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Under the background of carbon neutral, hydrogen-fueled Wankel rotary engine (WRE) with high power density and compact layout has the potential to become a substitute for current fossil engines. Ammonia as a carbon-free renewable energy is considered alternative energy used in vehicles, however, its application on WRE is lacking. Therefore, the present work aims to investigate the performance of ammonia-enriched hydrogen-fueled WRE to analyze the feasibility of ammonia applied in hydrogen-fueled WRE. The main conclusions are as follows: Under test conditions, the maximum brake torque and thermal efficiency are obtained at about 30% and 40% volume fraction of ammonia in fuel, and compared with pure hydrogen conditions, achieving relative improvements of 21% and 29%, respectively. Slightly rich combustion is recommended to increase power and decrease NO emissions with an acceptable economic loss. In addition, ammonia enrichment can prevent the occurrence of abnormal combustion. [Display omitted] • Performance analysis of NH3-enriched hydrogen-fueled Wankel rotary engine. • NH3 enrichment can improve dynamics and economy with penalties for NO emission. • Slightly rich combustion is recommended for NH3-enriched hydrogen-fueled WRE. • NH3 enrichment is an effective way to prevent abnormal combustion. [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-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]

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]

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]

• The application of O 2 enrichment in a hydrogen Wankel rotary engine is assessed. • O 2 enrichment can improve power output with penalties in efficiency and emission. • O 2 -enriched level can effectively accelerate the heat release process. • O 2 enrichment will lead to knock, backfire and pre-ignition. • Low O 2 -enriched levels has higher application value than high O 2 -enriched levels. Hydrogen-fueled Wankel rotary engines (WREs) with excellent dynamic and emission characteristics have the potential to become a substitute for current fossil-fueled reciprocating piston engines. Oxygen enrichment can be employed to solve the lower power of hydrogen-fueled internal combustion engines, however, it has not been applied in hydrogen-fueled WRE at present. Hence, the goal of this work is to experimentally assess the application of oxygen enrichment in the hydrogen-fueled WRE. This work is conducted at 1500 r/min, wide-open throttle and 2.0 excess air ratio. The work found that within the test range, oxygen enrichment can effectively improve the brake torque of hydrogen-fueled WRE, however, penalize the brake thermal efficiency and NO emission. When the oxygen fraction in air increases from 21% to 29.1%, the brake torque is increased by about 45% while the brake thermal efficiency is lowered by about 8.5% and NO emission deteriorates by three orders of magnitude. Oxygen enrichment can accelerate flame development and propagation process. Besides, as the oxygen-enriched level is increased, abnormal combustion, including knock, backfire and pre-ignition, began to occur and affected the normal operation of hydrogen-fueled WRE. In general, compared with the high oxygen-enriched level, which has severe penalties on performance except for power output, low oxygen-enriched level has higher application potential. [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]

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-fueled Wankel rotary engine, with few available research currently, has excellent power and emission characteristics, however, with lower efficiency. With increasing attention to low-carbon emission, it is of great significance to explore methods to improve the efficiency of hydrogen-fueled Wankel rotary engines. This work aims to study the effect of qualitative control coupling variable engine speeds at the wide-open throttle on the power control. The comparative effect of qualitative control coupling engine speeds from 1000 r/min to 1500 r/min under the wide-open throttle and quantitative control at 1500 r/min on the combustion and emission characteristic of hydrogen-fueled Wankel rotary engine is investigated. The results show that compared with quantitative control, qualitative control coupling variable engine speed can achieve excellent performance. The brake thermal efficiency can be maximally increased by 43.5%, an absolute increase of 6.22%, as well the volumetric efficiency with a maximal 105% improvement. The thermal load and risk of knock can be greatly reduced. Moreover, NO emission also can be reduced by more than an order of magnitude or even by zero. Although there is an increase in cyclic variation, the value is no more than 4%. In addition, qualitative control coupling variable engine speed allows flexible matching of appropriate engine speed and excess air ratio based on the actual requirements of efficiency, stability, durability and emission. • The power control method of the hydrogen Wankel rotary engine is investigated. • Qualitative control achieves better engine performance than quantitative control. • Qualitative control coupling engine speed meets the power and high-efficiency demand. • Lower thermal load, knock tendency and NO emission can be obtained. • Much high excess air ratio isn't recommended due to high cyclic variation. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled Wankel rotary engine, with few available research currently, has excellent power and emission characteristics, however, with lower efficiency. With increasing attention to low-carbon emission, it is of great significance to explore methods to improve the efficiency of hydrogen-fueled Wankel rotary engines. This work aims to study the effect of qualitative control coupling variable engine speeds at the wide-open throttle on the power control. The comparative effect of qualitative control coupling engine speeds from 1000 r/min to 1500 r/min under the wide-open throttle and quantitative control at 1500 r/min on the combustion and emission characteristic of hydrogen-fueled Wankel rotary engine is investigated. The results show that compared with quantitative control, qualitative control coupling variable engine speed can achieve excellent performance. The brake thermal efficiency can be maximally increased by 43.5%, an absolute increase of 6.22%, as well the volumetric efficiency with a maximal 105% improvement. The thermal load and risk of knock can be greatly reduced. Moreover, NO emission also can be reduced by more than an order of magnitude or even by zero. Although there is an increase in cyclic variation, the value is no more than 4%. In addition, qualitative control coupling variable engine speed allows flexible matching of appropriate engine speed and excess air ratio based on the actual requirements of efficiency, stability, durability and emission. • The power control method of the hydrogen Wankel rotary engine is investigated. • Qualitative control achieves better engine performance than quantitative control. • Qualitative control coupling engine speed meets the power and high-efficiency demand. • Lower thermal load, knock tendency and NO emission can be obtained. • Much high excess air ratio isn't recommended due to high cyclic variation. [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 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]

[Display omitted] • Optimized injection strategies affect CA0-90 by varying NH 3 -H 2 premixing degree. • Multiple injection mode is more suitable for an ammonia-hydrogen dual-fuel engine. • Oxygen enrichment strategy improves the flame kernel formation and cyclic variation. • Oxygen-enriched levels of about 34.3% can elevate BMEP by more than 42%. • A hypothesis is proposed for the use of NH 3 -H 2 engines in hybrid power systems. Ammonia with low reactivity still faces some challenges as the main fuel for engines. Hydrogen and oxygen, as two combustion improvers, are expected to solve ammonia's reluctance to combust. To date, the application of oxygen enrichment strategies in ammonia-hydrogen dual-fuel engines is very rare, which motivates this study. The main objective of this investigation is to explore optimal hydrogen injection strategies and evaluate the effectiveness of oxygen enrichment strategies for the engine. Optimizing hydrogen injection parameters can vary the ammonia-hydrogen premixing degree, thus improving the flame kernel formation and flame propagation velocity. This is supported by the shortened CA0-90, lowered cyclic variation, and elevated thermal efficiency and power. Oxygen-enriched strategies enhance the heat release process and mitigate the negative effects of ammonia on the ignition process, which reduces cyclic variation and prominently increases power. However, the elevated oxygen concentration may increase the adiabatic flame temperature of the mixture and thus heat transfer loss, which decreases brake thermal efficiency. Moreover, a hypothesis that an ammonia-hydrogen engine may be employed in a hybrid power system in the future is proposed. The results of this work can be used as raw data for developing a hybrid power system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Consider using rich-burn strategy to control the performance of an ammonia-hydrogen engine. • The rich burn strategy can reduce NOx of ammonia-hydrogen internal combustion engine. • The rich burn strategy can increase the exhaust temperature of the ammonia-hydrogen engine. In order to achieve climate goals as soon as possible, it is necessary to reshape the energy mix to replace fossil energy with carbon–neutral fuels as soon as possible. Ammonia is a good medium for energy storage, has a large industrial scale and a well-developed infrastructure. To address the problems of NOx emission and low exhaust energy of ammonia-hydrogen engines, this study proposes a rich burn strategy. Firstly, a commercial spark ignition (SI) internal combustion engine (ICE) was modified to achieve a dual fuel supply of ammonia and hydrogen. The engine was run at a steady state of 1500 rpm with the intake 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 experimental results showed that the rich burn strategy could significantly reduce the NOx emission and increase the exhaust temperature of the ammonia-hydrogen engine when the excess air coefficient was below 0.8. The rich burn strategy can appropriately increase the engine power output, but it has some influence on the brake thermal efficiency (BTE). [ABSTRACT FROM AUTHOR]

Wankel rotary engine (WRE) is welcomed due to its high power density and compact layout and is criticized due to its low efficiency and poor emission. To develop low-carbon emission and high-efficiency WRE, the present work discusses the potential of hydrogen/methane dual-fueled WRE with varied fuel components. The test was conducted at 1500 r/min, 80 kPa manifold absolute pressure and stoichiometric ratio. The results indicate that compared with pure hydrogen or pure methane WRE, hydrogen/methane dual-fueled WRE can achieve higher thermal efficiency and output power, the maximum values of which are 30.7% and 12.4 kW under test conditions, respectively. Besides, it also has satisfactory NO, CO2 and HC emissions. In particular, there is a linear relationship between NO emission and methane volumetric percentage with a 0.9949 determination coefficient in logarithmic coordinates, which means that the NO emission can be easily predicted. In addition, the cyclic variation is acceptable at maximum-efficiency CH4% and the knock can be almost eliminated in hydrogen-methane dual-fueled WRE when the volume ratio of methane exceeds about 40%. In general, WRE is recommended to be fueled by a mixture of hydrogen and methane with a high methane volumetric percentage. • Discussion on the potential of hydrogen/methane dual-fueled Wankel rotary engines. • Dual fuel is better than single fuel in terms of power, efficiency, emission, etc. • There is a linear relationship between NO and CH4% in logarithmic coordinates. • Knock can be almost eliminated when the CH4% exceeds about 40%. [ABSTRACT FROM AUTHOR]

Hydrogen and ammonia, as two renewable carbon-free combustibles, are considered to be potentially excellent alternative fuels for internal combustion engines. Since hydrogen and ammonia have opposite and complementary combustion characteristics, the co-combustion of ammonia with hydrogen is reasonable. However, there are few detailed experimental studies of load control strategies for an ammonia‑hydrogen engine. In this study, four load control strategies including the variation of ammonia volume share (AVS), qualitative control, quantitative control, and adjustment of intake variable valve timing (VVT) are proposed and assessed for their applicability in an ammonia/hydrogen dual-fuel engine. The ignition and combustion characteristics of ammonia do not allow for high thermal efficiency, so the Miller cycle is introduced to improve engine performance. The engine employs fuel supply modes of ammonia port injection and hydrogen direct injection at 1500 rpm. In general, the engine achieves higher brake mean effective pressure (BMEP) and brake thermal efficiency (BTE) at different AVS. Moreover, qualitative control and adjustment of intake VVT allow the engine to own a wide regulation range of BMEP and maintain the BTE of more than 37% under most conditions. However, quantitative control appears to be an inadequate strategy for the engine due to the lower BMEP and BTE. • The co-combustion of ammonia with hydrogen and hydrogen direct injection are crucial. • Four load control strategies are proposed for an ammonia/hydrogen dual-fuel engine. • Technologies of the Miller cycle and variable valve timing are used in the engine. • Higher cyclic variation should be avoided when using various load control strategies. [ABSTRACT FROM AUTHOR]

• The effect of intake valve timing on hydrogen-rich ammonia engines was studied. • Delaying the valve timing to increase the power of hydrogen-rich ammonia engines. • Advanced valve timing to improve BTE of hydrogen-rich ammonia engine. • Increasing valve overlap increases the cycle variation of hydrogen-rich ammonia engines. To reduce carbon emissions and increase the efficiency of internal combustion engines (ICEs), it is necessary to use renewable alternative fuels and to use Miller cycle technology. Ammonia is an energy storage medium as well as a hydrogen storage medium. In this experiment, a Miller cycle ICE using ammonia-hydrogen fuel was investigated at 1500 rpm and 60% throttle opening. The excess air factor of 1 and the ammonia volume share of 70% were maintained during the experiment. The objective of the experiment was to vary the intake valve timing and investigate its effect on engine performance. The results show that when the intake valve timing is advanced and the valve overlap angle is kept larger, the combustion stability deteriorates, the flame development period (CA0-10) is prolonged, the cycle variation increases, COV pmax will exceed 25%, and the brake thermal efficiency (BTE) quickly dropped from 32.8% to 30%. When the intake valve timing is retarded by 25°CA, IMEP and BMEP can be increased by 8% and 16% compared with the minimum values. Generally speaking, the Miller cycle hydrogen-enriched ammonia engine is not suitable for the early opening strategy of the intake valve. [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]

• The Miller cycle and variable valve timing are used in a DI hydrogen engine. • The effects of variable valve timing on the airflow exchange process are analyzed. • The engine can reach 42.22% maximum brake thermal efficiency. • Positive effects of exhaust valve timing on a DI hydrogen engine cannot be ignored. The favorable physicochemical characteristics of hydrogen make it an excellent alternative fuel for IC engines. Compared with hydrogen port injection, hydrogen direct injection provides multiple degrees of freedom for injection strategies and mixture formation, which can effectively suppress abnormal combustion and enhance engine performance. Variable valve timing (VVT) and the Miller cycle, as advanced and effective means of improving engine performance, are less investigated based on a direct-injection (DI) hydrogen engine. Hence, to optimize the performance of a DI hydrogen engine, strategies of the Miller cycle and VVT are introduced. In this work, a 1.5 L direct-injection engine with a VVT system was employed to evaluate the effects of the Miller cycle, intake valve closing (IVC) timing, and exhaust valve opening (EVO) timing on the airflow exchange process, in-cylinder charge characteristics, heat release process, dynamics and emission characteristics. The engine speed was stabilized at 1400 rpm and the strategies of lean combustion and late injection were used. The main results are as follows. As the IVC timing is delayed, the ratio of pumping mean effective pressure to indicated mean effective pressure (PMEP/IMEP) first decreases and then slightly increases, and the engine tends to achieve a high level of brake thermal efficiency (BTE) when PMEP/IMEP is lower. The optimization of intake and exhaust VVT can shorten combustion duration and reduce cyclic variation by increasing volumetric efficiency and improving mixture distribution at ignition timing. Furthermore, the engine reaches 42.22% BTE and a high level of brake mean effective pressure by the cooperative control of IVC and EVO timing. NOx emissions can be controlled below 100 ppm by employing the synergistic effect of the Miller cycle and VVT. It is worth mentioning that exhaust VVT as a way to compensate for the performance of the DI hydrogen engine with the Miller cycle should not be ignored. [ABSTRACT FROM AUTHOR]

The present paper introduced an experiment survey focusing on exploring the impact of hydrogen enrichment on improving the lean-burn performance of an n-butanol rotary engine. During the test, the engine speed and intake pressure were roughly set at 4000 rpm and 35 kPa, respectively. A constant spark advance of 45 °CA was adopted through the test. Hydrogen volumetric fraction of the total intake was severally kept at 0% and 3%. The testing results manifested that the brake thermal efficiency and peak chamber temperature were heightened with hydrogen addition. Besides, the ignition delay and rapid combustion duration were both reduced with the hydrogen additive. Engine running stability gained an improvement by hydrogen supplement. Moreover, HC and CO emissions from the original n-butanol rotary engine were reduced after hydrogen adding. NOx emissions were increased with hydrogen enrichment while reduced with the increase of excess air ratio. This indicated that NOx emissions from both the n-butanol and hydrogen-blended n-butanol rotary engines could be reduced by lean combustion strategies. [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]

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]

This study aims to explore the combined effect of injection angle and injection pressure in a gasoline direct injection rotary engine through numerical simulation. Simulation results show that with the increase of injection angle, the fuel impinging position changes from the rotor recess to the middle cylinder block. At the same injection angle, the fuel enrichment area turns to the front of the combustion chamber with the increase of injection pressure. The lower injection angle is helpful to obtain a more uniform fuel distribution. Especially, with increase of injection pressure, the in-cylinder inhomogeneity index presents a downward trend. Under different injection pressures, the flame propagation period of the 0° injection angle is the most stable. The stable flame development and propagation processes make it easy to obtain higher indicated thermal efficiency. The highest indicated thermal efficiency is obtained at 0° injection angle and 15 MPa injection pressure, which is up to 24.99%. Moreover, compared with the fuel intake port injection, part of direct injection strategies help to get higher in-cylinder peak pressure. The higher in-cylinder temperature promotes the formation of NOx, but the organization of the in-cylinder fuel distribution can effectively reduce CO and Soot emissions. • A rotary engine simulation model with gasoline direct injection was established and validated. • The combined effects of injection angle and injection pressure were investigated. • The lower injection angle can obtain higher in-cylinder pressure and indicated thermal efficiency. • Lower in-cylinder inhomogeneity index is helpful to reduce Soot and CO emissions. [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]

• 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]

• 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]

• The characteristics of a rotary engine are predicted by support vector machine model. • The coefficient of determination of the optimized models is greater than 0.98. • The effects of parameters on performance, combustion and emissions are studied. • The optimal thermal efficiency and nitrogen oxides are 18.12 % and 248.58 ppm. The purpose of current research was to implement an intelligent regression model and multi-objective optimization of performance, combustion and emissions characteristics for a hydrogen-enriched gasoline rotary engine. The brake thermal efficiency (BTE), fuel energy flow rate (E f), nitrogen oxides (NO X), carbon monoxide (CO) and hydrocarbon (HC) were predicted by intelligent regression model with hydrogen volume fraction (α H 2 ), excess air ratio (λ) and ignition timing (IT) as independent variables. The intelligent regression models were based on support vector machine (SVM) and optimized by the genetic algorithm (GA) to obtain the optimal parameters of the regression model. The data for training the SVM model were derived from the experimental results of a hydrogen-enriched rotary engine, in which the speed was kept constant at 4500 r/min, the absolute manifold pressure remained at 35 KPa, the variation of α H 2 , λ and IT were 0–6%, 1–1.3 and 24–42 °CA before top dead center (bTDC), respectively. After optimized by GA, the coefficient of determination of BTE, E f , NO X , CO and HC between the SVM model and the corresponding data were greater than 0.98, and the mean absolute percentage error were <1%. The performance, combustion, and emissions characteristics including BTE, E f , NO X , CO and HC were considered for multi-objective optimization to obtain higher performance and lower emissions, and were solved using the non-dominated sorting genetic algorithm II. For this study, when the Pareto-optimal solutions were obtained, the optimal operating parameters were further obtained by limiting the performance and emissions parameters with the α H 2 of 5.06%, λ of 1.09%, and IT of 34.27 °CA bTDC. [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]