C - Cylinder
Cylinder - An engine's pistons move up and down inside a number of cylinders cast into the block. As the piston moves down, it creates a chamber into which the fuel and air are drawn in and as it moves up again, these gases are compressed.
A spark from the spark plug then ignites the gases and a contained explosion forces the piston down again creating driving force. The burnt gases are expelled from the cylinder as the piston rises, and the cycle is repeated.