As the piston reaches top dead center at the end of the compression stroke，theignition system produces an electric spark. The spark sets fire to the fuel-air mixture. Inburning，the mixture gets very hot and expands in all directions. The pressure rises toabout 600 to 700 pounds per square inch. Since the piston is the only part that can move,the force produced by the expanding gases forces the piston down. This force，or thrust，is carried through the connecting rod to the crankpin on the crankshaft. The crankshaft isgiven a powerful twist. This is known as the power stroke（Fig. 3. 5）．This turningeffort，rapidly repeated in the engine and carried through gears and shafts，will turn thewheels of a vehicle and cause it to move along the highway.