Performance and NOx emission characteristics of intercooled composite cycle aero engines

Journal article, 2027

The intercooled composite cycle engine (CCE) is a propulsion system concept with potential for fuel burn reduction exceeding 10% compared to a future geared turbofan. However, nitrogen oxides (NOx) emissions are expected to exceed those of conventional combustion systems, and integrating a piston engine into the core introduces significant weight penalties. There is a lack of understanding of the underlying cycle design parameters that drive emissions and decrease core compactness, and how these can be traded off against efficiency for a more balanced design. In the present paper, we address this gap by systematically investigating the effect of key CCE cycle design parameters on thermal efficiency, power density, and NOx emissions. The analysis demonstrates that most of the design changes that increase thermal efficiency will also lead to an increase in NOx emissions, with clear trade-offs between specific power and efficiency. Multi-objective optimisations are conducted, generating Pareto fronts showing the efficiency-NOx trade-off. A 13.4% improvement in thermal efficiency relative to a future geared turbofan is possible. This optimum is achieved at the maximum admissible cylinder pressure and by only adding heat in the piston engine. Higher constraints on specific power are found to shift the Pareto front towards higher NOx and lower thermal efficiency. However, without the introduction of NOx mitigation techniques, the NOx emissions are estimated to be 5.3 times higher for the highest thermal efficiency design, highlighting the need for dedicated NOx reduction strategies.