During combustion, fuel reacts with oxygen and heat, then releases energy. Complete combustion happens in a plentiful supply of oxygen. Complete combustion releases more energy than incomplete combustion. Incomplete combustion occurs when the supply of air is limited, or poor. Incomplete combustion also creates carbon monoxide, and more soot. Several factors must be considered when choosing the best fuel for a particular purpose. Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released. About 21 per cent of the air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion. Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen. Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. Combustion is a high-temperature oxidation reaction.