A spark plug

A spark plug is what gives the cold fuel its hot behaviour and the energy to force the piston downward. It generates the electrical spark to ignite the fuel. The battery supplies the energy which is intensified by the ignition coils. The spark plug generates the spark by forcing the electrical charge to jump a gap in the lower part of the plug which is inside the cylinder.
To enable a spark to jump the gap, enough quantity of voltage must be made available by the ignition system.
In case you don't know, you can only find a spark plug in a petrol engine.