Investigation on strategies for reducing exhaust losses in opposed rotary piston engine and the impact of structure-induced internal EGR on engine performance

The opposed rotary piston (ORP) engine offers advantages such as compact size, high power-density, and structural simplicity, making it a promising candidate for long-endurance, heavy-load unmanned aerial vehicle (UAV) applications. Unlike conventional engines, the ORP engine eliminates intake and exhaust valves, instead controlling the gas exchange process through phase adjustments of the exhaust ports on the cylinder block. Investigating the impact of exhaust port structural parameters on exhaust behavior and the realization of internal exhaust gas recirculation (iEGR) is essential for reducing exhaust losses, mitigating knock tendencies, and improving emission performance. In this study, the structural parameters of the ORP engine's exhaust port were defined, and the exhaust characteristics under various port configurations were analyzed. A strategy for achieving iEGR through adjustments of the exhaust early closing angle (EECA) was proposed, and its effects on intake characteristics, in-cylinder combustion behavior, and overall engine performance were evaluated. Results indicate that an exhaust advance angle (EAA) of 18° yields the highest indicated work, representing a 13.75 % improvement over the baseline configuration. Increasing the exhaust tilt angle (ETA) reduces expansion losses but simultaneously increases exhaust losses, resulting in a net decrease in indicated work, which declines by 2.71 % when the ETA reaches 30°. The EECA influences the residual gas content by altering the exhaust stroke termination timing and shows a positive correlation with the iEGR rate and a negative correlation with charging efficiency (CGE). At an EECA of 50°, the iEGR rate reaches 15.91 %. An EECA range of 10–20° is recommended for optimal fuel economy and combustion stability. Conversely, if emission performance is the primary concern, a range of 20–30° is more appropriate.