## 9 Operating Characteristics Of Stirling Engines

introduction

A Stirling engine consists essentially of two spaces of variable volume and different temperatures connected by a duct. The spaces arc (illed with the working fluid and the duct is provided with apparatus for adding heat (heater), abstracting heat (cooler) or storing heat (regenerator). The system can be arranged in any number of ways, in single or multiple combinations, with the volume variations caused by reciprocating or rotary motion. Whatever form of mechanical arrangement is adopted certain common factors and considerations prevail which affect the system performance. These are discussed below.

ideal maximum power and el higiency

For any Stirling-engine system the maximum power and efficiency would be achieved if the ideal Stirling cycle described in Chapter 2 could be followed. This requires that all the working fluid in the system is. at any instant, in the same condition (thermodynamic equilibrium) and all the heat added to the cycle or rejected from it is transferred at constant temperature. Similarly, at any particular location in the matrix, all the heat transferred between the regenerator and the working fluid occurs at constant temperature.

Such conditions are not, of course, achievable in real engines. It is simply not practical to have all the working fluid in one space or the other at the same temperature. There would be no 'dead' space in the engine, that is, no volume in the connecting duct with all the heat transfer apparatus. Equally unrealistic is the requirement for constant temperature (isothermal) heat addition and rejection. It could only be achieved with infinite rates of heat transfer or with a working fluid having a heat capacity that was zero.

The ideal theoretical cycle does define the absolute upper limit of performance and might be represented, for example, by the lines *A-A' drawn in Fig. 9.1. This shows the power output per unit mass of working fluid and thermal efficiency as a function of some parameter, let us say, the pressure of the working fluid or the speed of the engine.

the real cycle