ScavengingScavenging is the process whereby air at pressure higher than atmospheric is used to push out the exhaust gases from the cylinder. Unlike the four stroke engine, the piston does not push out the exhaust gases from the cylinder. Instead inlet air at pressure higher than the remaining exhaust gas pressure enters the cylinder around BDC and sweeps or scavenges the exhaust gases from the cylinder.

Two stroke engine with exhaust valve in the cylinder head are known as uniflow scavenged engines. This is because the flow of air is one (uni) direction.

Some two stroke engines do not have exhaust valves. As well as scavenge ports in the cylinder liners, they are fitted with exhaust ports just above the scavenge ports. As the piston uncovers the exhaust ports during power stroke, the exhaust gases start leaving the cylinder. After sometime, when the scavenge ports are uncovered, the scavenge air loops around the cylinder and pushes out the remaining exhaust gases out of the cylinder.

This type of engine is known as loop scavenged engine. In these engines, the piston skirt is much longer than uniflow scavenged engines because the piston skirt has to seal the scavenge air and exhaust gases from mixing, when the piston is at TDC.

Loop scavenged engines are simpler in construction with less moving parts but they are less efficient than uniflow scavenged engines as scavenging is less than 100% efficient and less fuel can be burnt per stroke. Also, as the stroke of engines are increased, scavenging efficiency keeps falling down. All modern two stroke crosshead engines are being built today as uniflow scavenged engines.