The cooling system and the lubricating oil system are so dependent upon one another for proper operation that they should be considered together. The function of the cooling system is primarily to dissipate the excess heat remaining in the engine after combustion.
Basically, the cooling system operates as follows: the water is taken from the bottom of the cooling radiators into a water circulating pump, which may be driven by the engine or by the auxiliary motor. The pump forces the water around each of the cylinder liner water jackets; then to an outlet water header, which directs the water into the radiators for cooling. The system has a water-expansion tank to hold the excess volume of water when it expands from heat. The expansion tank generally has a vent pipe to take care of overflow and a sight gauge to judge the amount of water in the system. The outlet header holds a thermometer to indicate the water temperature at the control panel in the operators cab. The thermometer reading is an indication to the operator to open or close the radiator shutters to restrict the flow of cooling air as a means of controlling water temperatures. The air is forced over the radiator core by a motor-driven fan powered from the auxiliary generator.
Considerable heat is taken up by the lubricating oil, particularly when piston cooling by means of oil is adopted, and this necessitates provision for cooling the oil before it is recirculated through the engine. This is usually done by providing a separate set of radiator elements in the main radiator or by the fitting of a special oil radiator with its own cooling fan.
The lubricating oil may be considered the life blood of the diesel engine. When metal surfaces, under pressure, have a sliding contact, heat and wear will result from the friction, but if a fluid is maintained between these surfaces the friction will be negligible and heat and wear will be at a minimum.
Generally speaking, modern locomotive diesel engines are lubricated on a forced system using high pressure oil. The oil is forced to the main bearing and is directed through drilled passages in the shafts to the crankpin bearing, then to the piston pins; it is forced up under the piston to act as a cooling agent and lubricates the camshaft, gears, and other moving parts of the engine. Free return oil is supplied to cylinder walls and camshaft driving gears or chains. The engine crankcase sump forms the collecting point for the returned oil, which is then passed through filters and circulated through the air cooled radiator or heat exchanger by means of a scavenging pump. The oil in the diesel engine is collected, filtered, cooled and re-circulated for many hours before it deteriorates to such an extent that it must be drawn off and replaced.
The stability of the lubricating-oil system is directly dependent upon the cooling system for its proper and efficient operation. The lubricating system, on the other hand, assists the cooling system by removing the heat from the underside of the pistons and by providing a film of oil on the cylinder walls. It can be seen that if the cooling system fails to function as it should, the temperatures of the cylinder walls will rise to a point that will cause the lubricating oil to burn and the thin film of oil will disappear, allowing a metal-to-metal contact between the pistons and the liner.
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Exercise 3
Answer the following questions:
1 What cooling system has the diesel engine?
2 What is the function of the cooling system?
3 How does the cooling system operate?
4 What is the water circulating pump driven by?
5 Where is the water directed to for cooling?
6 What is the water expansion tank used for?
7 What device is used to judge the amount of water in the cooling system?
8 What do we call the device for measuring the water temperature?
9 Where is this device mounted?
10 How does the operator control the water temperature?
11 By means of what is the air forced over the radiator core?
12 What parts of the engine must be cooled?
13 What lubrication system has the diesel engine?
14 What parts of the engine are lubricated?
15 Why must the parts of the engine be lubricated?
16 Where is the lubricating oil stored?
17 What are the filters used for?
18 By means of what is oil circulated through the heat exchanger?
19 When must the oil in the diesel engine be drawn off and replaced?
20 What happens if the cooling system fails to function as it should?
