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

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

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]

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

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

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]

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]