The tables in the booklet complement the property tables in the appendices to Thermodynamics: Concepts and Applications and Thermal-Fluid Science: An Integrated Approach by Stephen R. Turns. In addition to duplicating the SI tables in these books in both SI and US Customary units, the present booklet contains property data for the refrigerant R-134a and properties of the atmosphere at high altitudes.

This chapter continues the examination of the limits on Stirling engine performance by taking into consideration, with the mechanical losses already covered, thermal limitations and losses from which real Stirling engines suffer. First covered is limited heat transfer rate into and out of the working fluid of the engine. This is modeled here just as Curzon and Ahlborn did for Carnot engines (Curzon & Ahlborn, 1975). In addition, introduced later in the chapter is an internal heat leak through the engine from the hot to the cold section governed by the same heat transfer regime. This simulates in a general way the various internal thermal losses occurring in real Stirling engines.

HEAT EXCHANGE

Thermal energy must be transferred into and out of a Stirling engine via heat exchangers at the hot and cold ends. A temperature gradient is required to drive the transfer; in other words, there must be a temperature differential between the source reservoir and the working fluid when it receives thermal energy. Likewise, a temperature difference is required between the engine working substance and the sink reservoir in order for the engine to reject thermal energy. The larger these differences, the greater the rate of energy transfer. This aspect of heat transfer is modeled in a general way by Newton's Law of Cooling (Bejan, 1996b).

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.