Gasoline

Gasoline, or petrol, is made of a mixture of hydrocarbons, which are molecules composed of carbon and hydrogen atoms. Typically, in standard gasoline, the hydrocarbons consist of carbons chains that are 5–10 carbon atoms long. The exact mixture of hydrocarbons depends entirely on the specific sample of gasoline (type of oil that it was made from, company that refined it, additives that were added, etc.). Gasoline is more volatile than diesel oil, or kerosene, not only because of the base constituents, but also because of additives. Volatility is often controlled by blending with butane, which boils at –0.5°C. The volatility of petrol is determined by the Reid vapour pressure (RVP) test. The desired volatility depends on the ambient temperature. In hot weather, petrol components of higher molecular weight and thus lower volatility are used. In cold weather, too little volatility results in cars failing to start.

In hot weather, excessive volatility results in ‘vapour lock’, where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines, rendering the fuel pump ineffective and starving the engine of fuel.