Comparison of Piston Concept Design Solutions for Composite Cycle Engines—Part I: Similarity Considerations

Composite cycle engines comprising piston engines (PEs) as well as piston compressors (PCs) to achieve hecto pressure ratios represent a target area of current research surpassing gas turbine efficiency. An unclear broad range of design parameters is existing to describe the design space of piston machines for this type of engine architecture. Previously published work focuses on thermodynamic studies only partially considering limitations of the design space. To untie the problem of PE design, a dimensional analysis is carried out reducing the number of parameters and deriving two basic similarity relations. The first one is a function of the mean effective pressure as well as the operating mode and is a direct result from the thermodynamic cycle. The second one is constituted of the stroke-to-bore ratio and the ratio of effective power to piston surface. Similarity relations regarding the PC design are based on Grabow (1993, “Das erweiterte “Cordier”—Diagramm Für Fluidenergiemaschinen und Verbrennungsmotoren,” Forsch. Ingenieurwes., 59, pp. 42–50). A further correlation for PCs is based on the specific compression work and the piston speed. In Part I, data of existing PEs have been subjected to the above similarity parameters unveiling the state-of-the-art design space. This allows a first discussion of current technological constraints. Applying this result to the composite cycle engine gives the design space and a first classification as a low-speed engine. Investigating various design points in terms of number and displacement volume of cylinders confirms the engine speed classification. Part II will expand this investigation using preliminary design studies.