No abstract

Hecto pressure composite cycle engines with piston engines and piston compressors are potential alternatives to advanced gas turbine engines. The nondimensional groups limiting their design have been introduced and generally discussed in Part I [1]. Further discussion shows, that the ratio of effective power to piston surface characterizes the piston thermal surface load capability. The piston design and the piston cooling technology level limit its range of values. Reynolds number and the required ratio of advective to diffusive material transport limit the stroke-to-bore ratio. Torsional frequency sets a limit to crankshaft length and hence cylinder number. A rule based preliminary design system for composite cycle engines is presented. Its piston engine design part is validated against data of existing piston engines. It is used to explore the design space of piston components. The piston engine design space is limited by mechanical feasibility and the crankshaft overlap resulting in a minimum stroke-to-bore ratio. An empirical limitation on stroke-to-bore ratio is based on existing piston engine designs. It limits the design space further. Piston compressor design does not limit the piston engine design but is strongly linked to it. The preliminary design system is applied to a composite cycle engines of 22MW take-off shaft power, flying a 1000km mission. It features three 12-cylinder piston engines and three 20-cylinder piston compressors. Its specific fuel consumption and mission fuel burn are compared to an intercooled gas turbine with pressure gain combustion of similar technology readiness.

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.