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Crude oil is the most commonly used source of energy in the world today. In total, roughly 97 percent of crude oil is made up of carbon and hydrogen atoms, primarily in the form of hydrocarbon bonds - the energy obtained from oil during combustion comes from the energy released as these bonds break down, and the carbon atoms in the oil mostly bond with oxygen in the air to form CO₂, while the hydrogen also bonds with oxygen to form H₂O gas.

Depending on the source of crude oil, the sulfur content of oil can be as high as six percent (crude oil from the U.S. contains roughly 1.4 percent), and this can be converted into sulfur dioxide during combustion - therefore the removal of sulfur during refining is very important for both fuel efficiency and the environment.