2 - Background

2.2 - Specific examples

2.2.2 - Mechanical design of heat engines

2.2.2.2 - Four-stroke and two-stroke reciprocating gas oil engines

The reciprocating gas oil engine uses a non-uniform air-fuel mixture.

Mixing takes place in the cylinder or in a chamber arranged in the cylinder head and opening into the cylinder. The Ricardo chamber is an example of this type of chamber.

The engine operating cycle is comparable to the petrol engine, except that:

- the air is compressed to 30 to 55 bars;

- the compression temperature varies between 700º and 900º C. in order to exceed the auto-ignition threshold;

- injection takes place slightly before the top dead centre;

- peak pressure can sometimes rise up to 200 bars, but is more generally around 150 bars, although pressure has risen to over 240 bars in some laboratory engines;

- the effective average pressure does not exceed 30% of the pressure obtained at the end of the compression stroke in gas-oil injected engines whereas it is limited to the pressure obtained at the end of the compression stroke in engines with supercharged air.

Gas oil injection engines have to work with a greater safety margin than petrol engines, to take into account auto-ignition misfiring which increases peak pressures in the following cycle.

Therefore, taking into account the high compression pressure in gas oil engines, the high peak pressures caused by spontaneous ignition require a relatively heavy engine unit.

Engines with non-uniform mixing must operate with a large air surplus, so even at full charge, gas oil engines have a low power density.

Advantages of the gas oil engine:

- design mastered for a long time;

- commercial structures based on the use of this engine;

- the piston/cylinder configuration performs better at high pressures.

Disadvantages of the gas oil engine:

- high production cost;

- poor to very poor p_max/p_me ;

- low power density, gas oil engines are generally 1.5 times bigger than petrol engines for the same power;

- poor performance: only around 30% of the thermal energy in the fuel is transformed into mechanical energy;

- polluting emissions: soot and unburned hydrocarbons;

- noisy to very noisy;

- requires a large gearbox.

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