Mechanical Efficiency of Kinematic Heat Engines

ABSTRACT:
This paper presents a general conceptual and basic quantitative analysis of the mechanical efficiency of kinematic heat engines. Typically, engineering studies of the mechanical efficiency of heat engines are made on a case by case basis. In ordinary practice, kinematic analysis and

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computer simulation of specific engine mechanisms coupled with calculated or measured pressure-volume cycles usually can be effectively used for evaluating and optimizing engine designs. Nevertheless, an overall insightful view of mechanical efficiency would be of great value in practical settings and it is of obvious importance for theoretical work. However, no general treatment of mechanical efficiency of heat engines has been available. This is in sharp contrast to the situation regarding the thermal efficiency of heat engines. Classical thermodynamics treats the subject of thermal efficiency in great generality. Its results, although obtained in a highly idealized setting, are of profound importance to engine theorists, designers, and operators. This paper approaches mechanical efficiency of engines at a similar level of ideality and generality. The paper first identifies and introduces a conceptual basis for the essential energy transfers which take place among the basic components of reciprocating heat engines. This illuminates the interplay of engine characteristics quantitatively determining mechanical efficiency. Second, it is shown that the ideal Stirling engine has the highest mechanical efficiency potential; this yields an absolute upper limit to the mechanical efficiency of all kinematic engines. This limit is given explicit mathematical expression in terms of only three basic engine parameters : the ratio of the temperature extremes between which the engine operates, the volume compression ratio and the effectiveness of the mechanism of the engine in transporting energy between the piston and output shaft. This result is the mechanical correlate of the Carnot limit to thermal efficiency.

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