Development of a Combustion System for a New Generation of 2-Stroke Spark Ignition Engines

Conventional 2-Stroke Spark Ignition engines are characterized by very high power to weight ratios and low manufacturing costs, but also by very low thermal efficiencies and high pollutant emissions. The last issues can be fully addressed by adopting an external scavenging pump and a direct or semi-direct injection system. The implementation of these solutions requires a strong support from CFD simulations, in particular for the optimization of air-fuel mixing and combustion. The paper presents a theoretical study on a new 2-Stroke, three cylinders, 1.3 L, Spark Ignition engine for light aircraft. The power-unit also includes an electric motor connected in parallel with the thermal engine. The latter features a supercharger and a two-stage injection system, made up of a set of low-pressure fuel injectors installed on the transfer ports, and a high-pressure gasoline injector on the cylinder head. While a previous paper [1] describes the general design guidelines and the overall performances predicted for this engine, the current study is focused on the development of the combustion system, driven by 3D-CFD multi-cycle simulations. In particular, the paper reviews the main steps followed for the set-up of the injection and ignition parameters at the condition of maximum power, as well as for the design of an "open"pre-chamber. The simulation results show that the proposed system, with an optimized combination of dual stage injection, piston-controlled ports and open pre-chamber, can be a good basis for achieving a regular and efficient combustion at all the operating conditions of interest for an aircraft piston engine. The concept can be extended also to other types of 2-Stroke high-speed SI engines, suitable for motorcycles, small boats, snow-mobiles et cetera.