Optimization of combustion chamber geometry for a two-stroke spark-ignited direct-injection opposed-piston rod-less aviation kerosene engine

Spark-ignited direct-injection opposed-piston rod-less kerosene engines are highly suitable for small and medium-size unmanned aerial vehicles (UAVs) because of the high power-to-weight ratio, self-balancing ability, and safety of such engines. The design and optimization of combustion systems are crucial for power, fuel economy, and knock control. To meet the increasing demand for UAV power systems and fill the research void in this area, reported here for the first time are the design and optimization of a swirl-wall-guided combustion system including the spray pattern and pit structure on the top surface of the piston, with the dynamic, fuel economy, and knock intensity as the evaluation indicators. The results indicate that the spray pattern mainly affects the fuel escape rate and inhomogeneity. After it is injected into the pit, the fuel is guided to the combustion chamber by the steering role of the guide arc. If the spray drop point is located away from the pit, it leads to increased fuel escape rate and inhomogeneity. The turbulent kinetic energy in the spark-plug zone decreases as the pit diameter decreases, fluctuating by 48 %, while the average value remains relatively constant. The distribution is influenced primarily by the enveloping effect of the pit structure on the airflow. Under low-speed conditions, as the pit diameter decreases, the ignition delay period increases and then decreases, and the combustion duration decreases monotonically. The fluctuation of indicated power is relatively small (2.64 %), and the fuel escape rate decreases monotonically, resulting in the same variation pattern of indicated specific fuel consumption (ISFC). When the pit diameter is 43 mm, the fuel escape rate and ISFC reach their lowest values. Knock combustion near the wall is influenced mainly by two factors: the temperature rise rate and the flame propagation speed. The temperature rise rate increases significantly with decreasing pit diameter. When the pit diameter is less than 50 mm, knock combustion begins to occur. Under high-speed equivalent combustion conditions, as the pit diameter decreases, the indicated power changes by 5.21 %, and the ISFC decreases monotonically.