Let’s discuss an important terminology and process of marine engine i.e. Scavenging, types, their advantages, and drawbacks.
It is the process of removing exhaust gases from the cylinder after combustion and replenishing the cylinder with fresh air.
In the scavenging process, the air at a pressure higher than atmospheric is used to push out the exhaust gases from the cylinder. Scavenging process is different in 2- stroke and 4- stroke unlike the four-stroke engine, the piston does not push out the exhaust gases from the cylinder. Instead, inlet air at a pressure higher than the remaining exhaust gas pressure enters the cylinder around BDC and sweeps or scavenges the exhaust gases from the cylinder. Scavenging is very important for 2 stroke engine because in 4-stroke there is sufficient overlap between the opening of the inlet valve and closing of the exhaust valve, but in 2 – stroke overlap is limited. 4 – stroke engine having one complete stroke to wipe out exhaust gases which is not in 2 – stroke engine.
Why do we need scavenging?
- Efficient scavenging is necessary for good combustion of fuel inside the engine cylinder.
- The better is the fuel combustion and power output of the engine.
- Reduces fuel consumption per unit power
- Reduces the lube oil consumption
- The passage of scavenging air will also assist cooling of the cylinder, piston, and valves.
- Time available for scavenging process in 2 stroke engine is less than 4 stroke engines.
The whole process completes in three stages –
1st stage – This stage is blown down of exhaust gases into exhaust manifold due to sudden opening of the exhaust valve.
2nd stage – After blowdown of exhaust gases, scavenge ports opens and fresh air swap out remaining exhaust gases from the cylinder. In this matter pressure difference of fresh air and exhaust gases matters.
3rd stage – In this stage, all the efforts made to contain the air mass into the cylinder.
Type of Scavenging
Generally are three type of scavenging
In all above scavenging methods, purpose and functions are the same in all three. The difference in these is only, the path of flow of fresh air inside the cylinder. This path may be loop, cross or uniflow. Let’s discuss one by one with their advantages and disadvantages.
As shown in the diagram, in this method the fresh charged air passes over the piston crown and rises to form a loop inside the cylinder which expels out the exhaust gases through the exhaust port. Here both scavenge and exhaust port is at the same side of the liner. The exhaust port is above the scavenging port.
Here ports are angled to give rotary effect to the incoming fresh air. Generally used by the engines having stroke-bore ratio 1 – 1.75
Cross Scavenging –
As shown in the diagram, in this method the fresh charge air passes are directed upwards in the cylinder, passing under the cylinder cover and down the opposite side. Fresh air is entering from one side and expelling the exhaust from the other side. This method is wiped out and no longer in use. This was used by an engine having stroke-bore ratio 2 – 2.2
Advantages of Loop & Cross Scavenging-
- Do not require exhaust valve as ports are there
- Simple and reliable cylinder cover design
- One centrally located fuel valve
Disadvantages of Loop & Cross Scavenging-
- There is a possibility of intermixing of exhaust gases with fresh air
- Temperature gradient exhaust to scavenge port is high cause thermal stress
- Liner design become complicated due to ports cause an increase in cost
- Uneven wear of piston ring due to ports
- Efficiency is less than uniflow
- Do not scavenge all area of cylinder combustion chamber
Uniflow scavenging –
As shown in the diagram, in this method the fresh charged air enters through the scavenge ports in the lower part of the cylinder liner and the exhaust gas is expelled out through the centrally located exhaust valve in the cylinder head. In this method also ports are angled to give rotary effect to scavenge air.
Advantages of uniflow scavenging –
- Fitting of the exhaust valves gives the advantage of the modern design of variable exhaust timing control.
- Scavenge efficiency is higher than loop and cross scavenging
- Intermixing of the gases is very less
- Good for higher stroke bore ration engine
- Reduces the number and size of ports leads to reduces the problem of liner lubrication
- Arrangement for sealing the bottom of cooling water space become simple.
Disadvantages of uniflow scavenging –
- Complicated cylinder head design
- Required more maintenance
- More than one fuel valve needed in instated of central single one