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The hydrogen-fueled Wanke rotary engine is a promising power system that has both high power and eco-friendly properties. This work investigated the effect of ignition timing on a dual-spark plugs synchronous-ignition hydrogen-fueled Wankel rotary engine under low speed, part load and lean combustion. The results show that with delaying the ignition timing, CA0-10 is shortened first and then lengthened and CA10-90 is consistently shortened. When the CA50 is located between 35 and 40°CA ATDC, the maximum brake torque can be realized. Besides, the selection of ignition timing needs to consider the "trade-off" relationship between the combustion phase and corresponding in-cylinder pressure. The maximum brake torque ignition timing is between 5 and 10°CA ATDC. And there is also a "trade-off" relationship between stability and thermal load when ignition timing is selected. In addition, HC and NO emissions will not become the problem limiting the power performance of hydrogen-fueled Wankel rotary engine under this operating condition. • A modified synchronous ignition hydrogen Wankel rotary engine was used. • The effect of ignition timing on performance of rotary engine was studied. • The power depends mainly on the in-cylinder pressure near 135°CA ATDC. • Selecting ignition timing needs to weigh the stability and thermal load. • Suitable ignition timing makes CA50 located between 35 and 40°CA ATDC. [ABSTRACT FROM AUTHOR]

Three axisymmetric 2D numerical model including a spherical and two cylindrical constant volume combustion vessel with different diameter to length ratio are constructed to explore the geometric effects and possibility of adopting cylindrical vessel in measurement of laminar burning velocity (LBV) by constant volume method (CVM). Two methods are adopted. First, the exact combustion properties from simulation are used to extract the LBV. Second, the estimated combustion properties based on the experimental techniques are applied. The results show that the combustion pressure and pressure rise rate are almost same in three different chambers before flame touching the wall, but in cylindrical vessel, flame surface area is changed by cylindrical confinement. The violation of spherical flame assumption applied in CVM results in the obvious overestimation of LBV. However, the discrepancy of LBV is not shown when the estimated combustion properties commonly adopted in practice are used, because the simplified properties do not capture the true burned mass fraction. Therefore, the pressure data before flame hitting the chamber wall in cylindrical vessel could perform same accuracy with that of spherical vessel in experiment. The flame impacting timing could be easily identified by the sudden drop of pressure rise rate. The only concern is that the final pressure used in calculating the burned mass fraction may be reduced by heat loss in cylinder vessel. It is suggested that before the pressure date in cylindrical vessel is utilized in determination of LBV, the final combustion pressure obtained in cylindrical vessel should be compared with the data in spherical vessel, at least compared with ideal constant volume equilibrium pressure to evaluate the deviation. [ABSTRACT FROM AUTHOR]

Highlights • A CFD model of H 2 -gasoline fueled rotary engine was established and validated. • The CFD model was modified by installing the compound intake ports. • The mean in-cylinder pressure and indicated thermal efficiency were increased. • Combining compound intake with HDI obtained a better engine performance. • CO emission was decreased, but NOx emissions were increased. Abstract The compound intake possesses large port area. Applying it in Wankel rotary engine (WRE) could increase volumetric efficiency and reduce pumping loss. To investigate the effect of the compound intake on the mixture formation and combustion process in gasoline WREs with hydrogen port and direct injected (HPI and HDI) enrichment, a three-dimensional computational fluid dynamics (CFD) model was established and validated. Investigation results showed that the peripheral-ported intake flow plays a leading role in the compound-ported WRE. Because the peripheral-ported intake flow has the same direction with the rotor movement, the formation time of mainstream flow field is shortened and the mean flow speed is increased, the flame propagation in the same direction with mainstream flow field is accelerated. Meanwhile, the improvements in volumetric efficiency and quality of in-charged mixtures result in the rise of temperature and pressure at spark timing, which could also improve the initial thermal conditions. Therefore, the compound intake effectively accelerates the combustion process in WRE. In HPI conditions, compared with the side-ported WRE, the peak in-cylinder pressure and indicated thermal efficiency in compound-ported WRE are respectively increased by 4.5% and 3.9%. In HDI conditions, the flame could propagate to the rear combustion chamber and eliminate the unburned zone, the maximum in-cylinder pressure and indicated thermal efficiency in side-ported WRE with HDI are 84.7% and 6.6% higher than those of in side-ported WRE with HPI. At the same time, the compound intake could further increase the in-cylinder pressure and indicated thermal efficiency by 6.8% and 2.5%, respectively. However, the improved combustion performance increases the in-cylinder temperature, which provides a suitable thermal-atmosphere for nitrogen oxide (NO x) formation. The compound intake increases mass fractions of NO x emissions from 0.111% to 0.141% in HPI conditions and those from 0.268% to 0.284% in HDI conditions. Considering the compound intake promotes the combustion characteristics and effectively decreases carbon monoxide (CO) emission from 0.043% to 0.033% in HPI conditions, and from 0.008% to 0.003% in HDI conditions, it is a feasible way to improve the performance of WREs. Especially, combining it with HDI could obtain a better engine performance under low engine speed and part load conditions. [ABSTRACT FROM AUTHOR]

Highlights • A CFD model of H 2 -gasoline fueled rotary engine was established and validated. • H 2 was direct injected in the combustion chamber under different strategies. • In-cylinder pressure increased with retarded HIT and extended HID. • The lowest CO emission was achieved in HIT of 110 °CA BTDC, HID of 40 °CA. • Higher NO x emissions were acquired in retarded HIT and extended HID. Abstract In Wankel rotary engine (WRE), high mainstream velocity in combustion chamber blocks flame propagating to the end of combustion chamber, which causes high emissions and low combustion efficiency due to unburned mixtures. To solve this problem, a three-dimensional dynamic simulation model of a hydrogen-gasoline blends fueled WRE is built and validated. The in-cylinder mixture formation and combustion process are investigated under different hydrogen injection timing (HIT) and duration (HID) conditions. The study results show that the concentration of hydrogen distributed between the spark plug region and rear combustion chamber increases with retarded HIT and extended HID. Faster flame speeds are obtained for HIT of 110 °CA BTDC and HID of 40 °CA. At a fixed HID of 20 °CA, compared with HITs of 210 and 160 °CA BTDC, the peak in-cylinder pressures for HIT of 110 °CA BTDC are increased by 14.3% and 6.8%, respectively. At a fixed HIT of 110 °CA BTDC, the peak in-cylinder pressure in HID of 40 °CA is 52.3% and 9.21% higher than HIDs of 20 and 30 °CA. The highest in-cylinder pressure is achieved with the hydrogen injection strategy that HIT of 110 °CA BTDC, HID of 40 °CA. However, as the temperature increases with pressure, nitrogen oxide emissions are also increased with retarded HIT and extended HID obviously. Considering the lowest carbon monoxide is achieved and the unburned zone in the rear region of combustion chamber is eliminated in HIT of 110 °CA BTDC, HID of 40 °CA. The hydrogen direct injection strategy that HIT of 110 °CA BTDC, HID of 40 °CA acquires the best engine performance in this research. [ABSTRACT FROM AUTHOR]

This paper aims to reveal the effect of intake closing timing, and engine speed on the flow field, flame propagation, combustion characteristics, and emissions formations of a small-scaled side ported hydrogen-fueled Wankel rotary engine. For this reason, a three-dimensional dynamic simulation model was established using a reasonable turbulent model coupled with a kinetic reaction mechanism and validated by the experimental data. Simulation results show that an earlier intake closing timing increases the volumetric efficiency and increases intake loss. The increasing speed improves the volumetric efficiency while causing a higher intake loss. There are two peaks of the turbulence kinetic energy during the intake stroke. The first one is caused by the airflow hitting the wall, and the other is due to the backflow. During the flame development period, due to the strong unidirectional flow in the combustion chamber, which is not conducive to flame propagation backward. This phenomenon is more pronounced at higher engine speeds, resulting in a merged flame front not exceeding the trailing spark plug. The lean combustion leads to a lower in-cylinder combustion temperature, deteriorating the NOx generation environment. This paper provides a feasible method for matching the operating conditions and intake system. • The shape and area of the intake port varies with the intake closing timing. • Dual-spark plug flame propagation pattern is proposed. • The leakage of gas to adjacent cylinder is reduced at 8000 rpm. [ABSTRACT FROM AUTHOR]

A two-zone quasi-dimensional model is developed and validated for predicting the performance of a hydrogen-enriched methanol engine. The fractal-based turbulent entrainment model was applied to hydrogen-enriched methanol combustion simulations with a laminar flame speed correlation of hydrogen-methanol-air mixtures. The model accuracy was evaluated under different hydrogen volume fractions, equivalence ratios, loads and speeds. Satisfying agreement between numerical and experimental results was achieved. The validated model was applied to investigate the flame propagation speed, exhaust loss and brake thermal efficiency of hydrogen-enriched methanol engines. The results showed that the hydrogen enrichment could improve the flame propagation speed, reduce the exhaust loss and enhance brake thermal efficiency. Moreover, the results suggested that, for the actual vehicular engine operation, the hydrogen addition should be coupled with the lean burn strategy under low speed and part load conditions. [ABSTRACT FROM AUTHOR]

The hydrogen-enriched gasoline engine is a promising option for future spark-ignited (SI) engines. However, the lack of quasi-dimensional combustion models for hydrogen-gasoline blends restrains the efficient prediction of its performance for varying operating conditions and hydrogen enrichment strategies. In this paper, a quasi-dimensional model was developed and validated for hydrogen-enriched gasoline engines with a new laminar flame speed expression. Correspondence between the calculated and experimentally measured engine combustion parameters was observed for different hydrogen addition levels. The comparison results indicated that the proposed quasi-dimensional model is suitable for engine performance prediction of hydrogen-enriched gasoline engines. [ABSTRACT FROM AUTHOR]

The hydrogen-fueled Wankel rotary engine with excellent power and emission characteristic is under spotlight, while the leakage is still the major problem for Wankel rotary engine, especially the leading spark plug leakage. The peak pressure is increased from 3.34 MPa to 3.52 MPa and the indicated thermal efficiency reaches maximum value of 38.29% when the moving distance of leading spark plug is −6.5 mm, and the mass of leakage fresh mixture is reduced from 0.00311 g to 0 g. When leading spark plug is moved to minor axis, the flow field structure of working chamber is enhanced. However, the peak pressure and indicated thermal efficiency decrease when the moving distance of leading spark plug exceeds −6.5 mm. The excess leakage residual gas has negative effects on combustion. The optimum moving distance of leading spark plug is −6.5 mm at 3000 rpm with λ of 1.6. • The leakage mechanisms of spark plugs are analyzed. • The influence of leading spark plug location on gas leakage is analyzed. • The influence of leading spark plug location on combustion is analyzed. • The influence of leading spark plug location on flow field is analyzed. • The optimum location of leading spark plug is given. [ABSTRACT FROM AUTHOR]

• Effect of Spark plug position on hydrogen rotary engine is investigated. • Leading spark plug can obtain higher power and low cyclic variation. • Hydrogen rotary engine's spark plug should locate the front of the minor axis. • Hydrogen rotary engine has excellent emission under lean combustion. • Best CA50 of hydrogen rotary engine locates between 35 and 40°CA ATDC. As a promising power system, hydrogen-fueled Wankel rotary engine with excellent power and emission characteristic have received attention increasingly, which makes it of great interest to investigate it. To make up for blanks, this work was conducted to experimentally investigate the effect of spark plug position on the combustion and emission characteristic of hydrogen-fueled Wankel rotary engine under 1500 r/min, 66 kPa manifold absolute pressure and 1.5 excess air ratio. The results indicate that compared to the trailing spark plug, the leading spark plug is more suitable to hydrogen-fueled Wankel rotary engines, which can realize a wider ignition range, higher maximum brake torque, lower cyclic variation and better nitric oxide emission. The maximum brake torque of adopting trailing spark plug is 31.2 N·m, which is only 87% of the maximum brake torque of 36.0 N·m with adopting leading spark plug. At the maximum brake torque ignition timing, adopting leading trailing spark plug achieves lower cyclic variation and slightly higher thermal load than adopting trailing spark plug. Besides, the optimum central heat-release point of both spark plug positions locate between 35 and 40°CA after the top dead center, and that of leading spark plug is later 2°CA than that of trailing spark plug. Moreover, both spark plug positions obtained 0 nitric oxide emission at maximum brake torque ignition timing, which means that emission will not be a problem to limit the selection of spark plug position under tested conditions. [ABSTRACT FROM AUTHOR]

As a hydrogen carrier, ammonia can suppress knock and enhance thermal efficiency of the hydrogen-fueled Wankel rotary engine (WRE), and achieve zero carbon emissions. This research established a three-dimensional fluid dynamics model coupled with detailed reaction kinetics of ammonia and hydrogen and verified it based on experiments. Incorporating 10% volume fraction of ammonia into the hydrogen-fueled WRE eliminates knock and decreases the excess air ratio (λ) from 1.8 to 1.4, effectively improving the indicated mean effective pressure (IMEP). The results indicate that when λ exceeds 1.4, flame propagation accelerates with higher concentration of the mixture. This enhances peak in-cylinder pressure and heat release rate, but it results higher NOx emissions. As λ varies from 1.8 to 1.4, NOx emission levels rise by 47.4%. At λ ≤ 1.2, the rapid flame propagation leads to short combustion duration, diminishing the power output. At this stage, the NO formation is dominated by H radicals, and the NOx production reaches its minimum value at λ of 1.0. In summary, the ammonia-hydrogen WRE achieves optimal performance at λ of 1.4 and ignition timing of −5 °CA after top dead center, the indicated thermal efficiency reaches 36.9% and the IMEP achieves 0.683 MPa. • The effect of ammonia addition on hydrogen-fueled WRE is analyzed. • The addition of ammonia is an effective way to prevent abnormal combustion. • The effects of excess air ratio on combustion and emission are analyzed. • The optimum ignition timing corresponding the optimal performance is given. • The optimum IMEP and ITE is obtained when the excess air ratio equals 1.4. [ABSTRACT FROM AUTHOR]

The present work proposes the application of CH 4 and H 2 synergy to improve the performance of the Wankel rotary engine (WRE). The whole work was experimentally conducted at 1500 r/min. The results indicate that CH 4 WRE can achieve similar power and significantly higher thermal efficiency than gasoline WRE based on quantitative control, with a maximum absolute efficiency improvement of 11.4%. At the same time, it also can achieve 64% and 77% maximum reduction of CO and NO emissions, respectively. CH 4 –H 2 synergy can further improve the performance of CH 4 -fueled WRE, which can broaden the lean flammability limits and make qualitative control application possible. Compared with CH 4 WRE with quantitative control, qualitative control hybrid fuel WRE can achieve a maximum relative improvement of 6.54% in brake thermal efficiency and significantly reduced NO and CO emissions. However, the extent of qualitative control is limited by cyclic variation. Overall, CH 4 –H 2 synergy can be a potential alternative to elevate the performance of WRE, the key to which is the reasonable match of synergy level and qualitative control extent. • Comparison of performances between CH4 and gasoline Wankel rotary engines. • CH4 has significant advantages in efficiency, CO and NO emission than gasoline. • CH4–H2 synergy can further improve the performance of the CH4 Wankel rotary engine. • Blending H2 coupling qualitative control is a good load control model in CH4 WRE. [ABSTRACT FROM AUTHOR]

To investigate the property of the promising and eco-friendly hydrogen-fueled rotary engine, the effect of excess air ratio on the combustion and emission characteristic of it was explored by experiment. The test was conducted under 1500 rpm and 5 CAD ADTC ignition timing. The test results demonstrated that with the decrease of excess air ratio from 2 to 0.85, the thermal efficiency of the hydrogen-fueled rotary engine increases first and then decreases. Besides, increasing MAP is beneficial to improve thermal efficiency. Among the tested condition, the highest brake thermal efficiency is realized when the rotary engine operates at 1.4 excess air ratio and 88 kPa MAP, about 18.34%. And the excellent HC and NO emissions can be obtained at the highest efficiency point. Besides, with the decrease of excess air ratio and the increase of load, the stability and flame development period gradually decrease. With a decreased excess air ratio, the flame propagation period decrease first and then increases, whereas work capacity and thermal efficiency increase first and then decrease. For NO emission, it will increase sharply near the equivalent ratio and gradually decrease after rich combustion. Also, according to the analytical model, it is found that the power performance of the rotary engine depends on the trade-off relationship of in-cylinder pressure and its angle of action. • A rotary engine was modified to run on hydrogen. • The effects of excess air ratio on combustion and emission were studied. • The best efficiency is obtained when the excess air ratio equals 1.4. • An analytical model is presented and the reasonability also be proved. • The emission corresponding to the best efficiency point is excellent. [ABSTRACT FROM AUTHOR]

This paper aims to numerically investigate the effects of high compression ratio (CR) on the performance of ammonia-hydrogen engines. In this work, four CRs from 10.7 to 13.7 with scanning spark timing (ST) from 28°CA to 0°CA BTDC were analyzed. The main results are as follows: As the CR increases, there is a trade-off relationship between the dissipation rate of turbulence and the turbulent kinetic energy (TKE). Initially, the TKE rises as the CR increases. As the CR continues to rise, the tendency for an increase in TKE diminishes, while the turbulent dissipation rate consistently rises. Additionally, there is an escalation in heat transfer loss. Therefore, there is a trend of rising and then falling in the flow velocity and turbulence intensity with the increase of the CR. Ammonia-hydrogen flame propagation is susceptible to temperature and flow field, and a high CR can improve ignition stability, shorten combustion duration, minimize cooling loss, and enhance output power. Unfortunately, the emission of NOx gradually rises as the CR increases. At high CR, the combustion performance is optimized by adjusting ST, and the maximum IMEP and ITE are 4 bar and 38.3 %, respectively. The ST for maximum braking torque (MBT) should be gradually delayed toward TDC as the CR increases. • As the compression ratio increases, the flow velocity first rises and then decreases. • The turbulent dissipation rate increases with increasing compression ratio. • A high compression ratio improves ignition stability and combustion velocity. • The NOx gradually increases with the increase of the compression ratio. • A high compression ratio improves the power and economy of NH3-H2 engines. [ABSTRACT FROM AUTHOR]

Hydrogen is a renewable fuel with excellent combustion characteristics. However, the direct use of hydrogen in existing engines faces obstacles such as abnormal combustion and high NOx emissions. This study proposes a strategy for controlling the combustion and emissions performance of a port-injection (PI) hydrogen internal combustion engine using ethanol direct injection (DI). The test conditions are 1000 rpm, 1500 rpm, and 2000 rpm respectively, the excess air coefficient is 1, 1.5, and 2, and 3%, 6%, and 9% ethanol is added. The results showed that the addition of ethanol can significantly reduce the pressure rise rate of the hydrogen engine and prolong CA0-10 and CA10-90. The addition of ethanol can promote the BMEP and BTE of the hydrogen engine. The addition of ethanol can reduce NOx emissions under lean burn conditions by 18%. The disadvantage is that the addition of ethanol increases the hydrocarbon emissions of hydrogen engines by about 50%, but the total amount is less. After the addition of ethanol, the flashback phenomenon of the hydrogen engine was significantly improved. [Display omitted] • Proposed ethanol-controlled hydrogen engine backfire strategy. • Studied the effect of ethanol addition on hydrogen engines. • The addition of ethanol increases CA0-10 and CA10-90. • The addition of ethanol improves engine BMEP and ITE. • The addition of ethanol reduces NOx emissions from hydrogen engines. [ABSTRACT FROM AUTHOR]

Syngas is a fuel with a huge market and low carbon emission, which has been proven to be well applied to internal combustion engines. WRE with high power density can effectively improve the power characteristic when syngas is employed as fuel in engines, however, which has not been reported publicly. This work aims to provide insights into the performance of syngas-fueled WRE under different CO/H2 ratios and thus gives theoretical guidance for its application. The test is done at 1500 r/min and part load. It is found that with the increase of the volumetric ratio of CO in syngas (CO%), the efficiency, torque and carbon emissions are deteriorated, however, the NO emission is significantly improved and the knock level is also reduced. In addition, on the premise of constant CO%, a high brake thermal efficiency, low NO emission and knock intensity can be achieved at lean combustion. In particular, when CO% is 10%, the combustion efficiency of CO is independent of the excess air ratio, while when CO% is 20%, that is decreased with increasing excess air ratio. In summary, the syngas-fueled WRE is recommended to adopt the qualitative control and the syngas with low CO%. • Insights into the performance of syngas-fueled Wankel rotary engines. • The trade-off relationship among power, efficiency and NO emission needs to be considered. • Qualitative control and low CO% are recommended in the syngas-fueled WRE. • The combustion efficiency of CO at varied λ is closely related to CO%. [ABSTRACT FROM AUTHOR]

Ammonia and hydrogen are two carbon-free alternative fuels for engines. Considering that there are few studies on improving an ammonia-hydrogen engine's lean combustion performance, this investigation based on an engine with the Miller cycle employs hydrogen direct injection (HDI) and NH 3 port injection and then proposes an adjustment strategy to vary the start of injection (SOI) of HDI for affecting the engine combustion process. When ammonia volume share (AVS) is greater than 50%, the engine shows poor performance. Nevertheless, the stratification effect of hydrogen jets may be used to elevate the hydrogen concentration around spark plug by properly delaying SOI and then creating the locally rich mixture with a high hydrogen share, which prominently promotes the flame kernel formation, shortens CA0-90, reduces C o V P m a x , and enables the engine to reach approximately 36% BTE. It is notable that very late SOI can slightly deteriorate the heat release process and lower engine performance. • Ammonia-hydrogen engines have poor combustion process under lean conditions. • A strategy to improve the lean combustion performance of the engine is proposed. • Effects of NH 3 –H 2 premixing and H 2 stratification effect on the engine are balanced. • Properly delaying SOI enhances the ignition and combustion process of the engine. [ABSTRACT FROM AUTHOR]

Variable valve timing (VVT) and Miller cycle are advanced technologies employed to optimize engine performance by improving airflow exchange, which are seldom investigated based on the direct-injection (DI) hydrogen engine. The objective of this study is to assess the effects of intake valve closing (IVC) and exhaust valve opening (EVO) timing on the gas exchange performance, combustion, and emissions of a DI hydrogen engine, after which a synergistic control strategy of IVC and EVO timing is proposed. This work is conducted under wide-open throttle and 1500 rpm. The results indicate that the synergistic control of IVC and EVO timing can increase volumetric efficiency by more than 40%, enhance gas exchange performance, shorten combustion duration, and reduce cyclic variation, resulting in approximately 43.15% brake thermal efficiency. Furthermore, brake mean effective pressure can be increased by more than 60% and NO emissions are controlled to less than 20 ppm by optimizing valve timings. • A synergistic control strategy of intake and exhaust VVT is proposed. • The airflow exchange and combustion of a DI hydrogen engine are optimized. • The regulation of valve timings can improve gas exchange performance. • The engine can reach 43.15% maximum brake thermal efficiency. [ABSTRACT FROM AUTHOR]

Driven by issues such as global warming, there is currently great interest and significance in developing zero carbon-emission internal combustion engines. Limited by structural and sealing requirements, there are no experimental studies related to combustion velocity measurement of the hydrogen-fueled Wankel rotary engine (HWRE) with excellent power performance, which is important to investigate its knock characteristic. In this work, a combustion velocity measuring method of dual spark plug HWRE is proposed, besides the obtained combustion, flow and flame velocities are correlated with the knock to explore their influence on the HWRE knock characteristics. The study found that adopting dual spark plugs in HWRE is necessary to reduce the negative impact of the flame not being able to propagate against the flow field. Besides, excess air ratio affects the combustion velocity of HWRE by changing the flame velocity, while the engine speed affects the combustion velocity of HWRE by changing the flow velocity, which basically does not affect the flame velocity. The knock level of HWRE is influenced by its flame velocity and flow velocity, both of which are positively correlated to the knock intensity and the latter has a more significant effect. • A method of measuring Wankel rotary engine combustion velocity is proposed. • Quantifying the combustion, flow and flame velocity of hydrogen Wankel rotary engine. • Investigating the influence of excess air ratio and engine speed. • Quantifying the relationship between knock intensity and above three velocities. [ABSTRACT FROM AUTHOR]

• Experiments in a modified hydrogen-fueled rotary engine are presented. • Comparatively investigating the effect of asynchronous and synchronous ignition. • Asynchronous ignition achieves higher torque and lower tendency of knock. • Leading and trailing plugs act as primary and auxiliary spark plugs respectively. • The CA50 should be located at 35 to 40°CA ATDC to obtain the highest power. This work aims to investigate experimentally the asynchronous ignition in a hydrogen-fueled Wankel rotary engine. This work was carried out at 1500 r/min and lean combustion. The experimental results showed that a suitable asynchronous ignition may realize better power performance than synchronous ignition and earlier ignition timing of the leading plug than that of the trailing plug is more preferable. When the leading plug and the trailing plug spark at 5°CA ATDC and 20°CA ATDC, respectively, the highest torque is obtained, 36.4 N·m, higher than 35.2 N·m, the highest torque when the synchronous ignition is adopted. Besides, a declined tendency of knock also can be achieved by asynchronous ignition. Compared with synchronous ignition at 35.2 N·m, a reduction of 19.5% maximum pressure rising rate can be achieved by asynchronous ignition at 36.4 N·m. In addition, for hydrogen-fueled WRE, the CA50 should be located at 35 to 40°CA ATDC to obtain the highest output torque. [ABSTRACT FROM AUTHOR]

• Combustion characteristics of NH 3 /H 2 /air with SI and TJI methods were compared. • Effect of fuel composition and equivalence ratio was evaluated under TJI conditions. • Different ignition mechanisms of NH 3 /H 2 /air were found. • Effect of orifice number on ignition and combustion characteristics was evaluated. The mixture of ammonia (NH 3) and hydrogen (H 2) is the potential alternative fuel for internal combustion engines (ICEs). The turbulent jet ignition (TJI) system can provide high ignition energy and turbulent disturbance to promote the combustion of the mixture for NH 3 /H 2 ICEs. Therefore, the combustion characteristics of NH 3 /H 2 /air under passive TJI conditions were experimentally studied in the present study. The experiment was conducted in a constant volume combustion bomb, and the effect of fuel composition and equivalence ratio was investigated. The experimental results show that compared to spark ignition, the combustion of NH 3 /H 2 can be effectively improved by using TJI, especially under high NH 3 fraction conditions. The addition of H 2 has a significant positive effect on the ignition and combustion performance. With the addition of H 2 , the ignition delay and combustion duration decrease evidently. And the H 2 addition is beneficial for improving the ignition mechanism and achieving flame ignition. In addition, poor ignition performance may occur under rich conditions due to the fluid-dynamic quenching of the jet. However, the high flame propagation rate on the rich side still leads to lower combustion duration. Moreover, increasing the orifices number appropriately can enhance the ignition and combustion performance. The jet strength is weakened by the increased total orifice area and the ignition of NH 3 /H 2 can be improved. [ABSTRACT FROM AUTHOR]

• The model of HFRE with active pre-chamber TJI was established. • The combustion characteristic of HFRE-SC was analyzed. • The optimum HIT and HID was obtained. Hydrogen can be used as fuel to replace gasoline, with the benefit of reducing harmful emissions of rotary engine (RE). The swirl chamber (SC) coupling spark plug and nozzle will achieve diffusion combustion and higher power as compared to the conventional spark plug. In the current work, a CFD model of a hydrogen-fueled rotary engine with swirl chamber (HFRE-SC) is established to study the impacts of hydrogen injection timing (HIT) and hydrogen injection duration (HID) on combustion characteristics of HFRE-SC. Results reveal that SC combustion system may achieve more combustion efficiency and higher indicated power when compared to port injection (PI). Moreover, lean hydrogen in the rear of combustion chamber (ROCC) can result from retarding HIT and extending HID. In addition, when the rich zone in SC moves toward the spark plug, making it difficult for flames to develop and spread. The best performance can be obtained when using the HIT at 75 °CA BTDC and the HID during 35 °CA, with an 8.42 % up in indicated power compared to the PI. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled Wankel rotary engine (WRE) suffers from high NOx emissions, high risk of knock and uneven thermal load. The goal of this work is to investigate the application of quantitative control and qualitative control on a hydrogen-fueled WRE and find a suitable load control strategy for hydrogen-fueled WRE to address above-mentioned problems, which is conducted at 1500r/min. The results show that the combination of qualitative control and quantitative control is an effective way to obtain the excellent performance of hydrogen-fueled WRE, which can achieve higher thermal efficiency while effectively improving some drawbacks of hydrogen-fueled WRE, such as uneven thermal load, high tendency of knock and high NOx emissions. Besides, it also can flexibly select control logic according to the priority of emissions, efficiency, stability and power. Compared to quantitative control, the combined control can realize higher brake thermal efficiency under the same brake torque, and the highest relative and absolute improvements are 38.4% and 4.75%. In addition, it also keeps the engine running in an acceptable cyclic variation range. • Effect of quantitative and qualitative control on hydrogen-fueled WRE is studied. • HWRE has low thermal efficiency, high knock risk and NOx, uneven thermal load. • Combination of two strategies can effectively improve the problem of HWRE. • Combination of two strategies allows flexible control logic based on requirements. [ABSTRACT FROM AUTHOR]

Hydrogen internal combustion engines (ICE) will play an important role in reducing carbon emissions, but low power density and abnormal combustion problems are the main obstacles restricting the promotion of hydrogen ICE. Ammonia is a low-reactivity renewable fuel. The purpose of this study is to study the effect of different ammonia-added volume fractions on hydrogen ICE. In this experimental study, the combustion and emission characteristics of an engine fueled by a hydrogen/ammonia mixture were evaluated at part-load operating conditions. The experiment was carried out on a modified engine, the engine speed was 1300 rpm, the absolute pressure of the manifold was 61 kPa, and the volume fraction of ammonia added was 5.2%, 7.96%, and 10.68%, respectively. The test results show that the addition of ammonia changes the combustion characteristics of hydrogen. As the volume fraction of ammonia added increases, the flame development period and flame propagation period are both prolonged, and the peak heat release rate decreases. The addition of ammonia increases the power of the engine and reduces the indicated thermal efficiency. At the ignition timing of the maximum braking torque, as the volume fraction of ammonia added increases, the indicated mean effective pressure and indicated thermal efficiency increase. Adding ammonia volume fraction has little effect on Nitrogen oxides (NOx) emissions, and NOx emissions gradually increase with the delay of ignition timing. • Study the effect of ammonia addition on hydrogen-fueled engines. • The addition of ammonia improves the IMEP of hydrogen-fueled engines. • The addition of ammonia reduces the heat release rate of hydrogen-fueled engines. • Ammonia addition fraction has less effect on NOx emission. [ABSTRACT FROM AUTHOR]

Hydrogen (H 2) is a carbon-free fuel with many excellent combustion characteristics, but abnormal combustion is one of the main obstacles to the promotion and application of hydrogen-fueled engines. This experimental study aims to investigate the suppression of the heat release rate (HRR) of a hydrogen-fueled engine through the addition of ammonia (NH 3). The engine was run at 1300 rpm, with manifold absolute pressure (MAP) of 61 kPa and NH 3 addition ratio of 0% and 2.2%, under lean-burn conditions. The results showed that the addition of small amounts of ammonia reduced the combustion rate of the fuel mixture, prolonged the flame development period (CA0-10) and propagation durations (CA10-90) of the engine, and reduced the peak in-cylinder pressure and peak HRR under lean-burn conditions. The addition of ammonia increased the peak indicated mean effective pressure (IMEP) and the peak indicated thermal efficiency (ITE) of the engine. The addition of ammonia resulted in increased nitrogen oxides (NOx) emissions. • The effect of ignition timing of an H 2 /NH 3 dual-injection engine was studied. • Reduces the peak heat release rate after ammonia was added. • CA0-10 and CA10-90 were extended after ammonia was added. • The peak IMEP and ITE were increased after ammonia was added. [ABSTRACT FROM AUTHOR]

The development of alternative fuels is important in the fight against climate change. Both hydrogen and ammonia are renewable energy sources and are carbon-free combustible fuels. In a recent experimental study, the performance and emission characteristics of a spark-ignition engine burning a premixed hydrogen/ammonia/air mixture were evaluated. The manifold absolute pressure was adjusted to 61 kPa and the engine speed was stabilized at 1300 rpm. The difference between a mixture with a 2.2% volume fraction of ammonia and a pure hydrogen fuel was analyzed in comparison. Specifically, the addition of ammonia increased the ignition delay and flame development periods and reduced the rate of in-cylinder pressure rise. In conjunction with the ignition timing strategy, the addition of ammonia did not affect the engine performance. Nitrogen oxides emissions are increased due to the addition of ammonia. The experimental results suggest that ammonia can be used as a combustion inhibitor, which provides a new reference for the development of hydrogen-fuelled engines. • The effect of ammonia addition on hydrogen-fueled engines was studied. • Direct injection and port injection were used for H 2 and NH 3 respectively. • Reduces the rate of pressure rise after ammonia was added. • The addition of ammonia has little effect on peak IMEP and ITE. [ABSTRACT FROM AUTHOR]

The application of hydrogen direct-injection enrichment improves the performance of gasoline Wankel rotary engine, and the hydrogen injection strategy has a significant impact on combustion, knock, and emissions. The Z160F Wankel rotary engine was used as the investigated compact engine, and the simulation model was developed using CONVERGE software. The combustion, knock and emissions characteristics of the engine were studied with the different mass flow of hydrogen injection, i.e., the trapezoid, wedge, slope, triangle and rectangle type of gas injection rate shape. In the numerical simulations, the in-cylinder pressure oscillations were monitored using monitoring points, and the knock index (KI) was used as an evaluation indicator. The study revealed that the gas injection rate shape significantly affected the mixture of hydrogen and air, thus impacting combustion, knock and emissions. When the injection rate shape was rectangle, the flame speed was faster, the peak pressure in the cylinder was higher, and the corresponding crank angle was earlier, which led to higher pressure oscillations in the cylinder and larger KI. Based on the rectangle injection rate shape, the KI decreased by 75.81%, 33.47%, 26.46% and 76.58% for trapezoid, wedge, slope, and triangle, respectively, and the indicated mean effective pressure increased by 15.68%, 5.07%, 0.56% and 14.98%, respectively. Due to the small difference in maximum temperature, which resulted in very little variation in nitrogen oxides for each injection rate shape, the total hydrocarbon emissions of the trapezoid and triangle injection rate shape was high due to the delayed combustion phase. This paper provides a solution for direct hydrogen injection to improve the combustion, knock and emissions behavior of the rotary engine. [Display omitted] • A rotary engine model with hydrogen direct-injection enrichment was established. • Five different types of hydrogen injection rate shapes were simulated. • The behavior of the combustion, knock and emissions was analyzed. • The optimal indicated thermal efficiency can be achieved by trapezoid type. [ABSTRACT FROM AUTHOR]

Recently, ammonia is considered to be an energy medium for hydrogen or energy storage and ammonia/hydrogen can be a potential fuel blend for the spark-ignition engine. Nevertheless, the combustion properties of ammonia/hydrogen mixtures could be distinctive under different initial conditions. In this study, several essential combustion properties, including laminar burning velocity, minimum ignition energy, NOx and ammonia emissions, combustion efficiency, and mixture heating values of ammonia/hydrogen/air premixed combustion were extensively studied under a wide range of equivalence ratios (ϕ), hydrogen fractions (α) and different compression ratio using one-dimensional laminar planar flame and compared with stoichiometric methane, methanol, and ethanol combustion. The behavior of NOx generation changing with ϕ and α was analyzed in detail. Results showed that most properties of ammonia/hydrogen combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions, except for the mixture heating value is slightly lower in all conditions. Besides, the NO mole fraction non-monotonically changes with ϕ and α due to the competition between the effects caused by the reduction of nitrogen-atom species and the enrichment of H/O radicals. The NO mole fraction of stoichiometric ammonia/hydrogen could be even lower than that of hydrocarbons. Considering the properties of hydrocarbon fuels as a reference, promising working conditions for ammonia/hydrogen mixtures are ϕ from 1.0 to 1.05 and α from 40% to 60%. Besides, a high compression ratio is more suitable for ammonia/hydrogen mixtures due to the excellent knock resistance ability of NH 3 /H 2 and better improvement than hydrocarbon fuels under high compression ratio. • Combustion properties of premixed NH 3 /H 2 /air were assessed under engine conditions. • One dimensional steady flame was simulated under different initial conditions. • NOx generation of NH 3 /H 2 /air combustion is analyzed in detail. • Most combustion properties, including NOx are comparable to light hydrocarbon fuels. • Potential working conditions are suggested based on the analysis. [ABSTRACT FROM AUTHOR]

CO 2 dilution is an effective strategy for combustion control. To understand the detailed dilution effects of CO 2 (including physical effects and chemical effects) on CH 4 and H 2 combustion, an open-source CFD package laminarSMOKE was utilized to simulate the transient one-dimensional outwardly spherical flame combustion process in a closed chamber under 0–15% CO 2 dilution mole fraction (α), at 293 K, 1 bar, stoichiometric ratio. The overall CO 2 dilution effects decrease the unstretched/stretched flame propagation speed, maximum combustion pressure and prolong the combustion duration. In most conditions, physical effects play a dominant role and chemical effects amplify the dilution effects. Typically, the maximum combustion pressure of CH 4 -CO 2 -air flame at α = 15% reduces 1.15 bar compared with the undiluted case, the reduction percentage caused by physical and chemical effects is 12% and 1.5%, respectively. In addition, opposite overall effects of CO 2 dilution on Markstein length (L b) of CH 4 flames and H 2 flame are performed. The physical effects of CO 2 dilution increase the L b of CH 4 flames but decrease that of H 2 flames. Chemical effects are similar to physical effects for CH 4 flames, but non-monotonic behavior is performed for H 2 flame due to the combined effects of density ratio and mixture reactivity changes. • Chemical effects of CO2 dilution on laminar spherical flame were numerically study. • Physical effects play leading roles in all changes of combustion properties. • Chemical effects reduce flame propagation speed, maximum pressure, increase combustion duration. • Chemical effects on Markstein length behave differently in CH 4 and H 2 flame. [ABSTRACT FROM AUTHOR]

Highlights • A CFD model of gasoline/H 2 fueled rotary engine was established and validated. • The ignition system consisted of leading-spark plug and trailing-spark plug. • H 2 was direct injected in the combustion chamber under varying DSP arrangements. • The preferable combustion was achieved when the DSP positioned symmetrically. • CO emission notably reduced and NO x emission slightly increased. Abstract A gasoline rotary engine enriched with directly injected hydrogen could be regarded as an appealing option to improve combustion efficiency while reducing emissions. In this paper, a three-dimensional dynamic simulation model coupling with the chemical kinetic mechanisms was established using CONVERGE software and validated by the experimental data. The numerical model was implemented for evaluating the influences of varying duel-spark plug locations on flow field distributions, combustion processes and major emissions formation of a gasoline rotary engine with hydrogen direct injection enrichment. Simulation results showed that, a mainstream flow field formed during the end period of the compression stoke whose direction was identical to the rotor rotating direction. The variation of trailing-spark plug location resulted in slight influence on the mean flow velocity as well as notable impact on hydrogen concentration and turbulent kinetic energy distribution. It was conducive to initial ignition when the local equivalence ratio was intensified near the duel-spark plug on the leading side of the combustion chamber. Higher hydrogen concentration near spark plugs could effectively improve the combustion intensity. The preferable combustion and emission performance were achieved when the duel-spark plug positioned symmetrically with respect to the minor axis. Compared with the scheme of the closest duel-spark plug, the peak pressure increased by 24.4% and corresponding crank position advanced by 9.2°CA. Despite nitric oxides emissions increased slightly, carbon monoxide emission notably reduced by 57.6% versus the scheme of the farthest duel-spark plug. When the leading-spark plug remained constant, the trailing-spark plug arranged on the major axis of cylinder wall and kept an appropriate offset from the minor axis was recommended in engineering applications. [ABSTRACT FROM AUTHOR]

Abstract The influences of wall are important in practical combustion devices. In present study, the propagating processes of near wall ignited laminar methane/hydrogen/air flame were explored under different hydrogen fractions in a constant volume combustion vessel mimicking engine geometry. Results showed that both effects of heat losses and wall compression cause difference of local flame speed at different directions. The flow inside burned zone induced by compression accelerates local flame speed at direction opposing to the wall, makes the local flame speed higher than freely propagating laminar flame speed. Meanwhile, flame shape changing process was quantified by fitted ellipses. It was found that flame shapes are strongly affected by the wall compression but not obviously influenced by hydrogen addition. Hydrogen addition exacerbated flame instabilities, notably improved the local and global flame speeds due to both increase of laminar flame speed and flow velocity inside burned zone. The maximum local speed increase from 258 cm/s for 20% hydrogen fraction to 695 cm/s for 80% hydrogen fraction. Maximum combustion pressure and maximum pressure rise rate were slightly increased by hydrogen addition. On contrary, the combustion duration notably decreased nearly 3 times when hydrogen fraction increased from 20% to 80%. Highlights • Near wall ignited lean CH 4 /H 2 /air flame were studied in a constant volume chamber. • Compression induced flow inside burned zone played a role for near wall flame. • Hydrogen addition enhances the flame propagation and compression induced flow. • Effects of heat transfer are more obvious for later combustion stage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ammonia-hydrogen fuel used in miller cycle internal combustion engine. • The stoichiometric and lean-burn conditions of the ammonia-hydrogen engine were compared. • For Miller Cycle ammonia engines, about 20% of hydrogen is required. • Expanded ammonia energy ratio can improve engine BMEP and ITE. Reducing carbon emissions in the transport sector requires technology upgrades to engines and the use of carbon–neutral fuels. Ammonia is a zero-carbon renewable fuel that complements hydrogen. This study is based on a Miller cycle spark ignition engine using ammonia-hydrogen fuel. The effects of the mixture ratio of ammonia and hydrogen under stoichiometric ratio and lean burn condition on the performance of the Miller cycle engine are focused. First, a commercial Miller-cycle ICE was modified to allow a dual fuel supply of ammonia and hydrogen. The engine was run steadily at 1500 rpm with the manifold absolute pressure (MAP) maintained at 60 kPa, and the ammonia and hydrogen supplies were adjusted to achieve different mixing ratios and excess air ratios. The test results showed that an increased ammonia percentage under stoichiometric ratio conditions significantly improved the Braking mean effective pressure (BMEP) and Braking thermal efficiency (BTE) of the ICE. Under lean burn conditions, the changes of BMEP and BTE are small with the increase of the ammonia ratio. The exhaust temperature of the ammonia-hydrogen engine is relatively low, so it is necessary to consider matching a small inertia turbocharger. Ammonia ratios above 80 % by volume resulted in erratic engine operation and even misfires. Ammonia-hydrogen fuel blends lead to higher Nitrogen oxide (NOx) emissions. [ABSTRACT FROM AUTHOR]

In this paper, a rotary engine equipped with an n-butanol and hydrogen port-injection system was developed to investigate the combustion and emissions characteristics of a hydrogen-blended n-butanol rotary engine at part load and stoichiometric conditions. A self-developed hybrid electronic control unit was adopted to adjust the injection durations of n-butanol and hydrogen. The rotary engine was run under the conditions of 4000 rpm, a manifold absolute pressure of 35 kPa and a fixed spark timing of 45 °CA before the top dead center during the whole testing operation. The hydrogen volumetric fraction in the total intake was varied from 0% to 6.30%. The test results manifested that the brake thermal efficiency and chamber temperature were simultaneously increased with hydrogen addition. The hydrogen supplement obviously shortened flame development and propagation periods. Both chamber pressure integral heat release fraction versus crank angle were increased when the hydrogen fraction was enhanced. HC emissions were reduced by 54.5% when hydrogen volume fraction was raised from 0% to 6.30%, CO and CO 2 emissions were also reduced after increasing hydrogen blending fraction. NOx emissions were mildly elevated due to the improved chamber temperature. [ABSTRACT FROM AUTHOR]

This paper aimed to study the effect of spark timing on performance of a hydrogen-gasoline dual-fuel rotary engine. For this aim, a modified rotary engine equipped with a dual-fuel port injection system was developed. An electronic management module (ECM) was specially made to command the fuel injection, excess air ratio and hydrogen volumetric fraction. In this study, the engine was operated at 4500 rpm with a manifold absolute pressure (MAP) of 35 kPa. Hydrogen volumetric percentage of total intake was kept at 0%, 3% and 6%, severally. When the hydrogen volumetric percentage was changed, the gasoline fraction was also adjusted to keep the mixture at the stoichiometric. For a specified hydrogen volumetric fraction, the ignition timing was varied from 24 to 42 °CA BTDC (before top dead center) with a fixed interval of 2 °CA. Experimental results showed that for a specific hydrogen volumetric percentage, the peak combustion pressure and chamber temperature were increased, brake thermal efficiency was first increased and then decreased with the increase of spark advance. Advancing spark timing caused the increased flame development period, the decreased flame propagation period and exhaust temperature. Cyclic variation was initially weaken and then deteriorated after raising spark advance. HC and NOx emissions were reduced after retarding spark timing. Spark timing had little effect on CO emission. [ABSTRACT FROM AUTHOR]

Because of the limit of properties of gasoline and irregular design of chamber, the pure gasoline rotary engine generally encounters partial burning, increased noxious emissions or even misfire at lean conditions. This situation could be deteriorated at idle because of the high variation in the intake charge and low combustion temperature. Hydrogen addition is proved to remit the deterioration of performance of sparked-ignited (SI) engines at idle and lean conditions. This paper conducted an experiment on a modified rotary engine equipped with gasoline and hydrogen port-injection systems to explore the performance of a hydrogen–gasoline rotary engine (HGRE) at idle and lean conditions. An electronic management unit (EMU) was invented to manage spark and fuel injection. Excess air ratio ( λ ) and hydrogen volumetric fraction in the total intake ( α H 2 ) were also governed through the EMU. For this study, the HGRE was operating at idle and α H 2 was kept at 0% and 3%, respectively. For a specific α H 2 , gasoline flow rate was reduced to make the HGRE run at desired λ . Results indicated that engine fluctuation and fuel energy flow rate were both decreased after hydrogen addition. Combustion duration was cut down and central heat release point was advanced after hydrogen addition. Peak chamber temperature ( T max ), pressure and heat release were enhanced after hydrogen blending. HC, CO and CO 2 emissions were simultaneously reduced because of hydrogen enrichment. Specifically, at λ = 1.00, HC, CO and CO 2 emissions were respectively reduced from 42,411 to 26,316 ppm, 1.86 to 0.78% and 9.96 to 8.58% when 3% hydrogen was added. [ABSTRACT FROM AUTHOR]

Because of the unusual structure chamber compared with traditional engines, gasoline rotary engine always encounters partial burning and increased noxious emissions at the idle. Hydrogen-addition could enhance the characteristics of combustion and emissions of sparked-ignited engines at idle. A modified gasoline rotary engine with electronic spark control and fuel (gasoline and hydrogen) port-injection system was developed to study the impact of hydrogen-addition on idle performance of a gasoline rotary engine. A hybrid electronic control unit was invented to manage the spark, fuel injection, hydrogen volume fraction of the total intake and overall excess air ratio. In this study, the engine was operating at the idle speed of 2400 rpm and stoichiometric conditions. The hydrogen volume fraction was gradually varied from 0% to 6.8%. Results showed that the coefficient of variation in speed and fuel energy flow rate were both decreased after the hydrogen-addition. Flame development and propagation periods were shortened owning to hydrogen-addition. The peak chamber temperature was enhanced after the hydrogen-addition due to the high adiabatic flame temperature of hydrogen. Cooling loss was dropped when hydrogen was added into gasoline. HC, CO and CO 2 emissions were reduced by 79.4%, 86.0% and 25.9% when hydrogen volume fraction were raised from 0% to 6.8%. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled Wankel rotary engine (HWRE), as a high power density and eco-friendly internal combustion engine, has the potential to become an alternative for gasoline-fueled piston engines. Cooled EGR, as an effective means of improving engine performance, is less studied based on HWRE. However, due to the different operating way and structure, the flame development and propagation of WRE are significantly different from those of the piston engine, so may the effect of cooled EGR. Hence, the goal of present work is to analyze the effect of cooled EGR on the combustion characteristics of HWRE. This work is conducted under 1500 r/min and wide-open throttle conditions. The results show that when the ignition timing and excess air ratio are fixed at 5°CA ATDC and 1, the cooled EGR level has a significant influence on the combustion process and operating stability. In addition, when maximum brake torque CA50 is employed, within test range, whether stoichiometric or lean combustion, both the brake torque and brake thermal efficiency are monotonous to the cooled EGR level. And cooled EGR can achieve high brake thermal efficiency compared with lean combustion at the same brake torque. Compared with the hydrogen-fueled piston engine, HWRE allows for a higher cooled EGR level whether in terms of efficiency or power output considerations. In general, the cooled EGR can be used as an excellent load control means to achieve high efficiency of HWRE. • The effect of cooled EGR on combustion in hydrogen Wankel rotary engine is analyzed. • Cooled EGR has significant impacts on the combustion characteristic of hydrogen WRE. • Cooled EGR can be employed as an excellent means of load control of hydrogen WRE. • Cooled EGR is more suitable for hydrogen WRE than the hydrogen piston engine. [ABSTRACT FROM AUTHOR]

• Two calculation methods for the leakage area of spark plug cavities are given. • The leakage gas mass of three types apex seal leakage is calculated. • The effects of three types apex seal leakage on engine performance are compared. • Measures to reduce apex seal leakage are given. • The effects of the movement of leading spark plug on leakage are calculated. Under the background of carbon peaking and carbon–neutral, the hydrogen-fueled Wankel rotary engine with excellent power and emission characteristic is under the spotlight. The leakage problem, especially the apex seal leakage, is considered to be an important engineering problem restricting the development of hydrogen-fueled Wankel rotary engine. To make up for research gaps, this study was conducted to investigate the effects of three types of apex seal leakage on engine performance. Two methods for calculating the leakage area of spark plug cavities are proposed and compared, and the definition method based on leakage direction is more consistent with the experimental results. The leading spark plug leakage and corner seal leakage has the greatest impact on peak pressure and IMEP, which reduce the peak value of pressure by 0.249 MPa and 0.047 MPa, and reduce the IMEP by 8.68 % and 10.51 % respectively. In addition to reducing the leakage area, reducing the distance between leading spark plug and minor axis by 5.5 mm could increase the peak pressure by 0.2 MPa and reduce the leakage from the working chamber to the adjacent chambers from 0.055 g to 0.038 g. [ABSTRACT FROM AUTHOR]

• Hydrogen Wankel rotary engine and reciprocating piston engine are compared. • This work studies the comparison from combustion, emission and abnormal combustion. • HWRE can achieve excellent power, but with low efficiency and poor NO emission. • Backfire won't prevent the application of HWRE, but knock will. • Efficiency, thermal load and knock need to be improved in the development of HWRE. Hydrogen-fueled Wankel rotary engine (HWRE) as an excellent power device deserves more in-depth study. The goal of this work is to provide a more comprehensive understanding of the pros and cons of HWRE and what should be done when developing a hydrogen-specific WRE by comparing HWRE with hydrogen-fueled reciprocating piston engine (HRPE), as well as some gasoline-fueled conditions, in terms of combustion, emissions and abnormal combustion. The results show that HWRE can achieve higher power per displacement compared to the RPE fueled by whether gasoline or hydrogen, as well as slightly low brake thermal efficiency and extremely poor NO emission. At 2500 r/min, the maximum power per displacement of HWRE is 1.66 and 1.23 times that of hydrogen and gasoline RPE, respectively, however, accompanying absolute reductions of 4.96% and 3.06% in maximum brake thermal efficiency. In addition, the characteristics and mechanisms of abnormal combustion in HWREs are different compared to HRPEs. In particular, in HWRE, the backfire can be eliminated by the improvement of the spark plug hole, while the knock problem is more prominent. Overall, HWREs have the potential to win a place in the future of zero-carbon engines, however, some works, such as improving thermal efficiency, reducing thermal load and preventing knock, need to be done for the development of hydrogen-specific WRE. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a laminar burning velocity correlation was proposed for combustion simulation of hydrogen-enriched ethanol engines. This correlation was developed based on the flame–temperature-based mixing rule. Wide ranges of hydrogen mole fraction, equivalence ratio, unburned gas temperature, pressure, and residual gas mass fraction were simultaneously considered in the correlation to cover the burning conditions encountered in the spark-ignited (SI) engine combustion. Computational fluid dynamics (CFD) calculations were performed with the implementation of this correlation. This correlation was confirmed to be suitable for engine simulation since the CFD model could well capture the combustion behavior in hydrogen-enriched ethanol engines. [Copyright &y& Elsevier]

[Display omitted] • Five machine learning models are used to predict combustion phases of Wankel engine. • The 5-fold cross-validation and Bayesian optimization are used to optimize the model. • The Bayesian optimization algorithm can improve the regression ability. • The Gaussian process regression model shows the best regression ability. Combustion phases, such as the development period (CA0-10) and flame propagation period (CA10-90), are the critical parameters for hydrogen-enriched Wankel rotary engines. An accurate simulation model and a suitable engine management system are required to control combustion phases. In this paper, five machine learning (ML) models, including the linear regression (LR), regression tree (TR), ensembles of trees (EnTR), support vector machine (SVM), and Gaussian process regression (GPR), are initially applied to predict combustion phases. Experiments were performed with variations of the main fuel types (gasoline and n-butanol), loads (idle and part load), ignition timing, hydrogen volume fraction, and excess air ratio. The sample data were divided into training and testing data set, and the normalization method, 5-fold cross-validation, and Bayesian optimization algorithm were used for data processing and model optimization. Among five ML models, the training speed of the LR model was the fastest; the generalization ability of the TR model was the worst. The minimum leaf size of the TR model significantly influenced regression and generalization ability. On this basis, the EnTR model improved the regression ability, but required more training time. The GPR model showed the best generalization ability among the above model, while SVM performed well in a certain data set. For CA0-10, the coefficient of determination ( R 2) of the best LR, TR, EnTR, SVM and GPR models was 0.9910, 0.9912, 0.9985, 0.9984 and 0.9994, respectively; for CA10-90, the R 2 was 0.9348, 0.8974, 0.9873, 0.9916 and 0.9975, respectively. It is highly recommended to apply the GPR model to the combustion phases prediction and control system modeling. [ABSTRACT FROM AUTHOR]

Abstract: This paper numerically investigated the combustion process in a spark-ignited (SI) engine fueled with hydrogen–gasoline blends through the computational fluid dynamics (CFD) calculation. Satisfying agreement between the calculated and measured in-cylinder pressure traces was observed for all test cases. The calculation results demonstrated that, the hydrogen addition availed the increased flame propagation speed and the enhanced degree of flame wrinkling. Compared with the original engine, the peak flame propagation speed was raised by 37.18% and 60.47% at the hydrogen volume fractions of 3% and 6%, respectively. The peak average flame surface densities of the 3% and 6% hydrogen-enriched engines were 28.88% and 35.99% higher than that of the original engine, respectively. [Copyright &y& Elsevier]

• Experimental and numerical study of stoichiometric and lean combustion is conducted. • High-temperature combustion and its duration have crucial effects on burning rates. • The combustion initiation and combustion duration are shortened by H2 enrichment. • H2 addition with lean operation shows higher thermal efficiency and lower emissions. • There is a close correlation between early flame growth and combustion stability. Based on a retrofitted Wankel engine, fundamental experiments and numerical simulations were conducted to investigate the role of the hydrogen addition under stoichiometric and lean combustion conditions. The test-bench recording of in-cylinder pressure, heat release, cycle-to-cycle fluctuation, and exhaust emissions were used to characterize fuel combustion. The verified CFD model clarified the inherent mechanisms behind experimental data. Results showed that the introduction of hydrogen addition caused combustion temperature to increase, the timing taken for active radical formations was significantly advanced, and the peak concentrations increased. This contributed to the increased pressure and the shortened combustion phasing in experiments. There was a proportional correspondence of the early flame growth and cyclic variation. Hydrogen addition has a minimal effect on the turbulent intensity after spark timing. However, adopting a highly diluted mixture caused the enhanced turbulent intensity, which was responsible for improving the stability of the engine. Due to the synergistic effects of the work delivery, cyclic fluctuation, and exhaust temperature, a larger hydrogen addition with stoichiometric operation showed lower higher brake thermal efficiency. Nitrogen oxide emissions increased with hydrogen enrichment and reduced with a higher degree of mixture dilution owing to a close positive correlation between the mass elevated-temperature and NO x formation. Benefits from sufficient oxygen concentration and elevated combustion temperature, the chain transfer process can be accelerated. This explained why CO emissions decreased at a larger hydrogen-enriched and leaner condition in the experimental investigation. Increasing the hydrogen content provided improvement in the desired HC emissions for all operating points. [ABSTRACT FROM AUTHOR]

• A modified n-butanol-fueled rotary engine was be used to experiment. • The effects of idle speed reduction and hydrogen enrichment were studied. • Idle speed reduction is an effective way to improve the economy of the engine. • Hydrogen-enriched can improve combustion and emission performance. • High blending hydrogen level can offset the negative effect of idle speed reduction. This work aims to improve the idle performance of the n-butanol rotary engine by blending hydrogen and reducing idle speed. Hydrogen volume percentage (β H2) changes from 0 to 7.94% and the idle speed decreases from 2600 to 2400 rpm. The test results show that the engine can achieve better stability and economy due to the coupling effect of mixing hydrogen and reducing the idling. The total fuel flow rate reduces from 24.57 MJ/h at 2600 rpm when pure n-butanol is fueled to 19.35 MJ/h at 2400 rpm when β H2 equals to 7.94%. The period of flame development and propagation are observably decreased by blending hydrogen even if reducing idle speed has a few negative effects. Hydrogen enrichment is an effective way to reduce the emissions of CO and HC. Besides, the negative effects of reducing idle speed on HC and CO emissions are decreased to a negligible level when β H2 passes about 7%. Besides, NOx emission is maintained at an extremely low value in the testing range. Also, enrichment hydrogen can compensate for the negative effects on combustion and emissions of reducing idle speed. [ABSTRACT FROM AUTHOR]

• Hydrogen two-stage direct-injection is implemented on a gasoline SI rotary engine. • Injecting proper hydrogen in the second pulse after spark-ignition is investigated. • Two-stage injection strategies can realize desirable stratified conditions of hydrogen. • High thermal efficiency and good knock resistance are the benefits of the HTDI. • The HTDI allows significant CO and THC reduction with a slight NOx penalty rate. Hydrogen two-stage direct-injection enrichment is a novel injection strategy to utilize hydrogen more efficiently and effectively in gasoline rotary engines by inheriting the merits of hydrogen and direct injection simultaneously, such as flexible control, efficiency improvement, and emissions reduction. Based on the CONVERGE code with detailed chemistry solvers, a full-cycle CFD modeling including hydrogen jet-flow and combustion processes was presented and validated by experimental data. To understand the role of hydrogen two-stage injection in improving engine performance at part-load and lean-burn regime, six different injection arrangements for which two-stage injection strategies with variable hydrogen amount for each pulse (up to 30% for post-injection) had considered. The different effects on species evolution, combustion characteristics, knock propensity, and emissions formation were analyzed step-by-step. The simulation results showed that compared with direct-injected hydrogen for single-pulse, injecting adequate hydrogen in the second pulse after spark-ignition onwards performed a significant beneficial effect on the mixture stratification and flame propagation, especially for the trailing part of the rotor chamber, which contributed to the improvement in both combustion characteristics and thermal efficiency. The assessment of knock propensity demonstrated that the two-stage direct-injected hydrogen had the potential of mitigating the knock under lean operations. Using split injection accompanied by an optimized hydrogen allocation strategy allowed substantial unburned hydrocarbon and carbon monoxide reductions with a slight nitrogen oxide penalty rate due to the elevated combustion temperature. [ABSTRACT FROM AUTHOR]

To achieve more efficient and robust combustion, split direct-injected hydrogen as a new injection strategy for rotary engines was used to clarify the influences of various injection parameters, including hydrogen amount for each pulse, the dwell between pulses, and secondary injection pulse-width, on combustion performance and emissions formation by CFD modeling. The results of this investigation confirmed the benefit of split direct-injection as a useful means to govern mixture stratification which enabled the combustion more effective and rapid compared to single-injection mode. Enhanced combustion could be achieved by increasing the mass fraction of post-injection with an optimized dwell between injections. Further investigation on secondary injection pulse-width demonstrated that engine performance was substantially improved with a wider second injection pulse. From one side, as the secondary injection pulse-width was increased, the surrounding mixture distribution of the spark-plug location was richer, which was confirmed as a beneficial inhomogeneous circumstance of hydrogen to accelerate the burning rate. From another side, strong tumble level and turbulence intensity were of significant importance for facilitating the combustion process and getting optimal thermal efficiency. For emissions formation, split direct-injected hydrogen made a near-zero HC and CO emissions at the price of a comparable increase of NOx emissions. • Hydrogen split direct-injection is implemented on a gasoline SI Wankel engine. • Analyzing three factors for amelioration including first, post-injection, and dwell. • Split injection strategies can realize desirable stratified conditions of hydrogen. • Injecting proper hydrogen in each pulse with an adequate dwell is of importance. • Thermal efficiency can be notably improved benefits from the wide second injection. [ABSTRACT FROM AUTHOR]

• A thorough 3D CFD model of the H 2 -gasoline Wankel engine is built up and validated. • Burning-source connection of the twin-plug has a negative influence on flame spread. • Combustion is enhanced in the L-side of the rotor chamber by increased spark-energy. • The improvement of ignitability was gained in the H 2 -enriched ignition environment. • Maximum mass fraction of the temperature region is closely related to NO x formation. Due to the elongated shape of the combustion chamber and strong one-way flow characteristic, changing the location of enhanced combustion is an effective means to ameliorate the indicated efficiency of the Wankel engine. For this purpose, strengthening combustion can happen by allocating spark-energy in the desired location along with a favorable ignition environment. In this study, three-dimensional CFD simulations were implemented and the numerical results were compared with existing measured data, in which ignition environment variations were achieved by hydrogen enrichment within the intake manifold of a spark-ignition Wankel engine. Based on CONVERGE code with the SAGE detailed chemistry solver, the twin-plug engine model was then used to assess the location of strengthened combustion employing a spark-energy allocation from the perspective of species evolution, combustion characteristics, and emissions formation. The results indicate that enhancing spark-energy at the leading side of the rotor chamber has an overall higher burned volume rate and faster species evolution due to the later connection of two burning sources prior to quenching. Increasing the hydrogen content in the ignition environment causes richer active radicals and higher turbulence, ultimately promoting flame propagation and combustion intensity. A larger T-plug spark-energy decreases the indicated mean effective pressure, heat release rate, and thermal efficiency because of the intensified wall heat flux and the lower mass burning rate at the leading side. There is no doubt that the introduction of hydrogen enrichment presents a positive effect on both thermal efficiency and specific fuel consumption at the price of a slighter increase of nitrogen oxide formation due to higher mass fraction of high-temperature regions. It is recommended that enhancing local mass burning rate in the leading part of the rotor chamber under the hydrogen-enriched ignition environment can provide a great potential in the quest towards high-efficiency Wankel rotary engine. [ABSTRACT FROM AUTHOR]

• Implementation of oxygen enriched air on rotary engine combustion was performed. • Thorough 3D CFD models of rotary engines was built and verified by experiment data. • Increasing oxygen content stimulated the growth of initial flame and burned volume. • Oxygen enrichment markedly reduced the formations of CO, HC, and soot emissions. • The optimum combustion behavior can be achieved by α O2 = 30% and λ = 1.1 operation. The attractiveness of oxygen-enriched combustion advantages are making a comeback in the implementation of the rotary engine concept. In this connection, a numerical investigation on influences of intake oxygen enrichment combined with excess air ratio on combustion behavior of the rotary engine needs to be clarified. A three-dimensional computational fluid dynamics simulations coupling with chemical kinetic mechanisms was performed in this study, and the numerical results had been validated with existing experimental data. The species evolution, combustion characteristics, and pollutant formation were monitored to assess the oxygen-enriched combustion of the rotary engine. Results showed that there was a significant influence of intake oxygen enrichment for improving flame development and burned volume due to the reasonable species concentration and intense turbulence intensity. Increasing oxygen enrichment led to the enhanced peak pressure and advanced corresponding position, resulted in a rapid combustion period for improving heat-release efficiency and combustion efficiency, meanwhile made a higher indicated mean effective pressure at the price of a smaller increment of nitrogen oxide formation. The substantial reductions of carbon monoxide, unburned hydrocarbon, and soot were recorded, which is attributed to the higher-oxygen mixture with suitable lean operations. Considering combustion characteristics and emissions performance, the operation of intake oxygen volume of 30% accompanying the excess air ratio of 1.1 is the best choice for the engine used in the current study. [ABSTRACT FROM AUTHOR]

In this study, the laminar premixed CH 4 /H 2 /air combustion was investigated by the global reaction pathway (GP) analysis and sensitivity analysis of physical parameters for individual species under various hydrogen fractions (α) and pressures to further understand the blended fuel combustion and explore two questions: (1) whether extra GPs exist in the blended fuel combustion beyond the pure fuel reacting system; (2) whether there are insignificant species in pure CH 4 and H 2 combustion showing large influence in binary fuel combustion. Results show that different GPs dominate fuel oxidation under different α. The increasing radicals are responsible for the linearly increasing laminar burning velocity (LBV) at small α , and the transition from CH 4 chemistry to H 2 chemistry is responsible for the nonlinearly increasing LBV at large α. Extra GPs are shown and play a role in the binary fuel combustion, but they are still within the CH 4 chemistry. For most species, the sensitivity of transport and thermal parameters to laminar burning velocity were found has a positive relationship with their maximum mole fraction at different α. No species were found only important to the binary fuel, however, some species such as H 2 O show higher sensitivity for binary fuel combustion than that of pure fuel combustion. The changes of GPs with increasing pressure verifies that CH 4 chemistry is dominant for small α (20%) combustion while H 2 chemistry dominant is for large α (80%) combustion. Most physical parameter sensitivity will increase with increasing combustion pressure, which should be paid more attention in high-pressure combustion. [ABSTRACT FROM AUTHOR]

• A simulation model of the hydrogen-enriched Wankel engine was built and validated. • Single-spark ignition strategies consist of split ignition, high-energy ignition. • Twin-spark ignition strategies cover asynchronous ignition and energy allocation. • The preferable arrangement of the twin spark plug was T/L = 1.5. • Optimal combustion was gained as the leading plug was ignited earlier and stronger. For the hydrogen-enriched spark-ignited Wankel engine, the optimization of ignition strategy is conducive to improve combustion performance and specifically effective to lessen the unburned region due to the elongated rotor chamber. In this paper, the role of the number of the ignition source, twin-spark plug location, asynchronous ignition, and energy allocation in improving lean combustion was investigated through the three-dimensional computational fluid dynamics model coupling with kinetic mechanisms. The model was validated by experiment, and good agreements between measured and predicted combustion pressure and the heat release rate was obtained. Results showed that the improvements of engine combustion were limited by single-spark ignition strategies, and the twin-spark ignition configuration was capable of enhancing combustion efficiency drastically. The arrangement of the twin-spark plug determined the space for flame development, and it was favorable for the trailing plug to stand a greater offset from the minor axis of the engine. An earlier leading-spark ignition enabled flame propagation faster and occurred quenching rapidly, which contributed to higher pressure-output and better heat-release. The higher energy of leading-spark ignition made the mixture consumption faster, combustion pressure higher, and combustion duration shorter. The optimum strategy on combustion was expressed as follows: the location of trailing-spark plug is offset from the minor axis by 20.7 mm; the spark timing and discharge energy of leading-spark plug is 325°EA and 0.03 J, respectively; and those of trailing-spark plug is 335°EA and 0.01 J. It was recommended that the leading-spark ignition was set earlier and stronger for practical operations. [ABSTRACT FROM AUTHOR]