Fuels and Combustion

Fuels can be separated into two categories:

  • Fossil fuels – These include coal, oil, and gas
  • Renewable fuels – These include wood and biofuels


Combustion is a more scientific word for burning. When fuels burn, energy is transferred to the surroundings. The two types of combustion are complete combustion and incomplete combustion.

Complete combustion

In a complete combustion reaction, a fuel reacts with oxygen to produce carbon dioxide and water.

Illustration of the combustion process. Oxygen (O₂) molecules approach a burning hydrocarbon (CH) flame. As they react, the flame produces carbon dioxide (CO₂) and water (H₂O) molecules that rise upwards. The bottom part of the image shows a simplified chemical equation: Hydrocarbon (CH) plus Oxygen (O₂) results in Carbon Dioxide (CO₂) plus Water (H₂O).

For example:

Methane + OxygenCarbon dioxide + Water

Or, more specifically

CH4 + 2O2CO2 + 2H2O

Vehicles rely on various fuels, such as petrol and diesel, to function. These fuels are made up of hydrocarbons, which are compounds of hydrogen and carbon. When these fuels burn in the engine, they produce carbon dioxide and water as byproducts.

Sulphur dioxide can be produced during combustion when fuels contain sulphur. The equation for this reaction is:

Sulphur + Oxygen Sulphur dioxide

Nitrogen oxides can also be produced in a hot car engine, often referred to as ‘NOx.’ The equation for this reaction is:

Nitrogen + OxygenNitrogen oxide

Incomplete combustion

Incomplete combustion occurs when there is not enough oxygen for the fuel to burn completely. For example:

Methane + OxygenCarbon monoxide + Water + Carbon

This can lead to the production of harmful byproducts such as carbon monoxide and particulate matter (smoke and soot).

Carbon monoxide is a toxic gas that can be fatal. Meanwhile, particulate matter can contribute to global dimming by blocking sunlight from reaching the Earth’s surface.

Hydrogen Fuels

Hydrogen is an alternative fuel that is much cleaner when burned. As you can see in the chemical equation below, when hydrogen reacts with oxygen in a combustion reaction, the only product is water.

Hydrogen + OxygenWater

H2 + O22H2O

In this case, burning hydrogen does not release carbon dioxide, which contributes to global warming, or any other harmful gases that cause environmental problems.

Therefore, hydrogen fuel may be increasingly used for combustion in the future,

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