Scavenging is the process
whereby air at a pressure greater than that of atmospheric
pressure is used to push the exhaust gas out of the cylinder of
an engine. Unlike the 4 stroke engine, a two stroke diesel
engine does not use the piston to push out the exhaust
gas, instead, air enters the cylinder around bottom dead centre
and sweeps or scavenges the exhaust gas from the cylinder.
2 stroke engines with an exhaust valve mounted in the cylinder
head are known as uniflow scavenged engines. This is because the flow of
scavenging air is in one (uni) direction.
MAN B&W MC series uniflow scavenged engine
Some 2 stroke engines do not have exhaust valves; As
well as scavenge ports in the cylinder liner, they are fitted with
exhaust ports located just above the scavenge ports. As the piston
uncovers the exhaust ports on the power stroke, the exhaust gas
starts to leave the cylinder. When the scavenge ports are uncovered,
scavenge air loops around the cylinder and pushes the
remaining exhaust gas out of the cylinder. This type
of engine is known as a loop scavenged engine. Note that the piston
skirt is much longer than that for a uniflow scavenged engine. This
is because the skirt has to seal the scavenge and exhaust ports when
the piston is at TDC.
Although simpler in construction with less moving parts, these
engines are not as efficient or as powerful as uniflow scavenged
engines. The scavenging of the cylinder is not 100%, and thus less
fuel can be burnt per stroke.
All modern large 2 stroke crosshead engines now being built are
of the uniflow scavenged type.
SULZER RLA Loop Scavenged Engine