RPM as related to 15-18kw output

Discussion in 'Gen 2 Prius Fuel Economy' started by rustystew, May 2, 2011.

  1. rustystew

    rustystew New Member

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    I was under early assumptions that RPM is a good indicator of the "sweet spot" hobbit wrote about. Mikewithaprius was nice enough to set me straight on that. He informed me that the rpm will be different at 15kw output depending on your speed. Would it be safe to assume I could still hit this 15kw sweet spot if I knew what the corresponding rpm was for a given speed?

    For example if 15kw of acceleration at 50mph is around 2000rpm and at 60mph is 2300. I could still get a good idea of the optimal moving acceleration point to acellerate from 50 to 60 mph based on RPM. is this true or are there yet more factors that make RPMs an unreliable indicator of the sweet spot?
     
  2. mikewithaprius

    mikewithaprius New Member

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    I wish I could help more, I just don't usually look at RPM, but I'm sure someone here has an answer. One thing that could be simpler, if you don't have any other instrumentation for the car, is the instant mpg.

    Just from observing, here are a couple 15 kW relationships I've noticed:

    -At 20 mph, your instant mpg should read about 16-17.
    -25 mph, ~19-20 mpg
    -30 mph, ~25 mpg
    -35 mph, ~29 mpg
    -60 mph, ~45 mpg

    At higher speeds the relationship is more obvious, like you can see at 60 mph. When the instant mpg reads between 2/3 and 3/4 of your current speed, you're right in that spot, you probably don't even need a Scangauge to find it.
     
  3. uart

    uart Senior Member

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    Thanks Mike.

    BTW. In my head I just made a quick formula out of your data. :D

    RPM (/100) = 2/3 MPH + 5
     
  4. SageBrush

    SageBrush Senior Member

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    The fuel pedal is linear, and goes from 0 - ~ 60 kw.
    So 15 kw is 1/4 depression.

    Regarding Hobbit's focus on staying at 15 kw if the ICE is on:
    The SFC chart suggests a much wider range of torque*rpm ranges than just 15 kw, so I am inclined to think the trick is to help the car stay in WOT and not 15 kw per se.

    As a technique I try to first hit an rpm something below 2000 for a couple of moments, drop the rpm by 1000 or so, and then stay there. My intent is to let the Prius pick the best gear for my power output and then draw a bit less power. I have no proof this is a good approach for fuel economy.
     
  5. rustystew

    rustystew New Member

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    Mike you are doing plenty to help me out. I just wanted to move the conversation over to the forums where more people can be involved.

    Well, I'm working on installing a tachometer which is why I'm asking these questions. Trying to decide if it is at all useful. The impg (instant miles per gallon) is great and all, but the refresh rate is kind of hard to deal with. Especially with the kind of precision/constant adjustments you need to keep the car in the proper range.

    Is that not RPM = (2/3 MPH +5)*100? And how did you get that when Mikes chart has no indications of what the RPM is there? According to this at 60mph, the rpm is 4100, I have a feeling that is incorrect. But it's always fun to try! Actually this is kind of what I'm looking for although I'm thinking low end of RPM - high end of RPM to accelerate at various speeds (say 20, 30, 40, 50, 60, and 70). I could plot those points on a chart as two separate lines (high end, low end rpms) and the sweet spot RPM's would theoretically be in between them.

    Thanks, I actually had no idea what 15kw was referring to.

    I don't know what the SFC chart is or what WOT is. Could you explain those terms so I can better understand this statement?
     
  6. SageBrush

    SageBrush Senior Member

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    kw: a unit of power, like horsepower.
    WOT: Wide open throttle. Throttle closure restricts airflow. This is the mechanism a combustion engine in a car uses to produce low power, but it wastes energy.
    SFC: fraction of fuel burned turned into mechanical work. Internal combustion engines have a wide range of SFC dependent on torque and rpm.

    Have a gander at this site by Graham Davies to learn about the IC engine. The rest of his website is a goldmine for people wishing to understand the Prius.
     
  7. mikewithaprius

    mikewithaprius New Member

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    OK, well apparently I never noticed RPM that much, but should've. Just was watching this on 54 miles of roundtrip driving today. 15 kW starts at 1650 RPMs or so, was only marginally higher at slightly higher speeds (like 1700 something at 50 mph). If you stay below 2000 RPM you're probably below the top of the high 18 kW range. So between 1600 and 2000 RPM or so generally is good for acceleration.
     
  8. rustystew

    rustystew New Member

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    After the explanation of the acronyms this statement makes much more sense. And with my limited knowledge of how things works, it seems quite plausible.


    AHAH! I knew there had to be at least a loose correlation. Can anyone duplicate these observations?

    Also could a few people with the necessary equipment fill out the following chart and post the results in this thread?

    Starting Speed: 20MPH, acceleration @ 15kw = ~XXXX RPM
    Starting Speed: 30MPH, acceleration @ 15kw = ~XXXX RPM
    Starting Speed: 40MPH, acceleration @ 15kw = ~XXXX RPM
    Starting Speed: 50MPH, acceleration @ 15kw = ~XXXX RPM
    Starting Speed: 60MPH, acceleration @ 15kw = ~XXXX RPM
    Starting Speed: 70MPH, acceleration @ 15kw = ~XXXX RPM

    Starting Speed: 20MPH, acceleration @ 18kw = ~XXXX RPM
    Starting Speed: 30MPH, acceleration @ 18kw = ~XXXX RPM
    Starting Speed: 40MPH, acceleration @ 18kw = ~XXXX RPM
    Starting Speed: 50MPH, acceleration @ 18kw = ~XXXX RPM
    Starting Speed: 60MPH, acceleration @ 18kw = ~XXXX RPM
    Starting Speed: 70MPH, acceleration @ 18kw = ~XXXX RPM

    Once I get a couple sets of data I can put them on a chart and we can use this data to help find the sweet spot at various speeds with just a tach. At least in theory. Worst case senario it will be cool to see all the data in one place even if there is no strong correlation that comes out of it. The more completely filled out the better, but I imagine a pattern will arise and we can predict the next couple points based on that.

    Thanks guys!
     
  9. uart

    uart Senior Member

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    Oh yeah forget about that. It was late at night and for some reason I read Mikes data as mph versus RPM(/100) instead of mph versus mpg. :p
     
  10. ken1784

    ken1784 SuperMID designer

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    The assumption is not correct.
    The 1NZ-FXE engine tries to trace running map and there is single rpm and torque combination for the required power, such as 1750rpm and 82Nm for 15kW, or 2220rpm and 86Nm for 20kW.

    By the way, the best efficiency (below 230g/kWh) is achieved above 19kW power demand.

    [email protected]

    [​IMG]
     
  11. ystasino

    ystasino Active Member

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    Let me be a little difficult and suggest that you don't pay attention to RPM with relation to FE.

    The following hold for speeds between 45 and 60 MPH without significant wind. Essentially these conditions describe a complicated version of Warp Stealth or pulse and glide with the ICE working.

    Accelerate through straights or mild uphills at 12 kW
    Accelerate through moderate uphills at 15 kW
    Accelerate through strong uphills with as little power over 15 kW as possible
    Use the yellow arrow condition to glide downhill
    Maintain speed at 10 kW (IGN 15 for those using SHM)
    Shallow downhills at 6-8 kW (IGN 14)


    That should get you ~57 MPG on the highway in moderately hilly terrain, more in flat terrain with tires inflated to 44 psi.
     
  12. planetaire

    planetaire Plug in 20 kWh 85 km/h or > 208km range

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    Here is a graph showing relation between kW and rpm.

    kw_rpm.PNG

    You can see that the relation is not one to one.
    There are several rpm with the same kw output. But they are close.
    Of course they don't have the same injection timing.

    Also shown accelerator pedal from 20% to 60%.

    This chart is for a 2007 Prius.
    :)
     
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  13. rustystew

    rustystew New Member

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    Unfortunately I have no way of knowing what 12kW, 15kW or 10kW is. Is the Scan gauge the only way to get this information accurately? Which is why I'm trying to figure out what the different power states equivalents are in RPMs if there is a correlation at all.

    I think this chart is what I'm looking for, but I really don't know what exactly I'm looking at. So much so that I don't know how to phrase my question about it. I see the x axis is RPM, y axis is kW. Are the two lines independent of each other or a relationship between the pedal position and the ICE kW (is the ICE kW separate from the y-axis kW?) What is the y-axis kW if not the pedal position that Sagebrush was talking about? I simply don't understand this chart. Sorry.
     
  14. ystasino

    ystasino Active Member

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    What parameters can your instrumentation read?
     
  15. rustystew

    rustystew New Member

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    Right now I have no mods to my instrumentation. And my Prius is just a base model. So really all I have to go by are speed, instantaneous mpg and the feel of the car. I have purchased a tach which I have yet to install, with the intent that I would have some more real measurement to work with. Although it is seeming as though I don't really need it. As I am getting the message that RPMs are essentially useless for helping with judge fuel economy.

    Also I still don't fully understand the chart that was posted.
     
  16. planetaire

    planetaire Plug in 20 kWh 85 km/h or > 208km range

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    Yes there is a relationship between pedal position and kW on my chart.

    For exemple, in order to obtain 18kW press the pedal slowly to 34%.

    Test condition : SOC=63% constant during test. ICE_Temp >84°C (183°F)
    Data read on can bus

    Full ICE power is not 100% pedal position.
    100% is full power ice+mg2.
    MG2 can give 25kW more coming from Nimh battery during ...

    I believe that the chart will not be the same if SOC is 40% or less.

    :)
     
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  17. mikewithaprius

    mikewithaprius New Member

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    That's just cause there may be easier ways of looking at with a Scangauge or other device. Ystasino likes to use kW or IGN, etc., but from Planetaire's chart and what I just saw the other day, RPM is perfectly fine for normal driving. Since you have a tachometer, I think it's very useful for everyday economy-minded driving. Sure, if you wanted to win a hypermiling competition and needed to get over 100 mpg or something, but...

    Let's pretend you wanted 20 kW of output for whatever reason. On the right side y-axis, see the 20? That's the 20 kW line (it just so happens the left y-axis, measuring how far down the pedal is pressed on a percentage basis, uses the same numbers, so you can really look from either the left or right side).

    Go over now to see where the blue "line" crosses that 20 kW horizontal line. When it does, you can see that, from directly below on the x-axis, it's right around 2100 or 2200 RPM.

    So, if you imagine where 15 kW would be halfway between the 10 and 20 lines, and head over to that point, somewhere a little past the 1500 RPM mark, so his observations match mine that it occurs somewhere around 1600 RPM (a little above, in my case). If you go to 18 or 19 kW, that tops out right around 2000 RPM, so he had the same observations as I did.

    RPM fluctuating so much as they do, you'll actually be all set with your tachometer for good acceleration at any speed. It doesn't have to be exactly 1678 RPM or anything, as if that were even possible to hold. If you find yourself between 1600 (or 1650 from what I saw) and 2000 accelerating to any speed, that's the sweet range you're looking for. It's really as simple as that.

    Hills, obviously you have to go over that, but not a big deal.

    Then there're glides, warp stealth, and super highway mode, if you haven't seen those already. That's what really affects the mileage more than acceleration style.
     
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  18. rustystew

    rustystew New Member

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    Thanks for the explanation Mike! I didn't expect to whittle it down to a number so specific as 1678 RPM exactly. But I was looking for a range, which is exactly what you, and the chart, are giving me.

    As far as the driving techniques go. I'm getting a better handle on them. I'm also quickly realizing the difference between 75 mph and 55! It's pretty huge.

    Gliding I've got this pretty good, although I don't get the opportunity to use it very often. Since Warp Stealth feels exactly the same, I have that down as well, and I get to use it tons more. I feel like just last night on my way home I was getting Super Highway Mode figured out which is probably why I got such great results home and on my way back in this morning, coupled with 55 instead of 75 mph.

    On a side note, I was on a LARGE downhill the other day and when I put it in B mode I got into a glide state (no arrows) at 75mph for near 5 seconds. Which I remember in one of your posts you saw on a hill but wasn't sure how to replicate because you were in cruise control or something.
     
  19. 2009Prius

    2009Prius A Wimpy DIYer

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    These are all great info!

    planetaire has been very generously helping me with the CAN bus messages and it just so happens that I started looking into this business of "15 kW" today. I found there may be a ~ 3 kW offset between planetaire's graph (from CAN messages) and what ScanGauge reports. Apparently planetaire's graph shows the net power output into the HSD, and the ScanGauge adds in the "wasted" power just to spin the ICE.

    Since Hobbit uses ScanGauge for his number, his "15 kW" "sweet spot" should really be ~ 12 kW on planetair's graph.

    How about Ken's graph? Are those kW contours "net" or "gross" (including wasted power)? I am guessing "net". If so then there seems to be a big difference between Ken's "best efficiency" region of "above 19 kW" and Hobbit's "sweet spot" of 12 kW. How do we reconcile these?
     
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  20. planetaire

    planetaire Plug in 20 kWh 85 km/h or > 208km range

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    Hello, 2009prius.

    Yes on Canbus ice torque is received in two pid.

    -One passive (the lowest)
    -The other active. It is the one i use on my chart in message #12. This one can be negative, when MG1 and MG2 start the engine.

    You have a very good idea comparing passive pid and active, using kW.

    Here is a chart showing the difference betwen the two engine_torque in kW:
    Pertes.PNG

    Y axis is diference in kW
    X axis is ice rpm.
    This curve is experimental, result of some driving on a road. With better conditions i believe the curve would be more regular. :D

    The difference is not always 3kW.

    A very good result is that it is lower at lower rpm.
    And i remember having obtained 1,2 to 1,3 kW lost while engine is spinning at 990 rpm.


    :)
     
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