What is incomplete combustion?

Fuels are normally hydrocarbons - molecules that are made up of only atoms of hydrogen and carbon. When these fuels combust, these atoms react with the oxygen in the air to form oxides.

Hydrogen atoms will form water molecules

Carbon atoms will either form carbon monoxide or carbon dioxide

If there is plenty of oxygen then carbon will form carbon dioxide and we call this complete combustion. If there is not enough oxygen then carbon can only form carbon monoxide and we call this incomplete combustion.

Incomplete combustion is particularly dangerous as carbon monoxide is toxic and can cause death.

This is where there is a limited oxygen supply, less than required. So carbon monoxide is produced alongside carbon dioxide and water. So there are three products in the equation compared to two in complete combustion.

In normal or complete combustion with lots of oxygen around ( called excess oxygen) all hydrocarbon fuels burn to give just carbon dioxide and water. However, when there is not enough oxygen around this is called incomplete combustion and either carbon monoxide (C0) or carbon or a mixture of the two are produced. A good example of incomplete combustion is to watch a bunsen burner burning with a sooty ( yellow/orange) flame. To do this you need the air-hole closed. If you put the flame up against a flame proof mat you will see the soot ( carbon) of incomplete combustion quickly starting to form.

when a fuel burn in limited supply of oxygen it will produce carbon monoxide and water

Incomplete combustion is when the oxygen supply for the reaction is limited.

Water is still produced but instead of carbon dioxide, the odourless toxic gas carbon monoxide is produced.

Incomplete combustion is what happens when there isn't enough oxygen or the supply of oxygen is poor. Whereas in normal (complete) combustion you would have the following equation:

hydrocarbon + oxygen -> water + carbon dioxide

in incomplete combustion you would have:

hydrocarbon + oxygen -> carbon monoxide + carbon + water

because there isn't enough oxygen to make both the water and carbon dioxide. This means that carbon monoxide gets made instead and, if there's any carbon left when all the oxygen is used it becomes soot.

Hope this helps!

incomplete combustion is when the fuel does not burn in sufficient oxygen and therefore produces carbon monoxide, carbon in the form of soot and water