Research on lean combustion characteristics of opposed rotary piston engine based on intake strategy optimization and vortex-tumble synergy

Owing to its simple structure and high power density, the ORP engine minimizes installation space while achieving superior power performance, serving as an ideal power source for hybrid electric vehicles (HEVs) and unmanned aerial vehicles (UAVs). The absence of an effective intake flow-guiding structure results in weak in-cylinder turbulent activity, which is unfavorable for lean burn limit extension. This study systematically investigates the effects of three intake modes on in-cylinder flow field evolution, combustion stability, flame propagation and emissions. The results demonstrate that the high tumble-vortex intake mode significantly enhances turbulent kinetic energy (TKE) at ignition timing, promotes favorable fuel stratification near the spark plug, and substantially accelerates flame propagation. Compared with the low tumble intake mode (tumble ratio = 2.14, vortex ratio = 0.52), the tumble ratio increases by 61.7 % to 3.46, and the vortex ratio increases by 338.5 % to 2.28, and the lean burn limit expands from 1.17 to 1.44. However, the high tumble-vortex mode also leads to a 403 % increase in intake loss due to elevated flow resistance. At excess air ratio (λ) = 1.3, the indicated thermal efficiency (ITE) reaches 37.8 % under the high tumble-vortex mode, representing a 17.1 % improvement over the low tumble mode, while CO emissions are reduced to only 6 % of the low tumble mode. In this study, a feasible intake strategy is established to overcome the lean burn limit and fully exploit the high power density advantage of ORP engine.