Scavenging in engine

Scavenging is the process of removal of exhaust gases from the cylinder with adding fresh air in it.

Types of scavenging:

Uniflow scavenging
Fig: Uniflow scavenging
Loop scavenging
Fig: Loop scavenging
Cross flow scavenging
Fig: Cross flow scavenging

1. Uniflow scavenging:

Uniflow scavenging is a type of scavenging process in which the fresh air flows into the cylinder in the same direction as the exhaust gases are expelled. This creates a unidirectional flow of air and gas which helps to reduce residual gases and increases engine efficiency.

Uniflow scavenging

2. Loop scavenging:

Loop scavenging is a type of scavenging process in which the fresh air and exhaust gases form a loop shaped path before flowing out of the cylinder.

Loop scavenging

3. Cross flow scavenging:

Cross flow scavenging is the type of scavenging process in which the fresh air and exhaust gases flow from the opposite direction because the inlet and exhaust ports are on opposite sides.

Cross flow scavenging

Steps in Uniflow scavenging (used in two stroke main engine):

Refer to the valve timing diagram of two stroke engine

        Step 1: Blowing out of exhaust gases (exhaust valve opens 75 degrees before BDC).

        Step 2: Scavenging (Exhaust valve remains open and scavenging ports open 42 degrees before BDC).

        Step 3: Post scavenging (Scavenging ports close at 42 degrees after BDC but exhaust valve remains open till 60 degrees after BDC).

steps of scavenging

Why is Scavenging required in the engine?

Scavenging is required in an engine to remove the residual gases from the combustion chamber. During the combustion process, not all of the fuel-air mixture is burned, and some of it is left over as exhaust gases. If these gases are not removed from the engine, they can cause a number of problems, including reduced engine efficiency, increased emissions, and even engine damage.

In a two-stroke engine, scavenging is particularly more important, as the engine does not have separate intake and exhaust cycles, and the fresh air-fuel mixture must be able to enter the combustion chamber and push out the exhaust gases.

        Overall, scavenging is an important part of the engine’s operation, as it helps to maintain the engine’s efficiency and reduce emissions.

Which type of scavenging is better?

Uniflow scavenging is better because uniflow scavenging provides better scavenging efficiency and allows for higher power output and speed, which are important for these applications. Fresh air comes in from below and directly pushes the exhaust gases out of the cylinder.


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