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In the previous article we learned about scavenging, its importance and the types of scavenging systems that are used by marine diesel engines. In this article, we will discuss about few more types of scavenging methods and the advantages and disadvantages related to them.
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The Loop type scavenging is similar to the cross type of scavenging but has only difference that the inlet and exhaust ports are present on the same side of the cylinder. In this type also the exhaust port is located above the inlet port, and is the first one to be opened by the downward movement of the piston.
The incoming fresh air passes over the cylinder crown and rises towards the cylinder end, pushing the exhaust air out of the cylinder. The path of the air flow is shown in the figure.
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Disadvantages of Loop and Cross flow scavenging
Loop and Cross flow scavenging are no more found in any of the modern engines. This is because of the problems attached with having fixed ports.
The main disadvantage of having these types of scavenging is that it couldn't clear off the cylinder of all combustion products.
Also, the time for scavenging is too less for the scavenging period only lasts till the time scavenging ports are closed.Efforts were made to eradicate this problem by providing high velocity air, but the steeply angled ports couldn't allow the smooth entry and exit of the air.
Many new designs were introduced to eradicate this problem but they all proved unreliable due to some or the other drawbacks. In the new design there were chances of the inlet fresh air to shortcut directly to the exhaust port. The situation used to worsen when the scavenge ports were blocked due to carbon build up.
One more disadvantage of a cross flow is that it needs a piston skirt to be fitted in order to prevent the air or exhaust gas to escape when the piston is at the top of the stroke. This increases the load on crosshead bearings and also increased the wear of the liner.
The fitting of both inlet and exhaust port in the cylinder liner made the design and construction complex and also made the lubrication difficult. This lead to a higher need for lubricating oil and also increase in temperature.
In the loop flow, low temperature inlet air and high temperature gas causes severe temperature differences which causes detrimental effects on the cylinder liner material.
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Advantage of Loop scavenging
An advantage of loop system is that it eradicates the need of an exhaust valve. This results in simpler cylinder cover design and less stressed cam shaft. This also prevents the problem of thermal stressing faced due to the fitting of exhaust valve.
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This type of scavenging takes place not on the basis of piston movement but with the help of an exhaust valve fitted in the cylinder cover.
Fresh air enters at the lower end of the cylinder liner and leaves from the exhaust valve on the top. The inlet scavenge ports are so designed and angled that the rising column of air forms a rotational movement. As the piston rises upwards, it compresses the air through the centrally mounted exhaust valve.
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Advantages of Uni flow
The main and foremost advantage of this system is the elongated timing for the opening and closing of the valves. In the modern engines the timing can even be altered in order to suite the requirement. This modern design is known as "Variable Exhaust Timing" Thus the efficiency of combustion by effective compression can be adjusted according to the varying loads and engine speeds
This system increases the power output of the engine and makes the system more economical.
The fitting of the exhaust valve reduces the mechanical complexity of the engine. This also makes the lubrication system of the engine easy and less complex.
The reduced number and size of the ports reduces the consumption of oil.
It is the most efficient of all the systems.
The amount of scavenge air required is also less due to the efficient compression of the piston.
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References & Image Credits
Marine Auxiliary Machinery, 7th Edition, 1995 by McGeorge