Cooling System Classification

The temperature of gases in the cylinders of a running engine averages around 1000°C. During engine operation, the gases heat the walls of the cylinders, pistons, and cylinder head. If the engine had not been cooled properly, the film of lubricating oil between the rubbing components of the engine would have been burnt off, resulting in undue wearing of the components, possible seizure of the pistons because of their excessive expansion, and other troubles.

An excessive heat removal from the engine (engine overcooling) reduces the engine power and increases the consumption of fuel, because of poor air-fuel mixing conditions and increased friction losses due to poor lubricating properties of oil at low temperatures. Excessively low operating temperatures cause incomplete burning of the heavier fuel fractions, resulting in heavy carbon deposits accumulating on the combustion chamber walls, pistons, and valve heads, with ensuring seizure of the piston rings and valves.

Thus, the overcooling of the engine is as undesirable as its overheating. For a water-cooled engine to operate normally, the temperature of the cooling water must be in the range 80 to 95°C.

The cooling system serves to remove heat from the hot engine components and maintain normal temperature conditions of the running engine. The withdrawal of the excess heat in internal combustion engines is effected through their forced cooling by some liquid (liquid cooling) or the ambient air (air cooling).

ENGINES WITH A LIQUID COOLING SYSTEM are the more common. The cooling medium, or coolant, in them is either water or some low-freezing liquid, called antifreeze. The system includes water jacket for cooling the cylinder block and head, radiator, water pump, and fan, and also auxiliaries: coolant distribution manifold, thermostat, connecting hoses, drain cocks, and coolant temperature gauge (thermometer).

In diesel engines using an auxiliary internal combustion engine for starting purposes, the starting engine, as it runs prior to cranking the main engine, is cooled by natural convection. The cooling water flows, as a result of temperature difference, from the cylinder jacket to the cylinder head jacket of the starting engine and thence to the cylinder head jacket of the main engine, where it gives off its heat to the cylinder head, and then flows back to the cylinder jacket of the starting engine. The natural circulation of the water by convection is known as thermo-siphon cooling.

When the main engine is running, the cooling water is forced to circulate through the cooling system by centrifugal water pump. The pump draws water from the radiator lower tank, termed the collector tank, and forces it under pressure into cylinder block jacket where it cools the walls of the cylinder. The water then passes upwards through holes and ducts into the cylinder head jacket. The ducts direct vigorous flows of water around the exhaust guides and seats that are subjected to the most severe heating and also around the brass injector tubes or sleeves, to protect the fuel-injection nozzles against overheating and prevent their spray holes being clogged with carbon. While the engine is cold, the water leaving the cylinder head is directed by the thermostat to the inlet side of the water pump, so that it flows by-passing the radiator (along the minor coolant circuit), but after the engine has warmed up, the thermostat directs the water to the upper, or header, tank of the radiator (along the major coolant circuit). As the water flows through numerous tubes between the header and collector tanks of radiator, it is cooled by the air that is induced to flow between the tubes by cooling fan. The water leaving the collector tank is again forced by the water pump into the engine jacket.

Thanks to the relatively high rate of coolant flow maintained in the cooling system, the difference in coolant temperature between the outlet and inlet of the engine jacket is not very high (4 to 7°C), which is beneficial for a more uniform cooling of the engine.

Modern engines use a pressurized (sealed) cooling system in which the radiator is hermetically sealed and communicates with the atmosphere only if the pressure in the system goes too high or falls too low. To this end, the radiator filler neck is closed by what is known as the radiator pressure cap which is essentially a filler cap incorporating a pressure relief (control) valve and a vacuum (recuperation) valve. Pressuring the cooling system allows the coolant to circulate at a higher temperature without boiling, which improves the engine operating conditions. A further advantage of using a sealed cooling system is that the coolant losses through evaporation and surging are minimized.

In engines with an air cooling system, the removal of heat from the high-temperature surfaces of the engine is effected by inducing air to flow around the cylinders and their heads. The forced circulation of air around the engine is provided by a rotary blower consisting of rotor (impeller) with a large number of blades, or vanes, and stationary inlet vane device. Revolving with a high speed, the rotor forces the cooling air under sheet-metal air ducting, or shroud, that almost entirely encloses the engine.

The air cooling system is provided with a means for controlling automatically the engine cooling conditions, in accordance with engine cooling requirements, by varying the speed of the blower rotor. This takes the form of fluid coupling installed between drive pulley and the rotor of the blower, the amount of oil filling the coupling being changed by oil-feed regulator mounted in the cylinder head. The coupling comprises two vaned members: the driving member, called the pump, or impeller, and, the driven member-the turbine, or runner. The latter is rigidly attached to rotor and has no direct mechanical connection with the pump which is attached to the drive pulley.

The automatic device operates as follows. While the engine is being warmed up and the cylinder head temperature is too low, piston valve is closed and does not let oil flow from the lubricating system into the fluid coupling. As a result, there is no oil in the coupling, and the turbine with the blower rotor does not rotate. The engine warms up rapidly, and as the required warm-up temperature is reached, the sensing unit of regulator moves valve to let oil enter the fluid coupling. The oil flowing into the coupling is entrained by the vanes of the pump and is thrown into the turbine with great force. It hits the turbine vanes at an angle and in this way applies pressure to one side of the vanes. The push against the vanes forces the turbine to turn together with the blower rotor.

The housing of the fluid coupling is provided with holes (1.5 mm in diameter) through which oil continuously drains into the engine crankcase.

The higher the temperature of the engine, the greater the amount of oil filling the fluid coupling and the higher the speed of the blower rotor. As the engine temperature goes down, the valve of the oil-feed regulator is moved to restrict the flow of oil into the coupling, and the blower slows down.