Attention
The engine cooling system performs one of the most important functions in an internal combustion engine. Therefore, failure of the entire system or any element can lead to overheating and engine breakdown. Driving and operating a vehicle with a faulty cooling system is undesirable or prohibited.
Figure 4.31 Functional diagram of engine cooling system.
The cooling system is used for maintaining the operating temperature of the engine (within the range of 80-95 °C) by forcibly removing heat from the engine cylinders and transferring it to the ambient air. The need for a cooling system is due to the fact that engine parts that come into contact with hot gases get very hot during operation. In case if the internal parts of the engine are not cooled, then due to overheating, the lubricant layer between the parts can burn out and the moving parts will jam due to their excessive expansion.
Cooling systems of almost all modern cars do not differ from each other. A schematic, generalized diagram of the cooling system operation is shown in Figure 4.31, where the liquid, which is heated by the engine parts is marked in red; and the liquid, which is cooled in the system radiator is blue.
Water cooling system with forced circulation of liquid includes cooling jackets for the cylinder head and engine block (we wrote about the jackets above, while examining a single-cylinder engine), a radiator, lower and upper connecting pipes with hoses and a water pump with a water distribution pipe, a fan and a thermostat.
When the engine is running, a water pump driven from it creates a circular circulation of water through the engine cooling jacket, pipes and radiator. The water is first directed through the water distribution pipe to the hottest parts of the engine block. While passing through the cooling jacket of the engine block and cylinder head, the coolant washes the walls of the cylinders and combustion chambers, at the same time cooling the engine. The heated coolant enters the radiator through the upper pipe. There, branching through the pipes into thin streams, it is cooled by air, which seeps past the pipes under the action of the thrust, which is created by the rotating fan blades. Cooled coolant re-enters the engine cooling jacket.
Figure 4.32 Cooling system diagram.
Figure 4.33 Radiator.
This is a set of thin tubes on which thin plates are strung for increasing the surface area, which is intended for heat dissipation. The main function of a radiator is to cool the liquid, which circulates inside its tubes.
Figure 4.34 shows an example of a radiator section with various versions.
Figure 4.34 Styles of design of cooling system radiator.
Tanks, to which the upper and lower branch pipes of the cooling system are connected, respectively, can be placed on the upper and lower parts of the radiator. In case if the tanks are installed, then in the upper one there is usually a filler for filling in coolant. In case if the tanks are not installed, then the neck of the expansion tank shall be used for filling. The tubes of the radiator are made flat and arranged in rows in a checkerboard pattern for better cooling of the liquid. A large number of thin brass plates, that are called cooling fins, are installed across the tubes. They increase the cooling surface of the core and promote more intense heat transfer from the water to the air passing through the core.
In a closed-type cooling system, the radiator filler is tightly closed by a special plug with a double steam-air valve (see Figure 4.35). The air valve of the plug is loaded with a weak spring and allows passing of atmospheric air inside the radiator. This eliminates the possibility of a vacuum in the radiator reservoir, which appears during condensation of water vapor.
The steam valve is loaded with a stronger spring and opens in order to release steam only when the pressure in the radiator exceeds atmospheric and reaches 1.28-1.38 kg/cm2.
Figure 4.35 Radiator cap.
The water pump forces the coolant to circulate through the system. This pump is of centrifugal type. The pump rotates using a drive belt, which is mounted on the crankshaft pulley.
The pump has a rather simple design: a shaft, at one end of which an impeller is installed (shown in Figure 4.36), and a pulley for a drive belt is situated at the other end.
The shaft is supported by a bearing, which is installed in the pump cover. Often a cavity or afflux in the cylinder block is the pump housing. Water flows through the inlet pipe to the inside of the housing and is supplied to the center of the rotating impeller. In this case, the water is carried away by the impeller. It acquires a rotational motion. Under the action of centrifugal force, it is thrown to the walls of the housing and through the outlet channel under pressure enters the engine cooling jacket.
Figure 4.36 Water pump. Impeller.
Long ago, the fan was installed on the same axis with the water pump shaft. It was rigidly attached to the drive pulley and drove air for additional cooling of the radiator constantly while the engine was running, because it was actuated by the crankshaft. In summer, this may be good. But in winter, when the ambient temperature is already enough for cooling, additional cooling is not good. Also, when driving a car in summer, when you often have to spend time idle in traffic jams, and the engine runs at low speeds, the cooling will be insufficient due to the lack of normal air flow from the fan.
Note
The importance of a certain (rather narrow) range of engine operating temperature, regardless of the season or load during its operation is worth mentioning here. As a conclusion: overheating is bad, but overcooling is also far from good.
But progress did not stand and does not stand still, therefore, realizing that there is no benefit in a constantly “turned on” fan neither in winter nor in summer, designers decided to install a fan with an electric motor, which turns on at the command of the temperature sensor. This is convenient, because the car warms up quickly, and when a certain temperature is reached, the electric fan starts working. In modern cars, the electric fan also has two modes of operation: fast and slow. It is controlled by electronics.
But there is another way to make the fan work without electronics in specified operating modes - install a viscous coupling.
This coupling is driven by a belt from the crankshaft pulley. The fan "sits" on the axis and, if unnecessary, does not rotate. As soon as there is a need for cooling, this coupling is triggered and the fan starts rotating, as if connected through a drive belt to the crankshaft.
Thermostat is a valve, which is installed in the body. It opens when the coolant heats up to normal operating temperature. An example of the design and operation of the thermostat is shown in Figure 4.37. The engine cooling system is designed so that it has two circles of circulation - small and large. When the thermostat valve is closed, the coolant is circulated by the water pump only within the cylinder head and engine block, thus it warms up quickly (small circle). As the coolant, in particular, and the engine as a whole warm up, the thermostat valve starts opening, letting the coolant circulate through the radiator - a large circle.
Note
Excessive overheating of the coolant reduces engine power and efficiency. In case if the coolant, and therefore the engine, does not warm up, then the condensation of the fuel increases. It causes the lubricant to be flushed from the cylinder walls and thinning it in the crankcase, as well as increasing heat losses. This leads to a decrease in engine power and an increase in fuel consumption.
Figure 4.37 Function of thermostat.
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