Principles of Piston Cooling Using Lube-oil
As we have seen earlier; marine diesel engine pistons require to be cooled either by water or oil circulation.
There are several types of oil cooling, and a description of one type as used in the modern diesels such as the B & W main engines follows;
The piston crown has channels and a large void cast into it through which the oil is circulated. These are as close as possible to the sides, top surfaces of the crown, as well as behind the piston rings to attain optimum cooling of the high fuel oil combustion temperatures.
The oil is drawn from the main engine sump by the lube-oil pump being discharged to manifold bolted to the outside engine casing. From here an internal swinging arm piping system supplies oil to the cross head bearing and the piston crown cooling.
The cooling oil is delivered to the piston crown, through the centre of the hollow piston rod, circulating through the cooling channels in the crown. The oil returns through the piston rod via small holes drilled into the piston rod where it is collected in a tray at the cross head bearing. here the oil temperature and viscosity is monitored before spilling over the tray; cascading down into the main lube-oil sump.
A diagram showing one method of oil cooling a piston is shown below; please click on image to enlarge.
There is a more modern method used circulating the oil around the inside of the piston crown than the one previously examined. This method employs a nozzle plate that matches up with numerous nozzles fixed to the inside of the piston crown that is virtually hollow. This system uses a similar type of oil supply to the previous example, except in this case the outside holes in the piston rod are used as supply, the larger bore being the return. This allows oil delivery at a higher pressure to the nozzles; causing it be injected as a mist against the inside surfaces of the crown and providing a more efficient cooling system.