Hybrid-electric power unit for an ultralight aircraft

High power-weight ratio, reliability and efficiency are the main design goals for the propulsion system of ultralight aircraft (mass up to 650 kg). For power outputs beyond 40 kW, 4-stroke multi-cylinder SI engines are the most widespread solution, while pure electric powertrains do not appear at the moment as a practical proposition. Four stroke diesel engines would also be very attractive, in particular for military applications, but their weight is significantly higher than their gasoline powered counterpart. The goal of the study presented in this article is to develop a hybrid-electric power unit for ultralights, running on heavy fuels (diesel, kerosene, jet fuels). Reference is made to Falco EVO, by Leonardo, originally equipped by the Rotax 912 ULS/S engine (peak power 73.5 kW, weight about 60 kg): the proposed power unit has the same cylinders layout, smaller overall dimensions and almost equivalent weight. The thermal engine is an innovative two-stroke unit, coupled in parallel to a permanent magnet electric motor. The design and optimization of the hybrid power unit has been supported by CAE tools, including CAD and CFD simulations. The results of the study show that the new hybrid system can save about 30% of fuel mass at the typical cruise conditions, and it can increase the peak power output up to 20%, compared to the reference Rotax engine. The reduction of fuel consumption can be translated into an equivalent increment of the operative range of the aircraft, or into an increase of payload (+30%, considering the Falco EVO aircraft). Finally, compared to the Rotax engine, the hybrid power unit exhibits significantly lower CO2 emissions (from -12% to -37%), thanks to the improvement of fuel efficiency.