AC commits pre-meditated murder on MPG

On short trips starting out with a car that's been sitting in the sun we often won't bother with Air Conditioning, regardless of conditions. It's the least efficient scenario: just shut everything off and roll the windows down

 

I appreciate all of the numbers, computations, tips, etc. But I'm pushing retirement age and I'll take some comfort over a few MPGs. So, my A/C is set at the "cool me off, now" setting. Not in 2 minutes, not in 5 minutes, ... now. Except don't blow up my engine/battery (paid too much for that).

Mike

 

You guys must run your AC balls out.

I have sat in the car with the AC on idle for over 10 minutes and only dropped 2 bars on the battery and never had the engine come on.

Thats at 72-73 with recirc and the fan on 3.

 

I agree. I run mine on Auto at 74. I am not giving up comfort for a few tenths mpg, either.

I do like others, park with car facing the sun with a sunshade in the window, open the windows when first leaving the parking lot to cool down interior. By the time I hit the highway the cabin is almost cooled down and the fan and compressor are slowing back.

I can see the battery amps dropping as the compressor and fan speed start dropping back. 5 minutes later I can start gaining back some of the mpg I lost from the parking lot to the highway. That is in 90F and 95% humidity.

 

I see a big increase in MPG when I turn off the A/C (~ 3-5 MPG).

 

OK, I have some data that shows that moderate use of the AC does not impact mpg at all. I drive from my home in CT to Boston frequently, it's just under 140 miles. The last three trips have given HSI numbers of:
72.5mpg - AC off, with the windows rolled up as I recall
73.0mpg - AC off, windows cracked open I think
69.5mpg - AC off, windows open several inches, slightly different route at the Boston end due to a stop for medical appointments, my GF came along on this trip

The third was this past Tuesday. All three were in the same direction.

I ran the usual route again today. It was a bit humid and 82F (CT) to 86F (Boston), no wind, so I decided to see what effect moderate use of AC would have. I set inside temp at 77F, recirculate (except for a brief period at the beginning when I didn't realize that changing another setting had switched it to external air intake), fan on AUTO, MODE upper vents only. I was very comfy thanks to the dehumidification effect of the AC.

Today's result:

72.7mpg

I do realize that actual mpg's are 2-6% lower.

 

I like it cool ... my temp is always set at 67 ... recirculation on ... fan at lowest speed. Plenty cool as it gets very hot and humid here in the summer. Last night I filled up and averaged 47.25 mpg for 200+ miles (hand calculated) vs. 50.1 mpg displayed. I will take 47.25 mpg any day which is far better than my 2007 Nissan Altima SL V6 which only got 17 mpg when running the a/c! :cheer2:

 

Just curious if you'd try taking a video of this happening. I could try to do this and it would drop to pink.

Perhaps my battery has less capacity since it is 6.5 years old.

 

I just made a video. I can upload it tonight. Ran a timer, idle, 72 degrees with recirc and fan on 3.

Engine came on when the battery was at 3 bars to recharge.

Total time to pink was 6 minutes. Would have been less if the engine didn't recharge at 3 bars.

If I had left the car on auto A/C it would have gone to MAX. Even after the 6 minutes of 72 degrees A/C. I tested for that too.

 

Don't forget to compare results you must have the same weather outside including temperature, humidity, sun exposure and wind and the same set temperature on the AC. Otherwise your results will be incorrect.

 

Well, since it wasn't on auto and it was on manual and recirculate, we can probably neglect outside temperature, humidity, sun exposure, wind, etc.

Temperature on A/C was 72. "3" on the fan. Idle, started from regular battery charge.

 

I've got the Fiesta today but if I think about it I will give it a try.

10 minutes may have been an exaggeration, 6min isn't bad but you drained a lot. I may have had the perfect storm that day...it was the day they were announcing that the Atlanta Thrashers were folding so I left my office to listen on XM. I took a short loop trip around the block in hopes of hearing the announcement but they kept babbling so when I got back to work I just left it on and sat in the car.

It was probably well topped off from the drive and that may have been the factor.

I'm curious now to what reality would be so I will give it a shot for you.

 

It was surprising to me too, which is why I posted it. My speeds are very consistent (I've been doing this for a while!) but of course the temps, winds, etc DO vary. Yes I think AC DOES add an electrical load, the point I was making is that this load varies, and with light usage may be far less than 4-5mpg. Plus, at higher mpg's, the effects of small loads are more visible.

 

The A/C when running at the lowest load draws about 2.5 amps from the battery. This is about 500 watts.

I have seen it draw 4 - 5 amps. This is about 1000 watts. It probably draws way more than this because I have ECO mode on, and the fan is never at a high setting, and I run the A/C at a reasonable temp (never LOW).

The A/C does seem to draw amps from the battery during cruise, though when the battery is being charged it will take it from the inverter.

I postulate that the MG1/MG2 DC/DC conversion is most of the time incompatible with the AC inverter input voltage.

 

Translation for dummies ?

In the meantime, I'll just add a couple of "probably right, based on general principles:"

1. The AC compressor is ~ 30% efficient, so if there is X amount of heat and humidity in the cabin to get rid off, it is going to take ~ 3 - 4X the energy to get the job done.

2. Energy not spent on (1) is energy spent on MPG.

3. High MPG cars show a greater drop in absolute MPG using AC than low MPG cars all else being equal, because the ratio of AC_energy_consumption / energy_per_mile_consumption is higher.*

* This confuses a lot of people used to MPG and not GPM.

 

It may be clearer if you think of it as simply a mismatch of the spin states of the isokinetic warp nodes. Or not.

 

Are you sure? That is a hideously low efficiency compared to my home heat pump AC.