Describe Trunk piston engine cylinder lubrication and piston cooling functioning?

21 Jun '17, 05:28

June 21, 2017, 5:28 a.m.
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Lubrication of a medium speed trunk piston engine is achieved by three methods

1) Splash lubrication: - where oil from the rotating crank and running gear is splashed on to the liner surface and distributed over the length of the liner by the piston and rings. The oil control or scraper ring scrapes oil downwards back to the crankcase, thus preventing excessive LO consumption.

2) Cylinder lubricators:- fitted on the larger 4 stroke trunk piston engines. Oil of the same specification as that of the crankcase oil is supplied by an engine driven pump through drillings in the liner to the liner surface. The idea behind this is so that oil is picked up by the ring pack as it passes, aiding upper cylinder lubrication, combating the products of combustion and reducing wear.

3) The third method has been adopted by some engine builders. This method entails feeding the LO through drillings in the piston skirt to the cylinder liner. Because the piston skirt transmits the side thrust to the cylinder liner, this method puts oil where it is needed, and helps reduce the liner ovality caused by wear in this region.

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The piston on an engine must be efficiently cooled to maintain the strength of the material, prevent corrosion, erosion and burning, reduce the thermal stresses set up within the material and to limit thermal expansion with a corresponding reduction in running clearances.

On a medium speed trunk piston engine, the system oil is used as the cooling medium. The oil passes from the crankpin journal up the con rod to the piston pin. From here the oil is used to cool the piston crown. There are several methods of achieving this. On some engines the oil passes through a cast in cooling coil before being returned to the crankcase. A simple method is just to direct a jet of oil onto the underside of the piston crown.

A more positive method of distributing the oil up to the piston crown on a large bore medium speed engine is to use a spring-loaded oil catcher. The oil travels around a groove in the piston pin, and then through a drilling, where the oil catcher ensures the oil is fed up to the underside of the crown throughout the whole of the con rod swing. Oil passes through drillings to the circumferential space as shown before returning to the crankcase.

Oil flow/return temperature can be monitored, although this isn't always undertaken on these engines.

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21 Jun '17, 05:33

June 21, 2017, 5:33 a.m.
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