The heat from the combustion of fuel have to be taken away continuously otherwise the metal components will become damaged. The material properties of the engine parts can change when it reaches high temperatures. Thermal stress can occur leading to cracks, deformation and weaknesses in the material.
Continuous cooling of the engine is necessary. The temperatures have to be maintained at an optimum level. They must not be too hot or too cold. The normal engine jacket outlet temperature is usually maintained at about 68 to 70 degree Centigrade, the piston cooling outlet temperature about 55 degree Centigrade. The lubrication oil temperature is maintained to about 40 degree Centigrade. The temperatures are maintained as steady as possible. The control systems are designed so that fluctuations in temperatures can be measured and controlled easily. Huge fluctuations in temperatures not only cause undue thermal stress, but also can cause the rubber seals to leak.
The main diesel engine cooling systems consists of the following:
- Jacket Cooling Water System
- Piston Cooling Water System
- Lubrication Oil Cooling System
- Fuel Valve Cooling Water System
All the above are circulating systems.
Jacket Cooling Water System
The water circulates inside the engine compartment surrounding the cylinder liner and also the cylinder head. Some of the water will also be circulated through the turbochargers, and exhaust valves if there are any.
Because the circulating water is in a closed loop, an expansion tank is installed to cater for expansion and contraction of the water at different conditions of operation.
Outlet valve opening can control individual cylinder cooling water outlet temperatures, while the controller for the Jacket Cooler can control the incoming temperatures.
Piston Cooling Water System
The piston crown is subjected to intense heat from the combustion of fuel oil. It is therefore essential to cool this part. Because the piston moves up and down in a reciprocating manner, some means of supplying the water to the internal compartments of the piston must be available. Some engines use telescopic and stand pipe arrangements to supply water for cooling the piston crowns. Others may use the lubrication oil as a coolant although the cooling effect is not as good.
The water, after passing through the piston compartments is allowed to flow out to a collection tank outside the engine. If the engine uses oil for cooling, then the latter is allowed to drop into the engine oil sump tank.
Lubrication Oil Cooling System
The lubrication oil, after lubricating the moving parts of the bearings will finally drop down to the sump tank of the engine. From the sump tank, the oil is pumped to the oil cooler for cooling. The controller will ensure that the oil becomes cooled to the required temperatures.
Depending on the engine, there may also be a turbocharger cooling oil system that uses its own cooling system. (The oil is different from the main engine, so it must not mix)
There will also be a cylinder lubrication system that injects small quantities of oil into the cylinder liners. This is for lubricating the rubbing surfaces between the piston rings and the cylinder liner. The oil is not circulated, so there is no necessity for cooling.
Fuel Valve Cooling Water System
Although this system is small, it is nevertheless important for the engine. The fuel valve, or injector is the component from where fuel is injected through nozzles. This component of the engine is subjected to intense heat of the combustion and needs to be cooled. Passages are drilled into the nozzles to enable water to be circulated within the nozzle. The water is maintained at 92 degree Centigrade so as not to flash into steam.
Because of the close loop, this cooling system also has an expansion tank.
There are other cooling systems other than those for the main engine. There will be another set of Jacket Cooling Water System for the Electrical Generator Sets. Many of the smaller engines have built in cooling systems for lubrication oil, and others. Other auxiliary systems like air conditioning, refrigeration, steering gear, etc. use their own cooling systems.
Usually, the cooling medium is seawater. The above systems have very close control limits. Automatic control systems are installed so that the temperatures can be maintained in a close range. Some cooling requirements are not so stringent. These can be controlled manually. A very good example is the air intake cooler.
For good temperature control, the seawater can also be circulated within the engine room piping to maintain the heat. This is useful during wintertime in freezing conditions.