Just wondering how the AC in Vehicles work.
When you set the Air conditioning to less the max, does The AC still
at maximum capacity while mixing the really cold air with some hot air?
so, I would say that there is cold air is being wasted. It would
more sense to me if AC would turn on and off every few minutes, when
have it set to medium cold.
As far as I know the AC is either on or off. There is no such
as partially on. In my house the AC is on max and when the desired
temperature is reached, the AC turns off. Can someone clarify
Actually you are closer to the truth than you may know. The AC in your car
doesn't work full time, only the fan does. The AC compressor under the hood
has a clutch mechanism that engages and disengages the compressor whenever
it's needed or not. The amount of power the fan uses by having it run
constantly is negligable on the low and moderate speeds so it doesn't make
economic sense to add in a switch to turn off the fan while the AC
compressor is disengaged. In fact, I think the interior of your vehicle
would heat up faster if the fan wasn't running anyway, so you might as well
keep air moving over the coils which are still cool after the compressor
shuts off. Putting the controls on Max or Hi doesn't effect the compressor,
just the fan speed and recirculation vent. It's actually more efficient to
run your AC on "Recirc" where it draws in the already cool air from inside
the passenger cabin, and many vehicles these days have a dedicated recirc
button that can be activated regardless of fan speed.
Hope this helps.
Cheers - Jonathan
But don't run Recirc when temp outside is below freezing. What
happens then is the moisture from your breath accumulates in the air
and, when you turn off the motor, becomes ice on the interior of the
Then the next time you run your vehicle the ice from the windows gets
into the air and it's a vicuous cycle. F
I've seen any number of vehicles over the years where the windows are
frosted up all the way around due to this problem.
Many home AC system have a feature that shuts down the blower, as
well as the compressor, when the desired temperature is reached.
The HVAC systems in automobiles do not, generally, have that
feature. That does not mean they do not shut down however. The
blower in AUTOMATIC climate control system will cut back but,
generally, does not shut down. The compressor on both manually
and automatic operated systems in cars does shut down when the
desired temperature is reached, then restarts as needed.
Generally, in cars the MAX setting is NOT the temperature
setting, it is simply used to operate the AC in the most
efficient mode by re-cooling the dehumidified inside air, much as
it does in a home AC system. If not set to the MAX setting the
system, generally, is taking in outside air and cooling that air.
When using outside air the compressor operate more frequently to
maintain the same temperature setting because it must cool the
warmer more humid outside air than if inside air is the source of
the air being cooled.
Every car I've ever seen does not cycle the Compressor on and off to control
temperature, it does how ever throttle down the blower to lower the amount
of air passing over the cooling side of the coils.
try www.howstuffworks.com for a breakdown on how A/C works in general.
Then you're seeing old GM cars with a suction throttling valve. Some cars
also had thermostatically controlled expansion valves. The newer cars cycle
the compressor to hold the evaporator temperature just above freezing.
On 15 Jun 2005 11:36:32 -0700, email@example.com wrote:
You are correct, the way it's done now is somewhat inefficient and is
different then how it's done in your house. In the "old" days the AC
in cars did have a temperature control similar to your house that ran
the compressor all the time until the temp was lowered to your desired
temp and then it shut it off till the temp went back up. But because
your car has so much more window and heat load then your house, it was
very hard to maintain the temp within any reasonable degree of
fluctuation, you were always fiddling with the temp knob. A typical
car AC has the same cooling capacity as the home AC for a 1600 sf
house. To better minimize temperature fluctuations in the car, modern
AC basically runs at max compressor output all the time and simply
cycles the compressor off whenever the evaporator coil temp gets low
enough that there is a possibility of freezing of the condensed water.
(In the old days they would literally blow ice at you if you set the
temp control all the way down). The current way to control temp is to
change fan speed and/or move the temp lever so some of the cold air
gets reheated by running it thru the heater core. Of course there are
tons of variations on these themes with all sorts of combinations
having been tried over the years by cycling compressors by temp, by
pressure, and running them constantly but changing how the freon flow
was metered, how the heater core was operated (water valve vs diverter
door), size of components, temperature of components, etc.
Presumably, what we currently have is what evolved as "best" since it
seems all the makers have pretty much adopted the same basic system
operation now. One major benefit of the current, slightly less
efficient operation, is that it maximizes the amount of humidity that
gets removed from the air since the ac is always "on" in the sense
that the evap coil is run as close to freezing as possible without
giving you a headache or actually freezing up. That's another change
from the old days, they used to run the coil so cold the air blowing
on you was like a walk in freezer, now they seem to keep it a little
higher so it's not giving you an ice cream headache. There are lots
of variables they can play with....
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