Car Air Conditioning

I know someone who, on very hot (25+ C) days, runs her car air conditioner until the car becomes comfortably cool, then turns off the AC without turning off the fan or closing the vent. I think this is rediculous. I got to wondering, however, whether it would take more, less, or the same amount of energy just to run the air conditioner continuously than to turn it on and off while keeping the vents and fan going.

I know that it is difficult to judge without any specific numbers, but if anyone could shed any light on the matter it would be great.
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 Recognitions: Gold Member It definitely takes more energy (quite a bit, in fact) to run the AC than just the fan. The compressor is belt-driven by the engine, and costs at least a couple of miles per gallon when running. Turning it off disengages the electromagnetic clutch that couples it to the pulley, so that drag is eliminated.
 Mentor Her approach actually isn't bad - car air conditioners are not capable of varying their capacity without losing their efficiency, so running at full speed intermittenly is probably more effecient in the long run.

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Car Air Conditioning

It would depend on how your friend picks her set points compared to the car's AC built in set points.
This means when to turn the AC on and when to turn the AC off.
The car measures the air temperature, indirectly for some cars, or directly (in degrees) for other cars.
The big difference between indirect or direct is that the later shows an actual temperature number.

The car actually does exactly what your friend does.
It turns the AC off (including the clutch to reduce drag) when it is cold enough inside and turns the AC back on when it starts to get too warm. While the AC unit is off the car leaves the vent open and the fan running.
This is because "cold" gets stored in what is called the evaporator and it takes some time to transfer this to the air.

Now the set points selected by the cars manufacture tend to be chosen for best comfort rather than best efficiency, but perhaps some manufactures do pick for best efficiency now.

She could be lucky and picking more efficient set points or actually making it less efficient.
 Thank you all for your answers. The problem is that by keeping the fan on and the vent open, warm air from outside gets blown into the car. I was wondering if the AC shuts off at a certain temperature to keep the car at the same temperature, and, if so, would it be easier to run it constantly than to run it for a short time, blow warm air into the car, heating it, then cool it back down to that temperature, and heat it up again. Sorry for not being clearer in the original post.
 Recognitions: Gold Member Are you sure that it's an external vent that she leaves open? That would be counterproductive, as you pointed out. At lot, if not all, air-conditioned vehicles have a 'recirculation' vent setting. That takes the air that's already inside the vehicle and blows it around to give a cooling breeze effect, and so is more effective than turning the fan off. The symbol on the vent switch shows an arrow going into one vent and out another.
 Recognitions: Gold Member Science Advisor Staff Emeritus Actually, most cars do not have thermostats inside the cabin to turn off the AC compressor. Newer luxury cars have thermostats -- you select a temperature, and the control system does the rest -- but most cars simply run the AC compressor the entire time the AC button is activated. This is why, as the OP mentions, the cabin can become uncomfortably cold if the AC is left on. If the car has a thermostat, you should probably just let it do its job. If your car does not have a thermostat, then turning the AC on and off periodically -- varying the duty cycle of operation -- is definitely more efficient than leaving the compressor running all the time, and adjusting the temperature (reheat) control. - Warren
 Mentor I was just speculating, though, that car thermostats do not work by cycling the compressor. I didn't think of it before, but they probably work by reheating the air using engine coolant (the same way they produce heat in heating mode). The thermostat in manual temperature control cars doesn't control space temperature, it controls discharge air temperature. And varying fan speed (also manual) just dumps capacity from the a/c unit, losing efficiency. Leaving the vent open, however, would be a mistake if you are going to manually cycle the a/c. And in the summer, as Danger said, it should be closed anyway.

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 Quote by russ_watters I was just speculating, though, that car thermostats do not work by cycling the compressor. I didn't think of it before, but they probably work by reheating the air using engine coolant (the same way they produce heat in heating mode). The thermostat in manual temperature control cars doesn't control space temperature, it controls discharge air temperature.
I don't have a lot of experience with luxury cars, but my mother's car certainly does cycle the AC compressor. You can feel it engaging and disengaging.

- Warren

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 Quote by chroot most cars simply run the AC compressor the entire time the AC button is activated
Not around here, they don't.
The store parking lots get quite irritating with all of those compressers cutting in and out while the owners are shopping. You can always tell when one's starting to spool up, because the idle drops by 200 or 300 rpm. It'll run for a few minutes, then drop out again.

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 Quote by chroot I don't have a lot of experience with luxury cars, but my mother's car certainly does cycle the AC compressor. You can feel it engaging and disengaging. - Warren
Yeah --- far's I know, all, or nearly all do --- it's triggered by system pressure in the refrigerant loop --- saves bursting the line between the compressor and expansion valve on cool days. "Cabin" temperature is controlled by bucking cold air through the heater core (never off) with dampers in the underdash ductwork.
 Recognitions: Gold Member Science Advisor Staff Emeritus Okay, I guess I just haven't paid much attention to the cycling of my own AC compressor, which does not have a cabin thermostat. Sorry for the confusion! - Warren
 Recognitions: Gold Member Thanks for clearing that up, Bystander. I also thought that it was a thermostat system, but never looked into it.

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 Quote by Bystander Yeah --- far's I know, all, or nearly all do --- it's triggered by system pressure in the refrigerant loop --- saves bursting the line between the compressor and expansion valve on cool days. "Cabin" temperature is controlled by bucking cold air through the heater core (never off) with dampers in the underdash ductwork.
The heater core is turned off when the selected mode is AC.
There are valves and switches just for this.
If one of these fail and leaves the heater core on then your AC will either appear not to work or work very poorly.

While there are pressure sensing switches (high and low incase of leaks) to prevent damage there is also an actual thermometer that measures the evaporator temp (on all cars) and maintains it at a constant(see hysterisis) temp. I have replaced a few of these as the switches tend to fail from the constant cycling.

The air duct control doors move to flow more or less air over the evaporator as you adjust the temperature.

With less air flowing over the evaporator it takes more time for it to warm back up to the compressor on temp.

For more detailed information, the air conditioning maintenace and repair section of any car shop manual
is fairly comprehensive and with minor variations much the same.
 Thank you all for your responses.
 Recognitions: Gold Member Homework Help Science Advisor It occurs to me that the consol switch is not likely to be rated for constant cycling. There is a good chance that it will eventually break from this kind of usage.

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