What Is Scavenging?
For a better combustion of fuel oil inside a marine diesel engine, an adequate supply of fresh air is needed. The method by which sufficient amount of air is provided to the engine’s cylinder is known as scavenging.
Scavenging is generally provided by the engine’s turbo-charging system. The more efficient the scavenging, the better is the fuel combustion and power output of the engine.Turbochargers are provided with the engine to use the exhaust gases in order to supply a consistent flow of fresh air inside the main engine.
Scavenging for Two stroke and Four stroke Engines
Scavenging is not the same for both two stroke and four stroke engines. This is mainly because in the four- stroke engine there is adequate overlap between the opening of the inlet valve and closing of the exhaust valve. But in a two-stroke engine this overlap is limited and for this reason a slight mixture of exhaust gases and incoming air occurs inside the cylinder.
Also, in a four stroke engine the air is induced during a downwards stroke of one of the two cycles, i.e per power stroke, and the exhaust gases are removed in the preceding stroke.
But in a two stroke engine there is no specific cycle of scavenging. Air is introduced during the end of power stroke when the exhaust gases are removed. The time of scavenging is too less in two stroke engines, as greater the exhaust valves stay open lesser the time the piston gets for compression. To prevent the effects due to this phenomenon, more amount of fresh air is supplied. Thus, greater the scavenging better the combustion of fuel.
There are different types of scavenging methods that are used in both two and four stroke engines but the basic principle remains the same, i,e. the fresh air enters the cylinder by the opening of the port during the downward stroke of the piston and continues till the port is closed by the upward stroke of the piston. The different types of scavenging methods are based on the location of scavenge ports and design and shape of engine’s exhaust arrangement.
There are mainly three types of scavenging systems :
1) Cross flow
The main difference between the first two system and the uni-flow system is that the former ones require the movement of piston to open and close ports , while the later one requires an exhaust valve for the scavenging to take place. Nowadays, most of the modern ships use uni-flow scavenging system which comprises of a cylinder-head exhaust valve.
Cross scavenging takes place with the help of movement of the piston. The inlet and the exhaust ports are found on the opposite side of the cylinder with the exhaust port situated at a higher level than the inlet port. Once the ignition of fuel takes place, the piston experiences a downward movement due to the expansion of gases. The downward linear motion of the piston first uncovers the exhaust ports which results in the release of exhaust gas at a pressure more than the atmospheric pressure.
This sudden release of pressure from the cylinder is known as blowdown and its effects can be seen on the power card towards the end of the cycle. The piston moves further downward and opens the inlet port to let a fresh stock of air enter the cylinder. The piston again moves up, compressing the fresh air for the next combustion cycle.
In the next article we will learn about the other two types of scavenging systems and their advantages and disadvantages.
References & Image Credits
Marine Auxiliary Machinery, 7th Edition, 1995 by McGeorge