Aerodynamic drag from open windows vs. Power drain of air con

Discussion in 'Clarity' started by Akinto, Jul 21, 2018.

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  1. Akinto

    Akinto Member

    We had a 27C (80F) day today.

    Is it better to open the windows rather than run the AC?

    Does it depend on your velocity?

    How do you decide?

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  3. KentuckyKen

    KentuckyKen Well-Known Member

    The only hard data we have is one post that showed a loss of only 4 miles or so of EV range from a full charge’s range when using the AC. And I’m only noticing that my EV range has come down from high 60s to low 60s by using the AC. But both these observations were with the windows up.

    So in the absence of any hard data on windows down drag vs windows up and AC drain on battery, I’d say do what ever makes you the most comfortable. AC use doesn’t seem to penalize range nearly as much as heater/defroster use does. I attribute this fact to the very efficient electrically driven scroll type compressor Honda uses in the Clarity.

    Again, as in other areas, the Clarity can just be a “get in and drive” type of car and be almost as efficient as when we play with all the settings or try hypermiling strategies.
    Akinto likes this.
  4. insightman

    insightman Well-Known Member Subscriber

    If you're really worried about squeezing the most miles per watt, you can run the climate control system fan with the A/C off (windows up, of course). That's what I've been doing with my Insights for 19 years. On really hot days, I'll cave and run the A/C for a minute, then turn it off again (this is the first time I've admitted this in public).
    Akinto likes this.
  5. KentuckyKen

    KentuckyKen Well-Known Member

    While confession is good for the soul, you’d better hope none of the Insight crowd is reading this!
    insightman likes this.
  6. AnthonyW

    AnthonyW Well-Known Member

    In general and particularly at highway speeds, it's best to keep the windows rolled up. Stop and go around town is a toss up. The Clarity has a whole lot of glass "real estate". Best thing you can do is get ceramic tint on all the windows including front and back. Check your local laws to see how low you can go in regards to VLC. Key is to do the entire front windshield (if the local law allows). The other key is to do the entire front windshield (even if the local law doesn't allow it). If you get 70% or 80% VLC on the front windshield, someone would have to look really really hard to even see if it is there. I have ask quite a few of my colleagues to get in my car at work and let me know what they think of the tint job on all the windows. They all love it and think it is awesome. After a moment I will ask "do you think the tint on the front windshield is too dark" and they all swear to me that there is no tint on the front windshield.

    Anyway, once you have tint the car will be so much cooler. Get in the car an run the AC fan at max for about 5 to 10 seconds. After that you will be able to set the temp 3F to 5F degrees higher than you do now. Problem solved.
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  8. AaD

    AaD Member

    The word on tinting the front window here in MA is that you "have to know your state inspection guy" according to our installer. They check it mechanically, so even though it may look clear they can fail you when common sense doesn't prevail. We've had one inspection site that refused to inspect a car with a car seat installed and threw away a license plate frame that had passed many times, and another that passed a car with no issues and simply said "you should get that taillight fixed some time"... we stayed legal all the way around just in case.
  9. Atkinson

    Atkinson Active Member

    Could be my subjective observation, but the Clarity seems to be the quietest car I've ever ridden in with the windows open.
    The wind at highway speeds is a solid stream with very little buffeting or turbulence.
    Wind tunnel design or my imagination?
    If someone figures out the efficiency of windows down/AC off vs windows up/AC on, we are all interested.
    Personally, I think my HV MPG boils down to whether I have a favorable wind during that trip.
  10. insightman

    insightman Well-Known Member Subscriber

    This Business Insider article (citing a GM study) says roll those windows down and shut off that A/C. But YMMV with a Clarity PHEV.
    Last edited: Jul 22, 2018
    Akinto likes this.
  11. bpratt

    bpratt Active Member

    I did a test a couple of months ago where I sat in my car in the sun on a 95 degree day with the car turned on (engine not running) and the AC turned on to 69 degrees. I watched the EV mileage and found it would lose 1/10 of a mile every 2 minutes. It took 20 minutes to lose 1 mile. I would say that is very efficient.
    Akinto likes this.
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  13. Akinto

    Akinto Member

  14. jdonalds

    jdonalds Well-Known Member

    I gotta believe it depends on the car. The efficiency of the A/C varies quite a bit from car to car, and how the wind impacts drag on the car, windows open or not, will also very quite a bit.

    I've always followed the rule of windows up at highway speeds. Windows down at lower speeds but that depends on the ambient temperature. Our current daytime temp is running close to 105F which dictates windows up and A/C on.
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  15. MarkClarity

    MarkClarity Active Member

    Testing on a Tesla Model X (Youtube Reference) showed that it is still more efficient with windows open at 62mph / 100km/h. Maybe when you get above 70 mph A/C will be more efficient. It all depends on how much the drag increases with the increased speed.
    Akinto likes this.
  16. insightman

    insightman Well-Known Member Subscriber

    I'd love to know whether the Tesla A/C is as efficient as Honda's electric A/C. Does Tesla make their own A/C units?
  17. Hobbesgsr

    Hobbesgsr Active Member

    Great scientific discussion, but with the added wind noise, wind messing up the wife's hair and any road dirt/dust/bugs, we just leave the climate control to our desired temp.
    Maybe if we were in a low range/battery EV or hypermiling then I'd remember to roll down the windows..or sweat it out.
    insightman likes this.
  18. KentuckyKen

    KentuckyKen Well-Known Member

    It’s been 89s to 90s here for a while, but much needed rain brought the temps down to the mid to high 70s and I noticed the climate control do something new. I had it set on auto (green light in driver’s temp set knob was green) and at 72 when I saw it had changed from its usual recirculating mode to fresh air mode. I hit recirculate and the green auto light went out. Hmmm. I hit auto and it went back to fresh air. It seems the car senses outside and inside temp (and maybe humidity?) and chooses the mode for most efficiency? Never had a car’s climate control do this before.

    The auto system and scroll compressor are so efficient that I’m just setting the temp, leaving the windows up and only losing 4-5 miles of range (went from 66 to 68 to 62 to 63 w a/c).
    Akinto and Johnhaydev like this.
  19. Atkinson

    Atkinson Active Member

    "The humidity/in-car temperature sensor has a humidity sensor and an in-car temperature sensor. The climate control unit receives
    humidity information from the sensor to help control the electric A/C compressor operation. When it is humid, the climate control
    unit operates the electric A/C compressor to prevent the windshield from being fogged. When the humidity is low enough, the
    climate control unit reduces the operating time for the electric A/C compressor. A flexible hose connects the sensor case to an
    aspirator on the HVAC unit to detect the accuracy of temperature and humidity."

    No mention of recirc mode off when interior humidity is high, but that would make perfect sense.
    Johnhaydev, Akinto and KentuckyKen like this.
  20. Unless one really prefers having outside wind, I can't really understand why I, or anyone, would trade the comfort for saving the cost of a cup of coffee for blowing hot air and dust into face, and cabin, while sweating below the shoulder. Especially not when we're having 90-120F in Southern California now.
    It's not a convertible so I will just keep the windows closed.
    (I have a convertible but will keep the top on with ac during hot days, otherwise top off as much as possible)
    Last edited: Jul 23, 2018
  21. Atkinson

    Atkinson Active Member

    OMG, and I complain about New England summer heat!

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