Scavenging - marine diesel engine

The process of exchanging the gases present in the cylinder after completion of expansion with a charge of uncontaminated fresh air is termed as scavenging.
The performance of marine diesel engine depends to a large extent on the efficiency of the scavenging. There are few requirement's for that which are:

1.  The exhaust valves should open early,in advance of the scavenge ports for the effective removal of the exhaust gases.
2. Scavenge ports should be closed after the process of gas exchanging is carried out.
3. Loss of fresh air escaping through the exhaust valve should be minimized.

The Scavenging process is different for two and four stroke engines. It is so because in 4 stroke engine have in between opening and closing of exhaust and scavenge valves. But for the 2 stroke engine the overlap is limited and so there is always slight mixing of fresh air with exhaust gases inside cylinder.

Different method's of scavenging: Scavenging can be of many types which can be  used in both 2 & 4 stroke engines but the basics remain the same, i,e. fresh air enter the cylinder through opening of the port/valve in downward stroke of the piston and continues till the port closes in upward stroke.These scavenging methods are based on the location,shape,size of scavenge ports and engine's exhaust arrangement.

Uniflow Scavenging: The scavenge air enters through the scavenge ports in the lower part of the cylinder liner and is expelled through centrally mounted exhaust valve. In this system charge flows in one direction and so the intermixing is minimum leading to high scavenge efficiency.

Stroke to Bore ratio: 3.5-4.2

Loop Scavenging: The exhaust ports are placed above the scavenge ports on the same side of the liner. The air flows in a loop within cylinder, expelling gas through exhaust ports.

Stroke to Bore ratio: 1.75

Cross Scavenging: In this system the air is directed upwards, passing under the cylinder cover and down on the opposite side.

Stroke to Bore ratio: 2-2.2 

Difference between uniflow and cross/loop scavenging:

UNIFLOW SCAVENGING:Uniflow scavenging with a large centrally positioned exhaust valve is used in most modern slow speed large diesel engines.

Advantages:

1. High Stroke – Bore ratio.
2. More power can be developed.
3. Design of the cylinder liner is simple due to absence of exhaust ports. Cost of liner is less.
4. Effective length of Piston stroke is considerably more.
5. Arrangement for sealing the bottom of cooling water space is simpler.
6. The increased scavenging efficiency with this type of scavenging creates greater scope for increased economy and so all modern designs are based on the design.
7. The reduced no and size of ports reduces the problem of liner lubrication, leading to less oil being consumed.

Disadvantages:

1.  Require exhaust valves, additional cost and maintenance.
2.  Require two or more fuel valves to be fitted per cylinder.

CROSS/ LOOP SCAVENGING:

Advantages:

1. Do not require exhaust valves. Simple cylinder cover design and simpler camshaft and camshaft drive design.
2. Only one centrally located fuel valve is sufficient.

Disadvantages:

1. Design of Cylinder liner is complicated because of exhaust ports hence liner is expensive.
2. Greater possibility of mixing of gases.
3. Large temperature difference between small area of scavenge and exhaust ports leading to high thermal stresses.
4. More chance of piston rings leakage.

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Author marineGuru

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