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Old 07-16-2011, 07:53 PM   #1
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Turn Up the A/C Dial?

No this is not a revised song by Autograph.

So here's my question. My A/C dial has four speeds (low, med low, med. high and high). Obviously, when you turn the A/C on, you lose gas and it hurts your fuel efficiency. My question is this, though, the higher your turn your A/C dial up, does that hurt your gas mileage more. For example, if I dove 100 miles with the A/C on the low setting vs. driving 100 miles with the A/C on the medium high setting or high setting, would it make a difference? I figure it probably doesn't, but this has always puzzled me and I'm not mechanically inclined so I honestly don't know for sure.
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Old 07-16-2011, 08:33 PM   #2
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Re: Turn Up the A/C Dial?

I think once the compressor is on, its on and is a drag on mpg. The blower works on electricity so it is only a slight drag. On hot days this is what i do. I only use the ac once on the freeway and get the cabin cool. Once i do this I keep it on med low but turn off the compressor for awhile with the fan on. It will keep blowing the cool air in the system until it starts to blow warm, then i turn the compressor back on and repeat. It works really well when drafting.
I do this and still avg over 50 mpg. Hope this helps.
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Old 07-16-2011, 09:11 PM   #3
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Re: Turn Up the A/C Dial?

If the compressor is on, the compressor is on. Different fan speeds will have negligible results on economy, as large resistors are used to slow the motor, so energy not used by the motor in slower fan speeds is wasted as heat. The load on the engine is essentially the same.
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Old 07-17-2011, 01:15 AM   #4
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Re: Turn Up the A/C Dial?

I think you're all wrong. Arky is wrong in talking about the AC having four speeds. The fan has four speeds, which it uses for AC, heat, and non-AC and non-heat ventilation. Most modern fans draw nearly the same power at low speed as they do at high speed, since they simply add resistance to slow the fan speed at its lower settings. So there's almost no benefit to slowing the fan down. When my fan is on, it's on full speed.

The actual AC compressor operation in basic systems is on when the heater/AC slider or dial is in the AC position and the AC button is pushed, but the duty cycle of the compressor and the temperature setting on the temperature dial determine how often the AC compressor is working. It may run 80% of the time in the max AC position with the temperature dial set to its coldest position. It may run 10% of the time in the normal AC position with the temperature dial barely in the cold zone.

The way I drive my basic AC system is fan on full, temperature cold, vents closed (air recirculation on), normal AC selected, and I toggle the system on with the button. I try to turn it on under light load: when gliding, coasting down hills, etc. I try not to turn AC on when pulsing or climbing hills.

Cars with automatic climate control are a different and very inefficient animal. They run the AC a lot more than basic systems, mixing the cold AC air with air from the heater to maintain cabin temperature. They even run AC in the Winter to dehumidify the heater and defroster hot air.
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Old 07-17-2011, 05:12 AM   #5
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Re: Turn Up the A/C Dial?

I always got the impression that adjusting the temperature just mixes warm and cool air, but does not change the compressor's duty cycle. It seems that way to me, with its immediate response and such.

When I'm hot enough to use A/C I crank it max speed max cold recirc until I'm cold, then I turn it off until I'm hot.
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Old 07-17-2011, 06:20 AM   #6
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Re: Turn Up the A/C Dial?

I don't know about foreign cars, but my experience with American vehicles is that sliding the temperature control over just adds warm air to the air conditioned air. The purpose of this was to help defog the windows. Heating dehumidified air makes quick work of dissipating any fog, and helps maintain comfort by not making it too cool in the vehicle.
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Old 07-17-2011, 10:01 AM   #7
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Re: Turn Up the A/C Dial?

You guys are seriously spoiled. I've never owned a car with automatic climate control. Basic AC, which is all that any of my cars have had, works as I described, I think. I need to check THC's statement. He may be right about temperature control manually mixing heat with AC.
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Old 07-17-2011, 10:32 AM   #8
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Re: Turn Up the A/C Dial?

THC's more correct than I.

Quote:
Although there are many different types of A/C systems, each falls into one of two categories, depending on how the system is controlled: manual and automatic. Manual air conditioning systems have an on-off switch, a temperature control knob or slide switch and a knob or switch for adjusting fan speed. The driver turns on the A/C when he or she wants cool air and selects a temperature setting and blower speed. If the air gets too cold, the driver can turn down the blower speed or change the position of the temperature setting. If the air isn't cold enough, the driver can crank it all the way up.



The temperature slide switch is connected with cables or vacuum hoses to airflow control doors inside the HVAC unit under the instrument panel. Changing the temperature setting opens or closes the doors either to increase or decrease airflow through the A/C evaporator. It's a relatively simple control system that doesn't require a lot of complicated electronics.
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