The sustained combustion of organic carbon compounds requires breaking strong chemical bonds. A covalent bond is a strong link where atoms share their electrons. During combustion, the carbon and hydrogen bonds in the fuel break apart.
The atoms then rearrange to form highly stable bonds with oxygen atoms. In an ideal complete combustion, a fuel reacts with an abundant oxygen supply. This perfect reaction yields exclusively carbon dioxide gas and water vapor.
However, sometimes the available atmospheric oxygen supply is heavily restricted. When this lack of oxygen happens, incomplete combustion inevitably occurs instead. This leads directly to the unwanted formation of toxic carbon monoxide gas.
It can also create fine elemental carbon particles, which we call soot. Every spontaneous combustion process requires an initial spark of activation energy. Activation energy is the minimum heat needed to start a chemical reaction.
A small electrical spark or focused heat source usually provides this initial energy. After ignition, the reaction becomes self-sustaining because it releases so much heat. Controlled combustion constantly drives the engines inside of most modern cars.
It also powers massive fossil fuel power plants and fires modern weapons. The rapid expansion of hot gases in these machines generates immense mechanical work.