Air Conditioning and Gas Mileage

[KevinKernaghan]

Recently my girlfriend and I were discussing tactics to improve gas mileage and the subject of air conditioning came up. We have differing opinions on whether or not the fan speed setting will affect the total engine energy loss caused by the air conditioner. My question is, if the fan is moving air through the condenser at high speed, will it cause the compressor to work any harder? Or, once the compressor is clicked "on," it is just "on" and your car will work at the same efficiency if the fan setting is at "1" or "4?" If the latter is correct, it means I've been sweltering in my girlfriend's car for 2 years now because she refuses to turn the fan speed up above about halfway. Thoughts?

[congochris]

Good thing she doesn't watch mythbusters.. you'd be sweating your balls off, since they showed that windows down is more fuel efficient than running the AC.

[mws]

Turning up the fan will increase the load on the engine for two reasons:

 

1) The alternator has to work incrementally harder to generate the electricity to run the fan faster (maybe .001 mpg impact)

 

2) The compressor actually varies its output based on how much work it is asked to do. The simpler compressors have a clutch that engages and disengages. Others are variable displacement.

Bottom line for either case: Running the fan faster to make the interior cooler WILL require the compressor to do more work, adding load to the engine.

 

So, unfortunately, she is right.

But, From sweltering to comfortable may be 1 or 2 mpg in city driving, and .2 to .5 mpg in highway driving, so it may not be as much fuel savings as hoped for.

[94extreme]

imho, the fan speed has nothing to do with how hard the AC works. i get the same milage with windows down as i do with AC on. I also noticed that my truck cools the best when the fan is on 2 when super hot. i have no idea why but maybe the condenser is unable to keep up with the airflow when it's sweltering.

[Slick]

if she refuses to turn her a/c up...... drive yer own rig. f all that noise. when it's hot, i'll lose 1mpg and be 30 degrees cooler tyvm.

[rmd0311]

if she refuses to turn her a/c up...... drive yer own rig. f all that noise. when it's hot, i'll lose 1mpg and be 30 degrees cooler tyvm.

+1

[88pathoffroad]

Every test or review I've seen says the same thing, just turn on the AC and drive. The difference is negligible.

[QuismO]

yea i don't see much of a difference in mileage. i actually tried it last fall driving on two different trips to sacramento. one trip w/AC, one w/o. still ended up with a little under 1/4 tank when i got home.

 

EDIT: I'm sure i did consume a little more gas on the trip with AC...i'm not going to argue with the laws of physics, but it was definitely negligable.

Edited September 7, 2006 by QuismO

[Viper929]

the fan has nothin to do with how much work your AC is doing.... it eather on or off (I am "608" and "609" certified, for all of you who don't know what that means, Its a certification by the EPA to work on AC units) and as far as gas mileage you get better MPG with the windows down if your under 53MPH and above 53MPH with AC, it all has to do with aerodynamics, witch plays the major roll in MPG above 53MPH under that it rolling resistance ie tires

Edited September 7, 2006 by Viper929

[mws]

I have to respectfully disagree. No automotive compressor should ever be operating at 100% output 100% of the time.

 

They either cycle on and off via a clutch (WD21's use a clutch), or are variable displacement (the rotaries in GM's are an example.)

 

If you watch the AC clutch on the WD21 at idle, you can see it engage and disengage, and hear the load it applies to the engine when it engages. The more heat being extracted (running the fan faster will extract more heat), the longer the duty cycle of engagement. While it is engaged, it requires power from engine. While disengaged, none.

 

And I agree, it is not much, but work is work and the laws of physics are quite difficult to break.

Edited September 7, 2006 by mws

[94extreme]

I have to respectfully disagree. No automotive compressor should ever be operating at 100% output 100% of the time.

 

They either cycle on and off via a clutch (WD21's use a clutch), or are variable displacement (the rotaries in GM's are an example.)

 

If you watch the AC clutch on the WD21 at idle, you can see it engage and disengage, and hear the load it applies to the engine when it engages. The more heat being extracted (running the fan faster will extract more heat), the longer the duty cycle of engagement. While it is engaged, it requires power from engine. While disengaged, none.

 

And I agree, it is not much, but work is work and the laws of physics are quite difficult to break.

you sure about this mws? mine is on or it's off.. it doesn't cycle like that, ever. the only time i can tell through power is when i kill it on big uphills (highway.) the truck get's a decent surge.

[mws]

I'll double check... I might be confusing it with my GM van that does cycle. The Nissan may be a variable displacement. It's never failed, so I haven't had to learn the details of it. Yeah!

[94extreme]

variable displacement A/C compressor

[zonianbrat]

I do not think mine cycles on and off. I do know my compressor has a bad bearing in it somewhere because it whines like crazy. I want to fix it but I do not want to pay a ton of cash to replace the compressor when it still actually works. Hey viper do you still drive to ft. collins? maybe when it does go out I can pay you to replace my compressor instead of gettin raped by a mechanic.

[94extreme]

check your tensioner pulley, zbrat.