The engine only works properly under certain thermal conditions. If cylinder heads, cylinders, pistons and all parts overheat from contact with hot gases, wear increases due to lubricant burnout

Clearance reduction due to thermal expansion can cause the pistons to seize in the cylinders.

In addition, the fuel does not evaporate well, ignites difficultly and does not burn completely, which reduces the power and efficiency of the engine, and abundant carbon deposits during incomplete combustion of the fuel can lead to piston ring sticking and valve sticking.

Wear in a supercooled engine also increases, as combustion products condense, which, being in a liquid state, cause severe corrosion of cylinder liners, pistons and piston rings.

Due to the increase in the self-ignition delay of the fuel, the rigidity of the work increases.

The most favorable thermal regime of the engine is maintained by the cooling system, which removes excess heat from the parts and transfers it to the surrounding air.

Elements of the cooling system

The liquid cooling system (Fig. 1.) consists of a pump 2, a fan 4 with a controlled hydraulic drive (fan drive fluid coupling, fluid coupling switch), a radiator with shutters, an expansion tank 15, thermostats, instrumentation, cavities and channels in the cylinder block and heads, pipelines.

The cooling system is designed for the most severe conditions, when the engine is running at full load at high ambient temperatures.

There are control devices in the cooling system to prevent the engine from overcooling in other, lighter operating conditions, and at start-up to ensure its fastest warm-up.

Cooling is regulated by changing the amount of air and liquid passing through the radiator.

The air flow is regulated automatically by periodically turning off the fan, driven through the fluid coupling.

It is also possible to control the air flow by changing the position of the louver plates, which are located in front of the radiator.

Open and close the blinds from the cab from the driver's seat.

The amount of fluid passing through the radiator is controlled automatically by the thermostat.

Depending on the temperature of the engine, the fluid can circulate in two ways:

  • in a large vicious circle with the thermostat valves open (engine - radiator - warm engine)
  • or bypassing the radiator in a small circle (the engine is cold and the thermostat valves are closed.

The direction of movement of the coolant is shown in figure 2 by arrows.

Thus, when the engine is running, the optimum temperature (75 ... 98 ° C) of the coolant in the system is maintained automatically by thermostats and, if necessary, the fan is turned on as follows:

At 78°C, thermostats begin to open and coolant partially enters the radiator; at 85...90°C, the fan is switched on; at 95 °C, the thermostatic radiator valves open completely and the small circle closes.

YaMZ-236/238 thermostats

When the temperature of the coolant drops from 95 °C, the radiator valves of the thermostats begin to close, the bypass valves open and the coolant partially flows to the pump in a small circle, at 85 °C the oil supply to the hydraulic coupling stops and the fan turns off, at 78 °C they completely close thermostatic radiator valves and coolant does not enter the radiator.