Here, you will find useful basics on the topic of engine cooling in vehicles.

For the internal combustion engine to run efficiently and at low emissions, it must reach its operating temperature as quickly as possible and maintain it at all load conditions. This is ensured by the engine cooling, which simultaneously supplies the passenger space with heat. On this page, we will describe the functions of the engine cooling and its components. A video will additionally inform you about the professional replacement of a Visco clutch.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


Engine cooling with water

The temperatures generated by the burning fuel (up to 2,000 °C) are detrimental to engine operation. Therefore, the engine is cooled down to operating temperature. The first kind of cooling with water was thermosyphon cooling. 


The heated, lighter water rises into the upper part of the radiator through a manifold and is cooled by the air flow. It then sinks down and is returned to the engine. The water is circulating while the engine is running. Cooling was supported by the fan, but regulation was not possible. Later, a water pump accelerated the water circulation.


Weak points:

  • Long warm-up time
  • Low engine temperature during the cold season


In the further development of engines, cooling water regulators (i.e. thermostat) were used. The water circulation through the radiator is regulated depending on the coolant temperature. In 1922, it is described as follows: "The purpose of these devices is quick engine heating and prevention of cooling down of the engine." 


We are referring to a thermostat-controlled cooling system with the following functions:

  • Short warm-up time
  • Keeping operating temperature constant

Modern engine cooling

The thermostat was a decisive improvement to engine cooling and enabled short-circuit coolant circulation While the desired engine operation temperature is not reached, the water does not run through the radiator, but by-passes it and runs into the engine. The thermostat only opens the connection to the radiator once the desired operating temperature is reached. That control system has remained the basis of all systems to this day. The engine's operating temperature is not only important with regard to performance and fuel consumption, but also for low emission of pollutants.


Engine cooling uses the fact that pressurized water does not boil at a temperature of 100 °C, but only between 115°C and 130°C. The cooling circuit is under pressures between 1.0 bar and 1.5 bar. This constitutes a closed cooling system. The system has an expansion tank which is only around half filled. The cooling medium is not just water, but a mixture of water and coolant additive. We are now dealing with a coolant providing anti-freeze protection, has an increased boiling point and protects the engine’s parts and the cooling system against corrosion.


Due to the increasingly constraint engine compartment, installing the components and dissipating the enormous amounts of heat poses a great challenge. The cooling of the engine compartment places high demands on modern cooling systems and therefore great progress has been made recently in cooling technology.


The demands placed on the cooling system are:

  • Shorter warm-up phase
  • Fast passenger compartment heating
  • Low fuel consumption
  • Longer service life of the components


All engine cooling systems are based on the following components:

  • Coolant radiator
  • Thermostat
  • Coolant pump (mechanical or electric)
  • Expansion tank
  • tubes
  • Engine fan (V-belt driven or Visco®)
  • Temperature sensor (engine control/indicator)

Operating principle

The heat generated during fuel combustion, which migrates to the engine components, is transmitted to the coolant. The circulation causes heat to transmit to the external air, thus cooling down the coolant. One or several fans (mechanically or electrically powered) that can be installed in front of or behind the radiator, support the cooling down process. This occurs in particular at slow speeds or during vehicle standstill. For maintaining the temperature of the coolant and/or engine relatively constant, a thermostat controls the coolant inflow.

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