Different Types of Scavenging Process – With Advantages and Disadvantages

Different_Types_of_Scavenging_Process_image
Read: What is meant by Scavenging?

There are mainly three types of scavenging method on the base of the flow of air
(I)   Cross flow scavenging
(II)  Reverse or Loop scavenging
(III) Uniflow scavenging

Cross flow scavenging

Cross flow and Revere loop scavenging take place with the help of piston movement. Cross flow scavenging, transfer port (inlet) and exhaust port are situated on the opposite side of the cylinder. The exhaust gas is pushed out by cross flow. The piston head is designed to have a hump shape called deflector. The fresh air enters in the engine cylinder is deflected to the upward by a deflector and pushing exhaust gas down the other side. Before loop scavenging invented, almost all two-stroke engines use this method.

Advantages and disadvantages of Cross flow scavenging

Advantages:
  • Low manufacturing cost.
  • Good scavenging at low speed and part throttle.
  • Low engine volume for the multicylinder arrangement.
Disadvantages:
  • Heavy piston with very high heat absorption.
  • High tendency to knock.
  • Poor scavenging at high speed and full throttle.
  • Compulsory water cooling, difficulty in cooling piston crown.

Loop scavenging

Loop_scavenging_image_Schnuerle_porting
Similar to the cross flow scavenging, but the inlet ad exhaust port are placed the same side of the engine cylinder. The gases are encouraged to move loops. This type of scavenging used carefully designed transfer port (inlet) to loop fresh air rise towards the cylinder head on one side and pushes the burnt gas down to the exhaust port installed just above the inlet. It has a flat or slightly domed piston crown. This is the most used type of scavenging system.
Example: Schnuerle porting

Advantages and disadvantages of Loop scavenging

Advantages:
  • Low maintenance.
  • The low surface area to the volume of the cylinder (hence the heat loss reduced).
  • Good scavenging at full throttle.
  • Water cooling system not necessary.
Disadvantages:
  • Poor scavenging at part throttle operation.
  • Scavenging time is short.

Uniflow scavenging

Uniflow scavenging so called because both fresh charge and exhaust gas move in a same upward direction. In this method, fresh air enters from the lower side of the cylinder, and it pushes out exhaust through the exit valve situated at the top of the cylinder. This method is used in large two-stroke diesel engines.

Advantages and disadvantages of uniflow scavenging

Advantages:
  • Extended time for valve operation.
  • The possibility of mixing is reduced due to uniflow.
  • Increase power output.
  • Most efficient of all three methods.
  • Good scavenging at all speed ranges and throttle position.
  • Low fuel consumption compared to other scavenging types.
Disadvantages:
  • Elaborate and costly construction.
  • Difficulty in cooling the piston.

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