So...does using the A/C affect my mileage?

Discussion in 'Gen 2 Prius Fuel Economy' started by andreax716, May 5, 2008.

  1. andreax716

    andreax716 New Member

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    Sorry if this is a dumb question :confused::confused: I am a new owner of an '08 magnetic grey, and one thing I am always wondering is if using the A/C drains gas in my prius the same way it drained gas in my gas vehicles?

    My first tank avg. 42 mpg and I ran the A/C about 40% of the time (never on max)...I'm wondering if I will get better mpg if I don't use it?
     
  2. Bill Merchant

    Bill Merchant absit invidia

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    Not a dumb question, but not a simple answer either. Using the AC in your Prius will not hit your fuel efficiency as hard as using AC in a conventional car, but all the power in a Prius comes from gasoline so it will affect your milage. The Prius compressor is electric and runs off the HV battery, so you can get cooling with the ICE off. Also, it's a scroll compressor, so if you only need a little cooling, the compressor only works a little, rather than the full on/full off of a regular car's AC system

    NOT using your AC can also adversely affect your milage. Cabin air passes over the HV battery to cool it (or warm it in winter). The battery works best at temperatures you would be comfortable in. So if you think you'll get better milage by turning off the AC on that 90° F day, you won't. The battery will get hot and not put out as much electricity so the ICE will have to run more.

    The third thing to consider is drag. If it's 70° F and you're driving at 30 mph, turn off your AC and roll down the windows. But when you get on the interstate, turn on the climate control and close the windows for better fuel efficiency. The Prius has a very low coefficient of drag, with the windows closed.

    And welcome to PriusChat! :welcome:
     
  3. patsparks

    patsparks An Aussie perspective

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    Hey Bill, a top answer but I have a little correction.
    Scrolling compressors are used in automotive air conditioners a lot but the electric compressor in the Prius is supplied through the inverter with just enough power to maintain the cabin air temperature at the desired temperature all the time so when the air conditioner is on the compressor doesn't cycle like a conventional automotive air conditioner (where the compressor speed is determined by the engine speed) but the speed varies to maintain a stable temperature using the minimum amount of energy. This allows a Prius full cooling when stopped (normal systems don't) or at highway speed and with more consistent temperature control.

    This diagram is for a home air conditioner but it gives you the idea. No peaks and troughs.
    [​IMG]
     
  4. 9G-man

    9G-man Senior Member

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    Very, very interesting.
    I've never seen that.
     
  5. 9G-man

    9G-man Senior Member

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    As Bill pointed out, use of a/c has less negative impact on gas mileage in the Prius, than other cars. For steady speed highway driving it's use has little impact. An ideal set temp for the cabin is 77 deg. for a best balance of comfort and fuel economy.

    Use of a/c at lower speed inhibits you ability to attain and maxize glides, and stealth, if you are into that.

    But by the some token, when it is really hot, not using a/c can have a negative impact, because the batteries need to be kept cool, to allow extended stealth (electric operation.)

    Moral: to maximize your MPG, if it's not hot, don't use it. Just use vent air. If it is hot, use it, but set it to 77 deg for an ideal balance of comfort and economy.
     
  6. andreax716

    andreax716 New Member

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    Thanks for all the great info!!
     
  7. dwdean

    dwdean Member

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    Andreax,

    See the previous answers above to the "whys and wherefores", I want to put a little real-world practicality around this one.

    I live in south Florida. Today is a moderately hot day; the high will be around 85 F. Do I use the A/C? You bet (I've got it set to the mid-70's)!

    If doing that has put a dent in my mileage, I haven't seen it, my average right now is 53 mpg. OK, I know that's not anything to brag about compared to what some of the really serious FE guys are getting, but it's more than double what I was getting before (with our without the AC.)

    I haven't gotten brave and tried driving without the AC yet (I really don't want the car to start smelling like a gym.) So I can't say how big the AC "hit" is, but I'm happy with the FE that I'm getting with it running. Be comfortable and enjoy the new car.

    As an ex-pat Bay Area resident I know you're going to have less call for AC than I do now, so one of the AC "tricks" I've read about may help you somewhat. The trick is to set the AC about two degrees lower than the outside temp, that'll cause the exterior vents to open and you'll use the fan less. I tried that here, and well, setting it at 83 didn't do much for making the car comfortable. In your more temperate climate, it'd probably be OK. Can't really say if that impacts your FE any, but I think that there are threads on here talking about that relationship.

    Good luck!

    *****Edit added 05/19/08*****
    Experience with weather so hot that the AC couldn't keep up (i.e., the thing was running at Max for protracted periods of time) indicates that you can suffer a real FE hit. With outside temps in the 95-100 this weekend and the AC set to the standard 78, I was seeing about a 10% FE hit.

    Simply adjusting the set temperature up some took care of that and kept me comfortable at the same time, the AC just didn't "blow icicles". I keep forgetting that the AC on this car isn’t “set it and forget it” like other cars, you can actually change how hard the compressor works by adjusting the set temperature.
    *****End Edit 05/19/08*****
     
  8. morpheusx

    morpheusx Professor Chaos

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    If you are travelling at a steady speed with little to no stops you will notice practically no difference in your mileage. At stop and go driving you will take a good bite if traffic isn't flowing.
     
  9. mheinle

    mheinle New Member

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    What a great question! I too am a new owner and like a driver of a gas guzzler assumed the A/C would hit the MPG pretty hard... It is amazing how much info can come out of one not so stupid question.
    ~thanks
     
  10. berinker

    berinker New Member

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    OK, here is another stupid one. If I leave the A/C on in the manual mode
    (not auto) but it is set at a higher temperature than the outside air - such
    as start-ups in the morning so it is obviously not cooling - will it cut down on my mileage as if I manually switched it off?
    Bud
     
  11. Bill Merchant

    Bill Merchant absit invidia

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    The thing to remember is that the Prius has an automatic climate control system. If you set it manually, you get what you choose. If you set it to Auto, the car gives you what you want.

    Let's say you have chosen 75° F, Medium fan speed, and the AC compressor disabled; if the ambient outside air temperature (OAT) is 45° the heater will come on to raise the interior cabin temperature (CT) to 75° and the fan will run at medium, unfortunately, the windows start fogging from the interior moisture. You drive into the desert and the OAT is 95°; the heater stops since there is no need for heat, the fan blows at medium, and the CT goes up to 105° (solar loading through the hatch and windshield). You're sweating and the HV battery starts to overheat so it cuts power and you use more gas to move.

    Let's say you have chosen 75° F and Auto. The OAT is 45° so the fan comes on high along with the heater to quickly raise the CT to 75°; once it's there the fan slows and the heating is reduced to maintain the CT, the compressor runs slowly to remove the moisture and keep the windows clear, then shuts off. You drive into the desert, the sun is shining, and though the OAT is only 75°, the compressor starts to slowly run to keep the CT at 75°. As the sun climbs higher and the OAT goes to 95°, the compressor provides more cooling and the fan speeds up to keep the CT at 75°. Driving west, so the sun is pouring in the windshield, the air conditioning system switches the air vents to direct more cool air toward your face and the fan speeds up. The cabin air, flowing over the HV battery, keeps it running at peak efficiency.
     
  12. N3FOL

    N3FOL Member

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    At 77°, I'll sweat like a dog with the windows up :p. I never roll down my windows even when travelling at 30 mph. Vent is usually my first choice. Once it gets warm inside the cabin, I turn on the AC and set it at 68°.

    The first time I used my AC I thought it was going to stop working once the ICE turns off...but it didn't to my amazement. :cool:
     
  13. berinker

    berinker New Member

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    Thanks for the detailed information but you
    still didn't answer my question - I don't think?
    If it is cool outside (lower than the temperature inside)
    and I have the A/C turned on, you say the A/C air compressor
    will be off. Does that mean that if I switched the A/C to off
    would my mileage be the same as it was when it was on?
    I know that on some other (maybe older) cars, when the
    A/C switch is on the compressor is on all the time.
    Thanks,
    Bud
     
  14. JimboK

    JimboK One owner, low mileage

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    The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two diagonally opposed halfway down, and two 1" down.

    The attached chart shows results with 90F OAT, with and without ACC set to 75F (the temperature I find most comfortable in the summer), and all window position models. The models assume the following:

    • Steady speeds
    • Level terrain
    • No wind
    • 190' elevation
    • 30.00 barometric pressure
    • Smooth dry pavement
    • Michelin Hydroedges inflated to 50/48
    • E10 fuel
    • 300# load, driver + cargo
    The conclusions:

    • All windows up and no ACC gives the best results across all speed ranges -- for the brave soul who can tolerate it.
    • Two diagonally opposed windows 1" down is next best, and might be tolerated by some at high speeds.
    • Two windows halfway down is superior to ACC at speeds below the low 50s.
    • All windows down is superior to ACC below 40 MPH.
    Specific values of course will change somewhat with different combinations of typical summertime OAT and ACC thermostat settings, but I doubt the overall relationships change much.
     

    Attached Files:

  15. Sheepdog

    Sheepdog C'Mere Sheepie!

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    here's what I noticed. I drive to work and the avg mpg is at say 50.0 I park at 215pm and sit in the car until 230pm with the ac still on and the car in park. it may cycle to recharge the batteries once.

    By the time I turn off the car at 230pm to go in my avg mpg is at 49.8 !

    Seems like the gauge is telling me it matters!!
     
  16. Bill Merchant

    Bill Merchant absit invidia

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    As I previously said:
     
  17. abq sfr

    abq sfr New Member

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    If you want to see your mpg go down fast, leave your better half in the car on a hot day while you run in to Worst Buy and leave the AC on so she is comfortable. In only 10 minutes or less your battery will be down to the minimum with the engine coming on frequently to keep it above 40% soc, and when you drive away you will be on 100% gas engine for a while (and probably 20 mpg when accelerating) until the soc comes back up.

    Something else to know about the AC... the solar sensor on the left inside near the windshield. I was driving this weekend, there was blue sky and sun in between clouds, every time I went in the shadow of a cloud the AC blew slightly warmer, when in the sun, it blew cooler. I keep the thermostat at around 78 to 80, if you keep yours much cooler than that you might not notice. This is such an amazing car!
     
  18. maximus2112

    maximus2112 Junior Member

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    Well, the last couple of days (and the next couple of days) it has been
    99-102 in Raleigh, NC. I rarely go above the 2nd fan setting, but I had
    the AC blasting at MAX right after getting into the car. Needless to say
    I have been struggling to maintain my MPG. I'm struggling to hold at 47mpg
    right now. Its good to know it is not only the AC but the battery that is
    affect by the heat, not to mention the driver! :flame:
     
  19. bbald123

    bbald123 Thermodynamics Law Enforcement

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    My experience this winter showed to my satisfaction that turning the AC off on the climate screen, but leaving the system in automatic mode to heat the car as necessary, increase my mileage about 2%.

    The negative was no drying of the cabin air so condensation was an issue in some conditions. In the long run, as mileage improved with increasing temperature, I decided that leaving the AC on was an acceptably small hit to mileage and I leave it on at all times now.
     
  20. weatherguy726

    weatherguy726 New Member

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    I have a Gen III and, due to the recent heat wave, have been using my a/c fairly consistently. My fuel economy has been around 54 mpg. However, I filled up last week, and I am struggling to stay at 47 mpg, which is what I usually get in the winter. Could the gas be affecting the mpg? The temps have also surged from 90 to near 100 the past two days. The temp in my car is set to 73F.
     
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