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How to Reduce MPG loss Because of A/C Use

Discussion in 'Gen II Prius Fuel Economy' started by F8L, Sep 11, 2011.

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  1. F8L

    F8L Protecting Habitat & AG Lands

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    Ok since the loss of MPG through the heavy use of the Prius A/C system has become such a big subject I thought it would be nice to have a thread that focuses on mitigating or reducing those losses. I have posted some basic ways in which you can reduce the losses but I am hoping those smarter and more creative than I can help me expand the list so that other Prius owners are able to benefit from our knowledgebase here at PC.

    Problem:

    Heavy A/C usage in hot climates and significantly reduce your average MPG. Why?

    • The A/C system is electric and therefor requires energy. The energy essentially comes from your HV battery which then requires recharging. The energy for recharging is supplied by your ICE which runs on gasoline.
    • A/C systems inherently run less efficiently in hot temperatures. The hotter the ambient temperature the less efficient the system is and the harder it has to work to reduce temperature inside your car (or house).
    • The temperature inside your car is often much warmer than the outside ambient temperature by 30 degrees or more depending on many factors like interior color, number of windows and their construction, etc.. The A/C system has to do a lot of work to reduce those temperatures to your desired temperature. Imagine trying to reduce the temperature inside your car from 130 degrees to 70 degrees. That is a lot of energy! Where does that energy come from? Ohh yeah, gasoline! The lower your temperature setting, the higher the energy required.
    • I have measured an 1800watt continuous draw for over 10min while the A/C tried to cool the car! Even after 30min the draw was over 600w. This was measured by Torque app in my GenIII.
    The video below is an example of how much of a MPG hit you can take when initially starting your A/C system and trying to reduce cabin temperature. This effect can stay active for 30 miles or more depending on the cabin temperature and your desired A/C temperature!




    Methods for controlling heat buildup in your car:

    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. Crack your windows a few inches to allow warmer interior air to vent outside the vehicle. Assumes you are parked in a safe area
    3. Tint your windows
    4. Use a high quality windshield sun shade. Priuschat shop has a very nice one!
    5. 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.
    6. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside.

    Best Practices for A/C usage and maintenance:

    1. Use the A/C sparingly or use the crossflow venting technique (see below)
    2. Crossflow Venting - Roll the driver side window down approx. 3" then do the same for the passenger side rear window. This will create a nice cross breeze and will not adversely affect aerodynamics in a noticeable way. In the event this is still not enough, roll the driver's window completely down and stick your arm outside the window. This allows wind to flow through the arm hole in my shirt and flow out through the other arm hole and the bottom of my shirt. This can dramatically add to the the cooling effect and is still more efficient than running the A/C , even on the freeway at 60mph.
    3. Use a setting that is high enough for maximum efficiency but makes you comfortable. 78 degrees seems to be sufficient for most drivers
    4. Ensure your A/C system is charged to spec levels and operating efficiently. i.e. condenser is clear of obstructions, bugs, etc.
    5. Use Eco Mode if your car is equipped with the feature

    These tips will not only help reduce the fuel economy hit of the A/C system but it may also extend the life of your HV battery by reducing interior temperatures while the car is sitting in the parking lot. If your interior is 130F then your battery may be too!

    When outside temps are over 100F it is recommended to run your A/C on longer trips to try and cool down the HV battery. I've measured a 15F difference between ambient temps and battery temps when the A/C was running. After turning off the A/C the battery quickly returns to near ambient temperatures. Fuel economy is important but your bodily well being and HV battery life are more important.
  2. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    Just a comment that since the battery pack is also using the cabin air to cool the HV Battery during use, just 'toughing it out' is not that correct way to reduce A/C use.

    Lower MPG is cheaper than a HV Battery.
    imOCD4a_prius and secondspassed like this.
  3. SageBrush

    SageBrush Senior Member

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    And to futher Jimbo's comment, a hot traction battery is less efficient -- all the more reason to not sit in 90+F interior car cabin temperatures for periods more than a few minutes when say the car is being vented at the start of a drive.

    AC is also (at least for me) a safety issue on long drives. I get sleepy in hot temperatures.
    dbcassidy and imOCD4a_prius like this.
  4. F8L

    F8L Protecting Habitat & AG Lands

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    Sure but I don't want to be responsible for someone fainting due to heat exhaustion and crashing. :)
  5. SageBrush

    SageBrush Senior Member

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    Good advice, thanks!

    Where is the condenser ?
  6. SageBrush

    SageBrush Senior Member

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    lol

    A couple years ago I remember a guy posting that he would liquid cool his body with an ice pack in GA summers to avoid using AC.
  7. F8L

    F8L Protecting Habitat & AG Lands

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    In front of the radiator and probably full of bugs! lol
  8. SageBrush

    SageBrush Senior Member

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    I got it -- "just look for something really gross." :)

    If I remove the engine shield can I reach down to clean it off ?
  9. F8L

    F8L Protecting Habitat & AG Lands

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    Possibly. You can also remove the plastic lower grille if needed. It pops out but I wouldn't recommend it if you don't have to. It's REAL easy to break the clips. Where is Patrick? He will have a suggested method for cleaning. :)
  10. lech auto air conditionin

    lech auto air conditionin Member

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    It's easy, so simple its stupid, yes all the above things are good, but some people just can't live 10 min with out A/C. You could start out driving your first 5 min with all your windows down if your interior of your car is 120F to 140F then turn on then A/C. It takes much less energy to cool a 90F interior then it dose a 120+F .

    Back to basics A/C 101, the tiny super high efficiency A/C in the Pruis is working on the principal of ( The straw that brakes the camels back ). If the Pruis is just 2oz to 3oz under charged or over it will keep running all the time trying to cool but it can because of the under or over charge, it will suck down your battery in no time flat.

    All the other good things you are doing to try to keep your mileage up will get eaten up by a poor preforming A/C. You can preform all the test you want but if your A/C is eating up your battery you will get false data. Garbage in garbage out. later I will post more photos of burnt up Prius compressors and parts.
    pgannon and Jenpen like this.
  11. 2009Prius

    2009Prius A Wimpy DIYer

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  12. dhanson865

    dhanson865 Expert and Devil's advocate

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    I've been in the habit of setting my temp for 65 degrees and turning the AC on and off manually instead of setting it for 70 and letting the computer mix the air.

    There is a thread here on priuschat that mentions better MPG at lower requested temps with AC but I started doing this on conventional cars years ago.

    I'm going on the theory that it's better for the system to work full power then rest several minutes (when I can do something like 5 mins on 10 mins off). If the time on is more than the time off I let it run until my knuckles are freezing and then turn it off as long as possible.

    Then again I got out of my car into a 76 degree garage and thought it was cool in the garage so I must be leaving it rather warm in the car even with my normal AC usage patterns.

    fwiw I got about 65 MPG on a recent trip doing this, no ABA testing or other hard data. Just my anecdote.

    Feel free to bash my theory if you think it isn't sound.
  13. zenMachine

    zenMachine Just another Onionhead

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    When parked in my garage I always roll down all the windows. Even the garage can get really hot in the summer.
  14. zenMachine

    zenMachine Just another Onionhead

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    When the interior is really hot after sitting out in the sun for a while, I always drive for the first few minutes with A/C on low speed (manual mode) and all windows rolled down. This seems to help circulate the hot air out quickly while keeping myself cool at the same time. It also gives the engine time to warm up without burdening the electrical system with too much A/C requirement at the start.

    Btw, rainguards are extremely helpful; you can crack your windows open to allow some ventilation while parked.

    You can also add insulation material to your doors. I even have a layer above the headliner (but that's a little extreme for most people).
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  15. F8L

    F8L Protecting Habitat & AG Lands

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    FWIW, I am observing this big drop in my GenIII as well although it does not appear to last as long or it is because I have not been testing in 100F temps yet. :)
  16. dianeinreno

    dianeinreno Member

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    For me, when the temps get much above 84 or so I crank that aircon down to 72 and enjoy the cool air. You know, if I am stuck with "low mileage " of 35 mpg or so ... I can live with that I think. No way am I going to sit , sweat and be miserable while I "enjoy" the 75 cents I saved while on my way to work today.
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  17. ProximalSuns

    ProximalSuns Senior Member

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    There is a short version.

    1. Buy a white, heat reflective car.
    2. Purchase the solar sun roof option which will keep car at ambient temp.
    3. Keep the sun roof shade closed.
    4. Get in, turn on the AC and stay cool knowing you are getting great mileage, better than the 99%.

    We already did our human sacrifice by paying the premium for the Prius. Time to enjoy the cool karma we've earned.
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  18. F8L

    F8L Protecting Habitat & AG Lands

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    Then this thread is not for you.

    Personally I don't sweat with the AC set at 78F but everyone has different tolerance threshholds. It helps being thin too.
  19. ProximalSuns

    ProximalSuns Senior Member

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    I'm with you though at 84 I'm likely still having the windows open.

    Does your mileage really drop to 35 mpg? I didn't notice much mileage drop at all in my Escape Hybrid with the AC on, hoping I get same small to none result with the Prius.
  20. dianeinreno

    dianeinreno Member

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    I really haven't kept too close track , I've been using the AC and my feeling is that my mileage on short (5-7 mile) in-town trips drops from mid-low 40's to high 30's / low 40's. On longer trips I don't think that you lose more than 5 to 7 mpg give or take. Considering my last car was a Vette that got 16 mph in-town I feel pretty good about mid to high 30's (if it even dropse that low) :)

    Technically we all know that turning off the A/C and keeping the windows closed will give you the best mpg - but is that really practical? For some I suppose it is. For the extra 75 cents or a dollar that I might spend (if that) to be comfortable that day - I just think it helps not being too anal about the mpg when it comes to the a/c.
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