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Piston & Piston Crown Design & Material Thickness Considerations for Marine Diesel Engine

Piston crown and piston shape configuration 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. Below considerations should be taken designing a piston:

The shape of the top of the piston crown which forms one part of the boundary of the combustion space – so while designing this it has to be considered how it is going to vary the shape of combustion chamber. The compression ratio,live air ratio have to be given due to consideration In certain types, the piston crown is machine to provided channels and cavities for the favourable reception and redirection of jets of flame issuing from the precombustion chamber. In direct injection engines, the piston crown may be convex 9part spherical), part conical with central depression, hearly flat, concave (part spherical) etc.

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In 4-stroke, it may be necessary to arrange ‘out outs’ in the surface of the crown, to clear the heads of the inlet and exhaust valves during overlapped period at the top of the idle strokes.


Piston configuration

Fig:piston-configuration

The material and thickness of piston crown

Bearing in mind its pressure loading in piston crown and the stresses induced by temperature gradients. The provision of tapped holes for lifting, in view of their weakening effect. The material and thickness of the side walls of the piston, which accommodate the piston pressure rings, and transmit the pressure rings, and transmit the pressure load to the gudgeon pin (trunk engines) or the bottom flange of the piston body (crosshead engines)
spare-pistons
Fig: Spare pistons



The gudgeon pin

The design of the gudgeon pin for strength and bearing pressure; as well as the gudgeon pin bosses (trunk) and closing plates if required.
The proportions and clearances of piston walls and skirts
The accommodation of scraper rings at the mouth of the piston for controlling lubricating oil consumption.
The conveyance of coolant to and from the piston cooling space, which acts as a heat sink for the piston crown and ring belt



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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