Question:
My Dodge has an air conditioner button that turns on and off the air
conditioner. (A light in the button indicates whether it's on or not.)
This is independent of the fan-speed and temperature knobs. I know that air
conditioners reduce engine efficiency. What I'm wondering is if they do it
all-or-nothing, or in gradients. In other words, with the a.c. button on
and the light lit, and with the temperature setting at coldest, clearly this
is the maximum drain on the engine. However, I can adjust the temperature
knob higher and higher until what emerges from the vents is no longer cold
air, or even cool, if I turn it high enough. I can turn the heat on by
rotating the knob to a high enough temperature. Does the temperature matter
in terms of the drain on the engine? Or would the only thing that matters
be whether a.c. button is clicked on or off?
Answer:
I am an EPA 609 certified tech for mobile HVAC so I'll see if I can straighten
this out a bit. I don't do it for a living, I just studied and got the 609 so I
can buy R12 Freon and service my own older cars. These are very broad
explanations just to answer the original poster, not intended to be overly
precise. If you want to skip the boring tech information and get to the bottom
line, go to the last paragraph :-)
There are two main types of auto a/c: expansion valve and fixed orifice. There
are two classes of piston compressors, fixed and variable displacement. (There
are other kinds of compressors as well). An a/c system has a low pressure and a
high pressure side, divided by an orifice.
The a/c system is controlled not by the high pressure as someone posted, but
by the low pressure. Since the system has both liquid and gaseous refrigerant,
it is in equilibrium and if we know the pressure at one point, we know the
temperature, and vice versa. Some systems measure temperature, some pressure.
There is a high pressure compressor shut off, but it is not intended to be a
control, just a safety shutoff.
