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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