Reducing greenhouse gas emissions to limit global warming is becoming one of the key issues of the 21st century. As a growing contributor to this phenomenon, the aeronautic transport sector has recently taken drastic measures to limit its impact on CO2 and pollutants, like the aviation industry entry in the European carbon market or the ACARE objectives. However the defined targets require major improvements in existing propulsion systems, especially on the gas generator itself. Regarding small power engines for business aviation, rotorcrafts or APU, the turboshaft is today a dominant technology, despite quite high specific fuel consumption. In this context, solutions based on Diesel Internal Combustion Engines (ICE), well known for their low specific fuel consumption, could be a relevant alternative way to meet the requirements of future legislations for low and medium power applications (under 1000kW).The objective of this paper is therefore to perform a detailed comparison between several propulsion systems for the particular application of a rotorcraft, taking into account realistic constraints (power to weight ratio, fuel economy…). Dedicated piston-engine architectures are first proposed, including 4-stroke Diesel technologies and turbocompounding. An evaluation of fuel consumption in realistic flight conditions is then performed using detailed ICE models coupled with a helicopter simulator. The use of Diesel ICE leads to consumption gains of the order of 40% but also increase the possible payload for long-range missions, compared to present turboshafts. The paper finally allows to draw perspectives of developments for future green rotorcrafts but also more generally for light and medium weight aeronautic applications.