Oil cooling of pistons is required to prevent thermal stresses due to the high temperatures of combustion cracking and deforming the piston crown.
The following sections examine the principles of piston cooling illustrating how marine pistons are cooled with oil.
The first section looks at the assembly of the piston and the materials of the components.
Marine Diesel Engine Piston Overview
The marine piston is made up of two major components, the crown and the skirt.
The piston crown is subjected to the high temperatures of heavy fuel oil, combustion therefore it is cast from an alloy of chrome, molybdenum and carbon steel. The heavy fuel oil contains several corrosive elements the main one being sulfur and this can erode the surface of the crown. To combat this a layer of durable high temperature alloy is welded to the top of the crown.
Inconel is used for this, being an special alloy of cast iron, nickel, cobalt, chromium and titanium. The skirt is of cast iron and is fixed to the crown by waisted studs screwed into the body of the crown. These are high tensile studs that pass through the piston rod flange and are then tightened using hydraulic tensioning gear and the hexagon nuts.
The crown has oil cooling conduits cast into it and, it is through these that the oil is circulated. This is examined in more detail in the next section.
A typical marine diesel engine piston is shown below, please click on image to enlarge.
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.
Pros and Cons of Oil Cooled Pistons
- No contamination of lube oil in the crankcase by leaking water from the telescopic water pipe/ standpipe seals.
- No requirement for a separate storage tank, cooler or pumps to operate the piston cooling.
- Inhibitors are not necessary to be added in the lube oil to prevent corrosion in the system; already present in the lube-oil.
- Oil has a lower thermal capacity than water, so as piston cooling oil forms a part of the engine main lube-oil system, a larger sump will be required.
- It also has a lower maximum working temperature of 130°F so must be controlled below this to prevent the formation of carbon and lacquer on hot internal surfaces.
- There may be a slight increase in the oil deterioration due to increased thermal cycling of the lube oil through the piston crown.
By comparing the pros and cons of water and oil piston cooling, it can be seen why the major marine diesel engine manufacturers are changing their piston cooling medium to oil.
- Willie Scott: Piston Water Cooling
- Machineryspaces: Piston Cooling Methods
- Willie Scott: Heavy Fuel Oil Contaminants.
- Hightempmetals: Properties of Inconel 718 Technical Data
- Willie Scott: Modern Efficient Diesel Engines