Fuel And Its Effects On DRIVEABILITY: Volatility

Volatility is a gasoline's tendency to change from a liquid to a vapor. Volatility is crucial to engine performance. Liquid gasoline will not burn; only vaporized gasoline burns. Once liquid fuel is injected into an intake port and mixed with air, some of it should start evaporating immediately.

Any liquid's rate of evaporation changes with temperature. Temperatures in an engine vary widely. During operation, the combustion chamber temperatures may rise to more than 822°C (1500°F). Before start-up, on a very cold morning, the temperature may be zero. To burn well across the broad range, a fuel must consist of a mixture of hydrocarbons that vaporize at different temperatures.

The smaller hydrocarbon molecules vaporize at lower temperatures and the larger hydrocarbon with larger molecules vaporize at higher temperatures. The easily vaporized hydrocarbons, which are easy to burn, are called the "light-ends." They help with quick starts when the engine is cold. The less volatile "heavy-ends" contribute more to engine power; but because they are harder to burn, they can coat the cylinder walls without burning and in extreme cases may dilute the oil in the crankcase.

Lower volatility is desirable in the summer to prevent excessive premature evaporation of the fuel. In the winter, high volatility is desirable because the fuel must vaporize quickly for cold starts and good cold driveability.