How to Reduce MPG loss Because of A/C Use

Discussion in 'Gen 2 Prius Fuel Economy' started by F8L, Sep 11, 2011.

  1. radmangto

    radmangto Junior Member

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    There's very little diff to me between 50mpg vs 47mpg .... given the choice, I made it .... cool dehumidified air ... unless gas goes to $8/gal, probably will not consider changing .... choice brother, that's what it's all about ....
     
  2. F8L

    F8L Protecting Habitat & AG Lands

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    Exactly. That is why I don't understand why people come in this thread to tell us how they are happy to lose a few mpg so they can be comfortable. It's your choice and in many cases I would do the same. It this thread was not created for folks like you. It was created for those who would like to maximize mpg or at least reduce the amount of cool down time upon enter their car in the evening. :)
     
  3. SageBrush

    SageBrush Senior Member

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    Justin has the right of it. This thread is also meant to teach how to use AC most efficiently.
    The only small caveat I want to add is that heat is bad for the traction battery, so even the most stoic MPG fanatic should apply some measure of moderation in doing without AC.

    I think a message most of us can agree to and practice is cooling unrelated to AC. Smart heat blocking and ventilation can do a lot for comfort, and the energy use is nil to minimal. Then moderate AC use has a pretty modest fuel economy penalty, and the traction battery is happier, too.

    An off-the-cuff estimate for using AC only to remove heat coming in through the windows while driving is 100 - 500 watts, depending on window tint, sun angle and car direction. If driving averages out to 10 kw, the AC used wisely is 1-5% extra.
     
  4. Brett.

    Brett. Junior Member

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    A few things I do to reduce the A/C usage.

    1.) Keep the cargo area cover / shade in use at all times. It shades the trunk space where the traction battery is as well as prevents would be thieves from seeing what ever it is that I may have back there at the time.
    2.) Before entering the car, open the hatch and at least the 2 doors on the drivers side, all it takes is a momentary breeze and the interior of the car (except for the fabric and plastics which radiate some heat) will cool down 10-20 degrees almost instantly. Close up, and drive.
    3.) On days when the A/C would be nice, but isn't necessary. When I am in close proximity to the vehicle, or not entering a building for more than a couple minutes, I keep all 4 windows down about 2-3". Just enough to allow air to pass through, but closed enough not to allow anyones arm to enter and open the vehicle from the outside.

    Things I plan to do in the near future.

    1.) Use the sunshade when parked for extended periods of time.
    2.) Tint at least the rear windows, my daughter is always squinting anyway she needs some kinda shade.

    I'm the kinda guy that needs the air on LOW setting, high fan speed set to face / foot mode at all times when in the car on any days over 72 degrees.

    Totally off the wall.

    I noticed a comment in this thread earlier about being thin and being cool. Well I'm here to tell you as an avid outdoors man, being very fit and able to climb a mountain faster than most people can fall out of bed in the morning I still sweat bullets and I always feel too warm. You might see me wearing no coat in the winter, or working in 50 degree temps with just a tshirt still drenched.

    In conclusion.

    My mileage went from 50-52 avg with no a/c to 46-48 with a/c usage. 4mpg difference, not totally A/c related. I tend to mash the accelerator pedal a bit harder in the summer as I don't want to be in the car any longer than I have to.
     
  5. jdcollins5

    jdcollins5 Senior Member

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    For those of us in warm/hot climates need to be aware of the strain being placed on the HV battery by trying to gain an extra mpg at the expense of the HV battery temperature. Now that the average temps have been in the mid-80's in NC the battery temps climb between 95-115F while driving in town.

    I have noticed that when I run the AC, especially at near constants speeds without a lot of start/stop, the battery temps will at least hold more or less constant with the battery fan running. If I have the AC off for any reason the battery temps will continue to climb even with the battery fan running.

    Remember that if you are uncomfortable so are your batteries. Personally I am willing to give up a couple of mpg to extend the life of my HV battery.
     
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  6. F8L

    F8L Protecting Habitat & AG Lands

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    No argument there, brother!
     
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  7. imOCD4a_prius

    imOCD4a_prius Member

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    ****That reminds me of another unique A/C method I'd like to throw out here: a FAN BLOWING into/onto a keg of ice. After many hrs. of reading HERE--I've learned that we MUST keep the inside of the "P" cooled and that will result in thousands of wonderful miles by the HV. (the main battery never gets cooled from outside/external AIR FLOW) Did i pass my 1st quiz ??? Sincerely imocd4a_prius
     
  8. 2009Prius

    2009Prius A Wimpy DIYer

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    We are having a few days of hot weather so I decided to do a comparison: AC vs. no AC. No AC means the climate control was completely off, no fan. When I tested with AC on I tried to be conservative: the set point was at 75F in the beginning, then tweaked up to 84F, then back down to 80F. The trips were made along identical routes, speed limit ranging from 25 to 40 MPH, with some small hills but nothing major. The result is summarized in the graph below:
    [​IMG]
    The outside air temperatures (OAT) were not exactly the same for the two trips but close: 2 ~ 3 degree C difference on average. There was hardly any movement of the HV battery temperature in both trips. In fact the measurement shows that the HV battery temperature increased more when AC was on: the temperature rise was 2 degrees C with no AC, and 3 degrees C with AC on. One could argue that the HV battery has to work harder to provide extra power to the AC, resulting in the higher temperature rise. At any rate it's safe to say that under my driving conditions the HV battery temperature changes very little and is not affected by the AC being on or off. Another interesting observation is that even though I was quite conservative in the AC use, the hit in fuel economy was huge: 88.4 vs. 75.0 MPG. As we often say: YMMV. :)

    Update: Since people are interested in knowing how much extra power the A/C system is drawing, I did some more digging and summarized the findings from the same two trips in the graph below:
    [​IMG]

    The lower left graph shows 1.6 A of baseline current draw from the HV battery when the car was stopped near the beginning of the trip with no AC. The lower right graph shows that the current draw increased to 8 ~ 9 A with the AC on. Since it's near the beginning of the trip the compressor was probably running near max speed but I can not be sure. The upper left graph shows that some time later the baseline current dropped to ~ 1.2 A with AC off. The upper right graph shows the corresponding time period with AC on, during which the compressor was cycling on and off, and the current switched between 1.7 A and 5 A.
     
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  9. F8L

    F8L Protecting Habitat & AG Lands

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    Thank you for collecting these data. Do you feel pretty confident that you minimized variables and that your tests are repeatable?
     
  10. 2009Prius

    2009Prius A Wimpy DIYer

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    Yes I am pretty sure. This is my daily commute route and I go through pretty much the same pulse and glide routine that has been refined over time. Of course I can't control the actual air temperature or traffic pattern. Though as the data shows there was not much difference in temperature, and I didn't encounter any special traffic conditions. I did notice the AC drew about 5A from the HV battery when on, thus the idea of extra load on the HV battery.
     
  11. bisco

    bisco cookie crumbler

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    it's been in the mid 90's the last 3 days here and the first time i've used the a/c on the pip. while i can't offer any hard data, my commute is 15 miles round trip and i do it in all ev. turning the a/c on and off manually to stay comfortible, i was still able to make the trip in all e/v with no noticeable use of battery. now, if only electric heat were that efficient.:rolleyes:
     
  12. SageBrush

    SageBrush Senior Member

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    Markabele,
    What do you think of this experiment, meant to sort out if alternative AC modes to auto are more efficient:

    Alternate between auto and a higher temperature with a higher fan setting. If you have a route you drive often the same time of day, you could start one day with auto and then alternate every 5 minutes; and on other days start with the manual setting and then alternate along the same pattern.

    If the Torque app will also let you monitor traction battery temp, it would be great. The last thing we want to do is improve fuel economy while adding heat stress to the battery.
     
  13. ProximalSuns

    ProximalSuns Senior Member

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    Conditions are never identical. Typically folks have point of view on it and likely it changes their driving behavior to influence the outcome despite the best of intentions. Be interesting to see some good controlled tests.
     
  14. F8L

    F8L Protecting Habitat & AG Lands

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    That is why we (Markabele and Sage Brush) are working on collecting wattage data as well. This way we are able to rule out anything related to driving techniques and focus on power usage. This can later be converted into miles per gallon and applied to many driving situations.

    It's pretty obvious that the hit is there. That is why you see drastic cuts to EV range on electric vehicles. It will be nice to have some solid numbers to work with for the Prius. :)

    Questioning the Value of SGII | Page 3 | PriusChat
     
  15. ProximalSuns

    ProximalSuns Senior Member

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    At best it ends up as a lot of anecdotal info since so much depends on the driver and driver is going to have strong point of view and it will influence behavior.

    There is no free energy lunch so using more energy for AC will reduce the mileage but how much is the question.

    Seems easy enough to find out by putting car on dynamometer, controlling temps and speeds and measuring the difference.
     
  16. SageBrush

    SageBrush Senior Member

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    Yep, just put a little sun in the corner with variable clouds, rotate the car on the dyno, and add wind. Easy Peasy
     
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  17. F8L

    F8L Protecting Habitat & AG Lands

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    I disagree. The information will be useful regardless whether you fail to find it rigorous enough. If you would like to pay for dyno time, supporting equipment and set up the test parameters then by all means go for it. In the mean time some of us will collect data that is conveniently available and see how these compare to data collected by others.
     
  18. ProximalSuns

    ProximalSuns Senior Member

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    Controlling environmental factors is standard in dynamometer testing.
     
  19. SageBrush

    SageBrush Senior Member

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    Uh huh. Who do you think you are fooling, laddie ? I know you do not have a clue, as do a dozen others who have taken a moment to try and correct your nonsensical posts.

    Give it a rest, you are embarrassing yourself.
     
  20. F8L

    F8L Protecting Habitat & AG Lands

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    I don't know what kind of dynos you have been using but the ones I use are not designed to control wind, interior temperature, induce heat soak, or continuously variable loads. This is why we are looking at AC wattage consumption in normal driving conditions. Frankly you are not contributing in any beneficial way so I'll leave it at that. Let us know when you have something worthwhile to contribute. :)
     
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