Understanding Your Car’s AC System

Air conditioning (AC) systems are pretty simple. You turn them on, cold air blows out and soon the inside of your hot car is nice and comfortable. It’s almost magic but ever wonder how this all works? Exactly how do you make “cold air” anyway?

You’ve got to remove the heat

The AC system in your car doesn’t “make cold air,” what it does is “removes the heat” from the air. It then takes that heat and releases it outside into the atmosphere. Another way to think about this is to consider that cold air is just air without heat. Basically, if you can remove the heat from air, it becomes cold, which is exactly what air conditioners do.

How they work

Air conditioning systems aren’t terribly complex. We will illustrate the process with a diagram. The compressor (Item 4 on the diagram) compresses a gaseous refrigerant to a pressure of about 200 to 250PSI. The condenser (Item 1 on the diagram) is connected to the compressor by the “High Pressure Line” and acts like a radiator by giving off heat. When this occurs the refrigerant gas turns into a liquid.


From the other side of the condenser, the “Liquid Line” connects to the Orifice Tube (Item 2 on the diagram). At the orifice tube, the liquid refrigerant squeezes through a narrow passage yielding a temperature and pressure drop. After the liquid passes through the orifice, cold refrigerant flows to the evaporator (Item 3 on the diagram).

The evaporator is also set up like a small radiator with the refrigerant circulating thru it. A fan pushes air across the cold evaporator which removes the heat from the air. The cold surface of the evaporator also condenses humidity out of the air. At the outlet of the evaporator an accumulator (not shown) is used to store liquid refrigerant that may have made it thru the evaporator. The Suction Line at the bottom of the diagram connects the outlet of the accumulator and to the inlet of the compressor. This restarts the loop at the compressor.

About Refrigerants

The fluid inside the tubing of your car’s air conditioner is called the refrigerant. The technicians at Bob Fisher Chevrolet of Reading, a local Chevrolet dealer in Reading, PA, schooled us on the finer points of automotive refrigerants.

Two types of refrigerant have been used in cars and trucks: R-12 Freon and R-134a Freon. R-12 was the original refrigerant used since the 1940s. However, R-12 is chlorofluorocarbon (CFC) has been shown to attack the earth’s ozone layer which contributes to global warming. In 1990, The Clean Air Act agreed to cease production of R-12 refrigerant. The result is that vehicles from 1994 and on came with R-134a refrigerant. R-134a is tetrafluorothane which is a hydrofluorocarbon (HFC). HFC refrigerants are much less harmful to the ozone layer.

Refrigerant Oils

It is important to know that oils circulate along with the refrigerants. R-12 and R-134a each use a different type of refrigerant oil. Mineral Oil is used with R-12 refrigerant only. PAG or synthetic polyalkyline glycol is used with R-134a refrigerant systems. Your mechanic will know this.

What you can do to keep your AC system healthy

There are a couple of things that you can do to keep your car’s AC up to snuff. First, run the system a few minutes at least once a week no matter how hot the temperature outside might be. This ensures that the many hoses, valves and pumps throughout the system are kept well lubricated by the refrigerant oil.

Second thing to do is whenever you have the car serviced, make sure you ask the mechanic to check the refrigerant level and top off the system if necessary. The air conditioner is not always included in a standard log book service, so it can be easily neglected during servicing.

One last thing, it is good practice to get a full air conditioning service at least once a year. Try and do this before the hotter months start, for obvious reasons!


Related Posts

  • No Related Post

Leave a Reply

XHTML: You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>