It's getting hot... let's talk A/C

[jeff88]

Since Summer is upon us, the last week most people have been using their A/C. With that being the case, I thought it would be a good idea to start a thread to talk air conditioning!

I'll start with a few questions:

1) With the A/C on, does the fan speed change anything with regards to energy usage (other than the extra energy for the fan)? In other words, does the A/C work harder with a higher fan speed or does the A/C only have off/on capabilities?

2) A lot of times, I cycle the A/C on and off because when I turn it off, I still get cold air for a little while. I'm wondering, is this cold air residual from the system components, or does the A/C stay on for a certain amount of time after turning the button off?

3) How much slower would one need to drive to match the equivalent FE of A/C on vs. off? For example, if one drives 65 with no A/C, would driving 60 with A/C on be the equivalent FE wise? I know this will vary by car, but I figured a rough estimate would suffice for most people.

Let's talk A/C!!!

[oil pan 4]

1, yes you are increasing heat load.
2, depends on how over computerized everything is. Likely its just the cold coils of aluminum and the water that makes up the mass of the evap coil and ducting that blow cool air for a short time after the A/C is switched off.
3, the tests I remember reading about said that A/C decreases fuel mileage by around 10%.

[serialk11r]

The AC compressor pump turns the same speed regardless of your fan setting, but more fan will bring the evaporator temperature up and give the compressor slightly more work to do. I wouldn't worry about this though, because doing that should increase the efficiency of the system by lowering the temperature delta. You're adding more load to the engine (increases efficiency) via more cooling load to the compressor (also increases efficiency), so it should barely burn more fuel.

From what I logged, it seems like AC uses somewhere between 0.05-0.1 grams of fuel per second on my car depending on engine speed (increases MAF reading by 0.7g/sec at idle, increasing to maybe like 1.2g/sec somewhere in the 2000s range from what I recall). Going 65 vs. 60 burns WAY more than that per mile travelled.

[TimV]

I use this rule. No AC on backroads. Roll down your window. Only AC on on the highway. Open windows then increase draft so more fuel use then you would have with closed windows and AC on.

I burn 7% more fuel with AC on on the highway.

The AC cycles automatically on and off once the temperature in the cabine is lowerd.
When you cycle the AC, when you switch it of, you get some redt cold out of the system.

When i need to brake, or DFCO, I put the AC on on full blast. Getting some regenAC

[Hersbird]

The fuel economy hit to run the compressor varies by car and motor size. A Geo Metro is going to take a much bigger % hit then a 2500 Ram Cummins.
Also realize most cars will run the A/C in defrost mode even if the A/C setting is turned off. I hate that in Montana, that is useful only in very humid areas.
A/C is one thing I will gladly take energy hit on. My work truck lacks A/C and I waste a lot of time dreaming of redneck cooling ideas as I boil away in an aluminum box.

[user removed]

use dfco to get the coil cold then the fan speed to distribute. Go max recirculate, max fan, when slowing down using dfco, best way to get cool cheap.

Then chop the fan speed, compressor off, to keep cooling.

regards
mech

[elhigh]

I just got done making a run to Chattanooga over the weekend, about 260-280 miles round trip. It was pretty warm and steamy, and of course at highway speeds riding with the windows down is just too blustery and noisy.

I found that it was impossible to maintain 60mpg (indicated) at 60mph on the flat with the AC on. Couldn't do it. AC off, the instant indicator would jump up to a tick above 60, AC on it dropped to a tick below. I have no more precise measurements to offer than that, and I didn't think to repeat the test along the same stretch of I-75 on the way back - besides, the median between north- and southbound was so far that the slope of the road couldn't be guaranteed to be the same.

DAC (Downhill AC) allowed me to pulse-and-glide the air conditioning, turning it on only for the downhill stretches. It reduced my glide a bit but not enough that I was about to give up the cooling.

When I got home I had averaged 60.5 (indicated) for the entire trip. The real world numbers are invariable 7-10% less than the indicated ones, but for a real time barometer it gets the job done.

[jeff88]

I do the same as most, a/c on during dfco or slowing to a red light (really useful when you're watching what the lights are doing ahead!). Then when I need that extra oomph to go, I turn it off and let the residual cooling do its thing.

[roosterk0031]

At idle A/C takes an extra 0.1 gallon per hour per SG2 with my Cobalt. If I'm averaging 45 mpg without it, running it would drop me to about 42 mpg. $0.26 per hour, if its hot I use it. The lowest setting usually freezes me out, so at speed the cost might even be lower.