Petroleum as obtained from the oil wells is predominantly a mixture of many hydrocarbons with differing molec ular structure. It also contains small amounts of sulphur, oxygen, nitrogen and impurities such as water and sand. The carbon and hydrogen atoms may be linked in different ways in a hydrocarbon molecule and this linking influences the chemical and physical properties of different hydrocarbon groups. Most petroleum fuels tend to exhibit the characteristics of the type of hydrocarbon which forms a major component of the fuel.