Exhaust on Four Stroke Cycle
As the piston approaches 48° BBDC, the cam of the exhaust lobe starts to force the follower upward, causing the exhaust valve to lift off its seat. As shown in Figure above, the exhaust gasses start to flow out the exhaust valve due to cylinder pressure and into the exhaust manifold.
After passing BDC(bottom dead center), the piston moves upward and accelerates to its maximum speed at 63° BTDC(before top dead center). From this point on the piston is decelerating. As the piston speed slows down, the velocity of the gasses flowing out of the cylinder creates a pressure slightly lower than atmospheric pressure. At 28° BTDC, the intake valve opens and the cycle starts again.
