Final answer:
The spark plug ignites the fuel-air mix in the chamber and converts chemical energy to thermal energy. This energy is then converted to the mechanical energy of the piston.
Step-by-step explanation:
The spark plug ignites the fuel-air mix in the chamber. This converts chemical energy in the bonds of the fuel-air mix to thermal energy. This energy is then converted to the mechanical energy of the piston as the gas expands and pushes the piston down.