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

Engine cooling

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 information
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day 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.


Engine cooling started in 1905. The combustion temperature in an engine back then was between approx. 600 and 800 degrees Celsius. Steel radiators were used from the turn of the century until around 1938, when they were replaced by nonferrous metal radiators (copper/brass). The disadvantage: heavy, limited supply, leading to a high material price.


Radiator requirements:

  • High power density
  • Sufficient stability
  • Permanent resistance to corrosion
  • Low production costs
  • Environmentally compatible production



  • Water box made of GRP = glass fibre reinforced polymer
  • Increasingly made of aluminium



  • Cooling the coolant in the engine circuit



  • Accurate-fit installation for easy assembly
  • Optimal efficiency
  • Tailored to customer specifications (OEM)

Typical design

The oil radiator for the coolant radiator may also be a separate component. The individual components are assembled. This endows the coolant radiator with its form. Cooling is effected by means of cooling fins (mesh). The air flowing through takes heat out of the coolant. The coolant flows from top to bottom, which is called downdraft, or with a cross flow (right to left or vice versa). For both variants, sufficient time and a sufficient cross-section are necessary for the air to efficiently cool the coolant.


Two typical designs: soldered and mechanically fitted. Both are downdraft radiators. At first the radiators were equipped with brass water boxes, later with plastic water boxes. Cross-flow radiators are 40% smaller than downflow radiators and are used in passenger cars today where flatter type of construction is required. 


The water box is fastened and sealed with a wave-slot flanging developed by Behr. Another type of fastening is tab flanging. Downdraft radiators are installed in higher passenger cars (cross-country vehicles, etc.) or commercial vehicles. 


Manufacturing distinguishes between basically two different production methods: components can either be mechanically joined or soldered. The technical performance data of both manufacturing processes are approximately identical. The only real difference is that the mechanically joined variation weighs less. It is the vehicle manufacturers who ultimately decide on which process will be put into series production. The construction of the radiator tube geometry / fin geometry is decisive for the respective performance. It is also important to take into account the available space in the vehicle.

Soldered vs. joined radiators in comparison

The table below summarizes the main differences between the radiator versions.

  Joined Soldered
Tubes Oval or round Braze-clad flat tubes, folded for reinforcement or beaded.
Ribs Punched, pinned Rolled corrugated ribs
Connection By expanding tubes and pinning By soldering
Miscellaneous Light weight
  • A flat tube usually sufficient for entire system depth
  • Length of component decisive for number of flat tubes


Full aluminium radiator

As you can see, the full aluminium radiator has a considerably reduced mesh depth. This type of construction helps reduce the overall depth of the radiator module. For example, the entire full aluminum radiator of the Audi A8 is 11% lighter and has a 20 mm smaller depth.


This construction design has the following characteristics:

  • Top not needed
  • Mesh depth equals radiator depth
  • 5%-10% less weight
  • Higher operational stability
  • Bursting pressure 5 bar
  • Can be recycled as a whole
  • Transportation damage is reduced (overflow sockets)
  • Various tube types can be used
  • Circular tube with turbulence insert in the case of higher capacity
  • Oval tube (means more surface for cooling)
  • Flat tube mechanically fitted (more surface area yet only one row necessary)
  • Flat tube soldered without fluxing agent (best cooling, lamellas fit tight 100%), but expensive
  • Special aluminium alloy is used (mesh)
  • Temperature 600-650°C, then cooling down to around 130°C (tension is equalized)

Effects of failure

A faulty radiator can become noticeable as follows:

  • Poor cooling performance
  • Increased engine temperature
  • Permanent radiator fan operation
  • Poor air conditioning system performance


These are possible causes:

  • Loss of coolant caused by damage to the radiator (rockfall, accident)
  • Loss of coolant through corrosion or leaky connections
  • Poor heat exchange caused by external or internal impurities (dirt, insects, furring)
  • Soiled or old coolant


Test steps towards recognizing faults:

  • Check the coolant radiator for exterior soiling, clean with reduced compressed air pressure or a water jet, if necessary. Do not get too close to the radiator lamellas
  • Check the radiator for external damage and leaks (hose connections, beading, lamellas, plastic housing)
  • Check coolant for discoloring/soiling (e.g. oil caused by faulty gasket) and check anti-freeze content
  • Check coolant flow (blockage through foreign matter, sealing agents, furring)
  • Measure the temperature of the coolant as it enters and leaves the radiator with the aid of an infrared thermometer.

Coolant radiator replacement

If the coolant radiator is broken, it needs to be replaced.
A few things need to be observed though! In this video we show you what it is in detail.


To prevent local overheating of the components, the coolant circuit must not contain bubbles. The coolant enters the container at high speed and exits it at a lower speed, due to different diameters of the openings. For comparison, commercial vehicles have three chambers and a large amount of water, e.g. 8 liter coolant volume. The expansion tank holds expanded coolant from the coolant circuit. The pressure is relieved by a valve and thus the system pressure is kept at a set value.

Operating principle

High coolant temperature results in rising cooling system pressure as the coolant expands. The coolant is pressed into the tank. The pressure in the tank rises. The pressure relief valve in the valve cap opens and lets air escape.


When the coolant temperature normalizes, a vacuum is generated in the cooling system. Coolant is sucked out of the tank. This also generates a vacuum in the tank. Consequently, the vacuum valve in the tank cap opens. Air flows into the tank until the pressure is equalized.

Effects of failure

A faulty expansion tank or a faulty lid can be noticed as follows:

  • Loss of coolant (leak) at various system components or the expansion tank itself
  • Increased coolant and/or engine temperature
  • Ausgleichsbehälter oder andere Systembauteile gerissen / geborsten


These are possible causes:

  • Excess pressure in the cooling system on account of a faulty valve in the lid
  • Material fatigue


Test steps towards recognizing faults:

  • Check the level of coolant and the antifreeze content
  • Check whether the coolant is discolored/soiled (oil, sealant, furring)
  • Check thermostat, radiator, heat exchanger, hose lines and connections for leaks and function
  • Burst test the cooling system if necessary (pressure test)
  • Make sure no air is trapped in the cooling system, vent the system according to vehicle manufacturer's instructions if necessary.


If all the above points are carried out without complaint, the lid on the expansion tank should be replaced. It is very difficult to test the valve on the expansion tank lid.


Thermostats control the temperature of the coolant and thus the engine temperature. Mechanical thermostats have not changed much through the years and are still installed. The function is provided by an expanding wax element which opens a valve and returns coolant to the coolant radiator to be cooled. The thermostat opens at a certain temperature which is set for the system and cannot be changed. Electronically controlled thermostats are controlled by the engine management and open depending on the engine’s operating conditions. Electronically controlled temperature regulators contribute to reducing fuel consumption and pollutant emissions by improving the engine’s mechanical efficiency.



  • Reduction of fuel consumption by around 4%
  • Reduction of pollutant emissions
  • Enhanced comfort (by improved heating power)
  • Longer engine life
  • Preservation of the flow conditions and the thermodynamic conditions
  • Demand-oriented temperature regulation
  • Highest temperature change rate
  • Lowest increase in construction volume (< 3%)

Operating principle

The wax filling melts when heated to more than 80 °C. The volume increase of the wax moves the metal box along the working piston. The thermostat opens the cooling circuit and at the same time closes the short-circuit loop. When the temperature sinks below 80 °C, the wax filling solidifies. A restoring spring presses the metal box back into normal position. The thermostat shuts off the flow to the radiator. The coolant flows directly back to the engine via the short-circuit loop.


Water pumps transport the coolant through the circuit and build up the pressure. The water pumps are also affected by technical advance, but many passenger cars and commercial vehicles with belt-driven water pumps are still available. However, the next generation will be electronically controlled water pumps. Those water pumps are operated as required, similar to the compressor in the air-conditioning circuit. This optimizes the operating temperature.


Check here for additional technical information on coolant pumps.


The heat exchanger supplies heat which is transported into the passenger compartment with the air flow of the blower. If an air-conditioning system is installed, which is mostly the case today, a mixture of cold and warm air is generated by the climate control. Here, all three systems get together: heat, cold and appropriate control = air-conditioning of the passenger compartment.



  • Fully recyclable
  • Guarantees desired passenger compartment temperature
  • Soldered full aluminium heat exchangers
  • Low space consumption in the passenger compartment
  • High heating power
  • End bottoms soldered and not clamped
  • Installed in the heating box
  • Design – mechanically fitted
  • Tube fin system
  • With turbulence inserts for improving heat transmission
  • Gill fields in the fins enhance efficiency
  • State of the art as for the coolant radiator – full aluminium

Operating principle

The cabin heat exchanger consists of a mechanically arranged tube/rib system, as does the coolant radiator. The trend here also points towards an all-aluminum design. Coolant flows through the cabin heat exchanger. The air flow produced by the interior fan or the wind blast is routed through the heat exchanger which has hot coolant flowing through it. The interior air is heated up via the cooling fins (network) of the heat exchanger. The air flow generated by the cabin fan and/or natural airstream flows through the cabin heat exchanger circulated by hot coolant. This heats up the air and allows it to enter the vehicle interior.

Effects of failure

A faulty or poorly working cabin heat exchanger can become noticeable as follows:

  • Poor heating performance
  • Loss of coolant
  • Odor build-up (sweet)
  • Fogged windows
  • Poor air flow


  • These are possible causes:
  • Poor heat exchange caused by external or internal impurities (corrosion, coolant additives, dirt, furring)
  • Loss of coolant through corrosion
  • Loss of coolant through leaky connections
  • Soiled cabin filter
  • Contamination/blockage in the ventilation system (leaves)
  • Faulty flap control


Test steps towards recognizing faults:

  • Watch out for smells and windows fogging
  • Check cabin filter
  • Check cabin heat exchanger regarding leakages (hose connections, flanging, mesh)
  • Watch out for contamination/discoloring of the coolant
  • Check coolant flow (blockage through foreign matter, furring, corrosion)
  • Measure coolant inlet and outlet temperature
  • Watch for blockages/foreign matter in the ventilation system
  • Check flap control (recirculated air/fresh air)


The engine fan transports the ambient air through the coolant radiator and over the engine. It is driven by V-belts or in the case of the an electrical fan by an electric motor controlled by a control unit. The Visco® fan is mostly used in the commercial vehicle area, but also in passenger cars. The engine fan guarantees the flowing through of a sufficient quantity of air to cool the coolant. In the case of V-belt driven fans, the quantity of air depends on the engine speed. The difference to the condenser fan is that it is permanently driven. The Visco® fan control is dependent on the operating temperature.


Rigid (permanently driven) requires a lot of energy (BHP), is loud, has high consumption. In contrast, electrical fans (passenger car) consume less, are low noise and need less energy. The development goals were low consumption and low noise, e.g. noise reduction by means of shielded fan.


The further development into the electronic Visco® clutch yielded:

  • Infinitely variable regulation
  • Regulation by means of sensors
  • Regulator processes data such as coolant, oil, charge air, engine speed, retarder, air-conditioning


This means demand-controlled cooling, improved coolant temperature level, low noise and reduced fuel consumption. In passenger cars, the fans used to be 2-part, Visco® clutch and fan wheel were bolted together. Today, they are rolled and thus cannot be repaired.

Operating principle of Visco® fan:

The fan wheel is usually made of plastic and is screwed to the Visco® clutch. The number and position of the fan blades vary according to design. The housing of the Visco® clutch is made of aluminum and has numerous cooling ribs. Control of the Visco® fan may be accomplished by a purely temperature-dependent, self-regulating bimetallic clutch. The controlled variable here is the ambient temperature of the coolant radiator. The electrically-triggered Visco® clutch is another variant. This is controlled electronically and is operated electromagnetically. Here, the input quantities of different sensors are used for control.

Effects of failure - Visco® fan

A defective Visco® fan can become noticeable as follows:

  • Loud noise
  • Increased engine temperature or coolant temperature


These are possible causes:

  • Damaged fan wheel
  • Oil loss/leaks
  • Soiling of the cooling area or bi-metal
  • Bearing damage

Troubleshooting - Visco® fan

Test steps towards recognizing faults:

  • Check coolant level
  • Check the fan wheel for damage
  • Make sure no oil is leaking
  • Check the bearing for play and noises
  • Check the fastening of the fan wheel and the Visco® clutch
  • Check to make sure that the air-baffle plates/air cover are present and fitted tightly.

The electronic Visco® clutch

Primary disk and flanged shaft convey the power of the engine. The fan is also rigidly connected to it. Circulating silicone oil effects power transmission between the two sub-assemblies. The valve lever controls the oil circuit between supply tank and working chamber. 


The silicone oil flows from the supply tank to the working chamber and back between two borings, the return bore hole in the housing and the feed bore hole in the primary disk. The valve lever controls the engine management by sending pulses to the magnet assembly. 


The Hall effect sensor determines the current speed of the fan and sends the information to the engine management. A regulator sends a cycled control current to the magnet assembly which controls the valve lever which in turn controls oil flow and oil quantity. The more silicone oil is in the working chamber, the higher the fan speed. If the working chamber is empty, the fan idles. The slippage of the drive is approx. 5%.

Effects of failure - Visco® clutch

A defective Visco® fan can become noticeable as follows:

  • Increased engine temperature or coolant temperature
  • Loud noise
  • Fan wheel continues to run at full speed under all operating conditions


These are possible causes:

  • Lack of frictional connection through leaking oil
  • Loss of oil due to leak
  • Soiling of the cooling area or bi-metal
  • Internal damage (e.g. control valve)
  • Bearing damage
  • Damaged fan wheel
  • Permanent full frictional connection due to faulty clutch

Troubleshooting - Visco® clutch

Test steps towards recognizing faults:

  • Check the level of coolant and the antifreeze content
  • Check the Visco® clutch with regard to outer soiling and damage
  • Check the bearing for play and noises
  • Make sure no oil is leaking
  • Check the Visco® clutch by turning it by hand with the engine switched off. With the engine cold, the fan wheel should be easy to turn and with the engine hot it should be hard to turn.

fan wheel should be easy to turn and with the engine hot it should be hard to turn

  • If possible check the slip of the clutch using speed comparison between the speeds of the fan and the drive shaft. With full frictional connection, the difference may only be max. 5% for directly driven fans. An optical revolution counter with reflective strips is suitable for this purpose
  • Check electrical connection (electronically-triggered Visco® clutch)
  • Check air cover/air baffle plates
  • Make sure there is enough air flowing through the radiator

Professional replacement of Visco clutch

Beginning with inspection and diagnostics, we provide a step-by-step description for the professional exchange of the Visco clutch.


04:18 min

Electric radiator fan

Passenger cars mostly use electric fans. They are frequently used as extractor fans but also as pressure fans. As more air circulates the engine radiator when the fan is running, optimal coolant temperature regulation is ensured for every vehicle operating condition. The vehicle front usually houses additional radiators (e.g. charge air, steering, fuel, condenser), whose media (air, oil, fuel, coolant) are also cooled by electric fans.


The control of the single or double fan(s) occurs via pressure and/or temperature switches or a control unit. Depending on operating conditions, it is therefore possible to control the fan speed gradually (switch) or flexibly (pulse width-controlled). For electronically-controlled fans, the control unit is often situated near the fan unit. Thanks to a diagnosis device/oscilloscope, it is possible to read the error memory and/or control the drive. Possible error causes are mechanical damage (crash, storage damage, broken guide vane) and electric errors (contact error, short circuit, defect switch/control unit).


The single or multiple electric radiator fan(s) are usually mounted to fan frames. Those mostly have the task of directing the air from the radiator directly, and ideally without flow losses, to the fan. For this reason, the fan frame is mounted as closely as possible to the radiator.


The control of the single or double fan(s) occurs via pressure and/or temperature switches or a control unit. Depending on operating conditions, it is therefore possible to control the fan speed gradually (switch) or flexibly (pulse width-controlled). For electronically-controlled fans, the control unit is often situated near the fan unit. Thanks to a diagnosis device/oscilloscope, it is possible to read the error memory and/or control the drive. Possible error causes are mechanical damage (crash, storage damage, broken guide vane) and electric errors (contact error, short circuit, defect switch/control unit).


The single or multiple electric radiator fan(s) are usually mounted to fan frames. Those mostly have the task of directing the air from the radiator directly, and ideally without flow losses, to the fan. For this reason, the fan frame is mounted as closely as possible to the radiator.