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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]

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]

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]

• 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 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]

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]

[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]

Ammonia is considered as a carbon-free alternative fuel and hydrogen carrier to realize green and renewable energy conversion, but the oxidation mechanism of NH 3 still needs to be improved and validated by more experimental data. In this study, the laminar burning velocity (LBV) was measured in a constant volume vessel at various equivalence ratios, initial temperatures, and oxygen fractions up to undiluted condition. Thirteen latest NH 3 kinetics mechanisms were collected and compared with each other and the experiments. Results showed that large discrepancies of predictions presented between different mechanisms. The maximum LBV of undiluted NH 3 /O 2 mixtures could reach 125.05 cm/s. The maximum predicted LBV of NH 3 /O 2 is nearly two times higher than the minimum one for NH 3 /O 2 mixtures. The reduced Okafor (2019) and Zhang (2017) mechanism gave the best agreement with experiments for lean and rich mixtures, respectively. Sensitivity analysis of rate constant, species thermal, and transport parameters were adopted to identify the critical reactions and species in the premixed flame. It was found that because of the coupled nature of the properties mentioned above in the premixed flame, the species involved in the essential reactions also exhibit substantial thermal and transport sensitivities. [ABSTRACT FROM AUTHOR]