Unified thermodynamic evaluation of Composite Cycle Engines
The Composite Cycle Engine concept introduces piston engines into the turbofan core. It promises significantly improved thermal efficiency over the conventional turbofan, but adds weight to the engine. Hence, both concepts need to be compared consistently to identify the true improvement potentials. To this end, a unified set of methods was established. The main cycle parameters were optimised for a short- to medium-range application, in particular overall pressure ratio and combustor exit temperature.
The optimum combustor exit temperature is rather low at 1,400 K. In this way, most of the fuel is burnt in the efficient piston engine and only a small share in the conventional combustor. Efficiency increases monotonically with increasing pressure ratio, but is restricted by permissible piston peak pressure and exit temperature. With today’s technology, mission fuel burn reduces by 18 % on aircraft level, including cascading effects through resizing, although engine weight increases by 24 %.
Future benefits of the concepts were also analysed, assuming evolutionary improvement of component technology until year 2035. The turbofan mission fuel burn reduces by 9 % compared to modern turbofans. The Composite Cycle Engine with likewise improved technology retains its relative advantage. This shows that the advantage of the Composite Cycle Engine is persistent for future developments. The concept achieves a combined fuel burn reduction of 37 % against year 2000 technology standard, surpassing the year 2035 target (30 %). To improve the concept further, the use of a rotary engine instead of pistons is being investigated, too.
- Turbofan and Composite Cycle Engine architectures: Piston engines drive piston compressors in the core of the engine. Turbo components are thermodynamically combined as indicated by the colours to simplify the calculation procedure.
- Rotary engine architecture: Conceptual sketch of a Composite Cycle Engine with lightweight five-disk rotary piston engine in beneficial coaxial integration