Impact of hydrogen direct injection on engine combustion and emissions in a GDI engine

The combustion and emission characteristics of a hydrogen engine were investigated through experimental analysis using a GDI engine. To enable hydrogen in-cylinder direct injection, a specialized hydrogen gas injector was employed. A comparative analysis of the combustion performance between gasoline and hydrogen fuels in a spark-ignited engine was conducted. Additionally, the study experimentally explored the thermal efficiency and emission reduction potential of hydrogen engines in lean combustion modes. The results indicated a significant improvement in the combustion rate when hydrogen fuel was utilized in the spark-ignited engine. However, the effective thermal efficiency was found to be lower than that of gasoline fuel due to the delayed MBF50 under stoichiometric conditions. Furthermore, when compared to gasoline fuel, the reduction of CO and THC emissions was accompanied by an increase in NOx emissions. Nevertheless, optimizing the air dilution ratio in hydrogen engines led to an improvement in the effective thermal efficiency. Specifically, under medium load conditions, a Lambda value of 2.7 resulted in an effective thermal efficiency of 43.5%. Additionally, under ultra-lean conditions (Lambda > 2.3), NOx emissions could be reduced to below 50 ppm, reaching as low as 44 ppm. This study highlights the potential of improving combustion efficiency and reducing emissions by utilizing hydrogen fuel, particularly in lean combustion modes. It contributes to the continuous development of hydrogen engine technology and promotes the implementation of cleaner and more efficient energy solutions.


Introduction
The degradation of the earth's ecological environment and global warming have compelled researchers worldwide to concentrate on discovering environmentally friendly solutions in all research fields.In the transportation sector, biodiesel, alcohols, compressed natural gas (CNG), liquefied petroleum gas (LPG), and liquefied natural gas (LNG) have gradually found application in internal combustion engines. 1,2Currently, extensive efforts are being made to develop powertrain systems with near-zero carbon dioxide emissions in vehicles.To achieve carbon neutrality targets, utilizing energy or fuel that is free from carbonaceous substances appears to be an effective approach to meet these demands.Hydrogen, as one of the most vital alternative fuels, has garnered increasing attention from governments and researchers worldwide. 3,4Hydrogen can be generated through various methods, including coal gasification, solar photovoltaics, thermal decomposition, and electrolysis of water.Due to its renewable properties, hydrogen has long been a research focal point in the field of energy utilization. 1Consequently, employing hydrogen as a fuel holds great promise as an alternative means to achieve zero carbon dioxide emissions, particularly for light and heavy-duty vehicles equipped with internal combustion engines. 5From a total cost of ownership perspective, hydrogen engines exhibit several advantages over battery electric powertrains and fuel cell electric powertrains. 6ue to its higher laminar burning velocity compared to gasoline, hydrogen offers the potential for increased thermal efficiency. 7This is attributed to the complete sharing of fuel energy during the expansion stroke, resulting in lower exhaust gas temperatures compared to gasoline. 8Additionally, hydrogen's wide flammability limit and lower ignition energy enable combustion under ultra-lean conditions in engines. 9Research on the application of hydrogen in internal combustion engines was initiated by the US Energy Agency in the 1970s, 10 and since then, numerous research institutions worldwide have conducted extensive studies on this topic. 11,12Yu et al. and other researchers have achieved reduced energy consumption and emissions in gasoline engines by introducing hydrogen into the intake air mixture. 13,14One notable example is the BMW 750HL, which features a 5.4 L 12-cylinder spark-ignited hydrogen engine.The vehicle utilizes liquid hydrogen storage, enabling a driving range of 400 km.However, the specific power output of the engine is limited due to lower volumetric efficiency resulting from hydrogen port injection. 15Moreover, the lower ignition energy of hydrogen poses risks such as charge self-ignition during the intake stroke, leading to backfire or pre-ignition when the intake valves are still open. 16This phenomenon can also contribute to combustion knocking due to increased intake temperature during backfiring. 17herefore, hydrogen direct injection holds more advantages than port injection in improving specific power.Eichlseder et al. 18 compared different fuel injection modes and found that hydrogen direct injection improved indicated mean effective pressure (IMEP) by approximately 15% compared to port injection.Homan et al. 19 investigated the effects of hydrogen injection mode on combustion process and engine performance, demonstrating that a proper match between hydrogen injection and in-cylinder flow field improved combustion stability in hydrogen engines.The transition from port fuel injection (PFI) to direct injection (DI) and optimized injection timing have significant impacts on flow requirements. 20However, to prevent backfire and surface ignition, valve timing is also restricted, necessitating negative valve overlaps to prevent hot exhaust gases from flowing back toward the intake manifold. 21In such cases, ensuring a sufficient gas flow rate becomes crucial for the DI injector to operate within the limited valve opening duration.
Due to the flow rate limitation of traditional GDI injectors, meeting the fuel supply demands of hydrogen engines becomes challenging 22 Sun et al. 23 evaluated the hydrogen spray characteristics of an outwardly opening piezo injector using the schlieren method.They found that the appropriate application of hydrogen jet in the cylinder could optimize the mixture formation in hydrogen internal combustion engines.Wabro et al. 24 also conducted experimental and numerical studies on the thermal efficiency potential of hydrogen engines with a large-flow outwardly opening injector, combined with a hybrid system.They discovered that a hydrogen engine based on in-cylinder direct injection, in conjunction with a serial-parallel hybrid system, can achieve a driving range of 619 km under NEDC conditions.However, due to the higher flame propagation speed of hydrogen mixtures, a significant amount of nitrogen oxides is easily generated within the cylinder. 16White et al. 25 observed that hydrogen engines equipped with TWC (three-way catalytic converters) can achieve ''nearly zero'' NOx emissions when the excess air coefficient is close to the stoichiometric ratio, particularly under slightly richer conditions.Exhaust gas recirculation or lean combustion combined with control parameter optimization can effectively reduce NOx emissions. 26,27ased on the above literature researches mentioned above, the thermal efficiency of hydrogen direct-injection internal combustion engines still needs to be improved, and NOx emissions remain at a relatively high level, especially under high load and stoichiometric combustion conditions.Therefore, the paper aims to investigate the combustion and emission characteristics of hydrogen internal combustion engines through experimental research, laying the foundation for the development of high thermal efficiency and low-emission hydrogen internal combustion engines.To investigate the combustion and emission characteristics of hydrogen fuel in an engine, a state-of-the-art hydrogen direct injection injector was employed to enable in-cylinder hydrogen injection.This study compared the combustion and emission characteristics between gasoline and hydrogen fuels in a spark-ignited engine.Additionally, the study explored the thermal efficiency and emission reduction potential of hydrogen engines in lean combustion modes, focusing on the lean combustion process.

Experimental engine and test fuel
In this study, a comparative test was conducted on a three-cylinder, four-stroke gasoline engine originally equipped with a 35 MPa high-pressure fuel injection system.The main technical parameters are presented in Table 1.Since the original fuel injection system was designed for liquid fuel, it was not compatible with hydrogen injection.Therefore, custom-made direct injection hydrogen injectors and a hydrogen rail were employed to fulfill the requirements of hydrogen application in the test.To ensure an ample hydrogen supply, the hydrogen pressure was maintained at a stable level of 1.6 6 0.1 MPa throughout the test.The fuel injection quantity was adjusted by controlling the pulse width of the hydrogen injectors using an engine control unit (ECU).The hydrogen used in the test was highpurity hydrogen, and its pressure was regulated through multiple stages of decompression to meet the experimental study's requirements.

Engine test bench and test equipment
An eddy current dynamometer bench was utilized for measuring torque output in this research.A piezo-electric pressure sensor (Kistler, 6115) was employed to measure the in-cylinder pressure and an angle marker was used to determine crank angle.Additionally, the crank angle and in-cylinder pressure signals were acquired and analyzed by a combustion analyzer (AVL Indicom 602).To measure the flow rate of hydrogen, a gas flow meter (RHEONIK, RHM03L) was utilized in this test.The average value of the hydrogen flow over 10 s is used as a measurement.The lambda meter (ETAS, ES630) was used as an oxygen sensor to obtain the lambda value from the exhaust gas.An AVL FTIR exhaust gas analyzer was employed for the measurement of gaseous emissions.

Test conditions and data processing and experimental method
In this study, stoichiometric mode was adopted to ensure comparability in comparing the performance of gasoline and hydrogen fuel in engine applications.Four typical engine speeds, namely 1000, 1500, 2000, and 2500 r/min, which represent a significant portion of the operating frequency, were selected under vehicle driving conditions.The engine performance was compared at BMEP = 2 bar, BMEP = 5 bar, and BMEP = 8 bar.The engine condition with a minimum specific fuel consumption of 2500 r/min and BMEP = 12 bar was chosen as a representative case to study the combustion and emission variations with Lambda, particularly in lean combustion mode.Furthermore, the thermal efficiency and emission characteristics of the engine under the same speed and small load condition of BMEP = 5 bar were compared and analyzed.In order to maintain consistent condition, the temperature of engine cooling water and oil was 88 6 2°C and 90 6 5°C, and intake air temperature was 35 6 3°C during the test.Atmospheric temperature and pressure were 25°C and 0.1 MPa.To ensure the repeatability of test results, when the engine was stabilized, consecutive 200-cycle cylinder pressure data were recorded for each operating point in the test, and the values of key combustion characteristic parameters were obtained by averaging and filtering.In this paper, COV imep (Coefficient of Variation of IMEP) was used to describe cycle-by-cycle combustion variations.The COV imep can be calculated using equations (1), (2), and (3).
Results and discussion

Comparison of gasoline and hydrogen fuel on engine combustion and emissions
As stated previously, the objective of this study was to examine the performance of a spark ignited engine fueled by hydrogen.In this investigation, a comparative analysis of combustion and emissions characteristics between gasoline and hydrogen fuels was conducted experimentally.Figure 1 illustrates the variation of cylinder pressure and heat release rate with respect to crank angle for both gasoline and hydrogen fuels.The ignition timing was adjusted based on the maximum brake torque timing (MBT) for each fuel type.The maximum cylinder pressure exhibited a significant increase with hydrogen.At BMEP values of 0.2, 0.5, and 0.8 MPa, the maximum cylinder pressure with hydrogen was enhanced by 30.5, 21.2, and 23.3%, respectively.This phenomenon can be attributed to the formation of a hydrogen-rich zone near the spark plug, which resulted in a shortened fire core formation time, accelerated flame propagation, increased isochoric combustion, and elevated maximum cylinder pressure.
The rise in heat release rate was sharp, and the duration of main combustion was considerably shorter compared to gasoline.As the load increased, along with the corresponding elevation in cylinder pressure and combustion temperature, the peak of the heat release rate gradually increased and was delayed due to the later ignition timing, as indicated in the compression pressure curve.Comparing to gasoline, hydrogen engine has a higher pressure rise rate and heat release rate, which means the combustion rate of hydrogen is much faster than that of gasoline.Figure 2 presents the combustion characteristic parameters under different engine speed and load conditions.In this section, the combustion phasing, including ignition timing, CA10, CA50, and CA90, was investigated for both gasoline and hydrogen fuels in a spark-ignition engine.In this study, CA10 was defined as the crank angle at which 10% of the total heat was released, CA90 represented the crank angle at which 90% of the total heat was released, and CA50 indicated the crank angle at which 50% of the fuel mass was burned, providing an indication of the combustion phase.The flame propagation duration, represented by the CA10-90 of MFB (Mass Fuel Burned), was analyzed to illustrate the influence of hydrogen on the burn rate.
Based on the statistical values, it is evident that the in-cylinder thermal environment improves significantly as the load increases for various engine speed conditions.A higher chemical reaction rate leads to a shorter ignition delay.However, there are substantial differences between gasoline and hydrogen.When the engine load increases from BMEP = 0.2 MPa to BMEP= 0.8 MPa, the ignition timing is retarded by 24.8, 19.9, 16.7, and 15.5°CA under different speed conditions for gasoline.On the other hand, when hydrogen fuel is used, the ignition timing is retarded by 3.3, 4.1, 5, and 7.5°CA for the same load step.The main reason is that hydrogen has faster combustion rate and is less sensitive to engine speed.Furthermore, the change in CA50 varies with the increase in speed under different load conditions.CA50 experiences significant delay due to knock limitations when the load reaches BMEP = 0.8 MPa at lower engine speeds for gasoline.However, with the increase in engine speed, the improvement in charge motion within the cylinder allows CA50 to be maintained near 8°CA ATDC.In contrast, when hydrogen fuel is used, CA50 can remain at approximately 8°CA ATDC under different load conditions at low speeds, but it is retarded at higher speeds.The primary reason for these differences lies in the varying ignitability of fuels and their sensitivity to the combustion process, primarily reflected in ignition energy and burn rate.Hydrogen, with its minimum molar mass, exhibits a strong gas diffusion ability, allowing for the formation of a homogeneous mixture under turbulent flow conditions in the cylinder.Additionally, hydrogen has a lower flashpoint and ignition energy, leading to a shorter ignition delay compared to gasoline.Moreover, hydrogen possesses a faster flame propagation speed and shorter combustion duration compared to traditional gasoline fuels.Consequently, due to these factors, the ignition timing of an engine using hydrogen fuel needs to be further retarded than that of gasoline to prevent knocking combustion.Moreover, the lower ignition energy of hydrogen makes it easier for more oil particles in the cylinder to trigger knocking combustion under higher engine speeds with faster piston movement.To further elucidate the difference in fuel economy between engines fueled with different fuels, the break thermal efficiency (BTE) was compared under various engine speed and load conditions for hydrogen and gasoline, as depicted in Figure 3. BTE is defined as the ratio of the work produced to the amount of fuel  energy supplied.From the figure, it is evident that the thermal efficiency of hydrogen engines under different speed conditions is lower than that of gasoline, and this reduction becomes more pronounced with increasing speed.The primary reason for this discrepancy lies in the shorter quenching distance of hydrogen combustion flames, which reduces the thickness of the quenching boundary layer between the high-temperature gas and the cylinder wall.As a result, heat transfer losses increase, leading to a decrease in effective thermal efficiency.Furthermore, under higher speed conditions, CA50 is excessively retarded, and a portion of the fuel's energy is expended during the expansion stroke, failing to fully utilize the working capacity of the combustible mixture.Consequently, a larger decline in thermal efficiency can be observed.Therefore, for equivalent combustion, hydrogen is not superior in terms of BTE compared to gasoline.The advantages of hydrogen are mainly lean combustion.The difference value contour map of gaseous emissions between gasoline and hydrogen at 2500 r/min is depicted in Figure 4.As indicated in the figure, the combustion process with hydrogen leads to increased NOx emissions compared to gasoline, primarily due to the higher combustion temperature, especially at higher loads.The favorable conditions for NOx generation are high temperature and oxygen enrichment.Because the combustion of hydrogen at the same equivalent ratio is much higher than that of gasoline, NOx emissions are also much greater than that of gasoline.However, CO and THC emissions are significantly lower with hydrogen compared to gasoline, as hydrogen does not introduce additional carbon atoms into the combustion process.Therefore, carbonaceous emissions generated during in-cylinder combustion are primarily attributed to the combustion of lubricating oil.Gasoline will produce a large amount of CO and THC emission due to insufficient local combustion and extinguishing effect.

Emission characteristics of hydrogen engine under lean burn mode
Due to the higher flame temperature associated with hydrogen combustion in a stoichiometric mixture, a significant amount of nitrogen oxides (NOx) is inevitably generated.Additionally, hydrogen engines are more sensitive to ''hot points'' within the cylinder, which can lead to pre-ignition and violent knocking combustion.To mitigate these issues, a lean burn mode is employed, which lowers the combustion temperature and reduces NOx emissions while achieving a milder combustion process.In this section, further investigation is conducted to elucidate the emission performance of lean burn hydrogen engines.
The combustion phasing results at an engine speed of 2500 r/min for two different loads in a lean burn hydrogen engine are illustrated in Figure 5.It highlights the differences in combustion phasing between low and medium loads under various air-fuel ratios.In the lower load range, lambda (air-fuel ratio) can be extended up to 3.0 without any concerns regarding combustion stability.With more air dilution in the cylinder, the burn rate becomes slower, resulting in a prolonged burn duration (CA10-CA90).However, at a lambda of 1.5, the shortest burn duration is observed for various excess air coefficients, regardless of the load level.In this case, the excess oxygen in the cylinder increases the microscopic collision probability between hydrogen and oxygen molecules, leading to an accelerated combustion chemical reaction rate and a shortened combustion duration.Conversely, when lambda is less than 1.5, the collision probability between hydrogen and oxygen molecules decreases due to the reduction in oxygen molecules, resulting in a prolonged burn duration.It is evident that under medium load conditions (BMEP = 1.2 MPa), the overall combustion phase is advanced, and the tendency for knocking can be effectively reduced by the dilution effect of excessive air through a higher lambda.
Figure 6 illustrates the variation of coefficient of variation (COV) with lambda for different loads when using hydrogen fuel.The figure indicates that the combustion process becomes more unstable as the air-fuel mixture is diluted, especially when lambda exceeds 2.5 for lower loads.However, for medium loads, where boost capacity is limited, the maximum achievable lambda is around 2.7, which has not yet reached the combustion instability limit.On the other hand, if the mixture becomes richer, such as lambda = 1, the combustion phasing needs to be retarded to avoid knocking, resulting in a higher COV level.Additional boost technologies, such as electronic superchargers, may be effective in extending the lean burn limit for higher loads.
Figure 7 compares the BTE with lambda for the two selected loads.It is evident that thermal efficiency can be significantly improved under lean burn conditions.Under low load conditions, when lambda exceeds 3, the potential for thermal efficiency improvement is diminished due to slower chemical reactions caused by the higher dilution of air in the cylinder.However, for medium loads, the thermal efficiency demonstrates a monotonically increasing trend with an increase in  lambda.When lambda is set at 2.7, the break thermal efficiency reaches 43.5%.
Due to the faster burn rate and higher flame temperature of hydrogen fuel, a significant amount of nitrogen oxides (NOx) is easily generated during the combustion process.The study compared gaseous emissions under different degrees of air dilution, as depicted in Figure 8.The peak value of NOx emission occurs at approximately lambda = 1.4,which differs significantly from traditional lean burn gasoline engines.Under low load conditions, when lambda exceeds 2.3, NOx emissions decrease to less than 50 ppm.Similarly, under medium load conditions, NOx emissions decrease to approximately 300 ppm at the same excess air coefficient, but are limited by the capacity of the supercharging system.The maximum lambda that can be achieved is around 2.7, resulting in NOx emissions of approximately 230 ppm.It is evident that the ultra-lean burn mode of hydrogen engines contributes to the reduction of NOx emissions but requires a more advanced supercharging system.Increasing the dilution ratio enhances the oxidation of unburnt hydrocarbons and carbon monoxide, leading to a monotonic decrease in total hydrocarbon (THC) and carbon monoxide (CO) emissions.However, lower lambda conditions do not result in significantly lower levels of unburnt matter due to the low oxygen content in the air/fuel mixture.On the other hand, higher lambda values result in a substantial reduction in unburnt matter, particularly for CO, due to the excess oxygen in the exhaust gas.Consequently, as lambda reaches 2.7 compared to stoichiometric conditions, an approximately 98% reduction in CO emissions can be achieved.

Figure 1 .
Figure 1.Effects of hydrogen fuel on combustion cylinder pressure and heat release rate (@2500 r/min).(a) Cylinder pressure.(b) Heat release rate.

Figure 5 .
Figure 5. of combustion characteristic parameters with excess air coefficient under different load conditions.

Figure 6 .
Figure 6.Variation of COV with excess air coefficient under different load conditions.