, while the IMEP or torque needs to be limited. The relationship among IMEP, intake boost pressure, and EGR is discussed in the later sections.

Increase in engine torque by intake boosting Intake boosting has become a common practice to improve the power density of an automobile system. Down-sized engines with an intake boosting system have already been applied to many vehicles in the worldwide market. In this section, the effects of intake boosting on the engine performance were investigated. The injection timing tinj was set at 88 deg. CAaTDC which is the same as in the previous section and EGR was not adopted. Three different equivalence ratios f were set by varying the intake pressure pin and the amount of injected hydrogen. Fig. 9 shows the results of the engine performance with various spark ignition timings tign, and Fig. 10 presents the instantaneous in-cylinder pressure, temperature, and rate of heat release. As discussed earlier, NOx emission is strongly sensitive to f and tign regardless of pin. As seen in Fig. 10, the in-cylinder temperature became higher during and after the combustion process along with an

Exhaust gas recirculation has been one of the most common techniques to reduce NOx emission of diesel engine systems and recently has been applied even in spark ignition engines. The EGR adoption in a spark ignition engine can contribute not only to the reduction of NOx emission but also the improvement of fuel economy by reducing the throttling losses [24]. Nande et al. examined the impact of external EGR on the hydrogen engine performance. They found that a reduction in combustion knock as well as NOx emission were achieved by adopting EGR [25]. They also reported that EGR had some negative impact as the combustion duration was increased by a factor of 1.5 along with an increase of the EGR level from 0 to 35%. Therefore, that the amount of EGR has to be limited to avoid an efficiency drop, which makes aftertreatment systems, like a three-way catalyst or de-NOx catalyst, necessary for efficient reduction of engine-out NOx emission [25e27]. As described in the introduction, this study aims to simultaneously achieve lower engine-out NOx emission and higher thermal efficiency and IMEP than those obtained on the baseline diesel engine. In this study, as explained in the previous section, EGR was simulated by supplying nitrogen gas to the fresh air to protect the supercharger from water condensation. Fig. 11 indicates the effect of intake oxygen concentrations (EGR levels) on the engine performance. The experimental conditions of hydrogen injection and intake gas pressures were kept constant. The fresh air was diluted with nitrogen gas, and the intake-oxygen concentrations were reduced down to 11.6 vol% which is equivalent to about 42% of EGR rate. Surprisingly, unlike the results presented in the literature [25], small differences can be seen in the incylinder combustion process and parameters like CA50. Also, even when the spark ignition timing was changed from 0 to about 15 deg. CAaTDC, similar ITE, and IMEP were obtained regardless of the intake-oxygen concentrations. Only the emission characteristics of unburnt hydrogen and NOx were affected by the EGR levels. It is evident that NOx emission could be remarkably reduced with a decrease in the intake-oxygen concentration. Fig. 12 indicates the relationship between the intake-oxygen concentration and the engine performance as tign was set to achieve optimum ITE.

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Fig. 11 e Effect of intake oxygen concentration (EGR rate) on engine performance.

The NOx formation varied in proportion to the intakeoxygen concentration, while IMEP and ITE were insensitive to the intake-oxygen concentration. Fig. 13 shows the instantaneous in-cylinder pressure and rate of heat release

for operations with various intake oxygen concentrations. Although the rise of heat release from the onset of combustion slightly declined in the case that the intake-oxygen concentration was 11.6%, the histories of heat release for

Fig. 12 e Effect of intake oxygen concentration (best efficiency). Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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Fig. 13 e Histories of in-cylinder pressure, temperature, and rate of heat release with equivalence ratio with swinging intake oxygen concentration.

the rest of the cases look similar. It can be surmised that the LP-EGR gas of the hydrogen engine included almost nitrogen and oxygen so the in-cylinder temperatures before the onsets of combustion could not be affected by the intakeoxygen concentrations or EGR levels. Also, the onset of combustion of hydrogen mixtures was barely affected by the EGR level due to the significantly lower ignition energy of hydrogen than other fuels. Unlike the literature [25e27], since this study adopts the DI strategy, heterogeneity of hydrogen mixtures is supposed to be formed within the engine cylinder. It is planned as future work to investigate and discuss how heterogeneity of hydrogen mixtures can contribute to an improvement of the hydrogen engine performance.

Increase of IMEP while keeping low NOx emission and thermal efficiency by a combination of LP-EGR and intake boosting As discussed above, it has been found that the LP-EGR can effectively reduce NOx emission with little negative impact on combustion performance of the hydrogen engine. However, since the intake-oxygen concentration needs to be lower than around 12 vol% for reducing NOx emission under 200 ppm, the lack of oxygen availability for hydrogen combustion limits the engine torque and power. In this section, intake boosting is combined with LP-EGR to increase the IMEP or torque, while NOx emission should be kept lower than 200 ppm. Under

these conditions, there was a concern about abnormal combustion occurrence because, as described earlier, high intake boost pressures frequently lead to pre-ignition. Fig. 14 presents the combustion and emission performance under various intake boost pressures pin as a function of the intake-oxygen concentrations. The amount of hydrogen could be increased as much as possible by an increase of pin up to the point where abnormal combustion such as pre-ignition and knocking was not observed. In terms of emission characteristics, Fig. 14 shows that the intake oxygen concentration had a substantial effect on NOx emission and unburnt hydrogen even when the intake boosting was applied, and that NOx emission decreased and unburnt hydrogen increased along with a decrease in the intake-oxygen concentration. In terms of the relationship between combustion characteristics with the intake boosting and the intakeoxygen concentration, it was similar to the naturallyaspirated one shown in Fig. 12. This means that the ITE or IMEP was insensitive to the intake-oxygen concentration due to that combustion phases and durations which were represented by CA50 and CA10-90 were almost independent on the intake-oxygen concentration. In addition, it was found that a decrease in the intakeoxygen concentration allowed the amount of hydrogen and IMEP to be increased without abnormal combustion even when the intake boosting technique was adopted. Moreover, it is evident that the ITE was improved with the increased intake pressure because the intake boosting lowered f and the incylinder temperatures which resulted in a decrease in cooling loss. The maximum IMEP achieved in the experiments was 1.46 MPa (135 Nm@1000 rpm) with the engine-out NOx emission to be lower than 150 ppm (ISNOx < 0.55 g/kWh) under a boost pressure of 175 kPa and intake oxygen concentration of 12.5 vol% similar to an EGR rate of 50%.

Further improvement of IMEP while keeping low NOx emission and thermal efficiency by optimizing the engine specifications This study aimed to assess whether an optimized engine geometry could improve the efficiency of the hydrogen engine while achieving near zero emissions. The recent developments on gasoline engines for passenger vehicles by Honda R&D Co. Ltd. and Toyota Motor Corporation [28,29]. Have shown that extremely high brake thermal efficiency can be achieved. Both groups optimized the expansion/compression ratio and the stroke/bore ratio, which are higher than those of conventional engines, to achieve 45% of brake thermal efficiency. The so-called Miller cycle or Atkinson cycle has a combination of low compression ratio and high expansion ratio for reducing throttling losses and increasing expansion work. If a more extended stroke piston can be adopted, the volume to surface ratio of the combustion chamber at the top dead center is increased, and this is significantly effective for improving engine's thermal efficiency. In this study, since the base engine has a large volume to surface ratio, the Atkinson/Miller cycle was realized by using a specially designed engine head gasket and camshaft. The head gasket was designed thinner than the conventional one to increase the geometrical compression/expansion ratio from 12.9 to 16.5. The camshaft was designed to retard the intake and exhaust valves closing times which reduced the

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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Fig. 14 e Combination of high amounts of the LP-EGR and intake boosting.

effective compression ratio from 10.3 to 6.5 and decreased the effective expansion ratio from 11.0 to 15.0. Fig. 15 presents the relationship among NOx emission, ITE, and IMEP. The experimental data shown in Fig. 14 are also plotted in Fig. 15. The

retarded intake valve closing time results in part of the introduced gas to be pushed back as the piston is moving towards the TDC. As a result, higher boost pressures are required to achieve similar IMEP to those of the previous

Fig. 15 e Further improvement of thermal efficiency, NOx emissions, and IMEP. Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

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studies. In the experiments, the maximum intake boost pressure required was 262 kPa, and over 200 NL/min. of hydrogen was supplied to acquire 1.64 MPa of IMEP (150 Nm@1000 rpm) with under 100 ppm of NOx (ISNOx < 0.36 g/ kWh) and over 50% of ITE. In this study, the maximum IMEP was limited by the maximum in-cylinder pressure limit of the baseline engine (Max. in-cylinder pressure < 13 MPa). The recent production engines, particularly for large-scale power generators, have much higher durability than the engine used in this study. Therefore, it is likely that much more powerful hydrogen engines could be developed in the future, by using LP-EGR and high intake boosting as proposed in this study. Furthermore, it is believed that recent works of advanced mathematical models and artificial neural network technologies will promote translating a new engine concept into practical use [30e33].

Conclusions and future plans The present study was conducted to develop a large-sized hydrogen-fueled engine for a stationary power generator. The engine experiments were performed on a singlecylinder test engine which was equipped with a gas injector for DI, a simulated low-pressure EGR system, and an intake boosting system. The aim of the study was to achieve simultaneous clean, efficient, and high-power combustion performance. The most significant findings of the study are as follows;  This study adopted an internal mixture formation with lower injection pressure and longer injection duration in comparison with the previous research on DI hydrogen engines. Such slower mixing strategy tends to give inhomogeneous mixtures within the cylinder at the time when the spark ignition starts. It is presumed that inhomogeneous mixtures as a result of the slow mixing strategy could result in robust combustion which would be independent of the injection timing. However, NOx emission measured in this study is higher than the previous research findings because of the lower injection pressure with slower mixing implemented which results in hydrogenrich mixtures within the cylinder.  Even though strong inhomogeneity of mixtures is probably a result of the low injection pressure and long injection duration, the overall equivalence ratio influences the combustion performance, and a trade-off relationship between equivalence ratio and NOx emission is observed. In this study, when the overall equivalence ratio was kept at around 0.3, low NOx and good ITE could be achieved.  Regardless of the intake boost level without EGR, the overall equivalence ratio is strongly related to the NOx emission. Intake boosting frequently results in pre-ignition when spark ignition timing is advanced. Particularly in the case of a large-sized engine running at low engine speed which is the case in this study, it is likely that abnormal combustion limits the operative engine load limits. On the hand, small-sized engines, previously studied by other researchers, can realize high power operation with intake boosting at higher engine speeds.

 According to the previous studies on a hydrogen engine with external mixture formation, negative impacts of EGR on combustion performance are concerned. Unlike the previous studies, it is newly found in this study that only slight differences in combustion performance are seen even when the EGR rate is extremely high, and that similar levels of ITE, IMEP, and CA50 are obtained regardless of the EGR rate, while an increase in EGR rate remarkably reduces NOx emission. In addition, it could be concluded that the heterogeneity of hydrogen mixtures in the cylinder brings such robust combustion which is insensitive to the EGR rate.  Intake oxygen concentration has a substantial effect on the NOx emission and suppression of abnormal combustion even when high intake boosting is adopted. The maximum IMEP achieved in these experiments is 1.46 MPa with the engine-out NOx emission to be lower than 150 ppm for an intake boost pressure of 175 kPa and intake oxygen concentration of 12.5 vol% corresponding to an EGR rate of around 50%.  For further improvement of IMEP and thermal efficiency without an increase in NOx emission, the Atkinson/Miller cycle has been attempted by retarding the intake valve closing and the exhaust valve opening time which can decrease in effective compression ratio and can increase in effective expansion ratio. The test engine used in this study finally achieved an IMEP of 1.64 MPa with under 100 ppm of NOx emission and over 50% of ITE.

Acknowledgments This work was supported by the Council for Science, Technology and Innovation (CSTI), the Cross-ministerial Strategic Innovation Promotion Program (SIP), “Energy Carriers” (Funding agency: Japan Science and Technology Agency (JST)).

references

[1] COP21, URL: http://www.cop21paris.org. [2] International Energy Agency, “Technology Roadmap Hydrogen and Fuel Cells”, URL: https://www.iea.org/ publications/freepublications/publication/ TechnologyRoadmapHydrogenandFuelCells.pdf. [3] IEA medium-term renewable energy market report. 2016. [4] Toyota Motor Corporation, URL: https://newsroom.toyota.co. jp/en/detail/4196421. [5] Escher WJD, Ecklund EE. Recent progress in the hydrogen engine. SAE; 1976. No. 760571. [6] Bechtold RL. Alternative fuels guidebook eproperties, storage, dispensing, and vehicle facility modifications. SAE, Inc.; 1997. p. 73. [7] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Prog Energy Combust Sci 2009;35:490e527. [8] Szwabowski SJ, Hashemi S, Stockhausen WF, Natkin RJ, Reams L, Kabat DM, et al. Ford hydrogen engine powered P2000 vehicle. SAE; 2002. No. 2002-01-0243. [9] Berckmϋller M, Rottengruber H, Eder A, Brehm N, Elsӓsser G, Mϋller-Alander G, et al. Potential of a charged SI-hydrogen engine. SAE; 2003. No. 2003-01-3210.

Please cite this article in press as: Tsujimura T, Suzuki Y, Development of a large-sized direct injection hydrogen engine for a stationary power generator, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.178

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 5

[10] Mazda. Overview of Mazda hydrogen vehicles, vol. 4. DOE Hydrogen and Fuel Cell Technical Advisory Committee (HTAC); 2009. https://www.hydrogen.energy.gov/pdfs/htac_ nov09_08_mazda.pdf. [11] Tang X, Kabat DM, Natkin RJ, Stockhausen WF, Heffel J. Ford P2000 hydrogen engine dynamometer development. SAE; 2002. No. 2002-01-0242. [12] Rottengruber H, Berckmϋller M, Elsӓsser G, Brehm N, Schwarz C. Direct-injection hydrogen SI-engine e operation strategy and power density potentials. SAE; 2004. No. 2004-01-2927. [13] Wallner T, Scarcelli R, Nande AM, Naber JD. Assessment of multiple injection strategies in a direct-injection hydrogen research engine. SAE; 2009. No. 2009-01-1920. [14] Matthias NS, Wallner T, Scarcelli R. A hydrogen direct injection engine concept that exceeds U.S. DOE light-duty efficiency targets. SAE; 2012. No. 2012-01-0653. [15] Nitta S, Moriguchi Y. New methodology of life cycle assessment for clean energy vehicle and new car model. SAE; 2011. No. 2011-01-0851. [16] Verhelst S. Recent progress in the use of hydrogen as a fuel for internal combustion engines. Int J Hydrogen Energy 2014;39(2):1071e85. [17] Olavson LG, Baker NR, Hynch FE, Mejia LC. Hydrogen fuel for underground mining machinery. SAE; 1984. No. 840233. [18] Kawamura A, Sato Y, Naganuma K, Yamane K, Takagi Y. Development project of a multi-cylinder DISI hydrogen ICE system for heavy duty vehicles. SAE; 2010. No. 2010-01-2175. [19] Verhelst S, Maesschalck P, Rombaut N, Sierens R. Increasing the power output of hydrogen internal combustion engines by means of supercharging and exhaust gas recirculation. Int J Hydrogen Energy 2009;34:4406e12. [20] Verhelst S, Demuynck J, Martin S, Vermeir M, Sierens R. Investigation of supercharging strategies for PFI hydrogen engines. SAE; 2010. No. 2010-01-0582. [21] Unni JK, Bhatia D, Dutta V, Mohan Das L, Jilakara S, Subash GP. Development of hydrogen fueled low NOx engine with exhaust gas recirculation and exhaust after treatment. SAE; 2017. No. 2017-26-0074.

15

[22] Smith JR, Aceves SM, Blarigan PV. Series hybrid vehicles and optimized hydrogen engine design. SAE; 1995. No. 951955. [23] Aceves SM, Smith JR. Hybrid and conventional hydrogen engine vehicles that meet EZEV emissions. SAE; 1997. No. 970290. [24] Fischer M, Kreutziger P, Sun Y, Kotrba A. “Clean EGR for gasoline engines e innovative approach to efficiency improvement and emissions reduction simultaneously”. SAE; 2017. No. 2017-01-0683. [25] Nande AM, Szwaja S, Naber JD. Impact on EGR on combustion processes in a hydrogen fuelled SI engine. SAE; 2008. No. 2008-01-1039. [26] Verhelst S, De Landtsheere J, De Smet F, Billiouw C, Trenson A, Sierens R. Effects of supercharging, EGR and variable valve timing on power and emissions of hydrogen internal combustion engines. SAE; 2008. No. 2008-01-1033. [27] Yao H, Sun B, Tian H, Luo Q, Tang H. A study of hydrogen internal combustion engine EGR system. SAE; 2014. No. 201401-1071. [28] Ikeya K, Takazawa M, Yamada T, Park S, Tagishi R. Thermal efficiency enhancement of a gasoline engine. SAE; 2015. No. 2015-01-1263. [29] Nakata K, Nogawa S, Takahashi D, Yoshihara Y, Kumagai A, Suzuki T. “Engine technologies for achieving 45% thermal efficiency of S.I. Engine. SAE; 2015. No. 2015-01-1896. [30] Cirak B, Demirtas S. An application of artificial neural network for predicting engine torque in a biodiesel engine. Am J Energy Res 2014;2(4):74e80. [31] Jafarmadar S. A comparative analysis of two neural network predictions for performance and emissions in a biodiesel fueled diesel Engine. Int J Automotive Eng 2015;5(2):999e1008. [32] Rahimi-Gorji M, Ghajar M, Kakaee A, Ganji DD. Modeling of the air conditions effects on the power and fuel consumption of the SI engine using neural networks and regression. J Braz Soc Mech Sci Eng 2017;39(2):375e84. [33] Kurtgoz Y, Karagoz M, Deniz E. Biogas engine performance estimation using ANN. Eng Sci Technol 2017;20:1563e70.

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