How Stuff Works

Fuzz Townshend explains how aircon keeps you cool

Air conditioning

Air conditioning thoroughly wove its way into the car scene in the 1940s, when sweltering Americans became accustomed to refrigerated air being pumped around their homes and shopping malls.

Climbing into a sun-drenched sedan was an unpleasant change of climate and so, with the introduction of engine-driven air conditioning compressors, an almost seamless transition became de rigueur.

Nowadays, air conditioning is ubiquitous on all but the lowest specification cars and has remained basically the same since its introduction, although until the 1990s a different refrigerant was used. This old refrigerant, R-12, was a chlorofluorocarbon (CFC) and was found to be damaging to the planet’s ozone layer. It was replaced by the non-CFC, R134a refrigerant.

Essentially, the process is similar to a household refrigerator. The compressor draws in the refrigerant in the form of a gas and then compresses it.

In a similar fashion to when air is compressed, such as by a bicycle pump, the refrigerant gas heats up due to the proximity and consequent friction of its constituent compressed molecules. It then passes through the condenser, which looks and acts in a similar fashion to a radiator, in that air is passed between the tubes, cooling the refrigerant therein to a now pressurised liquid form, like steam cooling back to water.

Before the refrigerant can be used to cool the car’s interior, any water contained within it must be eliminated, as ice crystals would damage the compressor and block the system. So it’s delivered to the receiver-dryer, which uses a granular desiccant to remove the water.

The water-free refrigerant then passes through either a thermostatic expansion valve, or an orifice tube. The job of these devices is to reduce the pressure of the refrigerant before it enters the evaporator which, in turn, cools it down to around zero degrees Celsius.

The cooled refrigerant enters the evaporator, which resembles a radiator and is located within the car’s passenger compartment. Warm air from the passenger compartment is blown through the evaporator’s tubes and fins and the heat is transferred to the refrigerant.

As a result of this heat transfer, the refrigerant becomes a gas again and passes from the evaporator back to the compressor, and so the cycle begins again.

If an orifice tube is used in the system, an accumulator placed between the evaporator and the compressor prevents any refrigerant that hasn’t turned back to a gas from entering the compressor. The reason for this is that compressors can only compress gas.

Conditioning the air also reduces its moisture content. This moisture is collected and allowed to drain and this can often be seen dribbling harmlessly from the car, which often alarms its owner.

One disadvantage of having an air conditioning system fitted to a car is that the compressor saps some of the energy from the engine when the system is in use. This can be detected most obviously when you switch an air conditioning system on with the engine idling. The engine note dips noticeably, indicating that it is having to work harder, which in turn has a detrimental effect on the overall fuel consumption of the car.