Stupid A/C question... (any MPG effect of max. vs low setting?)

[JohnS]

Hey guys,

[Keep in mind that I'm pretty ignorant when it comes to engines, accessories and how they all work together once the key is turned. So if you can type an answer to this dummy's question between falling off your chair and laughing hysterically, I'd appreciate it.]

I've been running without my A/C turned on since learning more about hypermiling. BUT, last week we had our first 95* days up here in NJ. Stuck in traffic heading over the George Washington Bridge into Manhattan, that A/C had to be turned on. So here's my question...

Once you turn the A/C on, does it matter whether you run it at a low level as opposed to MAX? In other words: As far as gas consumption goes, does setting the controls to a higher temperature and lower fan speed use less gas than full-on cold and blast the fan? Or is it that once you turn the A/C on, it uses the same amount of fuel no matter what the setting?

Thanks in advance,
John

[user removed]

Use recirculate and max cool until you get it comfortable, then turn the fan speed down, but keep it on recirculate and max cool. If you can after you get it cool enough inside then you can cycle the AC on and off but keep the fan blowing until the evaporator looses all of it's "coolness" then cycle the AC back on to get the evaporator cold again.

You can also turn the AC on when you are forced to slow down, and downshift to keep your RPM up and possibly get free AC with DFCO when decelerating.

Got stuck on that same bridge picking up an Eldorado for my 83 year old mother. God what a mess.

regards
Mech

[JRMichler]

On my truck, like most vehicles, the AC compressor is wired to a pressure switch in the Freon high pressure side. The faster you run the fan, and the hotter or more humid the air, the more the compressor runs.

I run the AC fan on low with the dash vents blowing cold air straight at me. I'll cycle the either the AC or the fan switch off if it gets cool enough. The compressor only runs when the fan is on and the Freon pressure switch is closed. The compressor is off much more than it is on when the fan is on low.

When I run it that way, the gas mileage hit is too small to measure.

[mcrews]

First rule of thumb (imho) NEVER SUFFER!!!!!!!!!
as a general rule, when its's on low or high, it's on. I let mine get my car as cold as fast as it can. the I adjust the therosate (I have a digital screen) to 74 or what ever and forget it.

I have run tests on the highway at 60 mph. A/c on window up. then a/c off windows down. in a 99 vaden plaus jag and an 2002 infiniti Q45. both lost 3-4 mpgs with the windows down.

[Thymeclock]

Originally Posted by Old Mechanic

Use recirculate and max cool until you get it comfortable, then turn the fan speed down, but keep it on recirculate and max cool. If you can after you get it cool enough inside then you can cycle the AC on and off but keep the fan blowing until the evaporator looses all of it's "coolness" then cycle the AC back on to get the evaporator cold again.

You can also turn the AC on when you are forced to slow down, and downshift to keep your RPM up and possibly get free AC with DFCO when decelerating.

Got stuck on that same bridge picking up an Eldorado for my 83 year old mother. God what a mess.

regards
Mech

I'm no stranger to that sort of traffic jam and heat here in metro NYC. I usually drive a convertible in summer - but short of that, here's a similar strategy: Leave the fan running but switch the A/C off while accelerating or when it's adequately cool enough; turn it back on while decelerating, or at idle. An added benefit is that turning off the A/C while accelerating won't rob you of power when you need to move forward, to avoid having someone cut in front of you.

If I were to be snide I'd say that if you can afford the price of gas and the bridge tolls between NY and NJ, you can afford to leave the A/C on. (I'm half joking...)

[mort]

Originally Posted by JohnS

Hey guys,


Once you turn the A/C on, does it matter whether you run it at a low level as opposed to MAX? In other words: As far as gas consumption goes, does setting the controls to a higher temperature and lower fan speed use less gas than full-on cold and blast the fan? Or is it that once you turn the A/C on, it uses the same amount of fuel no matter what the setting?

Thanks in advance,
John

Hi John,
It depends somewhat on the car. There are 3 control systems used.
On smaller cars the A/C compressor cycles, only running when the cabin is warmer than the thermostat setting. Using the lowest fan and warmest setting that is comfortable saves fuel.
Some larger cars; when you call for A/C the compressor is engaged and cabin temperature is regulated by also running the heater.
On hybrids with high voltage systems the compressor is run by a variable speed electric motor. It runs just fast enough to give the desired cooling.
In all cases the A/C system uses less power and therefore less fuel if you stay on the edge of comfort. Running the A/C full blast uses more fuel.
An air condioner is a heat pump. It uses power, usually from the engine, to move heat out of the cabin air. The amount of power required from the engine is the amout of heat removed divided by the "coefficient of performance." The COP depends on system design and outside temperature.
Car A/Cs have COPs from 3 to 5 and usually about 50,000 BTUH capacity.
-mort

[bertb]

Not a stupid question at all. I was coming on this evening to ask it myself. I started getting into all this last fall, and this is my first tank that I've let myself use the AC. It's just gotten too hot here in OKC. Without the AC I'd walk into a client's office dripping sweat. Thanks for asking!

[SentraSE-R]

I recirculate the air as Old Mech suggests, but run the fan at full speed, cycling the AC compressor as Thymeclock suggests. Modern climate control fans draw nearly as much current running in the #1 position as they do running in the #4 position. They inefficiently wire in resistors to the #1, #2, and #3 fan switch positions to cut current to the fan, while drawing the same amount of power usage. Ergo, you're better off running the fan at full speed.

I switch the AC on when needed when gliding or holding speed on level ground, and turn it off as much as possible when pulsing or climbing hills. That way, I'm in control of when the AC compressor is working. The other recommendations let the car's computer control the AC compressor duty cycle.

But I have a dumb climate control system. If you have a luxury car, you're hosed, because it will run the AC all the time (within its duty cycle), mixing heat with the cold air in the Summer to regulate temperature, and running the AC to dehumidify the heater air during the Winter.

[California98Civic]

Originally Posted by mcrews

First rule of thumb (imho) NEVER SUFFER!!!!!!!!!
as a general rule, when its's on low or high, it's on. I let mine get my car as cold as fast as it can. the I adjust the therosate (I have a digital screen) to 74 or what ever and forget it.

I have run tests on the highway at 60 mph. A/c on window up. then a/c off windows down. in a 99 vaden plaus jag and an 2002 infiniti Q45. both lost 3-4 mpgs with the windows down.

But you still had the AC belt on when you were not using the unit. If you take the whole unit out, the situation changes, since the parasitic draw on the engine from the off AC is gone. For me, I installed dark, metalized, heat reflective tint. My windows are never more than cracked on the freeway. I run the fan full blast.

[PaleMelanesian]

Not all fan speed controls are inefficient like that. I've measured the voltage drop on mine. It draws proportionally more power with each fan speed setting.

Fan speed - volts
0 - 12.3
1 - 12.2
2 - 12.1
3 - 12.0
4 - 11.9