MPG hit for hot weather and AC use?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pasadena_commut, Jul 28, 2021.

  1. pasadena_commut

    pasadena_commut Active Member

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    Since replacing the 12V battery our 2007 had been averaging 47-49 MPG. This was during the spring when it was much cooler. However, since summer rolled around and the average daily temperature rose to ~95F, and the AC is in use all the time, the MPG dropped down to 40-41 MPG. This is for more or less the same sort of mostly around town driving. No codes, bad sounds, or anything else to indicate a mechanical issue. The efficiency loss seems reasonable to me given the hotter intake air, constant AC use, engine restarts when stopped, and the limited capacity of the original traction battery. Is it?
     
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    Yes. I can get a bit over 54mpg on a tank (500+ miles between fill-ups) when driving with mpg in mind and using absolute minimal AC. Going back to typical hammer down kind of driving (80+) during the daily commute and heavy AC use will reduce my mpg to around 42-43.
     
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  3. archae86

    archae86 Member

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    Over the range of temperatures I'm familiar with, the propulsion efficiency of the gasoline engine gets steadily better the higher the temperature. Assuming your trip mix and driving style have not changed, I think use of Air conditioning is causing MORE than all of your loss.

    I live in Albuquerque, but find that by parking in the shade whenever possible, using my windows a lot, and just tolerating somewhat elevated temperatures I can and do avoid use of AC the great majority of the time. Now Phoenix...
     
  4. bisco

    bisco cookie crumbler

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    completely normal, imo
     
  5. pasadena_commut

    pasadena_commut Active Member

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    The AC is mostly for the traction battery. If one wants to maximize the life of a high voltage high current battery it is best to keep it cool.
    I'm not sure that "hotter air into the engine is better" is the whole story on a car as complicated as a Prius. The resistance of copper goes up .4% per 1C and there is a whole lot of current moving on some of those wires. Not sure what happens inside the battery cells but I'm guessing that there too the resistance increases with temperature. So maybe the gas engine does a little better, the electrical system does somewhat worse, and who knows if that is a net plus or minus.

    In my particular case I'm pretty sure the dominant factors are the load from the AC, and as a side effect, that the engine often runs when the car is stopped at a light after the battery voltage drops down into the purple. With no AC the gas engine is always off when the car is stopped.
     
  6. bisco

    bisco cookie crumbler

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    i thought engines prefer a cold air intake
     
  7. archae86

    archae86 Member

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    Overall, most automobiles get a considerably poorer fuel economy in colder weather. For the Feds take on reasons why you might review:

    Fueleconomy.gov on cold weather effects

    I don't know the overall shape of the curve, nor whether it actually turns over at some point (for reasons other than air conditioner operation).

    [edited to add: my initial post was in error in implying that the widely known cold weather fuel economy loss was all due to degraded efficiency of the ICE itself. I, personally, have not seen plausible indication of degraded economy at higher temperatures above some inflection point, but we clearly see degraded economy from lower temperatures within the range annually experienced in Albuquerque.]
     
    #7 archae86, Jul 30, 2021
    Last edited: Jul 30, 2021
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