The Free-Piston-and-Turbine Compound Engine—A Cycle Analysis

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Abstract
A simplified cycle analysis is presented for a compound engine of the free-piston type. To allow ready algebraic formulation, the assumption of perfect-gas behavior with suitably averaged specific-heat magnitudes is employed. The influence of compressor, turbine, and mechanical “inefficiencies” as well as the effects of valve-pressure drops and coolant-jacket heat transfer are included. The calculated parameters of specific flow rate, specific fuel consumption, and turbine-inlet temperature are consequently realistic performance estimates, as shown by comparison with available test information. Results of the analysis clearly indicate the importance of component losses on turbine-inlet temperature and plant specific-fuel consumption and point out the unique characteristics of the cycle. Consideration also is given to the “supercharged-intercooled” and the “reheat” variants of the simple free-piston compound engine. A study of methods of load governing leads to the important conclusion that in order to realize a flat part-load economy characteristic, comparable to that of a diesel, it will be necessary to obtain a turbine design which achieves high part-load fuel efficiency. The reheat cycle has an excellent part-load fuel economy, but probably will require the use of a variable-area nozzle turbine.

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