This vacuum can also be recycled to draw any piston blow-by gases Exhaust Systems from the engine's crankcase. This is notorious as a closed crankcase ventilation or positive crankcase ventilation (PCV) system. This expedient the gases are burned with the fuel/air mixture.
As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse. The remaining exhaust gas forms the low-pressure tail component. This tail component may initially trial ambient atmospheric pressure, but the momentum of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the spent fuel from the cylinder and induct the intake stink during the overlap period when both intake and exhaust valves are partially open. The effect is patent as scavenging