Exhaust:

Figure: Exhaust

As the piston approaches 48° BBDC then the cam of the exhaust lobe begins to force the follower upward that causing the exhaust valve to lift off its seat. As displays in Figure, the exhaust gasses begin to flow out the exhaust valve because of cylinder pressure and within the exhaust manifold. After passing BDC, the piston moves upward and accelerates to its maximum speed at 63° BTDC. By this point at the piston is decelerating. As the piston speed slows down then the velocity of the gasses flowing out of the cylinder creates a pressure slightly lower than atmospheric pressure. At 28° BTDC, an intake valve opens and the cycle begins again.