Recognizing the attention currently devoted to the environmental impact of aviation, this three-part publication series introduces two new aircraft propulsion concepts for the timeframe beyond 2030. Part one focuses on the steam injecting and recovering aero engine concept. This second part presents the free-piston composite cycle engine concept. A third publication, building upon those two concepts, presents the project which aims for demonstrating the proof of concept with numerical simulation and test-bench experiments up to a technology readiness level of three.

The free-piston composite cycle engine concept is composed of a gas turbine topped with a free-piston system. The latter is a self-powered gas generator in which the internal combustion process drives an integrated air compressor. Here, several free-piston engines replace the high-pressure core of the gas turbine. Through the ability to work at much higher temperatures and pressures, the overall system efficiency can be increased significantly, and fuel burn as well as CO2 emissions reduce.

The proposed free-piston composite cycle engine design is described in detail, and the sources of thermodynamic benefits are stated. Concrete engineering solutions consider the implementation into an aircraft. The free-piston design enables lower weight and size compared to a crankshaft-bound piston engine, as no mechanical transmission and lubrication system is required. The absence of a crankshaft and connecting rods eliminates reactive forces, reduces mechanical losses, and allows higher mean piston velocities. Facilitated through air lubrication, higher cylinder temperatures are viable. The reduction of heat losses enables cooling of the piston-cylinder with core fluid. The use of a sequential combustion chamber can enhance operability and tailor the production of NOx in low-altitude operation. A discussion of emissions affecting the environment shows the potential to reduce the climate impact of aviation.

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