marinediesels.co.uk

Sample Questions

Chiefs Engineers Motor EKs
 

 

October 2000 Question 7.

With reference to medium speed engine cylinder liners:

  1. explain the cause and effects of polishing or glazing; (6)

  2. describe, with the aid of sketches, an anti-polishing ring and explain how it is fitted in the liner; (5)

  3. explain the action of an anti-polishing ring during the operation of the engine; (5)

Authors Note: This question appeared for the first time in Oct '99. I may be missing something, but part c of the question seems to be a give away for 5 marks.

a.

When a liner and piston rings have run in, the asperities, or high spots have been worn off the rubbing surfaces, and ideally, what is left is a microscopic corrugated surface which will hold the lubricating oil. Combustion, especially of residual fuels, will produce carbon which tends to build up around the topland of the piston. As the carbon builds up, it will eventually touch the liner surface, removing the lub oil film, and, because the carbon is abrasive, it will polish the liner surface, removing the microscopic corrugations, and giving it a highly glazed surface to which a lub oil film cannot adhere. Carbon build up can be so severe that it jams the top ring in its groove. The dual action of further carbon abrasion, and unlubricated contact between rings and liner leading to microsiezure will accelerate the wear rate of the liner leading to blow by, loss of power, possible seizure and danger of crankcase explosion.

b.

The anti polishing ring has a slightly smaller diameter bore than the liner in which it is located, and is slightly larger than the  top of the piston. When the piston is on top dead centre the bottom of the anti-polishing ring is just above the top surface of the top piston ring.

The anti polishing ring is a clearance fit in a step in the top of the liner, and can be replaced when it wears. On some engines it incorporates a cooling space, and so the anti-polishing ring  (or fireband) is fitted with O rings for sealing.

 

c.

As the piston reciprocates in the cylinder, any carbon build up forming on the top of the piston will be removed by the anti polishing ring. This ensures a good lubricant film on the liner surface, reducing wear and giving a more consistent lub. oil consumption.

 

click here to go to top of page click here to go back to the home page click here to go to the sample questions

DHTML Menu / JavaScript Menu Powered By OpenCube