Most automotive engines operate in a four-stroke cycle: intake, compression, power and exhaust. The intake stroke refers to the first downward motion of the piston in the cycle where the fuel-air mixture is drawn into the combustion chamber through the open intake valves. As the piston reverses direction and begins to move back up, the intake valves close. This upward motion of the piston is the compression stroke. Compression raises the pressure and temperature of the mixture. Near the top of the compression stroke, the spark plug ignites the air-fuel vapors. The mixture then burns and expands, driving the piston down for the power stroke. As the piston reaches the bottom of its movement and begins to move back up again, the exhaust valves open leading to the exhaust stroke. The upward motion of the piston pushes the unburned gases out of the engine into the exhaust manifold and eventually, out through the exhaust pipe. And then the entire process starts all over again.