Scavenging

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Scavenging

• Scavenging 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.

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• 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.

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• 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.

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• 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.

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• 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.

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