Introduction:
The air-conditioning system is responsible for cooling and dehumidifying incoming air, which helps create a comfortable environment for the vehicle occupants. In addition to promoting comfort, a pleasant climate also affects the driver’s alertness. Air conditioning is part of what is known as 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 are encountering the term HVAC more and more often.
The extent to which an air-conditioning system can lower the temperature can vary depending on various factors, such as ambient temperature, the system’s efficiency, 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 outlet air temperature to the desired level, it is recommended to activate recirculation mode. This function ensures that the already cooled air in the interior is routed through the evaporator again to cool it further.
The following image shows the control panel of a BMW 3 Series. The air-conditioning (A/C) button is indicated with a red arrow. The A/C is switched on.
This page provides a brief overview of how the air-conditioning system works. For each paragraph, you can click the topic to go to the page where more detailed information is provided about that topic.
Operation of the air-conditioning system:
In an activated air-conditioning system, refrigerant circulates through the various components of the system. This refrigerant undergoes two changes of state:
- condensing: heat is released to the surroundings. During condensation, the refrigerant changes from a gaseous state to a liquid;
- evaporating: heat is absorbed from the surroundings. Evaporation takes place in the evaporator, which cools the air flowing to the interior.
The image below shows an overview of the components of a modern air-conditioning system. The refrigerant circuit can be divided into two pressure areas: high and low pressure. In the image below, the red line is the high-pressure line and the blue line is the low-pressure line.
In the coming paragraphs, the above diagram is shown again, with each component individually highlighted with a green frame. For each component, a detailed explanation is provided of its operation, its location in the vehicle, and its interaction with other components. First, the components that are directly responsible for the refrigeration cycle are described. Each component has a subpage that goes into more detail about its operation. To do so, click the blue-coloured text.
Compressor:
The A/C compressor draws in the gaseous / vapour refrigerant via the blue line and increases its pressure. Increasing the pressure also raises the boiling point. The vapour is then routed via the red line to the condenser. The vapour remains superheated. On the A/C compressor there is a pulley that is driven by the serpentine belt. The compressor can be switched on and off using an electromagnetic clutch. In electric or hybrid vehicles, the drive can be provided by an electric motor in the HV system instead of the serpentine belt.
The A/C compressor in cars with internal combustion engines is located in the engine bay on the side of the serpentine belt, and is driven by the serpentine belt together with the alternator and, if applicable, the power-steering pump. In the case of electric vehicles, the compressor may be located in the interior and an electric motor drives the (electric) A/C compressor.
Condenser:
After the refrigerant leaves the compressor, the pressure and temperature have increased considerably. The condenser has the task of condensing the superheated vapour from the compressor into a subcooled liquid. When the temperature has dropped below the boiling point, the refrigerant becomes liquid. In the condenser, heat is dissipated to the outside air. While driving, ram air flows through the condenser. If the airflow and cooling are insufficient, the fan switches on to increase the airflow. The outside air flowing through the condenser is then warmed up.
The condenser is located at the front of the car in front of the cooling system radiator.
Expansion valve:
The liquid refrigerant arrives at the expansion valve under high pressure. The expansion valve causes a sudden pressure drop, causing the pressure, temperature, and boiling point to decrease. The liquid changes to saturated vapour. This is a mixture of vapour 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 restriction (capillary) or variable restriction (thermostatic expansion valve).
Evaporator:
The task of the evaporator is to cool the air flowing into the car’s interior. The interior fan blows outside air or recirculated cabin air through the fins of the evaporator. Heat is extracted from the passing air. The cooled air is then blown into the interior.
From the expansion valve, the vapour refrigerant flows to the evaporator. The temperature, pressure, and boiling point are low as it flows in. The airflow through the evaporator warms the refrigerant, causing it to start boiling immediately. When leaving the evaporator, the refrigerant is in a superheated state. After the refrigerant has evaporated at the moment it leaves the evaporator, the cycle starts again. The compressor draws in the gaseous refrigerant again to compress it again.
The evaporator is installed in the heater/ventilation housing behind the dashboard.
The last image shows the components that are not mentioned above.
- Service connections: these are outlined in blue. These are used to check the pressures with a manifold gauge, and to evacuate or charge the system;
- High-pressure sensor: the sensor outlined in orange measures the pressure in the high-pressure line. Among other things, the engine ECU can use this to control the compressor output;
- Interior fan: the interior fan outlined in purple blows air into the heater box and thus also through the evaporator;
- Cooling fan: the cooling fan outlined in yellow blows outside air through the condenser. Some cars have a separate fan for the condenser, while other cars use the cooling fan that also cools the radiator;
- Filter / drier: filtering dirt particles and drying (dehumidifying) take place in the element outlined in green. The filter / drier element can be mounted directly next to the condenser, but it can also be located inside the condenser.
Pressures and temperatures:
By measuring the pressures and temperatures at different points in the system, we can determine whether the system is functioning correctly. Because the pressure and temperature of the refrigerant depend on the outside air temperature, we use as a guideline a temperature between 25 and 30 °C and an increased engine speed so that the A/C compressor has sufficient capacity.
Due to compression, the temperature of the refrigerant upon leaving the compressor rises to about 70 °C, while the pressure varies between 12 and 15 bar. Via the drier/filter, the refrigerant reaches the expansion valve, where a pressure drop occurs and the temperature drops to slightly above freezing. When the refrigerant leaves the evaporator, it has warmed up a few degrees due to the air that has passed through.
On the page: Diagnosing air conditioning based on pressure and temperature, the most common faults, causes and solutions are described.
