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.

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