Hello Everyone! We all know, vultures are called scavengers, because they eat up the left-over of animals helping in keeping the environment clean. Scavenging in engines is something similar, done with the intention of cleaning. So, what are we waiting for! Let me take you on a ride where you will learn about the concept and types of scavenging.
In technical words, Scavenging is the process wherein a charge of uncontaminated fresh air at a pressure greater than that of atmospheric pressure is employed to throw out the exhaust gases present within the cylinder after completion of the expansion stroke. Now, you must be aware of processes in an IC Engine. Unlike a 4 stroke engine, a 2 stroke diesel engine does not use the piston to push out the exhaust gases, instead, air enters the cylinder around the bottom dead centre and sweeps or scavenges the exhaust gas from the cylinder.
Why is scavenging necessary?
Combustion of fuel is a very critical process in any IC Engine. Some amount of fuel needs to be burnt in significantly less time. To increase the chances of complete combustion, we provide the engine with excess air because the stoichiometric amount of air would lead to incomplete combustion in that limited time. Along with sending excess air, sending fresh air without contaminants is equally important.
In 4S engines, after the combustion, the flue/combustion gases are thrown out by the dedicated upward piston stroke. So, scavenging is generally not to be worried off in 4S engines. But still, in some engines, scavenge air is sent/preferred to mitigate chances of pocket formation.
However, in 2S engines, the eviction of combustion gases depends on the timing of the engine(opening of exhaust port/valve) and the scavenge air. If the scavenging is not present or ineffective, some pockets of combustion gases will be present in the cylinder. This will lead to improper combustion. This makes the scavenging a necessity for 2S engines.
The functions of scavenge air are:-
- To throw combustion gases out of the cylinder.
- To provide air for compression which will further burn fuel.
- To cool the components.
Types of Scavenging:-
The three main types for 2S Marine Diesel Engines are cross-flow, uni-flow, and loop.
- In cross-flow scavenging, the incoming air is directed upwards pushing the exhaust gases before it. The exhaust gases then travel down and out of the exhaust ports.
- In loop scavenging the incoming air passes over the piston crown then rises towards the cylinder head. The exhaust gases are forced before the air passing down and out of exhaust ports located just above inlet ports.
- In uniflow scavenging, the incoming air enters the lower end of the cylinder and leaves at the top via large exhaust valves.
Loop Scavenging
In this method the fresh charged air passes over the piston crown and rises to make a loop inside the cylinder which expels out the exhaust gases through the exhaust ports. Both scavenge and exhaust ports are at the same side of the liner. Generally employed by engines having stroke bore ratio 1-1.75.
Cross Scavenging
In this method the fresh charge air passers are directed upwards in the cylinder, passing under the cylinder cover and down the other side. Generally employed by engines having stroke-bore ratio 2-2.2.
Advantages of Loop & Cross Scavenging:
- No exh. valve required.
- Simple and reliable single cover design.
- One central fuel valve located
Disadvantages of Loop & cross Scavenging:
- There is a possibility of intermixing of exhaust gases with the fresh air.
- The temperature gradient from the exhaust to the scavenge port is high causing thermal stress.
- Liner design becomes complicated due to ports, causing an increase in cost.
- Uneven wear of piston rings due to ports.
- Scavenging Efficiency is less than uniflow.
- Carbon deposits in the cylinder is more.
- Longer piston required leading to a reduction in the power-to-weight ratio.
Uni-flow Scavenging
In this method the fresh charged air enters through the scavenge ports in the lower part of the cylinder liner and exhaust gas is expelled out through centrally located exhaust valve in the cylinder head.
Advantages of Uni-Flow Scavenging:
- Fitting of exhaust valves gives the advantage of the modern design of variable exhaust timing control.
- Scavenge efficiency is higher.
- Intermixing of the gases is less.
- The cylinder liner lubrication requirement reduces.
- Removes the necessity of longer piston.
Disadvantages of Uni-Flow Scavenging:
- Complicated cylinder head design.
- Requires more maintenance.
- More than one fuel valve needed.
- Increase in the number of moving parts contributing to frictional losses.
So, now we know scavenging is basically cleaning the combustion space and preparing it for next combustion process. Uni-Flow is a modern concept which is used in almost all of the engines now-a-days.
Authored By:- Cdt. Abhinav Malhotra, TMI
