Principles of Water Cooling
The main reason for using water or oil to cool the pistons in a marine diesel engine is to prevent undue thermal stresses to the crown area due to the high temperatures of combustion temperatures caused by the burning of the fuel oil. However water is used because it requires about a third of the quantity of oil cooling, having better cooling qualities than oil through being able to operate at higher temperatures
A centrifugal pump supplies the cooling water under pressure to the piston crown; where it is circulated through water channels that have been cast into the piston crown. These channels are positioned close to the walls and top of the crown, also behind the piston ring grooves to promote optimum cooling of the surfaces of the crown subjected to the high temperatures of combustion.
The water enters and leaves the piston through telescopic pipes that slide up and down on stand pipes as the piston recippricates. These pipes have rubber water seals between them to prevent leakage of cooling medium into the crankcase lube-oil. There can also be enclosures around the seals that incorporate tell-tale leak pipes that eject water if a seal begins to leak. The pipes are located at convenient locations outside the engine enclosure, usually at control platform level.
The water temperature is maintained by a dedicated seawater cooler, and a header tank allows the system water and additive to be topped up.
A cooling water storage tank can also be included in the system; here the water flows into the tank from the return line, where it passes through filters and weir plates to separate scum form the medium. The cooling water pump then draws the water from here, circulating the water through the pistons and cooler. The cooling water and soluble oil additive can be topped up through the inspection hatch on top of the tank.
A typical flow diagram for piston water cooling, incorporating a both header and storage tanks is shown below; please click on image to enlarge.