Which is worse: AC or windows down?

Discussion in 'Gen 2 Prius Fuel Economy' started by Mondrin, Jun 29, 2008.

  1. Mondrin

    Mondrin Junior Member

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    It's getting hot here in Oregon (finally) and it's brought up a question. If you are driving 55 will the FE hit for the AC be worse than that of having both front windows open at least half way. Doing the later will increase wind resistance and reduce FE but I don't know which gives the worse hit. Let me know if you have any information in this area.
     
  2. minkforce1

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    i'll guess AC is worse
     
  3. qbee42

    qbee42 My other car is a boat

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    Depends on speed, temperature, and humidity. Normally windows work better at low speed, while A/C is more efficient at higher speeds. With the Prius it's a bit more complicated for two reasons: 1) The A/C on the Prius is very efficient, and 2) The Prius needs to keep its traction battery cool, and it does this by pulling air from the cabin. If you let the cabin get hot, it hurts the efficiency of the battery.

    Tom
     
  4. JimboK

    JimboK One owner, low mileage

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    Posted elsewhere, but re-posted here for the benefit of the OP and others coming here looking for an answer to this (more specific) question ...

    AC use will affect Prius fuel economy more in driving conditions where the ICE is not needed for propulsion: stop & crawl, pulse & glide driving, and other low speed conditions. The AC runs off the battery, but eventually the ICE will have to run to recharge it. Opening the windows affects it more at higher speeds where aerodynamics become more important.

    The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two halfway down, and two diagonally opposed 1" down. This chart shows simulator results with 90F OAT, with and without ACC set to 75F, and all window position models.

    [​IMG]

    The models assume the following:

    • Steady speeds
    • Level terrain
    • No wind
    • 190' elevation
    • 30.00 barometric pressure
    • Smooth dry pavement
    • Michelin Hydroedges inflated to 50/48
    • E10 fuel
    • 300# load, driver + cargo
    The conclusions:

    • All windows up and no ACC gives the best results across all speed ranges -- for the brave soul who can tolerate it.
    • Two diagonally opposed windows 1" down is next best, and might be tolerated by some at high speeds.
    • Two windows halfway down is superior to ACC at speeds below the low 50s.
    • All windows down is superior to ACC below 40 MPH.
    Specific values of course will change somewhat with different combinations of typical summertime OAT and ACC thermostat settings, but I doubt the overall relationships change much.

    As Tom suggests, a hot hybrid battery muddies the waters. As OAT approaches 100F, battery temp can do the same. (But does it ever get that hot in Oregon?) In the peak of summer heat I always look for a shaded spot when I expect to be parked for an extended period, even if it means a longer walk. (The exercise sure won't hurt! :rolleyes:). I had an episode last summer of sluggish performance and a spontaneously aborted glide that was likely caused by high battery temperatures. The battery cools very slowly, whether with AC or windows open, and can heat up even more with operation. Better is to prevent it from getting hot in the first place.

    Tom mentions humidity. The effect of outside air humidity on aero drag is pretty insignificant. The simulator shows an increase of 0.3 MPG with an increase from 10% to 90% relative humidity at 55 MPH. However, it seems intuitive that the ACC would work harder in humid conditions. Maybe someone more familiar with the technical side of ACC can elaborate on how much.
     
  5. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    Hey, that's really cool!! You've described my driving style to a tee! And exactly in the order I practice them, temperature permitting. The difference is that I don't always go for diagonal windows but rather all three except the driver's (due to direct sunlight).
    I very rarely use the ACC because my commute contains only a few miles at speeds above 40mph. But one thing I do in high temps (muddying the waters?) is to row the back passenger window all the way down in an attempt to put more air in the battery vent.

    Thanks once again, Jimbo, for the excellent data and explanation.
     
  6. mcsenerd

    mcsenerd New Member

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    All I've got to say is stay far...FAR away from southeast Texas in late June to try these no AC experiments. Or at the very least...please take some of the fuel savings you reap and invest in a large supply of Right Guard or your favorite antiperspirant/deodorant!
     
  7. qbee42

    qbee42 My other car is a boat

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    Humidity is only a factor in comfort. Heat and humidity work together to create a sensation of heat. In weather circles it's called the Temperature Humidity Index (THI), or just Heat Index. HVAC engineers call it total enthalpy, which is a measure of the total heat energy of air. Each gram of water in the air requires the removal of 540 calories to condense into water, a figure referred to as the specific heat of vaporization for water.

    With higher humidity, you feel warmer, so open windows are not going to work as well for cooling. On the other hand, the A/C will work a bit harder to get rid of the moisture. Recirculation allows the drier air to remain inside the vehicle.

    Tom
     
  8. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    i used AC on the freeway, and basic conclusions on AC also means what temp its set at, how hot it is, level of direct sunlight etc...

    i usually use windows all other times except when SO is in car. she a wimp...wants AC...but she is pretty cold-blooded, so she usually cold most of the time anyway... also, a lot of the times, she does not want her hair messed up...so that eliminates the window option as well
     
  9. JimboK

    JimboK One owner, low mileage

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    She must be related to my fiancee.:p
     
  10. richard schumacher

    richard schumacher shortbus driver

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    Not when it's 100'F outside, I hope :_> The battery wants its cooling air to be cooler than that. The window noise may also keep you from hearing the battery fan running fast.
     
  11. patsparks

    patsparks An Aussie perspective

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    My theory without proof.
    Windows down if that is comfortable below 70km/h
    windows up and AC on over 70km/h or if too hot for windows down.
    If it's cool enough I set fan speed to middle and leave AC off with vents open and windows up.
     
  12. LenS

    LenS New Member

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    Really nice graph. It also points out the impact of speed on fuel economy. Loose 10mpg with speed change from 60mph to 75mph.
     
  13. seattlite

    seattlite Junior Member

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    Anyone know how to purchase the simulator. Can't find a link on the site, but LOTS of useful information.
     
  14. JimboK

    JimboK One owner, low mileage

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    Unfortunately, the developer took the download & purchase links down after he suspected the registration code had been hacked. He had a flurry of hundreds of downloads (trial version available for free) with no purchases. Maybe sometime in the future if he can get it figured out.
     
  15. Jack66

    Jack66 Kinda Jovial Member

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    In NO VA I get better FE with AC off and windows down while driving at highway speeds if I can stand the wind. I usually try the vent and fan first but the range of temperatures where that is OK is slim so I usually have to put the windows down.

    I probably get too hot before the battery does so it seems to be in any danger...
     
  16. mattpopp

    mattpopp New Member

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    sounds like you need more power under the hood.
     
  17. archae86

    archae86 Member

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    Most of the claims of A/C superiority that actually have any data behind them (a minority) seem to assume windows full down.

    My preferred window position when I am driving alone is to lower both of the windows on the passenger side an inch or two. Even at freeway speed the sound is tolerable, and at lower speeds not any problem. On both our Prius and our Audi A4 this configuration sets up a sort of circular air flow around the cabin, so the driver side actually gets better air flow than if the two driver side windows are down instead.

    And I am confident that the fuel economy impact is below that of A/C up to speeds faster than I ever travel (probably over 100 mph on the Audi, unattainable on the Prius).

    I'm not arguing that you should suffer, just that if you think you are saving fuel by using A/C instead of partial windows you are probably wrong.
     
  18. seattlite

    seattlite Junior Member

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    Hmmm....all this seems like a good justification on getting some side window deflectors. Anyone know how well those side window deflectors does on minimizing wind noise?
     
  19. Jack66

    Jack66 Kinda Jovial Member

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    I'll take your comment at face value...

    The Prius has more than enough power under the hood. Look at the subforum this thread is in and it says "Fuel Economy." The question is not whether the Prius has enough power to run the air conditioner -- it is whether you better fuel economy with the AC on and windows up or AC off and windows down (a drag vs power tradeoff).
     
  20. Highly ImPriused

    Highly ImPriused Impressive Member

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    Great chart as usual Jimbo! It convinced me to try and sweat it out on the way home yesterday. OAT was 82F and normally I would use the AC when it's that hot. I tried the technique mentioned in this thread of having the 2 passenger side windows down about 2" and it actually worked pretty well. I was able to feel a nice little breeze on the back of my next with the circular air flow and wind noise wasn't bad at all. I wouldn't go so far as to say that I was comfortable, but it was certainly tollerable; better than I expected (I wouldn't try it with any pasangers though). Anyway the results comport with the chart. I usually get right around 70 mpg on this route in similar weather running the AC. Yesterday I got a tad under 75 mpg (per ScanGauge).

    One question on the siumlator. Does it account for the variable speed AC compressor that the Prius has? As I mentioned in another thread, I have seen the battery current draw while stopped with the ICE off, AC on, and all other accesories off range from about 2 to 8 amps. With the AC off it was about 1 to 2 amps. So I figure the AC uses anywhere from about 1 to 7 amps. Once you get the cabin cooled down and you are in recirc mode, I would imagine it would be on the lower end of that spectrum even on a 90F day. But I haven't done much observation yet to try to confirm that (it's difficult to track while driving for a number of reasons).
     
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