Windows down: Is it really a "drag"?

[user 1096]

Here in upstate NY we have been having a blissfully moderate summer; that, combined with the fact that I seldom use AC, I was wondering if riding around town and on local expressways with windows down has caused any significant change in range?

Further, I assume that the power pack "conditioning" is done on its own and not relying on me to switch on the cabin AC. So, my question is does the aerodynamic drag of open windows exceed the AC's electrical "drag" on the battery pack?

If these lowered windows do affect range, I am going to be doubly angry because, under normal circumstances over the past 40+ years of BMWs, I've gotten along quite well with the sunroof providing open-air ventilation ...

 

[Zzzoom3]

Here in upstate NY we have been having a blissfully moderate summer; that, combined with the fact that I seldom use AC, I was wondering if riding around town and on local expressways with windows down has caused any significant change in range?

Further, I assume that the power pack "conditioning" is done on its own and not relying on me to switch on the cabin AC. So, my question is does the aerodynamic drag of open windows exceed the AC's electrical "drag" on the battery pack?

If these lowered windows do affect range, I am going to be doubly angry because, under normal circumstances over the past 40+ years of BMWs, I've gotten along quite well with the sunroof providing open-air ventilation ...

You'll be fine with the windows down. Any reduction in aerodynamic efficiency will be offset by the reduction in electrical load not running the AC.

 

[KurtEndress]

The answer to this question involves a lot of “it depends”.

The general advice is that it is more efficient to have the windows down at low speed, but to use the AC instead at highway speed. But it depends on the aerodynamics of the car and how high the AC is turned up, and of course, the exact speed. The more aerodynamic the car is the more penalty there is from rolling the windows down; hopefully the i3 is pretty aerodynamic and thus might suffer with the windows down. One study showed a 20% drop in efficiency with the windows down on a sedan at 55 MPH.

I suppose someone needs to do some tests with an i3 to see how much having the windows down at speed affects the efficiency of an i3.

 

[i3Alan]

You'll be fine with the windows down. Any reduction in aerodynamic efficiency will be offset by the reduction in electrical load not running the AC.

At some speed the aerodynamic drag from open windows will be a greater loss than running the AC. Most cars that I have owned with instantaneous MPG indicators seem to put the trade-off between 40 and 50 MPH. That is with both front windows all the way down and the rear windows cracked enough to eliminate buffeting. With the windows only open an inch or so, drag is much less, but at high speeds, still significant.

 

[jadnashuanh]

I think if you go into Eco Pro mode, you can monitor the current flow. IT might take a few passes over the same road to get a reliable average, but you should be able to determine this yourself with a fair amount of accuracy. Personally, I much prefer driving with the windows up, and the a/c on - it helps to keep it nice and quiet, and also keeps out a lot of dust and pollen, which some people can't tolerate well. Ever been hit by a big bug at 70mph? It's not fun, and if it happens to be a bee or a wasp, things can get ugly quick!

I'm glad that BMW had issues with the regulatory agencies over installing a sunroof on the i3 (US code requires some metal when you have a sunroof - this may get changed, but currently it would not pass even though the CFRP is stronger than the steel in most roofs). It gives you a bit more headroom and is one less thing that can leak, squeak, or give you problems.

 

[TomMoloughney]

I've talked to BMW engineers about this exact topic. I was told that if you're driving under 30mph the open windows are more efficient than running the A/C. Over 30mph however the energy needed to overcome the additional drag is greater than the energy the A/C will use regardless of driving mode (comfort or Eco Pro).

So unless you're driving very slow on back streets, running the A/C is more efficient, and this is coming directly from actual i3 engineers.

 

[pastyboy]

I've talked to BMW engineers about this exact topic. I was told that if you're driving under 30mph the open windows are more efficient than running the A/C. Over 30mph however the energy needed to overcome the additional drag is greater than the energy the A/C will use regardless of driving mode (comfort or Eco Pro).

So unless you're driving very slow on back streets, running the A/C is more efficient, and this is coming directly from actual i3 engineers.

I wonder if the same goes for the sunroof. Being in the UK I have ordered it with the s/r so that when we are stuck for two hours on a motorway (freeway) due to someone's accident we can let some heat out rather than running the A/C

 

[user 1096]

I've talked to BMW engineers about this exact topic. I was told that if you're driving under 30mph the open windows are more efficient than running the A/C. Over 30mph however the energy needed to overcome the additional drag is greater than the energy the A/C will use regardless of driving mode (comfort or Eco Pro).

So unless you're driving very slow on back streets, running the A/C is more efficient, and this is coming directly from actual i3 engineers.

AhHa Tom, Thank you very much...I had guessed 40 mph to be an efficiency/turbulence cutoff, but I'll take the engineers' 30 mph!

Maybe, we could align that wisdom with recuperation~if one is city driving where there is a lot of recuperation opportunity, it's probably more efficient to enjoy fresh air (to the extent 'fresh' air is available in town)...LOL