Heat Engines

The most numerous thermal – mechanical energy converters are, without doubt, reciprocating internal combustion engines (C.I.C.E.). They also have the widest power range, from a few watts to several tens of megawatts. These are gas engines that operate with a single heat source, like gas turbines, whose cycles and operating principle are presented in the dossier. . The working fluid does not undergo a thermodynamic cycle, but an open sequence of transformations: starting with air and fuel under atmospheric conditions, the result is exhaust fumes, which are produced at atmospheric pressure and generally at a still relatively high temperature. The chemical energy of the fuel replaces the heat input from the hot source during the cyclic evolution of the thermodynamic fluid in two-source engines.

However, the thermodynamic presentation of reciprocating internal combustion engines is traditionally based on two fundamental dithermic cycles:

• the Beau de Rochas (or Otto) cycle, which is an isochoric heat input cycle;

• the Diesel cycle, an isobaric heat input cycle.

Each of these two cycles is a simple model of fluid evolution in the two essential types of combustion engine: the spark-ignition or gasoline engine, and the compression-ignition or diesel engine.

However, as this basic model, based on the assumption of a perfect gas with constant thermal capacities, leads to results that are far too optimistic to represent the operation of a MACI, a more realistic model that is still simple to implement is presented at the end of this dossier.