Cooling of piston is necessary to remove excess heat from combustion and to limit thermal stressing.It also limits thermal expansion to maintain correct clearances between piston and liners and between ring grooves and rings. cooling is carried out by circulating internal passage in piston with a coolant.This may be either oil or fresh water.
Advantage of water coolant
1.The Piston cooling pump is combined with the lubricating oil pump and Piston cooling oil cooler thus make overall simplicity in the system .
2.internal stress within the material of piston is generally less in oil cooler Piston then in water cooled Piston. good design in water cooled Piston to improve condition of working .
3.no risk of oil contamination when Piston cooling oil conveyance piping is fitted inside the crankcase.
4.simpler arrangement for cooling oil conveyance piping with less risk of hammering in piping and bubble impingement attack
Disadvantage of oil coolant in piston
1.Oil has slightly lower thermal capacity than water and an adequate supply is necessary.
2.It also has a lower maximum safe temperature of 56 degree Celsius to prevent formation of carbon or lacquer on hot internal surfaces with consequent reduction in heat transfer,or choking of small oil passages.
Advantage of fresh water cooling of piston.
1.The main advantage of fresh water is higher thermal capacity than oil and higher outlet temperature up to 70 degree celcius may be sustained.
2. Easily counter check from drain tank way leakage on Piston sealing.
1.The Piston cooling water conveyance pipes and alternate gear must be kept out of the crankcase as far as possible, due to the degree of contamination of crankcase lubricating oil by water leakage.In other word ,the jacket cooling water and Piston cooling system should have separate which have their own pump.
2.the Piston cooling space should be drained of water,after the engine is shut down for an extended period. A drain tank is necessary for the same purpose cascade type filter is often incorporated for separation of oil and filter.
3.there is risk of scaling and corrosion if water is not suitable treated and maintained.