A/C Usage and Impact on FE
 

now that summer is here, and 3 digit temps(heat index) with it, it is suprising how much impact my car's a/c has on FE.

let me first break down a/c on vs a/c off on the scangauge. this is at standing idle only since variables are eliminated...

IGN on: 14, off: 12
LOD on: 20, off: 15
GPH on: .4, off: .2

i cannot comprehend that gal/hr is doubled at idle w/ the air on. obviously w/ throttle and driving situations the % is no where near that but...

according to SGII, there can be as much as 4mpg difference in the same trip, EXACT conditions. i ran the air on the way to work the past 2 days yielding ~36 and 37 mpg. typically i'll get ~40-42 air off.

i am accounting for a bit a variables, so i dare not claim a 6mpg difference. remember, everything has been nearly exact and the same--route, weather, breeze(none), etc. typically, this time of year, my only variables are traffic lights. i time them very well, and usually make them all but 1 or 2.

so, at 2mpg, the loss is ~5%, and 4mpg it is ~10%. maybe closer to 4 and 9% respectively. still, that is significant. am i figuring this correctly?

At idle it only needs to make enough power to overcome engine friction, which isn't much at idle RPM and closed throttle. I can see A/C being as much as that.

MPG impact is directly related to engine power.

Larger engines don't get affected as much by ac. I would venture to say that the MPG impact is about as noticable as the performance hit. That's what my experience has been anyways.

Are there any cars that kick off the A/C when they need more power? I used to do it manually when climbing the steep hills in Providence in my 87 Sentra.

i think you're spot on there. i don't remeber my V6 olds losing that much. besides if the % was the same, obviously the loss in larger engines and lesser FE would not be as noticable.

AFAIK, most modern cars turn off A/C at WOT.

I think larger engines don't suffer as much... I tested this a year or 2 ago. Drove my truck on its daily drive, with the A/C on max for a week, then drove it with the a/c off and windows up for a week and noticed no difference in my mileage.

-Jay

Chryslers turned the a/c off under heavy loads in the 80's. I hated it. Hit a good sized hill and the a/c would shut off. I'd hate to have to drive it up a mountain in the summer...

I hadn't considered that. Pretty ironic given where I live!

Yea, but you are in Florida too.

I have actually only needed to use mine a couple of times this year and mine makes no difference on the highway where 95% of my driving is.

Was about a 4mpg spread on the last car I had with a/c too, my '04 Focus. My Mom's Mazda3 only seems to take about a 2mpg hit when they drive it (they're of the more leadfooted persuasion), but it seems to take 6 or so mpg off the clock when I drive...

Larger engines are already at a disadvantage MPG wise! :D

A/C boost button
 

My 02 Focus, 2.0, auto, really get a horsepower hit when I have the A/C on. Its Baltimore and its hot and humid and after getting heat stroke twice in my life, I know the value of A/C. Since I got rid of my factory cold air intake, I feed off the engine bay heat and I got about a 4 mpg boost from that, but lost my ability to claw over the hills. So when I drive I kick on the A/C down hill and off up hill. Some times I forget and turn it off while going up hill and I can feel the extra 6 or 8 horsepower it takes. So the A/C button is kind of a boost button for me.

In some cases like the hypermiling Insight owners who average close to 100 MPG, the AC penalty is 30 MPG !!!!!!.

I drove my Insight to my step daughters this weekend. Started out with 72.5 MPG for 45 miles.

Drove it 76 miles at speeds not over 57 MPH and ran AC at minimal fan, reciriculate, and econ mode. Mileage dropped to 63.4 MPG when we arrived at her house.

Drove back later in the afternoon and did not run the AC. Mileage increased to 66 MPG overall.

Not sure how good my math still is, but from 72.5 @ 45 to 63.6 @ 121 is a significant drop. Normally I would average about 70 on that trip.

To increase the overall trip mileage to 66 @ 198 would require the mileage to be back near 70 MPG on the return trip.

A guess would be at least a 15 MPG hit for minimal AC use in the Insight, but remember this, still better than 55 MPG if the whole trip would have been with AC.

Thats a 15 MPG hit for minimal AC use.

Fan speed 1, recirculate, with econ mode which allows the engine to still shut off at stops. That is the absolute minimum AC use on the Insight, and it still costs almost 15 MPG!!!!!

Now you must consider that thei Insight will burn one gallon of fuel every two hours at a constant 40 MPH under ideal circumstances. That's the same amount of fuel an average V8 uses just idling!!!!!

Bottom line is the more efficient cars become the much greater the cost of running the AC. It's more exponential than linear and if we ever build mega mileage cars we will also have to address the gross inefficiency of conventional AC systems, even with the best modern designs of compressor.

regards
gary

Actually, I think this is a case where Distance Per Volume (l/100km) would illustrate the difference much more accurately than Volume Per Distance (MPG). Making the car more efficient doesn't add or subtract any cooling cost, it just changes the "driving" side of the driving vs. cooling costs ratio, which looks exaggerated when discussing MPG but not when discussing l/100km.

72.5mpg = 3.24433908 l/100 km
63.6mpg = 3.69834251 l/100 km
55mpg = 4.27662879 l/100 km
100mpg = 2.35214583 l/100 km
70mpg = 3.36020833 l/100 km

The 9mpg penalty you experienced would cost you 1/5 liter (which is .05 gallon) over the course of 60 miles. At $3/gallon, that adds 15 cents to your 60 mile trip; so A/C cost you 1/4 cent per mile or one cent per four miles.

For 1/4 cent/mile I'd turn on the a/c! Thanks for bringing this to our attention HC!

-Jay

Yeah and if you sit and idle the car with the A/C running for a while it does eventually cool off then it starts cycling on and off with the compressor and uses even less gas. But bottom line is that it does take power to run A/C in a car 9000 watt A/C is about 1 gallon per hour 1800 watts is 0.2gph. Use it on the down hill when you can . . . next I have to see if the new ScanGauge can give a proper reading with the A/C kicked in down hill no gas.

Lets demonstrate that a civilized debate can have two victors, with both positions being justified.

The fuel was free, a gift from Pop for chauffering him to lunch, as well as being there 24-7 for the last decade and about 30k miles. He once told me I was the only person who ever drove a car that he could ride in and fall asleep.

To me it's the principle. Most of my relatives think I go overboard with the fuel economy thing. I say it's that important.

Time spent with Pop to me is priceless. He lost his father less than 9 months after he came back from the war in Europe in the spring of 1945. He was 24 years old.

I've had Pop for 58 years, he turned 88 this May and is still one of the best drivers I have ever seen, but with the traffic and inconsideration of drivers I see daily around here, it will be my pleasure until we have spent our last seconds together.


regards
gary

a/c is much more taxing than even rain...

i drove home today in light rain, cracking a window, and using fan only. that yielded almost 44mpg.

i've often wondered if rain decreases rolling reisistance such that it offsets its own created drag. heavy rain is of course something else entirely.

Why would rain decrease rolling resistance? I can only imagine it making more rolling resistance as the tire has to displace the water...

1.Rain.
At the family holiday feast preparations, the kid asked his mom why she cut the ends off the ham then tucked them in the pan before baking. She replied that that is what she learned form watching her mom, and to ask grandma. Grandma's reply was the same as the daughter so she turned and asked the Matriarch of the family, the Great Grandmother who replied; small pan.

People still drive in the rain with the windows open.
Great Grandpa didn't have Air Conditioning that automatically ran with the defroster to remove the window-fogging humidity.......

Water on roads dramatically reduces free rolling. Ever hit a puddle at speed?
Better MPG might be because we drive slower?

2. Using the A/C.
I missed the part about the reduction in MPG caused by the drag of having open windows...even a little bit.

I didn't see the part where the ScanGauge was compared to actual arithmetic involving real miles and real gallons.


But, silly me...
I got rid of my Dakota because the A/C broke and it cost more than the truck was worth to fix it.
To me, the conversation is academic. I chose comfort.

Me too. Besides, it evens out in the end; I'm a more patient driver and more able to save gas when I'm more comfortable.

I totally agree. If I'm not comfortable there is no way I'm going to hypermile. I'm going to get to my destination as quick as possible.

-Jay

{Bottom line is the more efficient cars become the much greater the cost of running the AC. It's more exponential than linear and if we ever build mega mileage cars we will also have to address the gross inefficiency of conventional AC systems, even with the best modern designs of compressor.}

I believe this only applies to traditional belt driven ACs. The Prius uses a variable speed electric compressor. It doesn't directly suck off energy going from the engine to the wheels. There will be a mpg penalty of some type, since the energy it takes from the battery has to be replenished in someway, but then the battery is cooled by the cabin air. So AC use may keep it at a more efficient point.

I've heard BMW was experimenting with regenerative braking on non-hybrid cars to run accessories. With the regen brakes and a beefy battery, it may be possible to replace all the belt driven add-ons with electric.

Driving in rain helps mileage a little sometimes - a little water on the road makes it more slick and lubricates the tread reducting traction and lowering rolling resistance a little and raising the MPG - however if it gets too deep then it can slow you down. It also adds moisture to the combustion which helps generate more power in the engine. Using a little heat for the defroster keeps the windshield clear but since the cars have gone to a single stupid control for the vents you can't get warm air on the windshield and cold air on you like my old VW Rabbit with the separate fresh air vents that only put out cold air and separate control levers for feet and windshield. Ahhh those were the simpler days when a simple lever turned something on and off as much or as little as you wanted. Now you have dash - dash and feet - feet - feet and windshield with defrost - windshield with defrost.

Adding moisture to the air/fuel mix might help.

Rolling resistance, AFAIK, is not caused by tread friction on the road (and if it was, tires would never last 40,000 miles). It's caused by having to deform the sidewall at the bottom.

I sure would like separate controls for things.

i'm thinking light rain and/or roads w/ good drainage keeping standing water to little or nothing.

oh, and we call slick roads at the beginning of rainfall "black ice" in FL.

i can vouch for that, i take a hit of about 4-6mpg in my civic running the a/c. my explorer is almost un noticeable.

The differences are insignificant in my cars. The Matrix is small but I think the electric compressor used in it instead of a belt driven one helps. However, its cooling effect is not exceedingly strong at the same time.

Light rain may improve mileage since it cools down the road surface turning it less sticky than in a hot day. As you all know, tarmac may start melting if the temperature is high enough.

Heavy rain will decrease mileage even if the tarmac has cooled down, for the reasons mentioned.

Also remember though that the hotter the road the more inflated your tires 'become' from being heated.

i tested(scangauge verified) my Accord for the first time. it has ~30% increase in fuel consumption air on vs air off at idle. much better than doubling it w/ the prizm. but i've taken a big hit on total FE.

I have sat in the Dakota with the A/C on full for three hours a few times.
It affected my MPG, but I'd rather be comfortable.

i'm w/ ya fully. this is just a notation. in the summer, i generally do not run the a/c in the morning to work(still dark), but on the way home, i always run it(90*F+ often w/ heat index in the 100s).

I grew up in Arizona, and had crappy cars most of the time. I got used to the heat, sort of. I had a '66 Buick Skylark that had an Iron block V8 with Aluminum heads... Not good in 100+ degrees. After the 1st time I heard the "Tink" of the metals expanding at different rates, I started running the heater in rush hour traffic. I sat in about 170 degree heat for about an hour, 5 days a week. I usually had a headache, and was nauzious when I finally got home.
Now I have A/C, and it doesn't affect my car much (about 240HP/ 2400lb. car), and I never use it, just out of habit, and ridiculous FE interest.

One thing you guys can do to improve the efficiency of the air conditioner is to switch to a hydrocarbon refrigerant over using R134a.

Think about this;

I spent ten years driving around in Jeeps and Land Rovers with no sides.
Lots of wind whipping around my head.
Lots of wind noise.

Now the noise NEVER stops.
Tinnitus (pronounced /tɪˈnaɪtəs/ or /ˈtɪnɪtəs/, from the Latin word tinnītus meaning "ringing")

Go ahead and drive with the windows open to save a few cents of gas.

Well it has been proven that it's better to drive with the windows closed and the A/C on than with the windows open and the A/C off when cruising on the highway. But if you're just stopped in traffic or driving at a slow speed, then driving with the windows open and the A/C off is a better idea.

Well, it looks like the Geo will probably get its A/C back this year. Since my wife has decided to ride with me in the little car, and summer is coming, she is voting for fixing the A/C. (When I bought the car, the condenser was trashed, and the compressor was missing.) I voted against fixing it, so it looks like we'll have to compromise...I'll have to fix it.

Bet it'll cost more than the car.......