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Air Conditioning


  • Preface
  • Operation of the air conditioning system

The air conditioning system is responsible for cooling and dehumidifying incoming air, helping to create a comfortable environment for vehicle occupants. In addition to promoting comfort, a pleasant climate also influences the driver's alertness. Air conditioning is part of the so-called HVAC, which stands for: Heating Ventilation Air Conditioning. A vehicle with HVAC can therefore heat, ventilate and cool the climate system. In the literature we come across the term HVAC more and more often.

The degree to which an air conditioning system can lower the temperature can vary depending on several factors, such as the ambient temperature, the efficiency of the system and the desired temperature setting. In general, it is common for a properly functioning air conditioning system to cool the temperature in a passenger car by about 10 to 20 degrees Celsius relative to the outside temperature. To quickly bring the air outlet temperature to the desired level, it is recommended to activate the recirculation mode. This function ensures that the air in the interior that has already been cooled is recirculated through the evaporator for additional cooling.

The following image shows the control panel of a BMW 3 Series. The air conditioning (A/C) button is marked with a red arrow. The air conditioner is on.

This page provides a brief overview of the operation of the air conditioning system. For each paragraph, the subject can be clicked to go to the page where more detailed information is given about the relevant subject.

A/C control button (BMW 3 series)

Operation of the air conditioning system:
When the air conditioning system is running, refrigerant circulates through the various components of the system. This refrigerant makes two state changes along:

  • condensation: heat is released to the environment. During condensation, the refrigerant changes from gaseous state to liquid;
  • evaporation: heat is extracted from the environment. Evaporation takes place in the evaporator, which cools the air flowing through to the interior.

The figure below shows the overview of the components of a modern air conditioning installation. The refrigerant cycle can be divided into two pressure areas: the high and the low pressure. In the image below, the red pipe is from the high pressure and the blue one from the low pressure.

Overview of air conditioning system components

In the coming paragraphs, the above diagram will be shown again, with each part individually highlighted with a green box. For each component, detailed explanations are given regarding its operation, location in the vehicle and interaction with other components. First, the components that are directly responsible for the cycle process are described. There is a subpage for each component where its operation is discussed in more detail. To do this, click on the blue-colored text.


De air conditioning compressor sucks in the gaseous/vapor refrigerant through the blue pipe and increases its pressure. As the pressure increases, the boiling point also increases. The vapor is then led to the condenser via the red pipe. The vapor remains superheated. There is a pulley on the air conditioning compressor that is driven by the multi-belt. The compressor can be switched on and off using a magnetic coupling. In electric or hybrid vehicles, the drive can be provided by an electric motor in the HV system instead of the multi-belt.

The air conditioning compressor in cars with combustion engines is located in the engine compartment on the side of the multi-belt, and is driven by the multi-belt together with the alternator and possibly the power steering pump. In the case of electric vehicles, the compressor can be located in the interior and an electric motor drives the (electric) air conditioning compressor.

Air conditioning compressor


After the refrigerant leaves the compressor, the pressure and temperature have increased significantly. The condenser Its task is to condense the superheated vapor from the compressor into a subcooled liquid. When the temperature has dropped below the boiling point, the refrigerant liquefies. In the condenser, the heat is dissipated to the outside air. While driving, the wind flows through the condenser. If the flow and cooling are insufficient, the fan switches on to increase the airflow. The outside air flowing through the condenser is then heated.

The condenser is located in the front part of the car in front of the radiator of the cooling system.


Expansion valve:

The liquid refrigerant arrives under high pressure expansion valve. The expansion valve causes a sudden reduction in pressure, causing the pressure, temperature and boiling point to drop. The liquid changes to saturated vapor. This is a mixture of vapor and liquid particles. In the expansion valve the high and low pressure are separated from each other. 

The expansion valve comes in different versions: with fixed throttling (capillary tube) or variable throttling (thermostatic expansion valve).

Expansion valve (type: TEV)


The task of the evaporator is to cool the incoming air to the interior of the car. The interior fan blows outside air or recirculated interior air through the fins of the evaporator. The heat is extracted from the passing air. The cooled air is then blown into the interior.

The vaporous refrigerant flows from the expansion valve to the evaporator. The temperature, pressure and boiling point are low when flowing in. The airflow through the evaporator heats up the refrigerant, causing it to immediately start boiling. The refrigerant is superheated when it leaves the evaporator. After the refrigerant has evaporated as it leaves the evaporator, the cycle starts again. The compressor sucks in the gaseous refrigerant again to compress it again.

The evaporator is built into the heater ventilation housing behind the dashboard.


The last image shows the parts not mentioned above.

  • Service connections: these are framed in blue. These are used to check pressures with a pressure gauge, and to drain or fill the system;
  • High-pressure sensor: The orange-framed sensor measures the pressure in the high-pressure line. This allows the engine-ECU to control, among other things, the output of the compressor;
  • Interior fan: the purple framed one interior fan blows air into it stove house and therefore also by the condenser;
  • Cooling fan: the one outlined in yellow cooling fan blows outside air through the condenser. Some cars have a separate fan for the condenser, and other cars use the cooling fan that also cools the radiator;
  • Filter / dryer: filtering of dirt particles and drying (dehumidification) takes place in the green framed element. It filter / dryer element can be mounted directly next to the condenser, but it can also be located inside the condenser.
Various components

Pressures and temperatures:
By measuring the pressures and temperatures at different places in the system, we can check whether the system is functioning correctly. Because the pressure and temperature of the refrigerant depend on the outside air temperature, we take as a guideline a temperature of between 25 and 30 °C and an increased engine speed so that the air conditioning compressor has sufficient capacity.

Due to compression, the temperature of the refrigerant rises to about 70 °C when it leaves the compressor, while the pressure varies between 12 and 15 bar. The refrigerant reaches the expansion valve via the dryer/filter, where a pressure drop occurs and the temperature drops to slightly above freezing point. When the refrigerant leaves the evaporator, it has been warmed up a few degrees by the air that has passed through it.

On the page: Air conditioning diagnosis based on pressure and temperature the most common faults, causes and solutions are described.

Pressures and temperatures in the air conditioning circuit