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Hydrogen-fueled internal combustion engine is proposed to resist the threat of global warming. Wankel rotary engine (WRE) has been proven to be an excellent hydrogen-fueled power device, which can overcome the shortcomings of hydrogen, such as poor power, serious backfire and large storage volume, to some extent when it is used as fuel in the internal combustion engine. However, due to its unique structure, WRE suffers from severe knock. Therefore, the goal of this work is to investigate the impacts of knock type on the heat release process in hydrogen-fueled WRE. This present work is conducted at 2000 r/min and wide-open throttle. The main results are as follows: In hydrogen-fueled WRE, the peak knock pressure of knock caused by rapid and unstable combustion of hydrogen is usually earlier than CA50 and that of knock caused by spontaneous combustion of end gas is usually later than CA50. The sequence between the crank angle of peak knock pressure and CA50 combined with knock intensity can be used to determine the knock type in hydrogen-fueled WRE. Besides, a means for knock detection is proposed according to the distribution of crank angle corresponding to peak knock pressure. In addition, the distribution of CA0-10, CA50 and CA10-90 of 1000 consecutive cycles under two kinds of knock in hydrogen-fueled WRE are discussed in detail, and regular conclusions are drawn. In particular, the limitation of CA50 as a metric for evaluating knock level is also demonstrated. • The impact of knock type on the heat release of hydrogen Wankel engine is discussed. • The distribution of CA0-10, CA50 and CA10-90 with knock intensity are analyzed. • A method is proposed to estimate the knock occur. • A method is proposed to determine the knock type in hydrogen 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]

• 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]

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]

• 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]

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]

The application of hydrogen in internal combustion engines (ICE) has attracted widespread attention. However, due to the low ignition energy, high flame propagation speed, and wide combustible range of hydrogen, it is easy to cause abnormal combustion phenomena such as backfire in the port fuel injected (PFI) hydrogen-fueled engine. When a backfire occurs, the combustible mixture burns in the intake, resulting in a decrease in the volumetric efficiency of the engine, which may cause it to misfire or shut down in severe cases. Fast and accurate detection of backfire events is essential to take targeted control measures. In this research, a backfire detection system based on dual intake manifold absolute pressure (MAP) sensors were designed, with two MAP sensors installed on the intake manifolds of the first and fourth cylinders, and four gas injectors were installed on the intake manifold to convert the gasoline ICE into a hydrogen-fueled ICE. During the experiment, the engine speed was stabilized at 1000 rpm, the throttle valve was fully opened, and the intake pressure was maintained at 100 kPa.The test results show that the system can accurately determine the location and intensity of backfire occurrence. This method provides a basis for precise control of backfiring. • Set up two MAP sensors to monitor the backfiring of the hydrogen ICE. • MAP sensors allow accurate monitoring of backfiring. • Need to optimize valve strategy and injection strategy for hydrogen ICE. [ABSTRACT FROM AUTHOR]

An over-expanded (Atkinson cycle) reciprocating internal combustion engine is a crucial component of hybrid and range-extended electric vehicles. The realization of the over-expansion cycle with high efficiency in novel power devices with a compact, simple design, such as cycloidal rotary engines, is a promising alternative. The cycloidal rotary engine is an internal combustion engine that operates four strokes without a reciprocating mechanism and valve train. The over-expansion cycle is implemented in the cycloidal rotary engine by asymmetrical intake and exhaust port arrangements, resulting in open and closure timings. This study aimed to establish inherent characteristics of the over-expansion cycle in gasoline-fueled rotary engines with spark ignition. A numerical simulation of the gas exchange and combustion process in a wide range of over-expansion ratios was conducted. It was concluded that the over-expansion cycle is realized without typical piston engine methods such as variable valve timing and multilink mechanism. The thermal conversion efficiency is a peak function of the over-expansion ratio. In the over-expansion range of 1.1–1.2, the thermal conversion efficiency of the cycloidal rotary engine increased up to 32.7–34.5%. The benefits of fuel economy and carbon oxide emission can also be achieved within the specified range of over-expansion ratio. • Manual adjustment of port timings is used to realize over-expanded (Atkinson) cycle. • Variation of over-expansion ratio alters the gas-exchange and combustion processes. • The thermal conversion efficiency is a peak function of the over-expansion ratio. • The benefits of fuel economy and emission can be achieved by over-expansion. [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]

[Display omitted] • Proposed VVT as a method of load regulation for ammonia-hydrogen engines. • Comparing the load control effects of MAP, α NH 3 , λ and VVT. • Proposed load control strategy for ammonia-hydrogen hybrid system. The development of hybrid technology and the use of zero-carbon fuels are effective means to improve energy efficiency and reduce carbon emissions. In this study, the Miller cycle spark ignition engine was used, and four load control strategies of throttle, ammonia-hydrogen ratio, air–fuel ratio, and variable valve timing were used to conduct experimental research on the ammonia-hydrogen engine. The experimental results show that the throttle strategy provides the widest output range, and the BMEP is raised from a minimum of 1 bar to 7.4 bar. The ammonia-hydrogen mixing ratio strategy is suitable for cold start conditions, fully utilizing the good combustion characteristics of hydrogen, and at the same time reducing the amount of hydrogen used in the heat engine process as much as possible. The air–fuel ratio strategy provides the highest thermal efficiency with a maximum BTE approaching 40%. It satisfies the energy-saving demand under the single-point working condition of the engine in the series mode. The variable valve timing strategy can realize switching between BMEP between 3.5 bar and 6.5 bar, and keep BTE above 33%. The valve timing strategy can not only provide a wider load adjustment range but also maintain high thermal efficiency. Finally, combining the effects of the four control strategies and the typical operating modes of the hybrid system, this study suggests that the throttle strategy is suitable for the feed mode, the VVT strategy is suitable for the parallel mode, the ammonia-hydrogen mixture ratio strategy is suitable for the cold start condition, and the air–fuel ratio control is suitable for the series mode. [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]

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]