In the final exhaust stroke, the piston moves back up again and a second valve opens to allow the spent gas to escape. Then the cycle starts all over again. Typical car engines have either four, six, or eight cylinders. It is important that all of the piston movements are timed to move in an orderly way. Otherwise, the engine won't run smoothly.
Student Activity Build a piston system to convert linear to rotary motion.
For a car to move, the wheels have to turn in a rotary motion. Create a piston by copying and enlarging the illustration below. You will mount the parts on the other piece. Using the point of the nail, carefully punch out four holes in the crankshaft, one hole at each end of the piston rod, and one hole midcenter at the bottom of the piston head. Assemble the piston head, piston rod, and crankshaft using the brass fasteners.