2 - Background

2.2 - Specific examples

2.2.1 - Thermodynamics of heat engines

2.2.1.7 - Combustion

Combustion takes place in two different ways:

- cyclically in reciprocating piston engines, after each replacement of the charge;

- continuously in gas turbine engines.

Cyclic combustion presents the most difficulties because it has to take place with as little lag as possible. Hence, the cycle in a four-stroke engine running at 6,000 rpm will take place in 1/(6000/2/60) = 1/50 = 0.02 sec

However, as the cycle includes a suction, a compression, a reduction in pressure and an exhaust stroke and combustion can only take place during the expansion stroke, it can only last: 0.02/4 = 0.005 s.

These times are divided by around 3 in a two-stroke engine.

In uniform mixing and spark-ignition engines, the ignition phase is practically constant and only depends on the composition of the mixture. Heat release is determined primarily by the shape of the combustion chamber and the position of the ignition point.

Combustion speed is determined by the diffusion process in front of the flames, turbulence intensity and temperature change in the unburned fuel.

As spark duration is constant and has to take place before the expansion phase, a system must be provided for adjusting the ignition moment in the form of ignition timing so that the thermal energy in the fuel is at a maximum when it has to be transformed.

In engines with non-uniform mixing, auto-ignition can take place before the end of the compression stroke. The fuel is then injected into the highly compressed air and heated to between 700º and 900ºC.

Combustion takes place in two stages, in what is known as a Sabathe cycle:

- a phase during which the fuel injected before ignition ignites: This is the isochoric heat input according to the A. Beau de Rochas cycle;

- a phase during which the fuel injected after combustion has started burns: This is the isobaric heat input according to the R. Diesel cycle.

Continuous combustion in gas turbine or external combustion engines must be stable and steady. That is the main requirement.

2.2.1.8 - Thermodynamic performance

The thermodynamic performance of a heat engine depends on the quantity of heat dissipated, and therefore lost. It is equal to:

η = (θ′ - θ′′) / θ′′

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