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Puzzled by how Prius decides whether to use gas or not when coasting

Discussion in 'Gen 2 Prius Fuel Economy' started by wfz, Aug 2, 2012.

  1. wfz

    wfz Junior Member

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    I'm very puzzled by how prius (2008) decides whether to use gas when I coast with neutral (I only do that if there is a long stretch of the road that is going down the hill). Same roads, same driving style, but sometimes, the gas usage goes to 0 but sometime it continue to use gas. Sometimes if you just let it in the gear a little longer or just press the gas pedal, it would go 0 after switching to neutral. There is no consistency that I can come up with the best method to get it to not to use gas and just slide using the gravity and momentum.

    I can understand if it has to fill up the battery but when it uses gas at neutral, no transfer of energy among all parts. The diagram (wheel, engines, battery) shows standstill but it still uses gas. I don't have air on and turn the fan to the lowest and it doesn't make any difference.

    A few conclusions I had:
    It will always use a little gas at highway speed (55 to 70mph).
    It appears to use gas at higher humidity (not sure but the MPG is higher when it's humid).

    How can I control it so it will not use gas when coasting (assuming you have high enough electricty in battery)?

    Thanks,
     
    #1 wfz, Aug 2, 2012
  2. walterm

    walterm Active Member

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    As I understand it shifting to "N" leaves the ICE in whatever state it was in prior to the shift - if it was off it stays off and will never start on its own, if it was running then it will stay running, but not generate any recharging current.

    I use this trick in the winter to reduce idling when the heat is on - I stop at a light, turn off the heat, let the engine shut off, shift to "N", and then restart the heat. When the light changes I shift to "D" and the engine starts again, but at least it's not running while the car is sitting still.
     
    #2 walterm, Aug 2, 2012
  3. uart

    uart Senior Member

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    Hi wfz. Read up about the the stages of hybrid operation and learn about stage 3 versus stage 4 operation (for the Prius-II). This will explain most of what you need to know about where and why it will continue to inject fuel (or idle the engine) after you begin an in-gear coast.

    As for a neutral glide, the "rules" are pretty much the same as above. If it's spinning the engine when you begin the glide then it will continue and idle the engine after you switch to neutral. Conversely if you're gliding with the engine off when you switch over to "N" then it will continue without fuel injection.

    Apart from the "rules" governing the stages of hybrid operation, there are also conditions such as requiring heat or low battery state of charge that will cause it to continue running.

    Read : http://techno-fandom.org/~hobbit/cars/five-stages.txt
     
    #3 uart, Aug 2, 2012
  4. wfz

    wfz Junior Member

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    Thank you all for the information. I will check them out!
     
    #4 wfz, Aug 3, 2012
  5. F8L

    F8L Protecting Habitat & AG Lands

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    In a nutshell, don't use neutral unless you REALLY know what you are trying to accomplish. Gliding is really all you need 99% of the time.
     
    #5 F8L, Aug 3, 2012
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