Definitions of combustion vary somewhat, but when talking about hydrocarbons it is taken to mean a chemical reaction with oxygen that gives off heat and light, i.e. burning something. Combustion requires ignition, or an initial input of energy as heat. Once ignited, the reaction is sustained by the energy released in the reaction.
A hydrocarbon is any chemical compound of carbon and hydrogen. Apart from coal, which is carbon with different levels of contaminants, fossil fuels are hydrocarbons. Methane is the simplest, with the formula `CH_4` . Propane is another gaseous hydrocarbon with the chemical formula `C_3H_8` . Petroleum as it is extracted from the ground is a mixture of hydrocarbons. The process of refining mostly involves separating these into fractions, which are mixtures of hydrocarbons having certain ranges of properties, such as gasoline, kerosene, diesel, etc. Many of these products are burned as fuel, and the heat energy released is harnessed in different ways.
The products of complete combustion are carbon dioxide and water. A general chemical equation for the combustion of a hydrocarbon is written
`C_mH_n + O_2 -> CO_2 + H_2O`
Note that this chemical equation is not balanced. The “m” and “n” subscripts indicate that compounds containing different amounts of carbon and hydrogen all combust according to the same pattern.
The carbon dioxide produced in combustion of hydrocarbons is a concern today because it is known to be a “greenhouse gas”—that is, a gas that, in increased concentrations, changes the balance of radiative energy entering and leaving Earth’s atmosphere, causing a net warming of the atmosphere. Methane, which can escape into the atmosphere during extraction of hydrocarbons, is also a greenhouse gas. Concerns about changes in climate caused by atmospheric warming have motivated recent development in energy sources other than hydrocarbon fuels.