Introduction:
The air conditioning system is responsible for cooling and dehumidifying incoming air, contributing to creating a comfortable environment for vehicle occupants. Besides enhancing comfort, a pleasant climate also affects 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 thus heat, ventilate, and cool the climate system. In the literature, we increasingly encounter the term HVAC.
The extent to which an air conditioning system can lower the temperature can vary depending on several factors, such as the ambient temperature, the system’s efficiency, and the desired temperature setting. Generally, it is common for a well-functioning air conditioning system to cool the temperature in a passenger car by approximately 10 to 20 degrees Celsius compared to the outdoor temperature.a0To quickly bring the air outlet temperature to the desired level, it is recommended to activate the recirculate mode. This function ensures that the already cooled air in the interior is recirculated through the evaporator for further cooling.
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 air conditioning is turned on.
This page provides a brief overview of the operation of the air conditioning system. For each section, the subject can be clicked to go to the page where more detailed information about the respective topic is provided.
Operation of the air conditioning system:
In the activated air conditioning system, refrigerant circulates through the various components of the system. This refrigerant undergoes two state changes:
- condensation: heat is released to the environment. During condensation, the refrigerant transitions from a gaseous state to a liquid;
- evaporation: heat is absorbed from the environment. Evaporation takes place in the evaporator, cooling the passing air to the interior.
The image below shows the overview of the components of a modern air conditioning installation. The refrigerant cycle can be divided into two pressure zones: high and low pressure. In the image below, the high-pressure line is red and the low-pressure line is blue.
In the upcoming sections, the above-mentioned diagram is shown again, with each component individually highlighted with a green frame. For each component, detailed explanations are provided about the operation, location in the vehicle, and interaction with other components. First, the components directly responsible for the cycle process are described. For each component, there is a subpage expanding on its operation. Click on the blue-colored text for more information.
Compressor:
The air conditioning compressor draws in the gaseous/vaporous refrigerant through the blue line and increases its pressure. With the pressure increase, the boiling point also rises. The vapor is then routed through the red line to the condenser. The vapor remains superheated. On the air conditioning compressor, there is a pulley driven by the multirib belt. The compressor can be engaged and disengaged using a magnetic clutch. In electric or hybrid vehicles, the drive may be powered by an electric motor within the HV system instead of the multirib belt.
The air conditioning compressor in combustion engine vehicles is located in the engine compartment on the side of the multirib belt and is driven by the multirib belt along with the alternator and possibly the power steering pump. In the case of electric vehicles, the compressor may be located inside the cabin, where an electric motor powers the (electric) air conditioning compressor.
Condenser:
After the refrigerant leaves the compressor, the pressure and temperature have significantly increased. The condenser is tasked with condensing the superheated vapor from the compressor into a subcooled liquid.a0If the temperature drops below the boiling point, the refrigerant becomes liquid.a0In the condenser, heat is dissipated to the outside air. While driving, the airflow passes through the condenser. If the airflow and cooling are insufficient, the fan engages to enhance the air circulation. 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 at the expansion valve under high pressure. The expansion valve causes a sudden pressure drop, decreasing both the pressure, temperature, and boiling point. The liquid transitions to saturated vapor. This is a mixture of vapor and liquid particles. In the expansion valve, the high and low pressure are separated.a0
The expansion valve has different versions: with a fixed restriction (capillary) or variable restriction (thermostatic expansion valve).
Evaporator:
The task of the evaporator is to cool the incoming air to the car’s interior. The interior fan blows outside air or recirculated interior air through the evaporator’s fins. Heat is extracted from the passing air. The cooled air is then blown into the interior.
From the expansion valve, the vaporized refrigerant flows into the evaporator. The temperature, pressure, and boiling point are low when entering. The airflow through the evaporator warms the refrigerant, causing it to boil immediately. The refrigerant is in a superheated state when leaving the evaporator. After the refrigerant has evaporated upon leaving the evaporator, the cycle begins anew. The compressor draws in the gaseous refrigerant to compress again.
The evaporator is installed in the heater ventilation housing behind the dashboard.
In the last image, the components not mentioned above are shown.
- Service ports: these are blue-framed. They are used to check pressures with a manometer and to evacuate or fill the system;
- High-pressure sensor: the orange-framed sensor measures the pressure in the high-pressure line. The engine ECU can regulate the compressor output with this information;
- Interior fan: the purple-framed interior fan blows air into the heater housing and also through the evaporator;
- Cooling fan: the yellow-framed cooling fan blows outside air through the condenser. Some cars have a separate fan for the condenser, while others use the cooling fan that also cools the radiator;
- Filter/dryer: the filtering of dirt particles and drying (dehumidifying) occurs in the green-framed element. The filter/dryer 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 various points in the system, we can determine if the system is functioning correctly. Since the pressure and temperature of the refrigerant depend on the external air temperature, we take a reference temperature of between 25 and 30 9C and an elevated engine speed so that the air conditioning compressor has sufficient capacity.
Due to compression, the refrigerant’s temperature rises to approximately 70 C when leaving the compressor, while the pressure varies between 12 and 15 bar. The refrigerant reaches the expansion valve via the dryer/filter, where a pressure reduction occurs, and the temperature drops to slightly above freezing. When the refrigerant leaves the evaporator, it has been slightly warmed by the passing air.
On the page: Diagnosing air conditioning based on pressure and temperature, the most common malfunctions, causes, and solutions are described.
