Minimizing MPG loss While Using Car AC

Ok, lets say you want to run your car AC but at the minimum needed so as to minimize your MPG loss while driving.

Should you
a. Run the fan at a lower speed vs. a higher speed
b. turn the temperature setting more in the center vs. all the way into “cold.”
c. It makes no difference, all settings affect MPGs the same
d. other, please explain

I have been turning this one over in my head and really don’t know enough about the way car AC works to answer it.

An air conditioner pumps a working fluid between a cold sink (the outside air) and a heat source (the air that you are trying to cool down) - you evaporate your fluid at low pressure and low temperature to suck heat from your hot zone, and condense your fluid at high pressure and high temperature to reject heat back to your cold zone (note that you are probably rejecting heat to a sink that is hotter than the interior of your car - otherwise, you’d just run the ventillation fan). Turning on your car’s A/C engages the compressor, which starts pumping the refrigerant around the loop (I suspect that the compressor might not engage if the fan is in the off setting). The higher that you have your ventillation fan speed, the more hot air you are passing over the evaporator coils, the more energy you transfer into your refrigerant, and the harder your compressor needs to work the re-compress the fluid (first answer). Sliding the temperature knob/slider more towards warm increases the amount of engine coolant (the source of the ‘hot’ when you are running the heater or defroster) that you circulate into your climate control system - if you want to keep cool, don’t move the knob/slider off of full cold (second answer).

I don’t know whether there is a sensor to engage/disengage the compressor depending on the return temperature of the refrigerant - maybe someone else can chime in on that one. Are these answers clear? Thanks.

Er, kind of. Are you saying that keeping the slider on COLD using LESS energy than keeping in more towards the middle? Because my question is more about using the minimum energy than about being frosty-cold.

Keeping the windows up to reduce drag will save more energy than any diddling you do with the airconditioner controls.

The fan speed has essentially nothing to do with energy consumption, anyway. On older cars, slower fan speeds were created by putting large resistors in series with the motor so the motor get less voltage, but essentially the same amount of power is used to run the motor alone, or to run it with a resistor in line. Newer cars have either multi-winding motors or electronic speed controls, but the power difference is still negligible.

If you are dumping in extra heat from the heater coils, then it will take more engine work to achieve the same level of passenger compartment cooling. The basic reasons to run the A/C compressor with the temperature slider/knob off of minimum temperature are: 1) if you are trying to defrost the windshield or 2) the temperature is too cold even on the slowest fan speed.

If mileage is your primary concern, then it would seem to me that you should 1) use the A/C only when really needed, otherwise just run the fan - keep the windows open around town, but up when on the highway and 2) run the fan on the slowest practicable speed and direct the cooling air with the dash vents where it’s most needed and 3) use the “Max A/C” or “recirculate” setting once the passenger compartment has cooled down (and until the air gets too moist). Good luck.

The Master’s comments on the topic, sorta.

I think you’ll find that the energy required to run the circulation fan is miniscule when compared to the energy to run the A/C compressor - in magnitude, it’s comparing watts (fan) to kilowatts (compressor). And drag will only really become an issue at highway speeds - otherwise, one can keep the windows down to increase airflow through the passenger compartment.

[FONT=Arial]To make a long story short, the A/C in your car does not work like the A/C in your house. That is, lowering the temperature control in your car (as described by Schuyler) does something totally different that lowering the thermostat in your house, so doing so in the car won’t save energy.

Put the temp all the way to “cold.” Put the fan wherever it makes you the most comfortable. Setting “recirculate” or “max” allows the interior air to dry out more, making you feel cooler (and in my car it blows more air).

[Car Guys]Did you know that car A/C was invented by three Jewish guys from New York? They were named Norm, Hi, and Max.[/Car Guys]

This is not necessarly correct Cooking.
It depends on the type of airconditioning system in the car. Assuming you have a cycling clutch orifice tube system or a variable displacement compressor, running at a higher interior temp will lower the load on the compressor and impove gas mileage.

[Nitpick] Auto A/C was invented by Otto Tinky[/nitpick]

Just to clarify, I am ONLY asking about the most efficient way to run the AC NOT the comparison of AC to windows-down at various speeds. etc.

I am anticipating a 6-7 hour drive, with (crated) cats in the car, all at highway speeds, with expected exterior temps above 90F. The car is a 01 Hyundai Accent in case anyone wants to get all CarTalk on the specific cooling system of my car.

I’ve found that on a hot sunny day the cooling effect of the AC last less than 10 minutes once it has been turned off. So the idea of turning it on and off is somewhat pointless over the kind of drive I’m speaking if.

And FTR anyone who uses the AC without recirc engaged or with the windows down is basically just an idot.

With the windows down, I agree. If you run 100% of the time with recirc engaged IMNSHO you are a blithering idiot. I have had to deal with car owners that do this, and the results. It ain’t pretty.
If you run with 100% recirc all the time, the evaporator will start to grow some very interesting and very smelly, well the technical term is, crud.
if you want your new car to smell like old gym socks or old dirty jock straps run your recirc 100% of the time.
When you bring it in, the shop will clean the evaporator and tell you not to do that. The chemicals used for cleaning are very strong, and frankly a little hazardous. Full of EPA warnings for respirator, gloves etc.
Your car you can run your A/C however you want, but before warned, full recirc is not the best idea ever.
Getting back to your question, in all cases, that I can think of, you would improve your gas mileage more by slowing down 5 MPH then by adjusting your A/C.

I suppose that is true in some climes and for some people. Since I almost never run the AC under normal conditions (maybe 4 or 5 times a year) its probably ok for me to “always” use the AC with recirc. But I’m sure what you’ve written is accurate for people who frequently use their AC.

Quite so, but not my question.

Excellent response, Rick, and Hello Again - I’m thinking that the original question has been beaten to death, and Rick was only expanding on the answers. YMMV (but that was to point of the OP). :slight_smile:

The point I am trying to make is that the difference between different fan speeds or different temp settings are picking fly shit out of pepper. It does not even rise to the level of picking nits. We are talking much finer than that. The differences are so small as to be unmeasureable with the tools you have available. Your odometer which is not 100% accurate, and a gas pump/ fuel tank which gives you no indication filling the tank to exactly** the same level each and every time you fill up. neither of these are precision instruments.

I agree with Rick. Use whatever fan speed or temp setting you want, the difference in MPG will be negligible. However, using max/recirc constantly is much less efficient than using max when you first start up and drive away, then switching to norm after the interior has cooled down a little.

My girlfriend used to drive around with max/recirc on all the time, until I demonstrated to her how much cooler it gets when you switch to norm.

Personally, I use max/recirc for the first few miles, then switch to norm and control the temp by adjusting the temp control, leaving the fan on the third highest setting all the time.

Oddly enough, my truck (Chevy Silverado extended cab with the 5.7 liter V8) gets exactly 17.5 MPG, regardless of AC on/off or driving style. I check the mileage every time I fill up, and it is always the same.

This is simply not true. You can experiment with it on your own if you like. Try driving around at 60 mph with your windows open and closed. Is there any difference in the performance of your car? Now try driving around at 60 mph with the a/c on and off. Pay close attention to the tach.

Another fun experiment is to try driving up a long hill at a constant speed (say 60 mph). When you’re halfway up the hill turn on the a/c and see what happens. Hint: it’s going to be harder to maintain speed.

If you don’t believe me, Mythbusters did a segment on this a while ago, driving SUVs around a big oval track at (I think) 45 mph. They made a big deal about how they were weighted exactly the same. (Jamie is bringing 5 pounds of bizarre nutritious food-like substance? Well, there’d better be an equivalent 5 pounds in the other car!) End result: a/c burns gas way faster than open windows.

Interestingly, their computerized gas mileage gauges didn’t detect any difference. When actually tested by driving cars on the track, the results were dramatic.

In other words, “choice c, it makes no difference.” No need to get up in arms about it. I guess that’s the consensus, although it seems illogical to me.

BTW, on my car recirc, AC temp, and fan speed are independent controls. You can have the recirc on at any temperature or fan level.

No you are not understanding my point. It does make a difference, but not one you can measure with the tools you have at hand.

You must have missed their later “revisit” episode. They conceded that the two results are both correct. The computer measurement was performed at a higher speed (55mph?), where aerodynamics plays a larger role. So at high speeds, closing the window and using the AC is more efficient; at lower speeds, the opposite is true.

(That’s still an over-simplification, but not outright wrong.)

I should have said, it makes no practical difference. I can’t use the information to modify my behavior, so what difference does it make, do you see what I’m saying?

I re-read the entire thread, and I guess people agree that the question’s been answered, but I guess I don’t understand the answer. Can someone lay out the answer my question in extremely simple terms?