The Composite Cycle Engine (CCE) enhances the conventional Joule/Brayton cycle by replacing the high-pressure compressor with a high-quality piston-based gas generator that enables extremely high compression, combustion, and expansion of the working fluid before entering the classic Joule burner. This piston-based topping cycle unlocks much more efficient fuel utilization. This paper studies a CCE concept featuring a system of free double piston (FDP) units for a potential long-range (LR) application in 2045, benchmarked against an advanced turbofan engine representative of the same time frame. In-house-developed simulation tools for the piston system and the overall power plant, as well as aircraft non-linear trade factor analysis, are used for different levels of conceptual assessment. First, the cooling demand inside the FDP system is determined. An engine cycle parametric study is then performed for the design point top-of-climb (ToC). Off-design performance is further studied, demonstrating a 9.3% improvement in thrust-specific fuel consumption (TSFC) in cruise relative to the baseline engine. After incorporating the engine weight and nacelle geometry effects, the engine reaches a total mission fuel burn reduction of around 14.7% compared to the baseline engine. The concept evaluation shows the fuel burn potential of the CCE in the future LR aviation sector and lays the foundation for further climate impact analysis.

The composite cycle engine (CCE) offers a promising pathway for significantly reducing fuel burn in future aircraft. By integrating a piston system into the core of a gas turbine, the CCE can utilise higher peak cycle temperatures and pressures and thus, achieve a step-change in thermal efficiency. These conditions also promote increased levels of normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ formation. This paper presents a methodology for the preliminary estimation of normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ emissions within both the piston engine and the subsequent Joule combustor. Fast-responding parametric models capable of capturing key normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ sensitivities, particularly the impact of combustion vitiation through water injection and exhaust gas recirculation (EGR), are established. Piston normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ is modelled using a two-zone approach, which demonstrates good correlation with reference engine data from literature. For the Joule combustor, a stochastic flow description is employed to predict normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ formation, with validation against an in-service combustor confirming the model’s predictive capability. Initial studies indicate high baseline piston normal upper N normal upper O Subscript x N O x $\mathrm{NO}_x$ emissions, though significant mitigation is achievable through water injection or EGR. This work establishes a framework for future research into the emissions and necessary mitigation strategies for the CCE and other advanced combustion technologies.