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Is it possible to calculate speed from ICE RPM?

Discussion in 'Gen 3 Prius Main Forum' started by Rob Ferreira, Aug 19, 2020.

  1. Rob Ferreira

    Rob Ferreira New Member

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    Hello all,

    I have a strange task in which I need to calculate the speed of a Prius solely by the ICE's RPM,

    Is this even possible given the layout of the drivetrain?

    If one was hypothetically going down the highway at a constant 3,500 RPM, could there be a speed associated with that?

    I have done some research and found out the gear ratios within the PSD, but not really sure how they can be related to the engine's RPM

    I know this is a weird task, but I'm just not familiar enough with hybrid vehicles to figure this out,

    Thank you in advance
     
    #1 Rob Ferreira, Aug 19, 2020
  2. ChapmanF

    ChapmanF Senior Member

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    If you were monitoring the MG2 rpm, you could, because there is a fixed ratio between that and the wheels.

    From ICE rpm, no, because that ratio changes. It's not even like you've got five gears and you could have a list of the ratios for each one; it's continuously variable.
     
    #2 ChapmanF, Aug 19, 2020
  3. Grit

    Grit Senior Member

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    I’ve gone uphill 4000 rpms at 25mph and 25 MPGs @ 1200 rpms on flats.

    You can make the call ;)
     
    #3 Grit, Aug 19, 2020
  4. ChapmanF

    ChapmanF Senior Member

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    In a conventional car with several fixed gears, there would be a fixed relationship between engine speed and road speed—one for each gear. Which led to the classic sound we all think of for a car zipping away: starts from a stop, as the car picks up speed the engine winds up, then a shift, then the engine winds up again, then a shift ...

    In contrast, with the CVT, if you're accelerating steadily, you pretty much mash the accelerator and the engine spins up to a certain rpm and stays there and the car picks up from 0 to 60 while you hear about the same engine sound the whole time (then it tapers off when you reach the speed you wanted). In that way, it sounds more like an airplane or a boat than a car.
     
    #4 ChapmanF, Aug 19, 2020
    tvpierce likes this.
  5. Rob Ferreira

    Rob Ferreira New Member

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    I knew it had a continuously variable ratio, but I thought once you got to a constant speed it would level out to a fixed ratio, but I guess that's not the case,

    thanks everyone.
     
    #5 Rob Ferreira, Aug 19, 2020
  6. Attila Fekete

    Attila Fekete Member

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    It may have fixed ratio on constant speed, but it will immediatly change, when you have different load - due to going uphill or downhill, wind changing, etc.
     
    #6 Attila Fekete, Sep 5, 2020
  7. ChapmanF

    ChapmanF Senior Member

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    Even under a constant speed and constant load, if the ratio is staying constant it is only because the ECU sees no particular reason to change it just then.
     
    #7 ChapmanF, Sep 5, 2020
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