Why do we need air conditioning for our cars?
Equipping cars with air conditioning systems has become a standard practice for car manufacturers. The AC system should be regarded as a system that improves the overall active safety of the car, not just the thermal comfort of the occupants.
The comfort of the driver and passengers is highly dependent on the temperature and humidity of the air in the passenger compartment. The thermal discomfort of the driver can increase the risk of an accident due to the loss of concentration and the increase of fatigue.
The classic ventilation system, which uses only atmospheric air for cooling/ventilation, cannot satisfy the thermal comfort of the driver and passengers, for the following reasons:
- The atmospheric air can have a temperature above the comfort limit
- The atmospheric air is further heated on the intake path
- Using the fan at a high speed decreases humidity below normal values
Research in the field shows that a person’s concentration level decreases due to stress. Excessive heat or reduced humidity leads to increased stress on the body. The optimum temperature for the driver is between 20 and 22 ° C (68 – 72 F).
The sun’s rays can increase the cabin temperature by over 15 ° C (59 F) compared to the atmospheric temperature, especially in the head area. In addition to body temperature, the pulse increases in intensity. Breathing becomes difficult and the brain receives less oxygen.
Studies show that increasing the temperature from 25 to 35 ° C (77 – 95 F) reduces a person’s concentration and reaction rate by about 20%. Driving the car in high-temperature conditions (35 ° C or 77 F) is equivalent to an alcohol concentration of 0.5 ml.
Note that the boiling point (transformation from liquid to gas) of a substance depends on the pressure. The water is evaporated at 100 ° C at atmospheric pressure (1 bar). If the water pressure is 2 bar the boiling point increases to 120 ° C. At 5 bars the water evaporates only at temperatures above 150 ° C.
For heat transfer, air conditioning systems use a special substance called refrigerant:
- R12 (CCl2F2)
- R134a (CH2F-CF3)
These substances allow the cyclical operation of the air conditioning system, take the heat by vaporization and transfer the heat by condensation, at temperatures close to the ambient temperature.
R12 (CFC, chlorofluorocarbon) boils at -29.6 ° C (-21.28 F), at atmospheric pressure (1 bar). This substance has been banned for use in air conditioning systems due to the negative impact on the ozone layer.
Currently, R134a (HFC, hydrofluorocarbon) is used, which boils at -26.5 ° C (-15.7 F), under atmospheric pressure conditions. The advantage of this substance, compared to water, is the boiling point at a very low temperature. At a pressure slightly above atmospheric pressure, of the order of 1.2 – 1.3 bar, R134a evaporates at about -5 ° C (23 F).
Car air conditioning systems, regardless of the manufacturer, contain approximately the same basic components.
The car air conditioning system uses two heat exchangers (radiators):
- Evaporator: it is located in the vicinity of the passenger compartment, it takes the heat from inside the car
- Condenser: it is located in front of the car/engine, it dissipates the heat in the environment
How does car air conditioning system work?
The mode of operation is cyclical. The compressor (A), driven by the propellant (thermal/electric) takes the refrigerant in gaseous form, compresses it and sends it to the condenser (B). The atmospheric airflow (natural or created by an electric fan) cools the compressed refrigerant from the condenser and converts it into liquid.
Through the expansion valve (E) the refrigerant, in liquid form, enters the evaporator (G) where it turns into gas and takes up the heat from the air around the evaporator. The fan (I) sends the cooling air into the car cabin.
Role of AC compressor
The role of the compressor is to compress and recycle the refrigerant in gaseous form. The compressor is driven by the drive of the car through a belt, but it can also be electrically operated. The compressor is switched on and off using an electromagnetic clutch that is located inside the driveshaft.
The refrigerant also contains oil that ensures the lubrication of the compressor during operation.
The condenser acts as a heat exchanger, its purpose being to dissipate the heat collected from inside the passenger compartment into the environment.
It has two ports, one inlet, through which the refrigerant is brought in the gaseous form and one out, through which the refrigerant is sent in liquid form to the evaporator.
The dehumidifier is usually located near the condenser and can be integrated into it or separate components. The role of the dehumidifier is to clean the refrigerant from impurities and to remove water vapor which, at low temperatures, can lead to the appearance of ice.
The dehumidifier also acts as an expansion vessel for the car air conditioning system.
The expansion valve is the connection between the high-pressure circuit and the low-pressure circuit. It is used to control the refrigerant flow entering the evaporator depending on the refrigerant temperature at the exit of the evaporator. The flow control is made mechanically, without electronic control.
The evaporator has a similar construction as the condenser, being also a heat transformer. Its role is to allow evaporation of the refrigerant. The evaporator has a large air contact surface, which facilitates the transfer of heat to the refrigerant.
Due to heat absorption and pressure drop, the refrigerant is converted to gas.
Pressure sensor – this sensor is located after the capacitor, on the so-called high-pressure circuit. This sensor senses if the cooling agent is in the installation and so the computer knows whether or not to start the compressor, obviously if we do not have a cooling agent in the system it is useless to operate the compressor.
If this sensor is faulty the system will not work – check it with a diagnostic tester or even a multimeter if you have some knowledge and reference values.
The cold airflow from the evaporator to the car cabin is regulated by a fan. In manual air conditioning systems, the driver controls the temperature in the cabin by setting the fan at a higher or lower speed.
Modern air conditioning systems are integrated into the general ventilation system of the car. The evaporator, expansion valve and fan are part of the integrated system positioned between the engine compartment and the passenger compartment. The compressor, condenser and cooling fan are located in the engine compartment.
In addition to the thermal comfort function it provides, the air conditioning system must be regarded as a system that enhances active safety. It helps maintain an optimal driver concentration level and can make the difference between a preventive driving mode and a high accident probability.