Finding the sweet spot for engine RPM

Discussion in 'Gen 2 Prius Technical Discussion' started by nh7o, May 24, 2013.

  1. nh7o

    nh7o Off grid since 1980

    Joined:
    Dec 10, 2011
    1,686
    323
    0
    Location:
    Hawaii
    Vehicle:
    2009 Prius
    Model:
    II
    I have been aware that there are two or more solutions which the Hybrid control algorithm can settle on, in many driving situations. In his pages, Hobbit describes adding a tachometer to help with finding the most efficient RPM to run the engine, since the sound is so low that it doesn't always give enough feedback.

    The situation where I notice this, is when I coast down a significant hill, and then slowly press the accelerator at the bottom as I go up the next fairly steep incline. Often I find that the pedal feels quite unresponsive, where I can continue to depress the pedal without much additional power from the car. If I take my foot off the pedal, and then quickly reapply it, then there will be a new engine RPM, and much better response and power from the car. Doing this "reshuffles" the MG1, MG2, and ICE rpm's, and seems to find a better solution, even though the sound is often not so different.

    I will have to pay more attention to the Prius Dash nomograph to see if I can see a pattern.
     
  2. Agent J

    Agent J Hypoliterian

    Joined:
    Apr 27, 2010
    294
    109
    0
    Location:
    Sydney, Australia
    Vehicle:
    2007 Prius
    Model:
    N/A
    You may be on to something there... But I think that "better solution" from the control algorithm you're feeling is the sudden surge of traction battery assist not necessarily a new RPM level, given the new demand of your foot on the gas pedal. Lots of things to take into account and going on in the background which we could only guess how the engineers designed the algorithms.

    I take on huge hills everyday and still couldn't decipher the optimum RPM to tackle that hill.. to the point that I've given up looking at RPM and instead just looked at instantaneous consumption (MPG for you guys). I just gain as much momentum as i can preparing for the hill and try and keep a set speed in the middle section with slightly increasing pedal inputs (if needed) up to a point where the car naturally starts to slow down as I head to the crest and start easing off the gas, while I keep an eye on the instantaneous L/100KM, making sure that the total average consumption during that climb time is as low as possible (or in north american MPG terms, as high MPGs as possible).

    At one point i'd even reach around 3,800+ RPM, but I think it really doesn't matter because that's the particular RPM the ECU has chosen for that particular load/grade/speed/power demand. It will only be for a short while, though, as I slowly ease off the gas.
     
  3. 2009Prius

    2009Prius A Wimpy DIYer

    Joined:
    Mar 25, 2009
    2,702
    478
    63
    Location:
    USA
    Vehicle:
    2009 Prius
    IIRC Hobbit said in one of the posts that he found 15 kW engine output to be the best. Note that the RPM for 15 kW is not constant, but speed dependent.

    I have been glancing at the calculated fuel efficiency from time to time. I found it to be ~ 30ish percent regardless the RPM. So my take is that the car is very well optimized and there is not much difference in fuel economy we would see by varying the RPM.
     
  4. nh7o

    nh7o Off grid since 1980

    Joined:
    Dec 10, 2011
    1,686
    323
    0
    Location:
    Hawaii
    Vehicle:
    2009 Prius
    Model:
    II
    On the mainly gently sloping and fairly high speed conditions that most of you over there have, I would say that is the case. On the precious little bit of that type of driving I do, the car zeroes in on the right answer. My above comment results more from snaky hill climbing at slow speeds, of which I do plenty.

    Although I do have one question. On the MFD, I see two states shown often for similar conditions. One is with just one set of orange arrows from the engine to the wheels. The other is with a second set of arrows shown as well (yellow), which go to the MG glyph and to the wheels. (This is from memory, so I hope that is right.) I have not been able to determine what is the criterion that selects between these two conditions. If I ask for more power when in the first state (just one set of orange arrows), the second set of arrows goes on, without much ICE rpm change. There may or may not be much battery current happening in these cases, it is not the determining factor.

    I assume this is again just reshuffling the MG1/MG2/ICE rpm slightly. The amount of calculation that goes into making these cars go down the road is astounding, and IMHO a bit over the top.
     
  5. 2009Prius

    2009Prius A Wimpy DIYer

    Joined:
    Mar 25, 2009
    2,702
    478
    63
    Location:
    USA
    Vehicle:
    2009 Prius
    I see. I think the general approach is to use engine power going uphill and to glide downhill. Regarding the MFD arrows I am guessing it's related to the two power modes discussed in this thread:
    Introduction to Prius Power Flow | PriusChat
     
    nh7o likes this.
  6. nh7o

    nh7o Off grid since 1980

    Joined:
    Dec 10, 2011
    1,686
    323
    0
    Location:
    Hawaii
    Vehicle:
    2009 Prius
    Model:
    II
    I notice in the first reply that others have been playing with the pedal to find the sweet spot:

    So I see this is old news then, but new to me. PC is deep well of knowledge.
     
  7. Drafty01

    Drafty01 Junior Member

    Joined:
    Jan 18, 2013
    42
    6
    0
    Location:
    Auckland, New Zealand
    Vehicle:
    2006 Prius
    Model:
    N/A
    Hi nh7o,

    I'not sure there is actually a "sweet spot".

    Like you, I have a hilly, snaky kind of a commute for about half of the journey. The other half is suburban crawl, followed by a fast (60-65mph) 15 minute freeway drive - sometimes not so fast though, so I see a large variation of state of charge, ICE performance etc.

    I have the TORQUE App (with DTCs for the Prius) on my phone, which analyses the charge/discharge current, state of charge, engine revs etc a little more finely. If you don't have it already, I'd highly recommend you look at getting it. There's a lot of info on the forums about it, and if you have a smart phone, you'd be all set for a minimal cost.

    I think the ECU(s) do a pretty good job of keeping the battery charged up most of the time, but they can't see what's coming up in the next 3-5 miles, so can't optimize to take advantage of the up coming terrain to improve economy. It's often I'm near the top of a high, but not so steep a hill, with quite a full battery, so on the descent, the friction brakes seriously cut in about 1/3 to 1/2 way down (found this using the TORQUE App), when the battery is "fully charged" - in my case about 82%.

    I think the Prius III has different modes that cater a little to this, but would be a great retro fit if it were possible... anyone? Whether control would be manual, or GPS (Altimeter) driven is a technicality really. Maybe someone with a better head for this sort of thing could do some work on this - I'm sure there'd be a demand for it.
     
  8. hill

    hill High Fiber Member

    Joined:
    Jun 23, 2005
    15,744
    5,702
    54
    Location:
    South OC So Cal & Nashville, TN
    Vehicle:
    2004 Prius
    Model:
    IV
    Yes ... varying the RPM . . . . perhaps you'll find this helpful while tinkering with sweet spot concepts:
     
  9. nh7o

    nh7o Off grid since 1980

    Joined:
    Dec 10, 2011
    1,686
    323
    0
    Location:
    Hawaii
    Vehicle:
    2009 Prius
    Model:
    II
    I use PriiDash for those functions, which covers the possible data set well, enough to be dangerous, actually. Going up the hills here at less than 30mph/48kmh when I let off the accelerator and reapply, the rpm changes a bit, but I have not looked at it enough yet to get numbers. I don't drive often, and I have just started playing with this.

    Been to Aukland, great place!
     
  10. Drafty01

    Drafty01 Junior Member

    Joined:
    Jan 18, 2013
    42
    6
    0
    Location:
    Auckland, New Zealand
    Vehicle:
    2006 Prius
    Model:
    N/A
    I must have a look at PriiDash.

    Auckland is a pretty cool city - Been here most of my life.
    Not sure if you made it out to Piha (where I live). It's a very good - world class in fact - surf beach.
     
Loading...