Research on the in-cylinder combustion and emissions of opposed rotary piston engines over various altitudes

Opposed rotary piston (ORP) engines boast a theoretical power density that is double that of traditional four-stroke reciprocating engines. Additionally, the ORP engines are free of connecting-rod mechanisms, which contributes to the compactness and efficiency of ORP engines. This study explores the performance of ORP engine under variable altitudes using three-dimensional simulation. The findings revealed that the charging efficiency diminishes as the altitude increases. During the compression stroke, the vortex formed in the intake stroke progressively dissipates, forming a flow field suitable for combustion. The combustion rate decreases as the altitude rises. However, the intense vortex formation within the cylinder at an altitude of 1000 m obstructs the flame propagation process, resulting in slower flame propagation. When the ORP engine operates at 4000 m, the combustion is enhanced due to the rich mixture around the spark plug. Additionally, the post-combustion phenomenon emerged at an altitude of 5000 m. The reduced cycle charge is a contributing factor to the lower in-cylinder temperature and pressure at high altitudes. As the altitude increases by 1000 m, the power output decreases by approximately 11.8 %. In contrast, the fuel economy is less sensitive to altitudes than power output. Nitrogen oxides (NO_x) emission decreases with increasing altitudes as a result of the drop of in-cylinder temperature. NO_x and hydrocarbon (HC) factors approach zero when the ORP engine operates at an altitude of 5000 m. However, the emission of carbon monoxide (CO) increases due to the significant percentage of post-combustion.