2 - Background

2.2 - Specific examples

2.2.2 - Mechanical design of heat engines

2.2.2.1 - Reciprocating four-stroke and 2-stroke petrol engines

The reciprocating petrol engine requires a uniform air-fuel mixture and uses spark ignition.

Fuel is generally petrol but can also be a replacement product such as liquid petroleum gas.

The fuel is mixed either in the carburettor, or indirectly in the inlet manifold, or directly in the cylinder and is injected cyclically.

The mixture is periodically compressed in a work cylinder up to a pressure varying between 15 and 25 bars.

The resulting compression temperature varies between 400º and 600º C. and remains below the auto-ignition threshold so the mixture has to be ignited in order to transform the fuel into thermal energy.

Spark ignition takes place almost instantaneously. It generates an increase in pressure equal to about seven times the pressure obtained at the end of the compression stroke, whereas the effective mean pressure is roughly equal to the pressure obtained after compression

These values cannot be calculated accurately due to the number of variables that simultaneously come into play. For that reason, the manufacture of petrol engines is largely determined by empirical experience.

In four-stroke engines the charge is renewed through inlet valves and in two-stroke engines through orifices arranged near the bottom dead centre.

The useful work generated is transformed into engine torque by a con rod assembly at the end of the crankshaft. The con rod assembly is complex and requires engine idling speeds between 800 and 1200 rpm to prevent the engine from stalling. This in turn represents a significant consumption of fuel, especially where circulation is not fluid.

Therefore we should note that:

- the relationship between the maximum fatigue of mechanical components subjected to peak pressure and the mean fatigue of the same components is equal to p_max/p_me;

- These components are designed to withstand peak pressure plus a safety margin and not the effective mean pressure, which would require largely oversized components.

Advantages of the petrol engine:

-design mastered for decades;

- economic structures are based on this engine;

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

Disadvantages of the petrol engine:

- it requires sophisticated fuels;

- poor p_max/p_me ratio;

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

- relatively high toxic emissions of NO, HC, CO;

- noisy;

- requires a gearbox.

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