what percentage does running the a/c decrease mileage and why?

Discussion in 'Gen 2 Prius Main Forum' started by bagwell, May 12, 2006.

  1. bagwell

    bagwell Active Member

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    I find I lose a significant amount of MPG on my daily commute with the a/c on in the hot afternoons versus in the morning when I don't turn on the a/c and wondered if the compressor is just an old fashioned drag on the ICE or is it sucking up electricity from the hybrid battery which would leave less juice for the elelctric motor.

    anyone know?
     
  2. efusco

    efusco Moderator Emeritus
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    Well, it runs directly off of the HV battery, but the ICE still needs to run more to recharge the energy drained from the battery.

    I can't give a % number too you. Too many variables. Outside temp, direct sun or cloudy, AC temp, fan speed, speed of travel, wind direction (determines how hard the car must work to maintain that speed), weight in vehicle, number of people in the vehicle (more people create more heat).
     
  3. Charles Suitt

    Charles Suitt Senior Member

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    <_< Lawyers would term this an "imponderable." Such as "High high is the sky?"

    There are far too many variables involved for a definitive response: Temperature, hills, road surface, varying speeds, use of brakeing, wind direction & velocity, percentage of ethanol in the fuel, how much coasting, type of accelleration, etc. etc.

    Perhaps some forum members can share their experiences and W.A.G. information. Sorry, I have no data to assist. Personally, I can't tell any appreciable difference with/without the compressor unit running. Also, this would be in warmer or hot weather which would frequently tend to increase the MPG's compared to cold weather in which the ICE might well run more to provide warmth thus reducing MPG's.

    :D :D Have to laugh at myself... All that text, and I really didn't say anything. And.... I'm not a lawyer or politician.
     
  4. NuShrike

    NuShrike Active Member

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    <div class='quotetop'>QUOTE(bagwell @ May 12 2006, 12:06 PM) [snapback]254254[/snapback]</div>
    What are the temperatures outside, and are you using AUTO? Generally, if it's not too warm outside with something like 90-100Fs, then AUTO and keeping your temperature around 78-80F will keep you cool enough and not impact mpg. Good IR tint also helps.
     
  5. qbee42

    qbee42 My other car is a boat

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    It takes power to run an A/C compressor. It makes no major difference whether the power comes directly from the engine as in a conventional car, or from electricity as it does on the Prius. Either way it is sucking up power and reducing your MPG. With thermodynamics, there is no free lunch: you want to move heat from the inside of the car to the outside, you gotta pay for it. Technology can produce more efficient compressors, but it can't defy the laws of thermodynamics.

    Tom
     
  6. tomdeimos

    tomdeimos New Member

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    AC runs on electricity. This comes from the gasoliine so it is costly as in any car. But the extra cost from running it can be anywhere from 1 mpg to 50 or whatever your normal mpg is without it.

    It is especially bad for short commuting after the car has beeen sitting in the sun. it takes lots of power to cool the car down.
    Other than that the cost depends on the amount of sun, the temperatures outside, what you set the inside temperature at, and how fast you are going. In a traffic jam at 0 mph 100% of your gasoline goes to cooling and you get 0 mpg when you aren't moving.

    So the added cost from AC generally gets less the faster you drive. With minimal use on a long trip, and going over 70 mph you can get to the 1 mpg cost level.
     
  7. darelldd

    darelldd Prius is our Gas Guzzler

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    <div class='quotetop'>QUOTE(tomdeimos @ May 12 2006, 03:07 PM) [snapback]254385[/snapback]</div>
    Sorry... by now you all know that I have no self control.

    The "cost" of AC in the Prius is NOT the same as in any car. Maybe the same as in any traditional/gasoline car. But when I use AC in my electric Rav4, all the power for it comes from my batteries which are charged from my roof-mounted PV system - so just like with traction power, the energy for my AC is basically free at this point. It sure as heck doesn't come from gasoline. (as a data point, the AC in the Rav can reduce the range by about 2-5%. But it sure doesn't decrease the gas Mileage. ) :)

    Obviously I know exactly what you meant... but hate to miss an oportunity to show that we could and *should* have options to powering everything with gasoline.

    But back to the discussion....
     
  8. EricGo

    EricGo New Member

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    I had to stop reading your posts to my wife, Darrell. She says you are a snot.
    Ah well. I'll just enjoy them quietly.
     
  9. Begreen

    Begreen Member

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    <div class='quotetop'>QUOTE(EricGo @ May 12 2006, 05:34 PM) [snapback]254448[/snapback]</div>
    Arogant and righteously proud maybe. But he is (s)not! Darrell points out that there is hope on the horizon.
     
  10. kirbinster

    kirbinster Member

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    Energy usage in for a/c in a new Prius (post '03) is strictly a functions of heat load. In a classic prius or any other non-electric a/c it is a function of heat load and vehicle speed. At low speed it puts a big strain on the engine and costs a lot, at high speed there is almost no strain on the engine and it is "almost" free.
     
  11. darelldd

    darelldd Prius is our Gas Guzzler

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    <div class='quotetop'>QUOTE(EricGo @ May 12 2006, 05:34 PM) [snapback]254448[/snapback]</div>
    A snot that can't spell too! Good lord. The brain gets there much faster than the fingers... Had I known that anybody was actually reading my posts, I'd have taken more time to make 'em more wife-friendly. ;)

    And speaking of AC... I thought my compressor crapped out on me today. Left my home at 87 degrees, and cranked the AC on... an hour later, the AC is most definitely off, and nothing I do will get it back on again. I finally lower the window and realize that it is 65 degrees in San Francisco! Brrr. The Rav won't let you turn the AC on if the compressor will freeze up in the ambient temp!
     
  12. ken1784

    ken1784 SuperMID designer

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    <div class='quotetop'>QUOTE(darelldd @ May 13 2006, 08:50 AM) [snapback]254426[/snapback]</div>
    Let me comment about this...

    The sunshine may be free, but I believe the LCA or Life Cycle Assessment is required to know the cost from the PV system.
    It costs and produce some CO2 to build PV system and to carry it to your house by truck consuming some fuel and etc.

    [email protected]
     
  13. edstewbob

    edstewbob Junior Member

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    My perception is that the A/C in my 2006 Prius impacts my mileage less than my prior non-hybrid vehicles but I have no concrete evidence. What I see happen occasionally is that the battery charge indicator goes green and I assume that at this point no more energy can be added to the battery so additional regenerative braking energy is lost. In the case of using the A/C more of this lost energy can be beneficially used to run the A/C. I don't know if this is true but it makes me feel good to think that it is.
     
  14. darelldd

    darelldd Prius is our Gas Guzzler

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    <div class='quotetop'>QUOTE(ken1784 @ May 13 2006, 07:06 PM) [snapback]254972[/snapback]</div>
    Work with me here, Ken. My point is that my GAS mileage does not suffer. When others speak of their mileage hit with AC useage, they aren't talking about the cost of the diesel fuel in the tanker truck that delivers the gasoline to the station, nor the incremental wear on the tankers tires, nor the lifecycle assessment of the gasoline station pumping equipment, the oil drilling rigs, the refineries, the delivery trucks, nor the cost of electricity used to pump the gasoline into their car, nor the electricity used to refine and pump the goo... blah, blah, blah...

    AC uses more gasoline in a gasoline vehicle. It uses NO more gasoline in my EV. That's it. End of snotty point.

    ( I would be MORE than happy to debate/compare the amount of C02 produced and amount of fuel burned in the delivery trucks between a PV array and 30 years use of gasoline. - but in another thread if you desire. )
     
  15. tomdeimos

    tomdeimos New Member

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    <div class='quotetop'>QUOTE(edstewbob @ May 13 2006, 11:02 PM) [snapback]255000[/snapback]</div>

    The cost of AC in a Prius is greater at slow speeds or stopped, less at high speeds, compared to gas cars with regular fixed output compressors. The Prius can pump as much or as little as needed independent of speed.
    A belt driven compressor will lose capacity at idle and therefore cost less in gasoline than in the Prius, but you will have more cooling power in the Prius. At high speed the opposite occurs and the pump wastes much of the work it is doing trying to pump more than needed. Prius compressor can still run at low output.

    They do have some belt driven compressors with variable output, but I doubt even they can adjust as well as the Prius variable speed compressor.

    The extra regen mentioned above could happen once in a while, but overall should have little effect for most of us. My car fully charged the battery or nearly so just once in the time I've had it.
     
  16. seasalsa

    seasalsa Active Member

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    <div class='quotetop'>QUOTE(bagwell @ May 12 2006, 12:06 PM) [snapback]254254[/snapback]</div>
    According to The Idaho National Laboratory ( http://avt.inl.gov/hev.shtml ) The Toyota Prius was over 20 percent less efficient with AC running.

    They have done a lot of testing on all types of hybrid vehicles and have a great chart on this subject that I will try to attach.
     
  17. hschuck

    hschuck Member

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    <div class='quotetop'>QUOTE(seasalsa @ May 14 2006, 08:04 AM) [snapback]255180[/snapback]</div>
    Strange chart. The average is shown as lower than all but one of data points.
     
  18. KTPhil

    KTPhil Active Member

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    20%? Wow, I only lose about 3mpg, so that's only 9%.

    They must run it on MAX COLD to get numbers like that, which bypasses the energy-saqving feature of the variable speed compressor. Sort of like saying a car gets bad mileage if it stays in first gear for all tests. People just don't drive like that.
     
  19. seasalsa

    seasalsa Active Member

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    <div class='quotetop'>QUOTE(KTPhil @ May 14 2006, 11:04 AM) [snapback]255248[/snapback]</div>
    Oh well, it is a government testing lab after all.

    However it appears that they test all of the hybrids to the same standards.
     
  20. Walker1

    Walker1 Empire

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