We begin the operating cycle with the piston at its position of closest approach to the top, or head of the cylinder. We turn the crank. The intake valve opens and the piston moves down, away from the head, creating a partial vacuum in the cylinder. A mixture of air and fuel rushes in through the open intake valve. When the piston reaches the bottom of its travel, the intake valve closes. Now the piston reverses direction, rising toward the head, compressing the fuel-air mixture. When the piston has nearly reached its top-most position, a spark is sent across the spark plug’s electrodes. The intense heat of this spark ignites the compressed fuel-air mixture, burning it and releasing heat. This heat causes the pressure of the burned gases to rise by about 7 times, to a high pressure. This pressure now drives the piston down again, but with great force, giving the crank-shaft a strong spin. Attached to the crankshaft is a flywheel, whose purpose is to keep the engine rotating smoothly even though power is delivered by the piston only intermittently.