Scavenging is the process that occurs in ic engine during the overlapping of valves (Opens at the same time) in which burnt gases are released out of the cylinder with the entering of fresh charge inside the engine at the same time.
The good scavenging process is significant for the engine’s proper fuel combustion and better power output.
The four stroke engines have enough valve overlap time for the scavenging while in the two-stroke engines, where one cycle is completed in two strokes of piston hence it doesn’t have enough time for scavenging. Therefore 3 types of scavenging methods are used in two-stroke engines.
The 3 types of scavenging used in two stroke IC engines are as follows:-
- Uniflow scavenging
- Cross-flow scavenging
- Loop or reverse scavenging
1] Uniflow scavenging:-
In this scavenging method, the one or two intake ports are located at the sides of the cylinder and the exhaust port is located at the cylinder head.
When the charge enters from both side intake ports, the charge flows upwards and after the combustion, the burnt gases go out of the cylinder from the exhaust valve which is located at the top of the cylinder.
Less air loss occurs and has lower fuel consumption hence gives better engine efficiency as compared to other methods.
Good scavenging obtains during the low speed and the high speed using the uniflow scavenging method.
The uniflow scavenging commonly is used in large-sized two strokes engines such as in two-stroke ic engines of big marine ships.
2] Cross-flow scavenging:-
The cross-flow type scavenging has an inlet port and the exhaust port that are located on the opposite sides to each other. The deflector piston (hump-shaped piston head) is used in this type of scavenging.
When the charge enters into the engine from the intake port, due to the hump shape of a piston, this incoming charge moves upwards and pushes the upper burnt gases to the downside and then these gases goes outwards through the exhaust port.
Therefore, the hump on the piston assists in the engine’s scavenging operation.
This method of scavenging performs well during the low speed of the engine while bad scavenging obtains at the high speed of the engine or at full throttle time.
3] Loop or reverse scavenging:-
In this scavenging method, the intake port and exhaust ports are located at the same side in the engine.
In this method, the size of the intake port is a large size to enter a large volume of charge inside the cylinder, and the size of the exhaust port is small (increases velocity) to flow out burnt gases quickly.
When the fresh charge enters from the intake port, the fresh charge has a swirling motion (as like a loop) hence this fresh charge moves upwards and pushes back the burnt gases. Hence in such a way, these burnt gases flows out of the cylinder.
This method of scavenging performs well during the high speed of the engine or at full throttle time while bad scavenging obtains at the low speed of the engine.