You might be surprised to know that the piston is not straight but slightly tapered in the modern day marine diesel engines. Learn about the concepts of piston clearance and piston-ring clearance in context of engines.
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In this series in which we are taking a look at various components of marine diesel engines, we have studied bedplate, crankshaft, pistons and piston rings till now apart from taking a look at the functions of piston rings, not to forget the important method of taking crankshaft deflections which help to gauge the accuracy and alignment of the crankshaft. But it is not only the crankshaft which needs to be measured for deflections, but the piston ring clearance is an important check to ensure smooth functioning of marine diesel engines.
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What is Piston Clearance?
Well logically you might think that the piston diameter has to be uniform otherwise how can it perform the continuous oscillatory motion within the cylinder liner. But let me tell you that actually it is not so. The pistons of most modern marine diesel engines are tapered and not straight. If you are having difficulty in grasping this concept better take a look at the picture below thoroughly before we discuss it further.
As you can see in the diagram the figure shows the piston clearances across various heights along the cylinder liner wall. This figure shows a particular engine with a bore of 76 cm and the associated piston clearances at various sections. Apart from being tapered the piston also has slightly ovality at the wrist pin land area which if of the order of the fraction of an mm like say 0.5 mm.
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Piston Ring Clearance
Apart from the piston clearance, the piston rings also need to have both axial and radial clearance in the groove. The axial clearance of the piston rings also depends on their position from the top in the grooves and this clearance ought to be more in the top couple of rings vis-à-vis the bottom ones.
The diagram above clearly shows the use of feeler gauges to measure the clearance of piston rings. Another check is to measure the end to end clearance or the opening between the gap ends of the piston ring which it is inserted inside the cylinder liner and the readings are compared both at the top end and the bottom end of the cylinder liner. Of course this can only be done in the case of smaller auxiliary engines and not with the main propulsion engine rings simply because of the size of the engine.
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Image of Piston Clearance: Sanyal, D.K. (1998) Principles & Practice of Marine Diesel Engines. Mumbai: Bhandarkar Publications.
Image of Ring Clearance: Calder, N. (2007) Marine Diesel Engines - Maintenance, Troubleshooting and Repair. Blacklick: McGraw Hill International