Combustion system optimization and knock characteristics of a two-stroke rod-less opposed-piston engine

Abstract

Recently, the use of aviation kerosene in small unmanned aerial vehicles has attracted widespread attention because of it being safer than gasoline during transportation, storage, and use. However, for increased power density, further research is required on (i) detonation under low speed and high load and (ii) combustion instability under high speed and low load. Here, a direct-injection combustion system is designed, then how the fuel injection and spark strategy influence the combustion and knock intensity of a two-stroke rod-less opposed-piston spark-ignition kerosene engine is evaluated under different conditions. The results indicate that the flame propagation speed, equivalence ratio distribution, and flow field interact with each other. The equivalence ratio distribution and flow field influence the ignition and flame propagation processes, while the pressure waves generated during flame development also affect the equivalence ratio distribution and in-cylinder flow field. When the start of injection is at −65° and the spark timing is at −15° crank angle after inner dead center (CA AIDC), the indicated power (8.94 kW) and combustion efficiency (94.1%) are maximum under high speed and low load. The intrinsic mechanism is influenced mainly by the mixture distribution, which further affects flame propagation. Knocking combustion occurs where the flame reaches last, corresponding to the region of reverse swirl near the spark plug due to the higher swirl intensity. In this area, the CH₂OH concentration increases, and the mixture undergoes low-temperature spontaneous ignition.