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MPG w/ AC Question

Discussion in 'Gen 2 Prius Main Forum' started by ack714, Jul 3, 2011.

  1. richard schumacher

    richard schumacher shortbus driver

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    The dealer probably saw the tires hot and reduced their pressure to the factory value. Naturally when they cool down their pressure drops further.

    I get 46 MPG in mixed driving in Dallas at 100'F+ and the A/C blasting at all times. Squishy tires do more to reduce my MPGs.
     
  2. ack714

    ack714 Junior Member

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    Update:
    My son started school again last week and we are putting 90 miles a day on the car once again. Now my MPG is around 41 in the heat with AC blasting.
    I thought it was opposite; that mileage was better in the city. This drive is 23 miles each way 16 HWY and 7 stop and go. Thanks again!!
    Im happy just updating.
     
  3. F8L

    F8L Protecting Habitat & AG Lands

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    MPG loss due to AC usage is minimal on longer trips but on short trips where ambient temps is much hotter than your desired temps can be much higher. City trips with a lot of stop and go can really hurts mpg as well. This is especially true when it's 100 degrees out and you are trying to keep the cabin at 76 degrees.

    To Minimize the mpg hit on short trips I just drive with windows cracked or down all the way. The mpg hit is less than with the AC on during short trips.
     
  4. sub3marathonman

    sub3marathonman Active Member

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    Danny provided a great example. However, one question I have is the "0.2 gallons per hour" an actual observed value, or an example of a possible value?
     
  5. Danny Hamilton

    Danny Hamilton Active Member

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    It was just my guess at a reasonable sounding number. The math demonstrates the same effect whether the A/C uses 0.02 gal/hr or 2.0 gal/hr. The point is that higher speed travel sets up a larger numerator in the ratio, so the increase in the denominator has a smaller effect.

    In reality the actual value of gal/hr used for A/C varies significantly and depends on many factors including but not limited to:

    starting interior air temp of the vehicle
    exterior air temp
    starting temperature of various surfaces in the vehicle
    cloud cover and other shade
    temp set on the AUTO A/C system
    apparent wind speed
    altitude of the sun
    humidity
    number of people in the vehicle

    In other words given the 2 options:

    1) Driving alone in a garage parked white car with a white cloth interior at 80 MPH on a tree lined expressway in Maine during a cloudy 80 degree Fahrenheit morning with a 10 MPH head wind, 50% relative humidity, and the AUTO temp set at 75.

    2) Driving 3 people in a lot parked black car with black leather interior at 20 MPH on an unsheltered road in Brazil during a clear 105 degree Fahrenheit afternoon with 10 MPH tail wind, 90% relative humidity, and the AUTO temp set at 65.

    Option #1 will use much less gal/hr than option 2
     
  6. lech auto air conditionin

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    Yes the A/C will use a little more gas, but the biggest effect will be the lower your A/C refrigerant charge is the more AMP's your A/C compressor will use, the longer it will stay running to cool the car, the high the RPM's the electric compressor will spin = the more AMP's will be drawn from the battery = the more gas used!. this also true if the A/C refrigerant is over charged just 1 or 2oz will make a big difference and the hotter it is out side the bigger the difference!!!!. On a cool day or vary cold day not much difference.
     

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  7. F8L

    F8L Protecting Habitat & AG Lands

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    I'm happy to see an A/C tech in here. You can help confirm my A/C and MPG tests. :)
     
  8. lech auto air conditionin

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    YES you are right ( well operating, properly charged, correct oil in the A/C, no missing air dam-seals underlip of hood front or rear, air deflector under car ) has little effect on mileage. Compared to opening the windows @ 65 MPH that has a bigger reduction in MPG. Oh just a little known note just as if you use 20/50 oil in your Prius will lower your MPG, think about the universal A/C hybrid oil that is thick 100 viscosity like gear oil when the compressor requires a oil as thin as water.
    Ill put another not so common but large impact on A/C performance and MPG!!, this has caused many untold wasted hours trying to find both low A/C and MPG. ( #1: CHEEP non-oe after market A/C condenser installed after a front end accident. Will send the high side A/C pressure from the normal 100psi - 120psi to a high of 150psi - 220psi iv seen this many times. High head pressure= high AMP draw from your battery ( lots of AMPS all the time ).
    #2: After market CHEEP front wind shield with no IR (infrared inhibitor ) this is the part of the sun light you can not see but causes the heat you feel on your face as the sun strikes you. Some NON-OE glass are good, some not. That large back sloped front glass on the Prius lets lots of sun in to hit the interior = lots of BTU's = a massive amount of heat to remove by a vary small A/C evaporator. It just cant do it, I can explain more but i,ll keep it short. I think you get the point. How many TECH scratch their heads, kick them selfs in the but, pull out hair, replacing parts, adjustments, preforming A/C recharge, but nothing helps!!!:eek::eek::(:(
     
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  9. F8L

    F8L Protecting Habitat & AG Lands

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    On the first part I have to disagree. If the ambient heat and that inside the cabin are very high then the loss from driving with the window down is MUCH less than that if one used the A/C instead. The caveate to this point is that for very long trips say over 50 miles, the A/C will eventually cool down the interior to a point where the compressor can wind down and the MPG hit will be only about 1-3mpg. Prior to that the hit is very substantial and will remain so until the cabin is cooled down to the desired temperature. I've monitored this over the last couple of months, in 95+ degree weather, and even after driving for 30 miles with the windows down and activating the A/C the large mpg hit remains consistent over the next 30 miles. I will admit to not having my A/C tested BUT when ambient temps are closer to the desired A/C temp setting, the MPG hit is only about 1-2mpg vs. 10mpg.

    When testing MPG with windows up and windows down I could never determine the mpg hit because it is so low. On a very flat surface traveling at 65mph I could not weed out the mpg hit because minor fluctuations in mpg created too much noise in the data.

    Regarding your explanation in #2, I totally agree. Using a windshield heat barrier like the one offered at the PriusChat Shop helps a lot. It keeps my interior much cooler than without it. I even created a list of practices one could use to reduce the MPG impact of the A/C system.


    1. Park in a shaded area. Under a tree may help but a better tactic is to park on the eastern side of a large building. As the sun moves to the west in the later portion of the day, the building will block the sun from hitting your car. With your windows cracked, the heat from the earlier part of the day will dissipate faster than if you parked your car in the full sun the entire day.
    2. Tint your windows
    3. Use a high quality windshield sun shade. Priuschat shop has a very nice one!
    4. If you cannot park on the east side of a building try parking your car facing the west and use a quality windshield sun shade. This will reduce the number of windows directly facing the sun and thus not allowing as much of the suns radiation to enter the car.
    5. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside.
     
  10. SageBrush

    SageBrush Senior Member

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    Cheers, Justin!

    My wife and I have had the following conversation more than once:
    me: why is the car parked to maximize cabin temperature when I use it ?
    wife: What do you mean ? I parked in the shade !
    me: Arghhhhhhhhhhh

    Anyway, I tend to park facing south if I cannot plan for shade when I next use the car. That way most of the direct radiation is blocked by the sun shield, and the east/west sunlight is partially blocked by window tinting. If I routinely had to drive away in a car that had baked in the summer sun I would put up with the hassle of a whole-car cover.

    Leaving the windows cracked open for ventilation is key.
     
  11. F8L

    F8L Protecting Habitat & AG Lands

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    LOL thanks!

    You bring up a good point and one I mean to start using. I currently park facing south and the whole side of my car is expose to the afternoon sun. If I started paring facing the west I could reduce my profile and the only window facing the sun is the windshield. While the windshield is the largest window and it is not tinted, I have an awesome sun shield that will block much of the suns energy from entering the car. I'll have to add that to the list! Doh! I just looked down in the thread and noticed I already did. lol
     
  12. SageBrush

    SageBrush Senior Member

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    Are we using the same orientation ? When I say 'facing' I mean the front windshield is the face ;)
     
  13. F8L

    F8L Protecting Habitat & AG Lands

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    Yup. :) Southwest would be the best facing overall.
     
  14. SageBrush

    SageBrush Senior Member

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    SSW - SW sounds like a good rule-of-thumb for afternoon planning, although the amount of direct radiation from the west is latitude dependent. I tend to err on the south side, because 10:30 to 15:30 are the hours I am most keen to avoid cabin super heating.

    Obsessive-compulsives might enjoy planning with a sun chart (I do).
     
  15. F8L

    F8L Protecting Habitat & AG Lands

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    Damn good point! My suggestion wouldn't work for someone like PatSparks. lol
     
  16. SageBrush

    SageBrush Senior Member

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    Pardon the foray into geekiness ...

    Radiation is a sine function through the day, so there is something to be said for hedging the peri-noon hours unless you know when you are going to use the car -- e.g. at the end of a work-day.
     
  17. lech auto air conditionin

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    I did leave out the part about using the windows first to cool the cabin is better then using the A/C, I put that in another post.
    #2 I was not talking about a sun block or reflector. I'm talking about when you crack your wind shield and have it replaced. In the glass there is a IR inhibitor, it's invisible to the human eye. On some cheap NON-OE windshields its not pat of the glass formula. Cut cost.
     
  18. F8L

    F8L Protecting Habitat & AG Lands

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    I didn't know that some windshields didn't have the IR inhibitor although nearly all glass blocks some or a lot of UV wavelengths.