The composite cycle engine concept
Piston engines feature combustion in a closed volume, leading to a pressure rise during heat addition, and allow for very high maximum temperatures due to short exposure times. This results in tremendous efficiency advantages over turbo engines. In contrast, the stationary mode of operation of turbo engines leads to superior power-to-weight ratio and reliability. A novel concept that synergistically combines the benefits of both is being investigated at Bauhaus Luftfahrt: the composite cycle engine.
The concept resembles a conventional turbofan architecture at first glance, but replaces the high pressure section of the engine with a piston system. In this way, the lightweight turbo components provide engine thrust, pressurise the piston system and extract the energy from the core-engine flow. The piston system tops the turbo-engine cycle at pressures and temperatures unseen in turbofan engines. Twelve piston engines drive 24 piston compressors in a self-contained unit. With this implementation the piston system runs mechanically independently from the turbo shafts and can be placed freely within the engine core. At the same time, the low pressure turbine provides power for the fan only and gets smaller.
This novel concept makes it possible to achieve peak pressure ratios of over 300, compared to about 60 of advanced turbofan engines. This reduces fuel consumption of the aircraft by 15 %, allowing to meet engine efficiency targets for year 2035, although engine weight increases by 30 %. The engine core size remains constant compared to a turbofan. NOx emissions can also be reduced by 10 %, compared to advanced lean-burn combustion technology.
- The principle: The piston system operates in the centre of the engine, where it delivers the highest benefits. At high pressures and temperatures.
- The implementation: The piston system is situated within the core engine around the turbo components. The compact arrangement allows to keep core engine size constant.