Air Conditioning


  • Air Conditioning

    Air Conditioning

    Pushing the "A/C" button brings cool air into the cabin. All of the components that make it happen are hidden from view. | March 18, 2010

2 Photos

Just about every modern car, truck or SUV sold these days can be had with air conditioning. It's so common that most people take it for granted. You press the button for air conditioning in your car and — presto! — cold air starts to flow out of the car's vents. It's easy, it's simple, and it's a major convenience. Could you imagine driving to a job interview in Phoenix, Ariz., if your car didn't have air conditioning? By the time you got to your interview, you'd be a sweaty, stinky mess.

Have you ever wondered how the air conditioning in your vehicle works? If you're like most people, you probably haven't. But we're here to educate you painlessly. Air conditioning is the process by which air is cooled and dehumidified. The air conditioning in your car, your home and your office all work the same way. Even your refrigerator is, in effect, an air conditioner. While there are many physical principles that relate to air conditioning, this article sticks to the basics. It explains the general concepts of automotive air conditioning, the components used and what you need to know to keep your car's A/C system working properly.

Did you know that when you turn on the A/C in your car, you are burning extra gasoline to make yourself feel cooler? It's weird to think that by burning something you become cooler, but it's true.

Do you remember anything from your high school physics class? Don't worry; very few of the writers here at Edmunds.com do, either. Basically, air conditioning systems operate on the principles of evaporation and condensation.

Here's a simple example of evaporation. Imagine that you're swimming around in your neighbor's backyard pool on a summer day. As soon as you get out, you start to feel cooler. Why? The water on your body starts to evaporate and turns into water vapor. And as it evaporates, it draws heat away from your body, and you get goose bumps. Brrr! Now let's say your neighbor hands you a big glass of ice-cold lemonade. You take a sip and set it down on a table. After a minute or two, you notice that water has collected on the outside of the glass. This is condensation. The air surrounding the glass becomes cooler when it encounters the cold glass, and the water vapor the air is carrying condenses into water.

Both of these examples occur at normal atmospheric pressure. But higher pressures can also change a vapor (or a gas) into a liquid. For example, if you look at a typical butane cigarette lighter, you can see liquid inside it. But as soon as you push down on the button, butane gas comes out. Why? The butane is under high pressure inside the cigarette lighter. This high pressure causes the butane to take liquid form. As soon as the butane is released and it encounters normal atmospheric pressure, it turns back into a gas.

OK, those are the basic ideas. But how do they apply to making your car's vents blow cool air? The principles of evaporation and condensation are utilized in your car's A/C system by a series of components that are connected by tubing and hoses. There are six basic components: the compressor, condenser, receiver-drier, thermostatic expansion valve, the evaporator and the life-blood of the A/C system, the refrigerant.

Refrigerant is a liquid capable of vaporizing at a low temperature. In the past, R-12 refrigerant was used in cars. But this chlorofluorocarbon (CFC) is harmful to the earth's ozone layer. Consequently, all vehicles built after 1996 use R-134A, a more environmentally friendly refrigerant.

Here's how an air conditioning system and its components work.

Step One: The compressor is the power unit of the A/C system. It is powered by a drive belt connected to the engine's crankshaft. When the A/C system is turned on, the compressor pumps out refrigerant vapor under high pressure and high heat to the condenser.

Step Two: The condenser is a device used to change the high-pressure refrigerant vapor to a liquid. It is mounted ahead of the engine's radiator, and it looks very similar to a radiator with its parallel tubing and tiny cooling fins. If you look through the grille of a car and see what you think is a radiator, it is most likely the condenser. As the car moves, air flowing through the condenser removes heat from the refrigerant, changing it to a liquid state.

Step Three: Refrigerant moves to the receiver-drier. This is the storage tank for the liquid refrigerant. It also removes moisture from the refrigerant. Moisture in the system can freeze and then act similarly to cholesterol in the human blood stream, causing blockage.

Step Four: As the compressor continues to pressurize the system, liquid refrigerant under high pressure is circulated from the receiver-drier to the thermostatic expansion valve. The valve removes pressure from the liquid refrigerant so that it can expand and become refrigerant vapor in the evaporator.

Step Five: The evaporator is very similar to the condenser. It consists of tubes and fins and is usually mounted inside the passenger compartment. As the cold low-pressure refrigerant is released into the evaporator, it vaporizes and absorbs heat from the air in the passenger compartment. As the heat is absorbed, cool air will be available for the occupants of the vehicle. A blower fan inside the passenger compartment helps to distribute the cooler air.

Step Six: The heat-laden, low-pressure refrigerant vapor is then drawn into the compressor to start another refrigeration cycle.

As you can see, the process is pretty simple. Just about every vehicle's A/C system works this way, though certain vehicles might vary by the exact type of components they have.

The best thing about air conditioning is that all you have to do is press a button to make it work. Air conditioning systems are pretty reliable. On a modern and relatively new vehicle, it is rare to have problems. And if there are problems, they are pretty much one of two things: No cool air or insufficient cool air. If you own an older car and its A/C system doesn't seem to be working properly, here are some general troubleshooting tips:

No Cool Air

  • Loose or broken drive belt
  • Inoperative compressor or slipping compressor clutch
  • Defective expansion valve
  • Clogged expansion valve, receiver-drier or liquid refrigerant line
  • Blown fuse
  • Leaking component: any of the parts listed above or one of the A/C lines, hoses or seals

Insufficient Cool Air

  • Low refrigerant charge
  • Loose drive belt
  • Slipping compressor clutch
  • Clogged condenser
  • Clogged evaporator
  • Slow leak in system
  • Partially clogged filter or expansion valve

Most A/C repairs are best left to a repair shop. Recharging the refrigerant, in particular, requires special equipment that most people don't own. There are a couple things you can do, however. First, make sure to have the system checked regularly according to your vehicle's owner's manual. Second, if you live in a place with a cold climate, it might not make much sense to run the A/C during the winter months, but many shop technicians recommend running your A/C system regularly, because it contains a light mineral oil in the refrigerant to keep the compressor properly lubricated. The general rule of thumb is 10 minutes per month. Some heating, ventilation and air conditioning systems also engage the A/C compressor for defrost mode (for example, most GM vehicles).

So those are the basics behind air conditioning. The next time you're riding along in a car and the driver hits the A/C button, you can say, Boy, those evaporator tubes sure are cold. It's all thanks to R-134A!

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