In a cyclic heat engine, the mechanism plays a key and complicated role. Its main objective is to transport energy from the working substance to the output shaft. But it also functions to constrain and effect the movement of the piston in order that it carry out a certain thermodynamic cycle. This requires that the mechanism work in a bidirectional fashion. It must transport work from the piston to the flywheel and output shaft during some parts of the cycle, and from the flywheel to the piston in other parts. In practice, it is sometimes even more complex. For example, just after dead center in some engines, both the piston and the flywheel supply work to the mechanism, which is consumed by friction.

For analytic treatment, a comprehensive model of machines that reflects in detail all of the modes in which a mechanism is called upon to function in an engine is the natural first thought. However, such a model quickly becomes exceedingly complex, as the development in Appendix A shows. Rather, the main text of this monograph employs only very basic principles and examines best possible cases. As will be seen as the chapters unfold, a surprising number of interesting and practical insights about ultimate engine performance can be easily deduced through this simple approach.

Although the analysis presented in Chapter 7 is highly idealized, it is quite appropriate for providing some insight into the geometrical requirements of the ultra low temperature differential Stirling engine illustrated in Figures B.1–B.3. Nicknamed the P-19, this engine has proven itself capable of operating down to a temperature difference of just 0.5 °C (less than 1 °F) between its warm and cool sides. The P-19 was the first to run from heat absorbed while resting on the palm of a human hand. The P-19 was first publicly demonstrated at the 25th Intersociety Energy Conversion Engineering Conference held in Reno, Nevada, in August 1990.

BACKGROUND

A low temperature differential (LTD) Stirling engine may be characterized as one that operates more or less optimally with a temperature difference of less than 100 °C between its hot and cold end. Ivo Kolin was the first to design and build such an engine. At the Inter-University Center in Dubrovnik in 1983 he demonstrated the first of his engines operating with hot water as the heat source and cold water as the heat sink (Kolin, 1983). The engine continued to run until the temperature difference between the source and sink dropped to 15 °C.

Kolin's first engine inspired a number of research projects over the next decade to further develop LTD Stirling engines (Senft, 1996).

This book presents a general conceptual and basic quantitative analysis of the mechanical efficiency of heat engines. Typically, treatment of the mechanical efficiency of heat engines has been performed on a case-by-case basis. In ordinary practice, kinematic analysis and computer simulation of specific engine mechanisms coupled with calculated or measured pressure–volume cycles usually can indeed be effectively used for evaluating and locally optimizing engine designs. However, going beyond the specific and local requires broader insights that only a general theory can provide.

No general approach to mechanical efficiency of heat engines had been available until recently. 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 practitioners. This book presents a theory of mechanical efficiency at a similar level of ideality and generality.

The first results in this area were published in 1985 and further developed in a series of papers up to the writing of this book. The work modeled the interaction between the mechanical section of an engine and its thermal section at a level compatible with that of classical thermodynamics.