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Requirement of piston rings in a marine diesel engine

Piston rings are particularly vital to marine diesel engine operation in that they must effectively perform three functions: seal the cylinder, distribute and control lubricating oil on the cylinder wall, and transfer heat from the piston to the cylinder wall. All rings on a piston perform the latter function, but two general types of rings—compression and oil—are required to perform the first two functions. The number of rings and their location will also vary considerably with the type and size of the piston.

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Rings must have sufficient spring so that they will provide an initial seal with the liner. As pressure builds up gas acting on the back face of the ring increase the sealing effect. The spring must be retained under normal operating temperatures. They must not crack under high temperature and pressure ranges. Rings are generally of spherical graphite cast iron because of the strength and limited self lubricating properties. With modern long stroke engines the rings do considerably more rubbing than equivalent sections of the liner and so the rubbing faces are usually made slightly harder. This is achieved by a case hardening process (usually Nitriding) some rings are contoured on the rubbing face in order to promote faster running in.

Copper or carbon coatings are sometimes provided for the same purpose. When running in cylinder l.o. is increased to provide an additional flow to carry away metallic particles and a straight mineral oil without antiwear properties is used.

The ring axial depth must be sufficient to provide a good seal against the liner but it must not be so great so that an oil wedge does not form. The ring actually distorts in the groove to form the wedge but if they are too deep they cannot do so. Thin rings will distort easily and scrape the oil from the surface. Radial depth must be sufficient to allow adequate support for the ring in the groove when the ring is on max. normal wear for its self and the liner.

Piston configuration

Fig:piston-configuration

Rings must be free in their grooves and the correct clearance is required. Excessive clearance can allow rings to twist while insufficient clearance can cause jamming and prevent the gas pressure from acting behind the rings. Also the rings may tend to twist excessively. Radial clearance must be sufficient between groove and ring back to allow a gas cushion to build up. The butt clearance must be sufficient to allow for thermal expansion. If insufficient the rings may seize and if excessive can lead to excessive blowpast

Grooves are sometimes coated with chromium to restrict deposit build up. For reconditioning the bottom face of the groove is generally provided with a replaceable steel wear ring. As the rings maintain the gas seal there is a desire to position the top or firing ring as close to the piston crown as possible. However ,since the crown is highly stressed, thermally, this results in distortion of that zone. There is thus a desire to position the ring a long distance away from the crown. A compromise position is decided upon in each engine design.

In order to minimise wear, a film of lubricating oil must be maintained between the moving parts i.e. the rings and liner, and rings and groove. Also the lubricating oil must spread over the liner surface by the rings, this helps to combat acidic products of combustion.

Skirts fitted to pistons on some designs perform the function of sealing the exhaust ports at T.D.C. these extended skirts have bronze rubbing rings inset to provide a bearing surface during the running in period.


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.
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  2. Piston oil cooling method - basic principle

    3. 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) .

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  3. Function of piston ring in a marine diesel engine

    4. 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.

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  4. Requirement of piston ring

    5. 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.

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  5. Piston designing considerations

    6. 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.

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    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.

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  6. Detection of piston overheating

    7. 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.

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