In a 2-stroke diesel engine, an air pump or blower provides clean incoming air at pressure slightly above that of outgoing exhaust gases.
The blower delivers air to an air-box surrounding the cylinder.
When the piston uncovers the inlet ports, air enters the cylinder through a number of them around the cylinder walls.
This fills the cylinder with fresh air and helps blow out exhaust gases. Removing exhaust gases is called scavenging. Without effective scavenging, diesel engines lose efficiency.
How scavenging occurs depends on the design of the cylinder and ports. In this example, the air flows in one direction, towards the top of the cylinder. This is called uniflow scavenging.
Crossflow scavenging occurs when air enters on one side of the cylinder, and exhaust gases exit on the other side. No valves are used, and the airflow is across the cylinder.
In loop scavenging, air enters across the top of the piston and flows up in a loop before moving in the direction of the exhaust port.
