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Function of piston ring in a marine diesel engine

How piston ring works as effective sealing between the piston and liners
The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber.



In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.

The efficiency of the engine depends upon the effective sealing between the piston and liners. Leakage will reduced compression pressure and power will lost. Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls Three to six power or compression rings are fitted to the piston, the number depending on weather the engine operates on the 2-stroke or 4-stroke cycle.

The top ring position is controlled by the operating temperature and exhaust or scavenge port trimming. In 2-stroke cycle engines the port trimming usually demands a higher ring position than used on 4- stroke cycle engine The main function of piston rings are;

i) to provide a gas seal in the case of compression rings and to prevent excessive built up of pressure in the crankcase which will results inexcessive lube oil consumption in a wet sump engine and the risk of explosion in the case of a dry sump unit

ii) To act as a scrapper or oil distribution around liner wall which could result if lubricant is allowed to leak past the compression rings and be burnt with fuel oil


Piston ring sealing face

Fig:Piston ring sealing face

To ensure adequate lubrication the operating temperature should not exceed 200oC and this requires the rings to be fitted as low down the piston as possible. Lubrication of the rings is essential as it is needed to reduce friction and wear to augment gas sealing Ring will normally rotate on piston due to the variation in gas pressure. A typical rate of movement is 1 3 mm per cycle

The diameter of the piston rings, when free, is slightly larger than the cylinder bore. Consequently, when the ring is squeezed into the liner, it presses against the cylinder wall and tend to seal it.

This initial sealing action is greatly improved by the pressure either compression or combustion gasses against the top of the ring forces the ring down on to the lower side of the piston grooves. This leaves a clearance at the top which permits gas pressure to travel behind the back of the ring and press it against the cylinder liner wall making a firmer contact, thus improving the seal

There are three clearances which are important;-

i) Side or axial clearances
ii) Butt or Gap clearances
iii) Back clearances



Summarized below more guidance for marine diesel engine piston handling safety procedure:
  1. Construction of 4 stroke piston

  2. For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into prevent distortion.
    More.....

  3. Piston oil cooling method - basic principle

  4. The coolant used for removing and conveying the heat from a piston may be either fresh water, distilled water or lubricating oil. Water has the ability to remove more heat than lubricating oil ( specific heat of water approximately 4 and lubricating oil 2 and temperature difference 14 deg C for water and 10 deg C for lube oil) .

    More .....

  5. Function of piston ring in a marine diesel engine

  6. The efficiency of the engine depends upon the effective sealing between the piston and liners. Leakage will reduced compression pressure and power will lost. Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls Three to six power or compression rings are fitted to the piston, the number depending on weather the engine operates on the 2-stroke or 4-stroke cycle.

    More .....

  7. Requirement of piston ring

  8. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

    More .....

  9. Piston designing considerations

  10. Piston forms the lower part of the combustion chamber in a marine diesel engine. It seals the cylinder and transmits the gas pressure to the connecting rod.

    More .....

    Troubles of piston to be avoided
    i) Piston seizure; all too often followed by an explosion of the lubricating oil air mixture in the crankcase. ii) Cracking of piston crown and side walls, due to cyclic variation of pressure stress superimposed on stress due to temperature gradient.

    More .....

  11. Detection of piston overheating

  12. most common reasons of Piston running hot : i) Inadequate circulation of cooling media and or supply not sufficient. ii) Excessive deposit in cooling space (scale or carbon). iii) Lubrication not sufficient.

    More .....










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