Initial Performance Synthesis of a Hydrogen-Fueled Free-Double-Piston Composite Cycle Engine

Presentation held on 10/10/2024 in the project session MINIMAL at the 14th EASN International Conference in Thessaloniki. The Composite Cycle Engine (CCE) concept combines the benefits of a turbofan and a piston engine. In a CCE, the piston engine is integrated into the high pressure part of the gas turbine to increase the engine’s thermal efficiency by closed-volume combustion at high peak pressure and temperature. In previous studies [1-3], kerosene-fueled crankshaft- as well as Wankel-piston engine based CCEs were investigated and a fuel burn improvement of up to 15 % compared to state-of-the-art engine technology was stated. Besides these well-known internal combustion engine concepts, free-piston engines appear highly promising not only due to engine mass and efficiency improvements but also to simpler bearing and lubrication as well as to the decreased installation space. The Free-Double Piston (FDP) arrangement, presented in [4], is a particularly attractive concept for an extremely compact, ultra-efficient piston-based gas generator and was initially assessed in [5, 6] to exhibit improved propulsion system weight and efficiency characteristics compared to a crankshaft-based variant. This contribution presents thermodynamic design exploration studies of a hydrogen-fueled CCE architecture featuring a Free-Double Piston based gas generator. In a first step, design and performance characteristics of an isolated FDP unit are studied. Therefore, design parameter sensitivity studies are carried out to evaluate the FDP performance regarding e.g. power output, heat losses and fuel consumption. As part of this, the valve timings are being optimized. In a second step, the optimized FDP is integrated into the FDP-CCE, where thermodynamic cycle optimization studies are conducted on overall engine level. The proposed propulsion system concept is modeled in the Bauhaus Luftfahrt (BHL) in-house framework Aircraft Propulsion System Simulation (APSS) where the piston engine model is integrated via a surrogate model (feed forward neural network). The FDP neural network is generated using the BHL inhouse piston engine performance simulation tool, which represents a time-resolved 0D-model of a single piston compressor and engine. The used heat transfer model and the combustion characteristics were adapted to hydrogen combustion as well as calibrated and verified against literature data. The studies presented in this contribution form an important waypoint towards the design optimization of hydrogen-fueled FDP-CCE power plants for minimum climate impact. References: [1] S. Kaiser, S. Donnerhack, A. Lundbladh, A. Seitz, "Composite Cycle Engine Concept with Hectopressure Ratio", Journal of Propulsion and Power, Vol. 32, No. 6, pp. 1413-1421, 2016.[2] M. Nickl, S. Kaiser, "Evaluation of Piston Engine Modes and Configurations in Composite Cycle Engine Architectures", CEAS Aeronautical Journal, DOI 10.1007/s13272-019-00399-w, 2017.[3] S. Kaiser, H. Kellermann, M. Nickl, A. Seitz, "A Composite Cycle Engine Concept for Year 2050", 31st Congress of the International Council of the Aeronautical Sciences, Belo Horizonte, Brasil, 2018.[4] H. Klingels, “Wärmekraftmaschine mit Freikolbenverdichter”, DE patent 10 2012 206 123 A1, 2013.[5] S. Kaiser, O. Schmitz and H. Klingels, “Aero Engine Concepts Beyond 2030: Part 2—The Free-Piston Composite Cycle Engine”, J. Eng. Gas Turbines Power, vol. 143, no. 2, p. 021002, 2021.[6] S. Kaiser, “Multidisciplinary Design of Aeronautical Composite Cycle Engines”, Dissertation, Technical University München, 2020.