Thermomechanical converters - Gas cycle engines: Stirling and Joule

As with thermal generators, there are two main types of fluid involved in engine operation: gases and two-phase fluids (liquid-vapor). On the other hand, in some engines, heat transfer from the external environment to the thermodynamic fluid is replaced by a chemical combustion reaction inside the machine itself: these are internal combustion engines. So, in addition to classification by fluid type, engines are also classified into internal combustion engines and external combustion or external heat input engines. The type of cycle used is also influenced by another factor. This is the fluid flow mode within the engine. The flow is either continuous, as in the case of turbines, or discontinuous, as in the case of reciprocating engines.

The study of gas engines is the subject of two dossiers.

In this dossier, we deal with maximum efficiency cycles on the one hand, and machines whose basic cycle is the Joule cycle, i.e. gas turbines and turbojets, on the other.

While it's possible to imagine an infinite number of maximum-efficiency cycles, i.e. with efficiencies equivalent to those of the Carnot cycle, we've shown that the reality is less interesting, and that corresponding applications such as Stirling or Ericsson engines, for example, are far from reaching these efficiency values. However, these alternative engines could benefit from further development, given that the heat input is external, so the energy sources can be varied, notably based on renewable energy.

While Joule's engine cycle and its derivatives, particularly with heat recovery, are a simple way of modeling gas turbine operation, it should be noted that most of these continuous-flow machines are actually internal combustion machines. In this case, the fluid flow is no longer cyclical, and combustion replaces the heat input from the hot source. The link between the two types of operation - heat input from a hot source and combustion - is highlighted in this dossier, where exergy analysis, which also plays a role, shows that the Carnot cycle associated with this type of machine has an efficiency equal to unity.

The following folder is an integral part of this dossier. It deals with machines that are extremely present in our environment and that make a complete break with two-source machines: these are reciprocating internal combustion engines.