MPG: open windows vs. AC

[djc]

Rolling down the windows doesn’t appear to put more drag on a car’s aerodynamics. “The effect of opening the windows at 65 mph did not measurably reduce fuel economy."  So says Jake Fisher, auto tester at Consumer Reports.  He says they have tested multiple vehicles with same general result.  AC, on the other hand, costs 1-4 mpg on an 85 degree day, depending on vehicle.

 

http://www.consumerreports.org/fuel-economy-efficiency/fuel-economy-face-off-driving-with-windows-open-or-ac-running/

 

I am guessing: mpg hit for turning AC on is greater the smaller the engine  (and the higher the vehicle mpg).  Also I would expect the energy needed to run AC is a linear function of the difference between cabin and ambient temps. 

In any case, C-max doesn't like to have open windows at speed - makes loud buffeting noise.

 

[obob]

 

In any case, C-max doesn't like to have open windows at speed - makes loud buffeting noise.

 

That noise that I think you mentioned - Very generally, lots of variations, my experience is that it goes away if three windows are opened, even if just a couple of inches.  At lower speeds only two windows need to be opened.

 

http://jalopnik.com/why-do-slightly-opened-car-windows-make-that-awful-soun-1447498738

 

https://www.reddit.com/r/askscience/comments/26vjj4/youre_driving_and_you_have_only_one_window_down/

Edited June 18, 2017 by obob

[ptjones]

The big hit with A/C is the initial cooling the car down, This is why A/C is a MPG killer on short trips. :sad:  My experience with HWY driving after the initial big hit in mpg's to cool the car down is about 2 mpg. This would vary with outside temps and humidity.

 

Paul 

[obob]

 This would vary with outside temps and humidity.

 

 

And light reflectiveness of the color of your car.

 

You reminded me to start using my windshied reflector.

[jestevens]

Yes, I remember the dealer told me that "Ice Storm Metallic" is actually a "green" paint.. too bad the interior is black leather and there's a giant glass hole in the roof (which I love). 

 

I hate the buffeting noise, which seems to happen with all such aerodynamic cars..as others have already said it can be reduced with opening a few windows but at 65MPH I find the road noise and all that wind to be even more irritating so use A/C on highway.  

 

Around town or late at night in the summer going 45MPH or below I love to leave the windows open and drive silently around town.  I also find that there is a difference in having the windows ALL the way down vs. part way.  All the way down seems to promote an open feeling to the car and the wind doesn't knock me around as much.

[fbov]

Rolling down the windows doesn’t appear to put more drag on a car’s aerodynamics. “The effect of opening the windows at 65 mph did not measurably reduce fuel economy."  So says Jake Fisher, auto tester at Consumer Reports.  He says ...

 

This is patently false, as we'll see, again demonstrating the true technical competence of CR. 

 

The only justification of the statement is "He says...." This is called "argument from authority." Jake is a "tester" at CR, so he must know what he's talking about. In truth, if he's any good, he can PROVE either result beyond any doubt. Without data and the test plan behind it, this means nothing. 

 

Here's a documented proof that AC gives better mileage, thanks to Mythbusters. 

 

And documented proof that AC gives worse mileage, a conclusion they accept. I can't find the test I remember, where they found no difference, so they decided to drive slower to they'd get this conclusion. 

 

The problem is poor experimental design, ignoring known facts, specifically the effects of speed. This guy does a good job explaining.

http://physics.info/drag/

Key bits are the drag force dependence on velocity raised to a power - velocity^n - where that power depends on conditions. Turbulent drag depends on v^2, while laminar flow depends on v^1, which is just v. 

 

In our cars, we have laminar flow just about everywhere above the wheel wells 

http://fordcmaxhybridforum.com/topic/3129-tuft-testing-a-c-max/

 

That means most of the drag force is a v^1 dependence - v^2 doesn't kick in until you open the windows. 

 

The big hit with A/C is the initial cooling the car down, ...

Bingo #2.

 

Mythbusters tested an SUV that never hit 12 MPG in their test. When on, AC ran full time, full load all the time because that's how Detroit classically designed it. When you get too cold, you don't turn down the air, you add HEAT! CR is similarly clueless, failing to understand that Volvo gave AC a knob so you could turn down the AC.

 

Current electric AC technology uses variable capacity compressors that are extremely efficient at low load. We added AC to our house last year, and the electric bill didn't change (fans use a lot of power). In our cars, akin to the ICE, most of the AC drain comes getting to operating temperature. 

 

So...

To prove windows-open is more efficient, drive a high-drag vehicle at low speed with high-capacity AC on max.

 

To prove AC is more efficient, drive a low-drag vehicle with modern AC at higher speed for a longer time. 

 

HAve fun,

Frank

[fbov]

A few links to basic research...

 

https://www.researchgate.net/publication/290771543_Analysis_on_car_aerodynamic_drag_for_open_windows

Modeling, identifies critical velocity (90 km/h) where break occurs with their assumptions

 

http://www.inderscienceonline.com/doi/abs/10.1504/IJVSMT.2014.066502

Modeling, confirmed with wind tunnel testing, showing greater drag when windows are open. 

 

https://www.quora.com/How-the-aerodynamics-of-a-car-change-with-the-opening-of-each-window-sunroof

And a good example of the kind of response to ignore....

 

Frank

[Redshift]

You guys are missing the fact that the A/C on the C-Max is ELECTRIC, thus no parasitic drag whether it's on or off.

[raadsel]

You guys are missing the fact that the A/C on the C-Max is ELECTRIC, thus no parasitic drag whether it's on or off.

 

Depends on what you mean by "parasitic." While the AC doesn't steal engine power, when cooling the car it does hit the battery very hard; which hurts mpgs, as you can't stay in EV mode for very long.

[ptjones]

Depends on what you mean by "parasitic." While the AC doesn't steal engine power, when cooling the car it does hit the battery very hard; which hurts mpgs, as you can't stay in EV mode for very long.

A/C doesn't run directly off ICE, but all energy comes from the ICE charging the HVB. This is probably a more efficient way to do it and I imagine some ICE cars are already doing it. I still know that running the A/C on long FWY trips I'm losing at least 2 MPG as compared to run with my windows down 2-3 inches. :sad: 

 

Paul 

[djc]

The use of energy by A/C is proportional to time.  It is the same whether you are sitting, going 30mph or 100mph.  Thus the A/C energy use has a decreasing impact on mpg the faster you go.  At 70mph it is half what it is at 35.  

[ptjones]

The use of energy by A/C is proportional to time.  It is the same whether you are sitting, going 30mph or 100mph.  Thus the A/C energy use has a decreasing impact on mpg the faster you go.  At 70mph it is half what it is at 35.  

This is a good point. The down side to that is I get twice the gas mileage at 35mph that I get a 70mph. :headscratch:  :)

 

Paul 

[Redshift]

Depends on what you mean by "parasitic." While the AC doesn't steal engine power, when cooling the car it does hit the battery very hard; which hurts mpgs, as you can't stay in EV mode for very long.

How can the A/C hit the battery hard? It runs of the high voltage battery that barely notices the drain.

[ptjones]

How can the A/C hit the battery hard? It runs of the high voltage battery that barely notices the drain.

It seems like the SOC goes down pretty fast when the A/C is running. IMO and the MPG's definitely is going down fast from my experience. :sad: With ScanGaugeII you can watch the SOC going down when A/C is on.

 

Paul 

[p38fln]

If you change the display to show the KW usage (where it shows "Climate" and "Other"), the AC will draw about 2 KW on a hot summer day when the car is first turned on.  It eventually goes down to about 0.25 KW. So yeah you get quite a hit when its first turned on, but if you use recirculate it eventually settles down and hardly draws any power at all. 

[fbov]

A/C doesn't run directly off ICE, but all energy comes from the ICE charging the HVB. ... I'm losing at least 2 MPG as compared to run with my windows down 2-3 inches. 

Yep; unless you have an Energi, all power comes from gasoline. The parasitic load is just routed through the Charger side of the power split device to the HVB. 

 

As to the second part, what were the controls on AC use? I only use AC when it's really hot. If you did too, you know why your conclusion isn't well founded. 

 

Have fun,

Frank

[plus 3 golfer]

If you change the display to show the KW usage (where it shows "Climate" and "Other"), the AC will draw about 2 KW on a hot summer day when the car is first turned on.  It eventually goes down to about 0.25 KW. So yeah you get quite a hit when its first turned on, but if you use recirculate it eventually settles down and hardly draws any power at all. 

In Phoenix it is significantly more.  In a sun baked car with ambient temps at 105F, upon initial startup the climate demand will be over 4 kW.  After maybe a minute or so, the demand may drop to about 2 kW and continue to drop to a steady state of between about 400 - 900 Watts depending on conditions.  I'll see about 250 Watts SS when temps are in the 80-90 F range.

You guys are missing the fact that the A/C on the C-Max is ELECTRIC, thus no parasitic drag whether it's on or off.

No, you are missing the point:  that the topic relates to the effect on FE of using AC vs no AC with windows down.  As many have said, using AC in our hybrid requires  additonal fuel to be used by ICE.  The load / energy requirements on ICE is not the same whether AC is on or off.  

How can the A/C hit the battery hard? It runs of the high voltage battery that barely notices the drain.

Again, the topic is about FE not about the % of AC load vs the capacity of the HVB.   Yes, a 0.25 kW load is hardly noticed by the HVB since the HVB can likely sustain about 35 kW of demand (hit then would be less than 1% of the HVB capability).  But, the effect on FE of the overall energy used for AC (gas burned) comprared to the gas used for other load while driving the car is likely significantly greater than 1%.  As djc says "The use of energy by A/C is proportional to time".  Thus, the longer it takes getting from point A to point B, the greater the effect of the AC on FE.  It's very easy to do the calculations to estimate this effect as I have done this using the climate demand screen, ForScan data, and the FE trip screen data.  I believe I posted on this several years ago.  

[joshg678]

If you change the display to show the KW usage (where it shows "Climate" and "Other"), the AC will draw about 2 KW on a hot summer day when the car is first turned on. It eventually goes down to about 0.25 KW. So yeah you get quite a hit when its first turned on, but if you use recirculate it eventually settles down and hardly draws any power at all.

I've seen 4kw the other day when it was 101 (just over 3 bar) but usually it's right under 3 bar for a few and goes to 2 bar for a while and steady at 1 or less.

 

The other morning when it was 75 but 100% humidity I ran it and jumped between no bar and .25 bar most of the trip and got my typical MPG of 56 for my morning drive.

[Redshift]

A/C doesn't run directly off ICE, but all energy comes from the ICE charging the HVB. This is probably a more efficient way to do it and I imagine some ICE cars are already doing it. I still know that running the A/C on long FWY trips I'm losing at least 2 MPG as compared to run with my windows down 2-3 inches. :sad:

 

Paul 

All the energy doesn't come from the ICE for charging. Don't forget regenerative braking. Also  don't forget the battery is charging whenever you are slowing down or going down hill.

[plus 3 golfer]

All the energy doesn't come from the ICE for charging. Don't forget regenerative braking. Also  don't forget the battery is charging whenever you are slowing down or going down hill.

How did the car get up to speed in the first place (neglecting external forces like a very, very strong tailwind ;)  :) ​).  by using ICE or energy from the HVB.   How did HVB get charged? Regeneration simply captures some of the energy of the moving vehicle which energy was at some time provided by ICE.  How did the car get to the top of the hill?  by energy provided by ICE.