How to Reduce MPG loss Because of A/C Use

Discussion in 'Gen 2 Prius Fuel Economy' started by F8L, Sep 11, 2011.

  1. nh7o

    nh7o Off grid since 1980

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    When I have a big hill to go down, one that will put the battery gauge up to green and more, I put the A/C to max and really chill things down, thanks to gravity power. Then at the bottom I set it lower again.
     
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  2. 2009Prius

    2009Prius A Wimpy DIYer

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    Good idea! :)
     
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  3. F8L

    F8L Protecting Habitat & AG Lands

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    This would work when you are not trying to glide or you have been gliding but now need to regen to keep your speed down. Otherwise it would be better to continue gliding and sweating instead of allowing the car to regen thus decreasing the distance you travel without fuel use. :)
     
  4. SageBrush

    SageBrush Senior Member

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    I think Nh7o was describing a condition I also encounter: my home is at 6300 ft elevation, and I often take routes that drop me 1000+ feet in the first five miles. If I coast down the hill the speed is OK, but the traction battery fills to max and then the ICE comes on to waste electric energy. Since that energy is not going to kinetic or chemical energy regardless, I use it for cooling instead.
     
  5. F8L

    F8L Protecting Habitat & AG Lands

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    I agree completely. I'd be cranking up the AC too! :)
     
  6. Oldwolf

    Oldwolf Prius Enthusiast

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    I guess I already use some of the efficient methods discussed in this thread. When my interior is hot, I ride with the windows down to exhaust the hot air and start the cool down of the interior surfaces. Then about 5 minutes into my drive I will turn on the A/C and roll up the windows.

    This all goes to the wayside if my wife is with me though. About 30 seconds of cool down and then the A/C is on, windows closed, so that her hair doesn't get fussy! o_O
     
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  7. borntexan76

    borntexan76 New Member

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    I've had my Prius a couple of months and recently had the 5k service. I saw a big drop in mileage driving to/from Dallas from my home in Houston with bikes on the rear. What had me worried was when I filled up recently and restarted the mileage - 40 or so miles into the new tank I'm getting less than 35 mpg despite my best efforts to go easy. Glad to hear its just the 95 degree temps eating my gas and not something the dealer did. We averaged 47 on the highway across West Texas, NM, AZ and a good bit of desert California and back. Really disappointed til I read this thread to find its just a/c use. I guess it'll get better come October...
    I know a lot of you are going to get another taste of Houston summer in your neck of the woods. Be glad that your mileage is nearly free and you are just paying to run the a/c. I remember my first car only got 4 mpg and had no air conditioner.
     
  8. edthefox5

    edthefox5 Senior Member

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    35 mpg's is not right. Something really wrong there. Unless its all in town stop & go. I use the ac constantly never off and traveling 60 miles an hour get 47 always. Be aware when dealer serviced they deflate the tires to door plate level usually 35 lbs. That will kill mileage. Inflate your tires to 42F-40R. Also make sure you use regular gas also 87 octane only.
     
  9. FL_Prius_Driver

    FL_Prius_Driver Senior Member

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    Those with a Gen II or III Prius need to see the effect of AC on Gen I to know how good they have it. The Gen I had a belt driven AC, requiring the engine run for max AC performance and the compressor had to run at engine speed, not optimum speed. The difference on the Gen I could easily be ~8 MPG. The difference on the Gen II/III would be closer to ~2 MPG for "rational" AC use.

    The missing part of the optimization discussion is what to do for thermostatically illiterate AC operators. (e.g. Those that believe AC only has two modes OFF and SPEW ICE......where "on" can only be max fan, lowest temp, and any other setting that uses more gas.) It that case, make the fan run at full speed while sneaking the temp up from 65 to 75.
     
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  10. F8L

    F8L Protecting Habitat & AG Lands

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    Like stated above, the mpg hit is not that big unless you are constantly making short trips which require the AC to work hard constantly. On longer trips the AC hit is lower because the system only has to work hard once over a larger number of miles.

    For example, If you drive 100 miles during 1 trip the AC hit may be large for the first 10miles or so but then it tapers off to a 1-2mpg hit for the remaining 90 miles.

    Now let's say you drive 100 miles but it takes you 5 trips to cover that 100 miles (think short evening commutes). The AC hit is large for the first 10miles and then tapers off but now you only have 10 more miles to go at the lower hit. You do this repeatedly over the course of that 100 miles and you can see how you have now spent more driving miles with the AC working hard (large mpg hit) than if you had a single 100 mile commute.

    When I was testing the effects in my GenII last summer with temps around 105F, it could take 20-30min for the interior to cool sufficiently for the compressor to wind down and reduce the mpg impact from 10mpg to 2mpg. Granted my AC system may have needed a recharge but it cooled pretty well. The Scangauge told the real story about how hard it was working despite how good it felt in the car. :)
     
  11. SageBrush

    SageBrush Senior Member

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    I've gotten in the habit of setting the cabin temp at 75-80F on hot days after I have ventilated the car, but I manually run the fan at a higher power than auto would, and I avoid recirc mode so that more air passes over the traction battery (or so I hope.) I wonder how my AC consumption is compared to a default setting of auto.

    Anecdotal MPG readings in my CTh are about 58 mpg from 30 mile trips starting from 90F in the shade, driving at 7pm on country roads at 55 mph.
     
  12. F8L

    F8L Protecting Habitat & AG Lands

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    You're getting 58mpg in your CTh?
     
  13. SageBrush

    SageBrush Senior Member

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    For that route, yes. Overall the last ~ 1500 miles are just under 52 mpg calculated. Not up to my G2 Prius' standards; but as you know better than me, a price is paid for a better suspension and beefier tires.
     
  14. F8L

    F8L Protecting Habitat & AG Lands

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    That
    That is still pretty awesome! Now slap some 15" Energy Savers on that beast and what your mpg climb! lol
     
  15. NinnJinn

    NinnJinn Member

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    Yes indeed!! I have only ran the a/c a couple times in my 02. And have found that on open highway i lose about 5-8mpg. In town ice runs non stop and mpg is pretty much non existant.

    Wife has already been warned, if you are going to ride in the prius, dont expect the a/c to be on.

    So she chopped her hair and donated it to "locks of love"
     
  16. SageBrush

    SageBrush Senior Member

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    Don't think I didn't consider it :)

    For now I'll stick with my stock 17 inch wheels, and buy LRR when replacement is needed. If in the future I need winter and non-winter wheel sets, I'll look for 16 inch wheels I can put 205 mm LRR tires on for the dry season, and stick with 17 inch, 215 mm for better traction in the winter.
     
  17. F8L

    F8L Protecting Habitat & AG Lands

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    Sounds like a plan! Michelin XIce for winter as they are great for traction and mpg the a 205 Energy Saver A/S for summer and you'll be set.
     
  18. radmangto

    radmangto Junior Member

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    I can only report my own experiences. I run with AC on 100% of time ... set @ 75 ... here in SW Fla it has averaged 90-92 for past many months and it not unusual to be in the Pri during late morning to early afternoon .... note my miliage below .... I am NOT a hypermiler ... I'm an old GTO guy so I run with any traffic .... sometimes, lead ... :cool: :>))

    Regards,
    radmangto
     
  19. F8L

    F8L Protecting Habitat & AG Lands

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    You could probably do even better with the AC off. The energy used to run the AC came from somewhere it wasn't free. So I would expect you to be at 50mpg or more. ;)
     
  20. SageBrush

    SageBrush Senior Member

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    AC use in S. Florida has to contend with humidity, not just heat.
     
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