CHAPTER 2 - PART 5 - ENGINE AUXILIARY SYSTEMS - COOLING SYSTEM

The cooling system maintains the engine's operating temperature within normal limits, generally between 80 and 95 °C.

During engine operation, thermal processes inside the cylinders cause the engine components to heat up beyond permissible limits. For this reason, a cooling system is needed to absorb and remove excess heat.

Cooling is indirect and uses a coolant that is forced to circulate through the system by a pump. In most modern motor vehicles, the system operates at a pressure higher than atmospheric pressure.

In a pressurised system (with a closed circuit), the coolant circulates in a sealed circuit, and the pressure is controlled by the expansion tank valve.

Coolant

Coolant is a mixture of water and antifreeze containing glycols that prevent freezing. It transfers heat between the engine components and the radiator. Coolant provides protection against freezing down to approximately -35 °C.

In addition to transferring heat, coolant also protects against deposits and corrosion, preventing the formation of limescale or other deposits that can reduce cooling efficiency.

Over time, coolant degrades and loses its properties, which is why periodic replacement is recommended according to the motor vehicle manufacturer's instructions. If the manufacturer does not specify how many years should pass before replacement, it can, as a guideline, be replaced at intervals of no more than 3 years.

The coolant level in the expansion tank must be checked regularly and maintained between the minimum and maximum limits.

The cooling system comprises:

• the radiator is the component that allows heat to be released to the surroundings. It consists of two tanks (upper and lower), between which are several thin tubes fitted with fins, through which the coolant flows.

The lower tank has a coolant outlet connection and mountings, and in some cases may also have a drain device;

• the water pump ensures forced circulation of coolant through the system. Automobiles generally use centrifugal pumps. The pump is mounted on the engine block and connects the radiator to the engine's cooling jacket.

Remember: breakage or loosening of the water pump drive belt causes the engine temperature to rise. In automobiles where the water pump, alternator and fan are driven by the same belt, its breakage is indicated by the battery charging warning light coming on;

• the thermostat is a valve that automatically regulates coolant circulation according to temperature. It maintains the engine's optimum operating temperature by opening and closing the coolant circuit. The engine's optimum operating temperature is generally between 90 and 95 °C;

• the fan ensures airflow through the radiator, helping to cool the coolant. It can be driven mechanically (by a belt) or electrically (electric cooling fan). During operation, the fan draws in outside air and passes it through the radiator;
• the thermal switch monitors the coolant temperature and activates the electric cooling fan when additional radiator cooling is needed. When the temperature specified by the manufacturer is reached, the thermal switch contact closes and the electric cooling fan starts to increase airflow through the radiator.