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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. uh60james

    uh60james Active Member

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    When turning the A/C on at idle I notice a rise in RPM's, however when driving along at 65mph I can turn the A/C and the tachometer will not move at all, the same thing goes for turning the A/C off while at speed. This leads me to believe that the only time the A/C affects gas mileage is when sitting at idle at a stop light, parked with A/C on, etc. Anyone know if this is true or not?
     
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  3. 410Fortune

    410Fortune Snow Season Staff Member Moderator

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    yes

    when the clutch on the ac compressor locks in the pulley is no longer free wheeling, it def robs some power (or more like takes power to turn)
    the rpm's only move at idle because the computer adjsuts for the added drag you just put on the serpentine system.

    Any pulley on that belt will use some potential power, if that pulley is used to pump or generate it will take even a bit more.
     
  4. shamaal

    shamaal Well-Known Member

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    At the lower RPM not enough energy is stored in the flywheel. When the compressor clutch kicks on, the PCM senses this and ups the rpm at the idle speeds. At higher rpm there is sufficient inertia that the rpms don't noticably drop on the tach. At WOT the compressor clutch opens.

    As 410 says, yes, any accessory extracting power from the engine is preventing that power from reaching the drive train.
     
  5. KPT

    KPT Well-Known Member

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    WTF? Flywheel has zero to do with the ac compressor, they are on opposite side of the engine. Flywheel doesn't store energy either.

    I loose 1.5-2 MPG with the ac on.

    When I turn the ac on at idle, the RPM's will rise slightly and then drop back down to normal.
     
  6. aldive

    aldive Elite In Memoriam

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    I loose no MPG with the AC on during highway driving.
     
  7. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    Sure about that??

     
  8. uh60james

    uh60james Active Member

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    Thats what I assumed to be true. It only affects idle consumption.
     
  9. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    I don't believe that's possible. The A/C compressor has two modes. One with the clutch disengaged and one with it engaged. There is NO WAY that the compressor requires the same HP to drive an engaged compressor as it does to turn the freewheeling pulley on the end of the compressor. The engine will use more fuel and you will have less available HP at the rear wheels with the compressor simple physics.
     
  10. KPT

    KPT Well-Known Member

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    I stand corrected. The flywheel does store some energy. It is more of a issue on smaller engines. The flywheel has stored enough energy at low rpm's though. If it did not, the engine might possibly stall when you turned the compressor on at idle.

    And yes you will lose MPG with the ac on at highway speeds. It is less than around town driving, but it is still at least 1 mpg.
     
  11. aldive

    aldive Elite In Memoriam

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    I beg to differ. I have tested this many times.
     
  12. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    Physics fail to apply to your vehicle Al?
     
  13. gensic11

    gensic11 Guest

    study has shown it does affect MPG but it's very minimal (as long as there are no other variables).. windows down cause an aerodynamic drag that's almost worse than having a/c on..
     
  14. Glacier991

    Glacier991 EF Tranny Guru Moderator Emeritus

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    I have read a study somewhere that showed the aerodynamic drag by windows down was a greater energy robber than AC with the windows up. I'll see if I can find it... but I swear I read that somewhere. Doesn't make it TRUE, but it looked legit to me.
     
  15. Rick

    Rick Pumpkin Pilot Staff Member Admin Elite Explorer

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    Mythbusters tried it and the vehicle with windows down got better economy than the vehicle with the A/C on and windows up. Their test used identical vehicles with the fuel line attached to a graduated cylinder filled with fuel. Others have mentioned that Mythbusters tested at 40mph and that the drag from rolled down windows wouldn't become apparent until 50mph or greater.

    However if you are just testing the parasitic effect of the A/C compressor at highway speed you would need to run the no A/C test with the windows rolled up to eliminate that variable.
     
  16. Glacier991

    Glacier991 EF Tranny Guru Moderator Emeritus

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    I've been a pilot for almost 40 years, so have had an interest in things aerodynamic for a long time. I had a college buddy studying to be an Aerodynamic engineer. I learned a lot of neat stuff through him as a result of my interest and his chosen field. I was very interested in parasitic drag and icing potentials related thereto. One thing I remember learning was that parasitic drag does not really start to mean much until a certain airspeed is reached, and it increases exponentially thereafter (well maybe not exponentially but not linearly). 40 MPH is not a fair test of parasitic drag from windows down on a car IMHO. I think what I read was a test done at freeway speeds. Around town I think there is no doubt AC uses more fuel than windows down. In a 500 mile interstate trip... I doubt it does. But then, what the h*ll do *I* know.

    ps. it was 102 and muggy in Sacramento today. I drove around town. Wanna guess which I chose? AC or windows down?

    DING DING DING we have a winner!
     
  17. aldive

    aldive Elite In Memoriam

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    This is true; I have tested it. However, only at 70 MPH in my testing.
     
  18. aldive

    aldive Elite In Memoriam

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    That is exactly how I have evaluated it.
     
  19. Homerexplorer

    Homerexplorer Active Member

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    Glacier991 is correct - drag is nearly linear at low speeds where the Drag Force is =~ -b*v, where b is a constant dependant on the fluid and v is velocity. At higher speeds, it does increase exponentially where drag force = -0.5*p*v^2*A*Cd. The v^2 indicates the exponential effect of speed on drag. I'm not sure of the transition point for an Explorer, but 40 mph definately qualifies for the high drag equation. Stick your hand out the window and this will give you an idea of how drag force changes with speed.

    When the AC is on, it has to have a negative impact on gas mileage, otherwise it defies every law of physics. Whether or not one is using equipment sensative enough to detect it is another story. As we all know, the compressor has to do work in order to recompress the fluid. It gets this energy from the engine, therefore some energy that would be used to move the wheels is now going to run the compressor. Unfortunately, you can't get something for nothing - you can only use as much as you generate, no more.

    Whether or not it is more efficient to run AC or have the windows open is going to depend on the vehicle drag properties and the evaluation speed. Assuming temperatures and humidity, etc. are all constant, the energy required by the compressor will be constant at all speeds (independent of speed). Drag, on the other hand, is going to increase with speed, so the net impact of having the AC on is going to decrease with speed. That's why you are going to notice a greater affect at lower speeds than highway speeds. Either way, I think most people would rather be comfortable than slightly more efficient.
     
    Last edited: July 22, 2006
  20. aldive

    aldive Elite In Memoriam

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    Your theory is sound. The ability to measure minute changes in gas mileage would require some elaborate testing equipment. In all of my testing by the usual method of calculating gas mileage, I have not observed any difference. I have made many long 300-500 mile highway runs testing this.

    Have you actually tested this?
     
  21. aldive

    aldive Elite In Memoriam

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    No, however real world testing does.
     

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