The pistons in large marine diesel can be oil or water cooled to prevent stresses in the piston crown due to temperatures induced by combustion.. Although water cooling has now been replaced mainly by oil cooling, there are still many ships main engines in use today using water as a piston cooling medium.
The following sections examine a typical method of water cooling the pistons; the first section giving an overview of the components and their different materials that make up the piston.
Overview of a Marine Diesel Piston
The main diesel engine piston is made up of two parts; the crown and the skirt. The crown is subjected to the high combustion temperatures so it is cast from an alloy of steel, chrome and molybdenum. The crown has cooling passages cast into it, through which the cooling water passes. The crown is bolted through the cast iron skirt into the forged piston rod using long, high tensile, waisted studs; the nuts being tightened using hydraulic tensioning gear.
The head of the crown can sometimes be coated with inconel; a hard, high temperature alloy, to prevent erosion from the burning of heavy fuel oil.
A sketch of a typical piston is shown below; please click on image to enlarge.
Additives to the Cooling Water
When I was an engineer at sea, I sailed on oil tankers propelled by Sulzer main engines whose pistons were water cooled and used an additive of soluble oil. There were several reasons for using this additive; I seem to recall we used a Shell product similar to that added to the cooling water used to lubricate and cool cutting tools on lathes milling machines and vertical drills.
In piston cooling an oil additave provides a lubricant for the seals on the telescopic arms as well as acting as an anti-corrosive; adhering to the surface on the inside of the piston crown waterways.
Water cooling has the advantage of having better stringency than chemical inhibitor counterparts; this helps avoid cavitation due to the reciprocating action of the piston.
Only disadvantage I can remember is that too much oil would corrode the rubber seals, so a low percentage of about 0.5% soluble oil was the safe maximum. We also used to change out the cooling water system at every yearly drydock; along with the jacket cooling water.
Nowadays however, most modern engines use lube-oil piston cooling as opposed to water cooling.
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.
- Shell oil: Piston cooing oil additive
- Free-Marine: Piston cooling
- Machuneryspaces: Piston cooling methods