Curious MPG loss with AC on. 10mpg drop!

Discussion in 'Gen 2 Prius Fuel Economy' started by F8L, Jul 21, 2011.

  1. F8L

    F8L Protecting Habitat & AG Lands

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    *please see below for results of a test I performed when the cabin temp was close to outside temp*

    Considering the recent discussion regarding MPG losses with the AC on I decided to play around with the AC while watching my ScangaugeII.

    Method:
    I started testing on the freeway. Speed was 65mph. I decided not to use CC due to major fluctuations in instantaneous MPG. I held my foot steady to maintain 65mph and chose the flattest section of freeway that I knew of. I would then turn the AC on with temperature set at 78degrees while monitoring the instantaneous MPG gauge onthe ScangaugeII. Then I would turn it off again while continuing to monitor the instantaneous MPG.

    Result:
    Every time I turned on the AC, the instantaneous MPG would drop by approximately 10mpg. I watched the MPG gauge drop from 72mpg to 62mpg. Sometimes it was not an even 10mpg drop and I would see numbers ranging from 8mpg-12mpg but this could simply be due to the changing road conditions. We all know how quickly the gauge jumps around.

    I then decided to take a back way home where the road is smooth, level, and I could go slower. I performed the same test but took video this time. I observed the same approx. 10mpg drop while maintaining a speed of about 58mph. I then tried turning down the fan setting to the lowest setting possible and nothing change. I continued to observe the 10mpg drop. After I finished taking video I rolled down the windows and the MPG stayed about the same as with the AC off.

    Summary:
    In the short bit of testing I performed I would almost be inclined to say that AC use may have a much more drastic effect than many of us realize and having the windows down may be better than running the AC when traveling at lower speeds (0-60mph). I would love to see someone perform a much more robust test to see if the MPG loss is as bad as it appeared during my test.




    * With the temperature set close to the outside temp (64deg outside 68deg inside) I tried cycling the AC on and off like I did yesterday and I only observed a 1mpg change. The change was very consistent. This is along the lines of what bisco is experiencing.
     
  2. Danny Hamilton

    Danny Hamilton Active Member

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    Your average MPG over the tank or multiple tanks would likely be affected even more than your instantaneous since instantaneous MPG doesn't take into account the reduction in SOC of the HV battery.
     
  3. F8L

    F8L Protecting Habitat & AG Lands

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    Very good point, Danny. I wonder if these people complaining about low mpg could be coerced into driving without their AC for a day or two.
     
  4. SageBrush

    SageBrush Senior Member

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    Nice game :)

    AC cooling starts off at high power to reduce the cabin temp to the set setting, and then throttles down as the setting is reached to the power required for steady state. I think you were only measuring the start-up power draw.

    Back of the envelope calc to make math easier ---
    60 mpg at 60 mph is 200 wh/minute energy consumption at the wheels. If initial AC power draw is 20% more, that is 40 wh/min = 2.4 kW. Sounds ballpark.

    People who want to spend less energy on AC cooling of the car should put effort into preventing the cabin from superheating while the car is parked. My experience, admittedly in hot but not humid weather, is that I can manage 60+ mpg with AC set to mid 70s F if the drive is not short, and the car starts out ambient or a bit lower.

    If the drive is short -- less than 5 miles, or sometimes up to 10 miles, I leave the AC off. I try to use it as a safety feature to prevent falling asleep on long drives.
     
  5. F8L

    F8L Protecting Habitat & AG Lands

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    Is there something else to the power of the AC beyond the fan speed? I did lower fan speed to minimum but the loss stayed constant.

    Give another setting to test. :)

    Once I establish ang average mpg for my commute I can try doing it within and without AC but I tend to run random errands which messes up the test.
     
  6. Danny Hamilton

    Danny Hamilton Active Member

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    Yes, there is, but it isn't directly controllable by the user. The A/C compressor is a variable speed compressor. The climate control system modulates the speed of the compressor based on the outside temp, inside temp, humidity, and solar input as measured by a sensor under the windshield.
     
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  7. Danny Hamilton

    Danny Hamilton Active Member

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    72 mpg at 65 mph for 1 hour
    65 mph X 1 hour = 65 miles
    65 miles / 72 mpg = 0.903 gallons

    62 mpg at 65 mph for 1 hour
    65 mph X 1 hour = 65 miles
    65 miles / 62 mpg = 1.048 gallons

    1.048 gallons - 0.903 gallons = 0.145 gallons per hour for A/C

    This 0.145 gal/hour is in line with the numbers I'm seeing from most of these "A/C effects my MPG" threads. Some a bit higher, some a bit lower, depending on various variables such as starting interior temp, cloud cover, outdoor temp, A/C set temp, humidity, and instantaneous vs trip average vs tank average vs multi-tank average.
     
  8. seilerts

    seilerts Battery Curmudgeon

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    It is the compressor that sucks the life out of the system, at least initially running on high, when it draws several amps at >200V from the traction battery via the inverter. After a few minutes, it will drop down to a lower speed. The fan is 12V so it can't do much worse than a couple hundred W.

    Cycling the compressor on and off during a trip is probably going to lead to higher energy consumption, since the compressor will only ever be running on high.
     
  9. bisco

    bisco cookie crumbler

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    what about when you run it manually instead of automatic? i have been testing this myself for the past week. with 200 miles on the tank i was averaging 60mpg. if i do my commute (7 miles one way) with a/c on, it drops to 59. if i do it with a/c off, it stays at 60 or goes back to 60 from 59.
     
  10. F8L

    F8L Protecting Habitat & AG Lands

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    Please keep in mind that I'm not suggesting that the AC drops mpg so drastically but it was rather interesting to see it drops by 10mpg every time it was turned on.

    Do you think it is worth trying the test again but after I have run the system long enough to cool the cabin down to the set temperature? I can also try a manual setting instead of auto.
     
  11. seilerts

    seilerts Battery Curmudgeon

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    You might try the reverse, have the a/c on for a good while, then see what happens to instantaneous mpg when you turn it off.
     
  12. mlg779

    mlg779 Junior Member

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    In the spring my average MPG was betwen 56 -58 MPG. Now in the summer in Georgia in over 90F constantly and MPG dropped at 52-53 MPG. My car stays in the sun. First I did not understand what is going on with the drop, but now I know why. I even tried 93 Gas, but nothing changes that drop in MPG.
     
  13. F8L

    F8L Protecting Habitat & AG Lands

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    Ok so I tested it again today and observed the following results:

    With the temperature set close to the outside temp (64deg outside 68deg inside) I tried cycling the AC on and off like I did yesterday and I only observed a 1mpg change. The change was very consistent. This is along the lines of what bisco is experiencing.

    So it appears that the larger the discrepancy between the temperature inside the car and what your AC setting is will determine the amount of MPG loss you experience. Once the cabin has reached your desired temp then the MPG should lessen to around a 1-2mpg loss. This is assuming freeway driving. City driving with the AC can experience other issues like the engine not turning off at stop lights as often as without AC. This could put an additional strain on the electrical system and result in further losses.
     
  14. Danny Hamilton

    Danny Hamilton Active Member

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    With an exterior temp of 64 degrees Fahrenheit and an interior set temp temp of 68 degrees Fahrenheit, your A/C compressor was just barely running (if at all) to control humidity. The climate control system wasn't trying to cool the cabin at all... it was trying to heat it! In this case I'm surprised you even saw a 1 mpg drop.
     
  15. F8L

    F8L Protecting Habitat & AG Lands

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    Aye, that is what i was pulling for. I wanted to see what happens when the compressor was barely working. I tried going to a lower temp setting but the cabin must have been much warmer than outside because when I dropped the setting the fan kicked into a higher flow.
     
  16. uart

    uart Senior Member

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    Wow that's almost exactly what I get mlg. :)

    I get around 56 to 58 MPG in spring and autumn and it drops to about 52-53 MPG in winter, especially if it's wet and windy. Same thing in summer, it typically drops from 56 -58 down to 52-53 once it gets really hot.

    Overall not bad MPG though. I can certainly live with it. :D
     
  17. F8L

    F8L Protecting Habitat & AG Lands

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    Agreed. The economic loss from the reduced MPG is chump change unless you drive 30,000 miles a year. :)
     
  18. F8L

    F8L Protecting Habitat & AG Lands

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    So does this large initial drops in mpg help make the case for the solar roof package? Not in economic terms but comfort and fuel saved.
     
  19. SageBrush

    SageBrush Senior Member

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    ^^ I"d instead argue for leaving the windows open a bit when parked ;)
     
  20. SageBrush

    SageBrush Senior Member

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    I have been mulling over the idea of a car cover that has a 3D stitch. It would reduce direct radiation on the car, and I could wet it down say 15 minutes before I intend to drive the car and let evaporation cool the car down, perhaps below ambient.

    I definitely have to replace my current front sunshield with one that covers the glass completely. The black dash is an amazing heat sink.
     
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