High-altitude performance and improvement methods of poppet valves 2-stroke aircraft diesel engine

Abstract

Poppet valves 2-stroke (PV2S) diesel engine is promising to overcome the problems of low combustion performance and deteriorated emissions of conventional 2-stroke aircraft engines due to the lubricant leakage into cylinders. However, the high-altitude gas exchange and power performance of PV2S diesel engine with complex supercharging systems have been rarely studied, and the corresponding experimental methods are expected to be improved. To address these issues, a reformative experimental platform and one-dimensional simulation model are proposed to accurately predict the high-altitude performance and explore the improvement methods of a PV2S aircraft diesel engine with a combined supercharging system. Tracer gas method and pressure-based mass flow acquisition method are innovatively conducted in the platform to measure the gas exchange characteristics, which are validated with satisfactory accuracy and feasibility. The results indicate the performance deterioration of the PV2S engine is more severe than conventional 2-stroke engines due to the additional decrease of trapping efficiency. As the altitude increases from sea level to 8000 m, delivery ratio, trapping efficiency, charging efficiency and available brake power decrease by 22%, 27%, 43% and 61% respectively. Adjustments of valve overlap duration, as well as exhaust manifold length and chamber volume, can effectively improve the charging efficiency and brake power, which increase by 36.73% and 30.38% at 8000 m altitude, whereas at sea level they decrease 5.68% and 8.39% respectively. The study reveals the feasible application of PV2S diesel engine in aircraft propulsion and facilitates its use at high altitudes.