Aircon - impact on fuel economy?

[vsonix]

Sorry if this has been touched on before but couldn't find anything when I searched. I've just had my aircon fixed (wasn't working when I got the car); the car is now icy cool when I want it to be; however I am worried if I start using it a lot my fuel economy will suffer. I don't mind maybe losing 1 or 2 mpg tops on long runs since the cabin is a quieter, nicer place to be with it on, but I need to know if I should be sparing with its useage I guess. Anyone found the ti's aircon to be better/worse than other cars in that respect?

[John Firestone]

The increased consumption depends on how much cooling you ask of the air conditioner. Typical averages for Germany might be 1-2 liters/100 km during cool down and 0.5-1 liters/100 km thereafter. How that converts to mpg depends on the car's realized mpg as those units are non-linear. If you are getting 10 mpg, the air conditioner won't be very noticeable. If you are getting 100 mpg, using the air conditioner might prove a financial hardship.

Using my long term average of 8.6 l / 100 km (32 mpg UK) for the base consumption, I convert the above to 4-7 mpg during cool down and 2-4 mpg thereafter. Perhaps someone someplace hot can give you some better figures from their consumption records. I'm not sure I use my air conditioner enough to have the data for a calculation

Judging by the last few days, this will be a good summer to have it in northern Europe. I think I'll be glad I got mine fixed.

[vsonix]

yeah, the last few days it's hit the low 20s in the afternoon, unprecedented this early in the year in these parts!

[pdxmotorhead]

In every test I've seen, the AC is lower impact than the windows being down.
I played with mine on a road trip, used the cruise and drove 100 miles on AC and 100 with windows and sunroof open. The windows cost me 3 MPG vs the AC.

The OBC even was able to tell.

Dave

Dave

[vsonix]

thanks Dave that's what I was kinda hoping.
Roughly what kind of speeds were you averaging? At the moment I tend to be windows open and aircon off up to about 60mph and above that the windows up and aircon on

[John Firestone]

The German automobile club (ADAC) needed (http://www.adac.de/infotestrat/technik-und-zubehoer/heizung-und-klima/default.aspx) another 0.2 l/100 km (less than 1 mpg) with the windows open, between 80 and 100 km/h. Using the A/C to modestly cool the car from 28 deg C to 22 was 2 to 10 times more costly in town.

Their test vehicles were a Ford Fiesta 1.4, a Mazda 6 2.0 MZR, an Opel Vectra 2.2, an Audi A4 1.8 T and an Audi A4 2.0 TDI. Using the AC was between

[pdxmotorhead]

thanks Dave that's what I was kinda hoping.
Roughly what kind of speeds were you averaging? At the moment I tend to be windows open and aircon off up to about 60mph and above that the windows up and aircon on

Hmm. Well,, I suppose the statute of limitations is up by now......
This was a unique area, I was setting the cruise at 90MPH its a REALLY out of the way road in eastern washington (Ie See for > 7 MILES!)

I had to drive it 4 times in 1 day 55 miles each direction... So I was able to use the same fuel and get a couple of nice long high speed runs on it. Got 24MPG on the OBC doing it... There is only 1 turn in the whole road that required dropping cruise and resetting.

Couple other interesting tidbits, the water temp stayed the same. The outside air temp was 95F and The car was empty except for me. I was running 32 rear and 28 front cold tire pressure. I got both runs done between 1 and 4 PM and the Air temp on the OBC stayed within a couple degrees all 4 trips.

Dave

[cooljess76]

Mythbusters actually tested this. Here's the thing though, according to the "quiz" on their website, windows up A/C on gets better fuel economy than windows down A/C off:
http://dsc.discovery.com/games-quizzes/driving-myths/

However, according to their test, it's the exact opposite by a significant margin:
http://dsc.discovery.com/videos/mythbusters-air-conditioning-vs-windows-minimyth.html

[pdxmotorhead]

Did they use a BMW? LOL

I have noticed that my car seems optimised for side windows up.
Bmw went to all the trouble to make that fancy little spoiler pop up in front of the sun roof, yet did not seem to do squat about the side windows, the buffetting ad the lack of cover on the seatbelt riser makes it NOISY as all hell at highway speed. I find that on vent with the sunroof in popup mode I'm the most comfortable.

I heard a great line from the engineer who designed the Audi diesel for the lemans races.

"Noise is energy, if you get rid of noise you save energy."

My little bonzai run is not a scientific test by any means, but what it proved to me was that It does not make enough difference for me to care.... :)

Damn the dinosaurs full steam ahead... !!

Dave

[John Firestone]

"Noise is energy, if you get rid of noise you save energy."Well, I suppose if you equate a symptom to the cause. An object that produced one acoustical watt, 0.0001% of the M42/M44 engine's 103 kW peak power, would rupture your eardrums, with the windows up or down.

As to the Mythbuster's quiz and the program (assuming I could view it), both could be true or false simultaneously, without recasting them as postulates in monetary economics. As the engineers found in the ADAC test I linked to earlier, which is more efficient, windows down or the air conditioner, depends, for a start, on the air conditioner technology and the vehicle speed.

[vsonix]

Hmm. Well,, I suppose the statute of limitations is up by now......

lol, if ever the long arm of authority tries to pull me up for anything said online the official line I stick to is that I was exaggerating to make myself look cooler ;)

[vsonix]

Bmw went to all the trouble to make that fancy little spoiler pop up in front of the sun roof, yet did not seem to do squat about the side windows, the buffetting ad the lack of cover on the seatbelt riser makes it NOISY as all hell at highway speed.

Dave

my fave design flaw is the way water comes in the windows when you wash the windscreen at >10mph :lol:

[Honolulu]

Unfortunately, I call the mythbusters "attempt" Bad Science. Nice try, but too many potentially significant variables have not been addressed. Even the test itself is flawed, thusly.


One: true that the engine (indirectly) measures intake air flow through the AFM. But the engine computer does know how much fuel the engine is getting, via the pulse width signal sent to the injectors. Some cars will report the mpg to the driver using that data... of which no mention was made.


Two: the "test" was only run once, with one car using AC, the other using windows. We can't/don't know the state of tune of these two vehicles, surely they are not identical. Therefore, for the test to be valid, the "AC versus windows down" test should be run with the vehicles reversed.


Three... even if the test was run correctly (see item two above) the test is only valid for the vehicles used. Aerodynamics for other vehicles will of course vary. The coefficient of drag for these SUVs was not mentioned. The results will be different for vehicles with a different Cd, ECU, state of tune, and numerous other factors.


Overall, this was a pretty lightweight attempt to address the question, and in my opinion the "results" are meaningless.

[M-technik-3]

The way we work it, if it's too humid and hot the AC is on. Otherwise around town windows open but for freeway the windows up and AC on.

We get 29-31 combined MPG so it's much easier to fill the Ti vs our Tahoe which gets 15/20 mpg and my E30 M3's the R12 has been dry for a couple of years so it doesn't matter in that respect.