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Does the A/C actually cause poorer gas mileage?

Discussion in 'A/C & Heater systems - HVAC' started by uh60james, July 21, 2006.

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  1. Homerexplorer

    Homerexplorer Active Member

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    Not the effect of AC vs. windows down. Yes, I have studied and tested the effects of velocity on drag under different conditions. The nice thing about the physics involved is that they apply across a broad range of circumstances.

    Long winded as my answer was, the purpose was to explain why one would notice a larger difference at lower speeds, not necessarily to question your real world experience. No intent to offend your findings.
     
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  3. aldive

    aldive Elite In Memoriam

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    None taken. I apprecialt your intellegent input.
     
  4. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    Al, I told you you should get an independent verification of your incredible fuel economy feats. Since you have what appears to be a one of a kind vehicle I think the auto manufacturers would love to find out why you get approx 10 MPG more than others with the same vehicle.... Now a couple MPG may be attributed to driving style, but 10MPG?

    Even EPA test cycles which do not use A/C, and average 48 MPH with a max speed of 60 MPH can't come close to your 30+ MPG claims.

    Sorry to be such a PITA, but claims like yours need to be verified.
     
  5. aldive

    aldive Elite In Memoriam

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    Thank you for your opinion.
     
  6. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    No problem... so when are we going to have the EF fuel economy proof in the pudding run?
     
  7. 410Fortune

    410Fortune Snow Season Staff Member Moderator

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    Holey Moley! Ask a simple question....
     
  8. Jason94sport

    Jason94sport Well-Known Member

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    On a stock 1st gen explorer you will loose MPG with the AC on & at HW speeds. I've had 2 & I've tested them on 1200+ mile road trips.
    You are talking about a 12+ year old vehical. A 160HP one that weighs 4K lbs. AC increases the engine load, now you need to compensate by more gas, more gas = less MPG. Anything that old you can't compare to newer cars, or freaks of nature like aldive's X.
    It does depend on the car. If you have 300hp the AC isn't going to make a difference. My acura that has 300hp gets the same milage with or without the ac. I've had other new cars that didn't see a difference & some that did.
     
  9. sn0border88

    sn0border88 Master Apprentice Elite Explorer

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    Real world testing fails to apply to your vehicle?

    May wanna reword that.

    But I will take real world physics over your testing, sorry to say. Would it be fair to say that the difference in consumption with ac on/ off is small enough to have slipped through your calculations.
     
  10. aldive

    aldive Elite In Memoriam

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    I already posted that the measuring w/o elaborate testing instruments might not be able to discern a difference. See post # 19.
     
  11. Homerexplorer

    Homerexplorer Active Member

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    I thought about this more and re-read my original post - my post was intended to support your findings (to some extent), but may not have been as clear as I originally thought so let me try again...

    The the 3 possibilities being discussed
    (1) windows up w/out AC
    (2) windows down w/out AC
    (3) windows up with AC

    Option (1) will always be the most efficient because drag is minimized and the compressor is not drawing any energy from the the engine. I don't think anyone is arguing this point.

    On to the bone of contention: which is more efficient (2) or (3)? It depends on speed. Remember, drag is exponentially increased as speed increases so the faster you go the more drag you experience and the more engine work you lose to drag. Opening windows increases drag - I hope everyone can agree on this so I won't go into it. Suffice to say that the increase in noise when the windows are down is an indicator that you are losing energy to drag.

    Unlike drag, the energy required by the compressor is (for all intents) constant, unaffected by speed. At some speed, the energy required to run the compressor is the same as the energy lost by the increased drag of the open windows (remember drag is increasing and compressor energy is staying the same). At this speed, neither open windows nor AC are any more efficient than the other. As speed increases beyond this point, drag causes more wasted energy than the compressor and the AC (3) becomes more efficient. At what speed this situation occurs is completely dependant on the car, and will change from car to car - ie, there is less drag on a Formula 1 race car at 70 mph than there is on the Explorer (pretty close to the drag characteristics of a 4x8 sheet of plywood) ;) . That being said, Al may have found a speed (70 mph) at which the compressor "drag" is less than or equal to the window "drag." To compare his results to those of a '93 Explorer are like comparing apples to oranges - different cars, different drag coefficient. If one is waxed, that too, affects the drag and would yield different results.

    I'm very surprised at the conclusions that have been drawn by some of these high profile tests (specifically, Consumer Reports and Mythbusters). I like both of them, so I was really surprised that neither of them made this distinction and generalized their findings across all cars at all speeds. Their findings will only pertain to the car they tested, at the speed they tested. I hope this explanation makes that apparent. If one knew the actual energy required to run the compressor (can be calculated, knowing the efficiency of the compressor, etc) and the drag of the particular vehicle, one could determine the speed at which the two are equal.

    Hope this makes sense. Great discussion.
     
  12. shamaal

    shamaal Well-Known Member

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    Not derail the thread by returning to the original question ...

    I think uh60james was speculating that because the tach dropped when the A/C was at idle and the A/C clutch engaged, and it did not appear to change when the when the engine was at a higher speed and the A/C clutch engaged, then the A/C engaging only affected gas milage at idle.
    At idle, my car dips 200 rpm at idle before coming back up.
    I believe that the stored engine energy in the flywheel, drivetrain, accessories, etc; at higher (2000) speeds prevent the compressor load from showing on the tach. The load on the engine is the same.
    Not being a racing person, do race cars have A/C, other than the ones with open cockpits of course?
     
  13. Homerexplorer

    Homerexplorer Active Member

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    To answer this question directly: Having the air conditioner on will always be less efficient than having it off, at any speed, at any time of the day, any day of the year, any temperature (assuming the AC system is working) ;) . No matter what, the AC system requires energy to operate and hence requires gas to run it - indirectly but true none the less.

    As was said before, the relative drain on the system at idle is larger than at highway speed, that's why it is more noticable (tach moves). At highway speeds there is enough energy being produced/stored where the drain from the AC is not really noticeable.
     
  14. Jason94sport

    Jason94sport Well-Known Member

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    On a low HP, high weight car you will notice it.
     
  15. Homerexplorer

    Homerexplorer Active Member

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    This just shows than none of have read your thread closely. Your gas mileage when sitting at a stop light or parked is 0.0 mpg, no matter what is turned on or off, including the engine :) . Problem solved.
     
  16. Homerexplorer

    Homerexplorer Active Member

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    Point taken. I guess it will always be "noticeable", it just depends on how closely and what instruments you are using for your observation.
     

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