So Air Conditioning in automobiles is still a bit of a mystery for me.
I was wondering, does it matter what setting you put the AC in regards to the MPG you lose?
Like, if I put it on the highest setting with the coldest temperature, does that affect my MPG more than putting it on the lowest setting at a mid temperature? Or as long as the AC compressor is turned on, then it will lose the same MPG?

 

It depends on a lot of factors, but generally speaking, the load will be much higher at idle than at cruising speeds, due to increased airflow over the condenser, which reduces the force that the compressor has to work against. So yes, there is not such thing as free energy, power must be consumed for the refrigerant to be moved from condenser to evaporator, the degrees of which depend on workload, and cooling capacity.

Many vehicles today use variable displacement compressors which minimize the losses, although the main reason is to more precisely control compressor displacement, which has the effect of better efficiency.

 

fwiw, in our old 90's hondas with their simplistic systems.. it doesn't matter. if the green ac light is on, the compressor is probably running, the fans are definitely running and it's sucking torque to do all of this.

i know with my b series i'll only loose a couple mpg's higway. but with mixed or city driving.. haa.. saying it was just a noticeable jump in fuel consumption would be an understatement.

that said, i only use it when:

-it's above 90
-i'll be in the car driving or sitting long enough for it to actually cool

 

The effect of temperature setting depends on how its accomplished. On some cars with advanced climate control, it does so by raising evaporator pressure by reducing refrigerant flow. This type of control will have a variable displacement compressor with electric control valve.

On most cars, vent temperature is raised by reheating the air with heater core, so if you're in recirculation mode, this adds load to compressor.

What makes the big difference is recirculation setting. Putting on recirc(often the MAX A/C setting) dramatically reduces A/C load, because it will operate like your house A/C. In fresh mode, you're using a lot of AC power to bring the outside temperature air to vent temperature and to condense out the water.

http://www.nrel.gov/docs/fy00osti/28960.pdf Page 9

The biggest and not a surprising factor is the minutes A/C used per mile.

Suppose the car takes 20hp to cruise along on freeway and compressor uses 2 hp. and driving like this uses 2.2 gallons per hour. let's use 60mph.

60 miles = 2 gallons = 30 mpg
60/(2 + (1 x 0.2gal/hr = 27.27mg
9.1 % reduction, but you're only using the A/C for a minute per mile.

The effect isn't all that great since the distance moved per minute the A/C is on is fairly large.

If you're not moving but the A/C is on it will still need 2 hp(condenser gets less flow,but compressor spins slower, so system capacity is reduced, so we'll assume it stays the same).

I believe the engine uses more fuel to make 2hp at idle than it does to tack on 2 hp on 20 hp at cruising speed so lets say you're now using 0.3 gallons per hour.

let's say you traveled 50miles in city and it would have been 25mpg without A/C use.
50/2gallons = 25mpg and you averaged 25 mph moving, but there is a lot of idle time involved.
This is the car you typically only drive at 8AM and 5PM rush.

Well, let's say its hot and now you're using A/C for that two hours.

50/2 = 25, a lot of traffic, stop and go.
50/ (2+ (0.3gal/h x hrs =0.6 gallons) = 19.23 mpg

19.23/25 = 23% reduction in mileage.
but you're using the A/C 2.4 minutes per mile driven.

The horsepower load of variable displacement compressor can be reduced by using recirc mode. When you reduce the evaporator it tells the compressor to reduce displacement thus reduce load on engine.

The total on-time of cycling clutch can also be reduced the same., which effectively cuts the compressor on time per hour driven with A/C button pushed in.