Scavenging and its types of scavenging system

There is no separate exhaust and inlet stroke in 2 stroke engines.
when we compared timing of a engine that operating on 2 stroke cycle, the timing is one fourth shorter then that of engine running with 4 stroke cycle.

"Scavenging is a process of replacing the burnt gases (present in the cylinder after combustion )with a fresh air."

There are three type of scavenging system:

1. Cross flow scavenging.
2. Loop flow scavenging.
3. uni-flow scavenging.

1. Cross flow scavenging:

• The exhaust and air inlet ports are arranged around the boundaries of the lower end of the cylinder liner.
• Inlet and exhaust ports are located opposite to each other.
• Simplifies the engine construction and minimizes the maintenance cost.
• This system is used in old sulzer engines.

2. Loop flow scavenging:

• The exhaust and air inlet ports are arranged around the boundary of lower end of the cylinder liner.
• In loop scavenging engines, the exhaust ports are placed on top of the inlet ports on same side.
• Incoming air passes transversely over the piston is forced to leave through exhaust ports.  

3. Uni flow scavenging:

• Uni flow scavenging engines are also known as poppet valve engines, air and exhaust gases passing through the cylinder do not change the direction.
• Fresh air enters from the inlet port which are situated at the lower end and exhaust gases comes out from the top through exhaust port.
• In the exhaust port the poppet valve is placed which controls the exhaust outlet, mechanically or hydraulically from a camshaft.

Modern engines are equipped with uni flow scavenging system which has :

  higher specific output.

  improved fuel efficiency.

  higher scavenging efficiency.