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Till now we have studied about two different types of lubrication namely for journal bearings and that used in crosshead bearings in marine diesel engines. In this article we will move further and know something about another equally interesting type of lubrication – namely cylinder lubrication
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We know that a piston moves to and fro within a cylinder and there are several piston rings inserted in the grooves on the piston which perform multiple tasks including sealing of pressure inside the combustion chamber lest it leaks from below. The speed of rubbing between these piston rings and the cylinder liner is quite high and apart from that there are extreme conditions of temperature, pressure and corrosive gases inside the combustion chamber to which any lubricating fluid between the liner and the piston would be subject to.
If you remember the 3 types of lubrication conditions we had studied in a previous article, the type of lubrication found inside the cylinder is of the thin film or boundary type of lubrication for most parts of the piston motion except for the upper and lower extremes of motion where this changes to imperfect lubrication as the speed reduces to zero at these points and direction of motion is reversed.
It does not require much reflection to imagine that lubricating oil used inside a cylinder liner must be able to withstand conditions of high temperature, pressure and should have good corrosion resistance as well. Just to give you a broad idea, typical lubricating oil used inside the liner should have viscosity in the range of 115-150 cst at temperatures of 50 degrees Celsius and this should not vary too much even at higher temperatures. Special additives could be added to the oil in order to increase its oiliness or stickiness. Apart from that the amount of oil supplied should be ideal because if too little oil will cause the surfaces to tear apart rapidly, too much of oil will also be a problem as it would interfere with normal combustion inside the chamber and also cause damage to various parts such as valves, valve seats and the exhaust pipe.
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In case of large two stroke marine diesel engines used for the main propulsion of the ship an independent lubricating system is used for the huge cylinder liners. Basically the system consists of a number of lubricating quills located around the circumference of the cylinder to which oil under pressure is supplied twice during each revolution of the crankshaft. I might devote a separate article to explain this system in detail but for the moment a simple diagram would suffice to explain its working. The circular section shows the top view of a cylinder liner with 8 lubricating quills fitted along its circumference which contained lubricated oil under pressure. Of course the entire system is much more elaborate and there is provision for indicating actual oil flow, not to mention the manual and automatic adjustments to vary the quantity of oil being supplied to the cylinder liner. (Next Article)