The Composite Cycle Engine: Performance and Emissions

Licentiate thesis, 2026

Currently, there is no viable solution to decarbonise aviation. It is therefore of outmost importance to develop the most fuel efficient engines possible. Current aero engines are reaching the technological limit of thermal efficiency, where further development yields diminishing returns. A novel type of propulsion system is the composite cycle engine that offers fuel burn reductions beyond what is possible with a conventional turbofan design. The rationale for the concept is to replace the turbomachinery and combustor in the high-pressure part of a turbofan with a piston engine. Combustion inside a piston engine is more efficient than a constant pressure burner, resulting in a higher overall efficiency of the propulsion system. Unfortunately, it also leads to higher emissions of nitrogen oxides (NOx). This thesis presents a framework for the thermodynamic modelling of the composite cycle engine together with a methodology to assess emissions of NOx. The developed framework enables design studies of the engine, where performance and emissions can be analysed. A parametric study is presented where the impact of key thermodynamic cycle design variables on efficiency, power density, and \ce{NO_x} is investigated. Prohibitive levels of NOx were found for all the parameters studied. Finally, an outlook for further work using the developed model is discussed, with possible techniques to reduce NOx.