Design of a Novel 2-Stroke SI Engine for Hybrid Light Aircraft 2021-01-1179

The trend of powertrain electrification is quickly spreading from the automotive field into many other sectors. For ultra-light aircraft, needing a total installed propulsion power up to 150 kW, the combination of a specifically developed internal combustion engine (ICE) integrated with a state-of-the-art electric system (electric motor, inverter and battery) appears particularly promising. The dimensions and weight of ICE can be strongly reduced (downsizing), so that it can operate at higher efficiency at typical cruise conditions; a large power reserve is available for emergency maneuvers; in comparison to a full electric airplane, the hybrid powertrain makes possible to fly at zero emissions for a much longer time, or with a much heavier payload. On the other hand, the packaging of a hybrid powertrain into existing aircraft requires a specific design of the thermal engine, that must be light, compact, highly reliable and fuel efficient. The last aspect has a direct impact on the performance of the aircraft, since the mission range depends on the capacity of the fuel tanks, which, in turn, is limited by the aircraft total weight.

The two-stroke cycle engine is far from a novelty for ultra-light aircraft; unfortunately, the specific fuel consumption and pollutant emissions of the conventional engines is quite high, in comparison to their 4-Stroke (4S) counterparts. The aim of the project presented in this paper is to develop a new type of 2-Stroke SI engine, able to match lightness, fuel efficiency and low pollutant emissions at a reasonable cost. The proposed ICE weights less than 60 kg, it delivers 110 kW@6000 rpm, along with a brake specific fuel consumption lower than 260 g/kWh in all the most relevant operating conditions.

The paper describes the design of the new engine, with particular attention to the optimization of the scavenging system (without poppet valves) and the design of a low pressure direct injection system. The process is supported by CFD 1D and 3D simulations. As far as the design of the injection system is concerned, the main goal was to obtain a fuel trapping ratio higher than 95%, along with a properly stratified charge at combustion onset, when considering the most critical operating condition (maximum engine speed and load). The main optimized parameters include the number of injectors, their locations, the injection timing and duration.