When fuels burn they react with oxygen in the air. Air is a mixture of gases made up of mainly nitrogen and oxygen gas.
When a hydrocarbon burns in a plentiful supply of oxygen, carbon dioxide and water are produced.
This is known as complete combustion.
This can be shown by the following experiment...
When a hydrocarbon burns in a limited supply of oxygen, carbon monoxide and fine particles can be produced which can affect health.
This is known as incomplete combustion.