Question:
My friend 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:
Most (not quite all...) car air conditioners are "dumber" than
the ones in a house window.
In your home, there's an actual thermostat control, so if
you set it to, say, 80 degrees, the compressor may kick
in for 1 minute out of three; if you lower it to 65 degrees,
perhaps 2.5 minutes out of three.
(Window units are an "all or nothing" for the compressor.
When you get into larger home units you'll start finding
ones with variable compressor outputs - but you'll pay
a lot more for them).
Car air condtioners, for the most part, don't have a
thermostat toggling the compressor on and off. Instead,
once you turn them on, the compressor stays "on" until
various pressure sensors and oter controls "tell it"
that full pressure has been reached and it shuts down
for a bit, then comes back on.
The "cooler/warmer" setting simply adds heat (from
your engine) into the cold air stream.
So once you've pressed that "a/c" button, you've
pretty much committed yourself to the full power drain
tthat the compressor will cause.
Note that's the case with most, not all, cars. Some
do, in fact, have thermostats in teh cabin.
Also... the calculations aren't quite as smple
as just "on/off". The various operating pressures
change as the temperatures shift around, so the
workload on the compressor does as well. And
tere are all sorts of other very hairy secondary
issues as well. But they're pretty much minor
stuff compared to the main ones.
