How is carbon monoxide formed in a car engine?

Carbon monoxide is produced in an engine when incomplete combustion happened. In a combustion engine intake valve, it combines the fuel and air (nitrogen and oxygen) and when spark plug ignites it pushes the piston downwards. After that piston will push itself upward and pushes the air that was generated by the combination of those components in the exhaust valve. This is a repeated cycle which keeps the engine running. In a perfect combustion process the air being released are carbon dioxide, water and nitrogen. But when an incorrect ratio of the combination of fuel and air in the engine is processed, carbon monoxide is being generated.