Too much light components may cause vapour bubbles in the fuel pump and fuel system of a car

Vapour pressure and the cold starting of the engine

The gasoline engine operates on a mixture of air and vaporised gasoline, or petrol. Therefore, sufficient evaporation of the gasoline under all circumstances is essential. This requirement sets limits to the boiling (or: distillation) range of gasoline and on the so called vapour pressure.

Volatility of gasoline and the distillation test

Gasoline is a blend of different kind of hydrocarbons (and sometimes other flammable substances) within a particular boiling range, approx. 35 to 210 °C. To provide a good engine performance, under cold as well as under hot conditions, the ‘spread’ of gasoline components is vital throughout the boiling range. 

The Front end evaporation is the percentage of gasoline evaporated at 70 °C

The Front end evaporation is the percentage of gasoline evaporated at 70 °C and indicates the summer and winter period. Too much gasoline evaporating at 70 °C indicates a high percentage of light components, favouring the cold starting.  On the other hand too much light components may cause vapour bubbles in the fuel pump and fuel system of a car, resulting in a shortage of fuel supplied to the engine, so called Vapour lock.

The Mid range evaporation specifies the percentage evaporated at 100 °C. This is the so called mid range of the gasoline, which is important for the warming-up and acceleration of the engine.

The Tail end evaporation indicates the heavy (high boiling) part of the gasoline (tail end) and is important for the fuel consumption. Components with a higher boiling point have a higher density and therefore a higher heat of combustion. When the components are too heavy, they may give combustion related problems as they burn too slowly and cause deposits.