How Does Car AC Work: A Guide to Cool Mobility

Given the tropical climate of India, an air conditioner in cars is as common as brakes. In fact, can you even remember the last time you sat in a car without AC? While we revel in this cool feature, let's take some time to understand its workings to be able to efficiently utilise it in our cars. Here’s a complete guide to how a car’s air conditioner works:

Simply put, a car air conditioner removes heat and humidity from the car’s cabin and replaces it with dry, cold air making the atmosphere pleasant to travel in. The basic principle of a car AC is similar to that of household AC’s i.e. heat exchange. In simple words, it means trading hot, humid air for dry, cold air. 

In technical terms, the car’s air conditioner manipulates a refrigerant into liquid and gaseous states. It uses this change of state to convert hot air into cold air by the principle of heat transfer. The AC changes refrigerant from liquid to gas by controlling its temperature and pressure. 

The Principle behind Air Conditioning: Heat Exchange

Why does ice melt? And when it does, why does the air around it get cooler? It is simply the universe’s way to balance itself. This is done by heat exchange. A simple example is if you put a hot object (heated coin) next to a cold one (cold coin), the hot coin will cool down while the cold coin will heat up until, eventually, both reach the same temperature. 

Here, the cold coin absorbs the heat radiated by the hot coin, which will increase its temperature. This cycle will continue until both variables reach the same temperature or room temperature. Now, the room is the third variable as it has a temperature different from the coins to begin with. Here, even the room played a part in heat exchange. 

Now think of the car’s air conditioner in terms of three temperature variables: Outside or ambient air temperature, AC temperature and cabin air temperature. In this case, by theory of heat exchange, the refrigerant uses the ambient air to cool down and then the cabin air uses the refrigerant to cool down. This cycle repeats until the cabin is significantly cooler than outside air. 

The Car AC Cycle: A Master Class in Refrigeration

The car’s air conditioner works in cycles, constantly expelling hot air from the cabin and replacing it with cold air. It works on five major cycles: Compression, Condensation, Dehumidification, Expansion and Evaporation. Let's take a deeper look at these cycles:

Compression

The car’s air conditioning system stores the refrigerant in a low pressure state. Once you start the AC, this gas first passes through the compressor. The compressor compresses the refrigerant turning it from a low pressure gas to a high pressure gas. This gas travels to the condenser though high pressure gas lines. 

Condensation

If you compress air, its temperature rises. It's the same case with the high pressure refrigerant gas. Once the gas reaches the condenser, outside air passes over the condenser coils with the help of fans. This helps cool the high pressure refrigerant gas, turning it into a high pressure liquid. A car’s air conditioner refrigerant gas converts into liquid between 32 to 49 degree celsius. 

Dehumidification

The refrigerant liquid then passes through a Receiver/Dryer. The dryer contains a desiccation or drying agent that removes moisture from the refrigerant. This prevents corrosion and freezing within the car’s air conditioning system. The dryer also filters out impurities in the refrigerant.

Some systems have an accumulator which is similar to a dryer but which is placed before the condenser. This accumulator removes liquid refrigerant droplets from the refrigerant gas before it enters the compressor. This is done to prevent clogging of the condenser system. 

Expansion

The cold, high pressure liquid refrigerant now passes through an expansion valve or a metered orifice tube. This tube lowers the refrigerant pressure which in turn significantly lowers its temperature. The refrigerant turns from a high pressure liquid state to a low pressure diphasic state or in simple terms, it turns into mist. 

Evaporation

The refrigerant mist now enters an evaporator which is a type of heat exchanger. A blower forces the cabin air over the evaporator. Heat exchange takes place between the cabin’s hot air and the cold refrigerant mist. This heat transfer cools the cabin air rapidly and this cold air is circulated through the cabin. The heat transfer also converts the refrigerant mist into a gaseous state. This gas is again sent back to the compressor and the cycle repeats itself.

Components Used in a Car AC

Now that we have gone through the process of a car air conditioner, let's take a look at the components used in it:

Compressor

The compressor sucks in a specific amount of refrigerant and compresses it. The compression mechanism could be a reciprocating, rotary or scroll type. The compressor is mechanically powered via a pulley and belt mechanism. It has an electromagnetic clutch that can engage or disengage the compressor when the car’s air conditioner is switched on or off. 

Condenser

A condenser is basically a heat exchanger. It is a radiator with fins and a network of coils running through it. Ambient air is passed through it via a fan. The ambient air sucks heat from the refrigerant running through the coils cooling it down. 

Receiver/Drier or Accumulator

A receiver/drier or accumulator is a dehumidifier that removes moisture from the refrigerant liquid. The receiver contains a desiccant to remove moisture as the moisture can convert into hydrochloric or hydrofluoric acid which are highly corrosive and can destroy system internals. The receiver also contains a filter which further removes any contaminants from the refrigerant. 

Expansion Valve

An expansion valve or orifice tube is also known as a metering device. It has two functions. One is to regulate the refrigerant flow into the evaporator and other is to reduce the refrigerant pressure which turns it into mist and further cools it down. The orifice tube’s conical shape helps reduce refrigerant pressure. It has a temperature sensor which, based on the temperature of the refrigerant, releases a metered quantity of refrigerant for it to turn into mist. 

Evaporator

The evaporator coil, like the condenser, is a heat exchanger as well. It is a small radiator through which cabin air is passed. The refrigerant absorbs heat from the cabin air and turns from mist into gas. The heat transfer cools down the cabin air. 

Blower motor

This motor blows the cooled air into the cabin through a series of vents. It sits behind the dashboard and can be speed controlled through switches or knobs on the centre console. 

Filters

A car’s air conditioner system has two types of filters. An AC inline filter sits between the AC receiver/drier and expansion valve and filters the refrigerant from debris. The other type is the cabin air filter which sits behind the dashboard. It is a paper type filter designed to prevent pollutants and debris from entering the cabin. 

Refrigerant

Refrigerants are chemical gasses that are preferred in car air conditioning systems for their fantastic heat transfer properties. There are three types of refrigerant gases. R-12 or Freon was initially used but replaced by R-134a as it was less polluting. Newer vehicles are using the R-1234yf refrigerant as it is more environment friendly. 

Common problems associated with car ACs

A car AC system has a mix of mechanical and electric parts and as such, is susceptible to issues. Here are some of the common problems associated with car AC systems:

Poor AC Cooling

Refrigerant leakage can cause poor AC cooling. The refrigerant in a car’s air conditioner system is pressurised and any leak will lead to loss of pressure and poor AC cooling. Common spots for leaks are hoses and joints in the system. To solve this issue, you need to identify the leak, replace the defective part and evacuate the refrigerant before replacing it with a fresh one. 

Another reason for poor AC cooling could be a blocked condenser where dirt lodged between the fins restrict airflow and heat dissipation. A pressurised wash to remove debris can solve this problem. 

Reduced Cabin Air Speed

This could be due to a clogged cabin air filter which blocks airflow and reduces blower speed. Replacing the cabin air filter should solve this problem. Generally, you should periodically replace the cabin air filter as per manufacturer’s recommendation to help AC efficiency. 

Zero Cabin Air Speed

A common cause is due to blown fuses or faulty wiring. Replacing them should mostly solve the problem. This could also be due to the blower not working. Common causes are blown fuses and faulty relays and in extreme cases, worn out motor brushes. 

AC Compressor Failure

There are many reasons for a car’s air conditioner compressor failure. It could be due to electrical issues, lack of lubrication or even refrigerant leaks which can cause the compressor to overheat and fail. Compressor issues can be a costly fix with the solution often being to replace the compressor unit entirely. 

Tips to Improve AC Efficiency

Running your car’s air conditioner efficiently will not only help your AC run optimally, it will also reduce your fuel costs and hefty AC repair bills in the future. Here are some tips to run your AC efficiently:

• Park your car in a cool spot or under shade. Direct sunlight heats up the cabin which means the car’s air conditioner has to work more to cool down the cabin. 
• Vent the cabin before starting the AC. Opening all windows for a few minutes will expel hot air and allow entry of fresh cooler air into the cabin. 
• Run the AC in recirculation mode. This helps put less strain on the AC as it will cool the already cooled air and recirculate it in the cabin. 
• Replace the cabin air filter periodically. This will not just help keep airflow optimum but will also help filter out pollutants and allergens more effectively. 
• Keep the car’s air conditioner condenser clean. A clean condenser will allow more airflow helping cool the cabin rapidly. 
• Service the AC periodically. Check for leaks, do pressure tests and top up refrigerant when required. This will ensure AC reliability.
• Ensure you run the car’s air conditioner for a while even when it is cold outside. Regularly running the AC will keep the AC system lubricated and prevent leaks as well. 

Conclusion

A car’s air conditioner has five cycles. These are Compression, Condensation, Dehumidification, Expansion and Evaporation. AC refrigerant is run through these cycles and changes from gas to liquid to mist in one cycle. This change in state is used for heat transfer and cool cabin air. 

The major components of a car’s air conditioning system are Compressor, Condenser, Receiver/Drier, Expansion Valve, Evaporator, Blower Motor and Refrigerant. A few common problems associated with AC systems include refrigerant leaks, blocked condenser, clogged cabin air filter, faulty compressor, faulty fuses and relays and faulty blowers.

It is essential to run the AC regularly to ensure longevity of the system. Cleaning the condenser and replacing cabin air filters periodically will ensure optimum AC performance. Parking your car in a shaded area, venting your cabin and running the AC in recirculation mode should ensure that your AC cooling is optimal.