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Construction of 4 stroke piston in a diesel engine

piston cooling process :Piston forms the lower part of the combustion chamber of marine diesel engine.Cast iron is the most common material used for piston. Cracking of cast iron piston much reduced by the use of iron castings of pearlitic structure, with less tendency to growth; as well as oil cooling of the larger sizes of piston.



Pistons for medium speed trunk piston engines which burn residual fuel are composite pistons; i.e the crown and the skirt are made of different materials. Piston crowns attain a running temperature of about 450 degree C and in this zone there is a need for high strength and minimum distortion in order to maintain resistance to gas loads and maintain the attitude to the rings in relation to the liner. The heat flow path from the crown must be uniform otherwise thermal distortion will cause a non-circular piston resulting in reduced running clearance or even possible contact with the liner wall. In addition to this thermal stress they are also subject to compressive stress from combustion and compression loads, as well as inertial loads.

Materials such as pearlitic, flake and spheroidal cast iron, alloy cast irons containing Nickel and chromium, and aluminium alloys may be used. The determining factor is the design criteria for the engine.

For a modern slow speed engine steel forging or castings of nickel-chrome steel or molybdenum steel are common. The weight of the material is not normally a governing factor in this type of engine although resistance to thermal stress and distortion is. Efficient cooling is a required to ensure the piston retains sufficient strength to prevent distortion.

Piston cooling

Fig:Path of piston cooling

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 the crown, although this may be omitted on smaller engines.Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface. Hard landings are inserted into the ring groves to keep wear rated down.Composite pistons may be used consisting of an cast alloy steel crown with an aluminium-alloy or cast iron body.

After casting or forging the component is formed of different material thicknesses. The thinner parts will cool more quickly thereby setting up internal stresses. Annealing removes or reduces these stresse as well as refining the grain structure.



Piston crown

Fig:Piston crown

The crown is a heat resisting steel forging which may be alloyed with chromium, molybdenum and nickel to maintain strength at high temperatures and resist corrosion. It is dished to form a combustion chamber with cutouts to allow for the valves opening. The topland (the space between the top ring and the top of the piston) may be tapered to allow for expansion being greater where the piston is hottest.

The skirt can either be a nodular cast iron or forged or cast silicon aluminium alloy. Aluminium has the advantage of being light, with low inertia, reducing bearing loading. However because aluminium has a higher coefficient of expansion than steel, increased clearances must be allowed for during manufacture. This means that the piston skirt clearance in the liner is greater than that for cast iron when running at low loads. The skirt transmits the side thrust, caused by the varying angularity of the con rod, to the liner. Too big a clearance will cause the piston to tilt.

The piston pin for the con rod small end bearing is located in the piston skirt. The piston pin floats in the piston skirt and is located in place by circlips. Depending on the material used for the skirt (esp. cast aluminium), a bushing may be used for the pin.

The piston rings may be located in the crown or in both crown and skirt. Normally, the rings are chrome plated or plasma coated to resist wear. Because the liner is splash lubricated, an oil scraper (oil control) ring is fitted to the piston skirt.

The piston is oil cooled. This is achieved by various means; The simplest is for a jet of oil to be directed upwards from a hole in the top of the con rod onto the underside of the crown




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) .

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  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.

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  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.

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  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.

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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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  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.

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