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Piston cooling methods - Advantage and disadvantages of water cooled and oil cooled pistons
Coolant used for cooling a piston of marine diesel engine :
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 14oC for water and 10oC for lube oil)
Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.
Fig:Path of piston oil cooling
Fresh water and distilled water piston cooling advantages :
i) The main advantages is the ability of water to absorb large amounts of heat
Relatively easy to obtain.
ii) Easily counter check from drain tank any leakage on piston sealing.
i) The piston cooling water conveyance piston and attendant gear must be kept out of the crankcase as far as possible, due to the danger of contamination of the crankcase lubricating oil by water leakage. In other word, the jacket cooling water and piston cooling system should have separate which having their own pumps , coolers, piping etc.
ii) The piston cooling space should be drained of water after the engine is shut down for an extend period. A drain tank is necessary for the same purpose cascade type filter is often incorporated for separation of oil and water.
iii) There is risk of scaling and corrosion if water is not suitable treated and maintained.
Lubricating oil piston cooling Advantages:
i) The piston cooling pump is combined with the lubricating oil pump and piston cooling oil cooler thus make overall simplicity in the system
ii) Internal stress within the material of the piston is generally less in oil cooled piston than in water-cooled piston. Good design in water-cooled piston to improve its condition of working
iii) No risk of oil contamination even when piston cooling oil conveyance piping is fitted inside the crankcase
iv) Simpler arrangement for cooling-oil conveyance piping with less risk of ‘hammering’ in piping and bubble impingement attack
i) Larger power requirements for pumping cooling oil
ii) Larger amount of lubricating oil required to give some cooling effect
Flow pattern of the coolant
The flow is such that piston cooling oil or water enters at the lowest part of the cooling space and leaves from the uppermost part to give even cooling effect without causing distortion due to unequal expansion.
The piston will always full of coolant and the underside of piston crown is always in contact with it
It is important in slow speed engine, as when the piston is running at dead slow speed the coolant in the piston is not ‘shaken up’ the way the engine in full speed.
If the cooling took place in the opposite direction, it would be possible at very slow speed for the coolant to drain from the piston and lose contact with the crown. The piston could become overheated.
Some water-cooled piston have the outlet for the water at approximately half the cooling space height. When running slow, the piston is half full of water and piston movement agitates this water in the piston – the water gets splashed on the underside of crown, piston wall
When the engine stopped a jet action from the piston cooling pipe nozzle directs cooling water onto the piston crown, thus removing residual heat and catering for an emergency stop at full speed.
The splash method of cooling is called “cocktail shaker cooling”
Fig:Water cooled piston
Requirement of piston ring
Function of piston ring in a marine diesel engine
Piston designing considerations
Troubles of piston to be avoided
Detection of piston overheating
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