Power Split Device

Discussion in 'Gen 4 Prius Technical Discussion' started by dstahre, Jun 27, 2016.

  1. Roger T

    Roger T Member

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    The engine must be revving really hard if you are going uphill and 80mph. :D
     
  2. bwilson4web

    bwilson4web i3 and Prime

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    No, the maximum rpm remains the same. Rather the traction battery provides the extra energy ... for a while.

    Bob Wilson
     
  3. CR94

    CR94 Senior Member

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    They were in a 1st generation, which I understand has greater tendency than the 3rd to spend battery energy ascending hills. I've never seen that behaviour either, but then I don't climb mountain passes absolutely flat out.
     
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  4. bwilson4web

    bwilson4web i3 and Prime

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    I benchmark all of our cars in a maximum, 525 ft (160 m), +8% grade hill. So far, all of them have reached 90 mph at the top and yes, the Gen-1 ran out of battery just as it crossed the upper threshold.

    Bob Wilson
     
  5. tzx4

    tzx4 Active Member

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    6.5 or 7? .
    MG2 provides instantaneous throttle response. I notice this with my former Prius c and my 2016. When the grade steepens on I-70 here in the mountains, other cars often lag an MPH or two relative to my Prii ,which shows a quick battery discharge almost always followed up with a recharge flow after the ICE catches up. In fact the c was even rock steadier than the 2016 which does seem to dip ever so slightly. No turbo to spool up, no down shifting transmission, no intake system delays, just instant power.
    This instant MG2 response is the same I experienced when I drove a Tesla, the difference being the Tesla snaps your head back with ludicrous acceleration rather than gentle Prius acceleration.
     
  6. tzx4

    tzx4 Active Member

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    The ICE does that, yes. What I entirely unexpectedly found is that on the steepest I-70 upgrades, the percentage of the time the battery is getting charged is like 80-90 %. The system uses the battery for increases in power and speed and immediately goes to work recharging the battery.
    I was expected MG2 to assist up hills until the battery discharged, and then the speed of the car would drop due to loss of the electric horsepower. I was wrong and quite impressed and pleased that I was.
     
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  7. ChapmanF

    ChapmanF Senior Member

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    The 1.5L engine in Gen 1 topped out at 70 HP at 4500 rpm, and I suspect that climbing Afton Mountain at 80+ mph is just a more-than-70-HP proposition inherently, so the battery was chipping in more or less constantly until the grade decreased. One nice thing about the Gen 1's 4500 rpm rev limit is that it doesn't really sound like extreme revving, just good honest work.

    This year I was back and making the same trip in a Gen 3, with the 1.8L engine and slightly higher rev limit, and the battery was getting charged some on the way up. (But also, I was driving this time, and closer to 70 than 80.)

    -Chap
     
  8. dangrass

    dangrass Junior Member

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    This thread has been super informative, but I still have a couple of questions. I've played around with the eahart model and watched videos showing how the various elements of the drivetrain play together, but I'm still a little unclear on one thing. Given that the ICE is not directly connected to the drive wheels, exactly how does it become coupled to assist MG2 in moving the car? It's clear in manipulating the eahart model that then ICE is rotating at the same speed as MG2 the energy from ICE is somehow assisting in turning the wheels (I think). I also understand how ICE rotates MG1. What I'm not entirely clear on is, other than generating electricity by rotating MG1 how is ICE actually mechanically coupling with MG2? Seems that MG1 is involved, but I'm not entirely clear as to how. Is it the case that if MG1 is spinning at the same speed as MG2 and ICE that it is effectively coupling ICE with MG2? Is MG1 also capable of providing additional direct motive force to MG2?
     
  9. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Lets start by visualizing the engine turning M/G1 to produce electricity. M/G1 will have drag, pulling magnets through a field.
    Since this unbalances the force on the engine, it exerts the same amount of force on M/G2 to stay in balance. So we see direct power by the engine on M/G2 and electricity produced by M/G1 both powering the wheels. the gearing in the Power split device ensures that the torque applied by the engine to MG/1 and M/G2 is 29% and 71% respectively.
     
  10. dangrass

    dangrass Junior Member

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    Ok, this is helpful. Is it correct to say that at full throttle (so to speak) that MG2, MG1 and ICE are all moving the car or is MG1 limited to coupling ICE to MG2? (and generating electricity)
     
  11. bwilson4web

    bwilson4web i3 and Prime

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    MG1 is powered to provide counter-torque that the ICE 'pushes against'. This feeds the rest of the ICE torque to the same shaft as MG2.

    Bob Wilson
     
  12. dangrass

    dangrass Junior Member

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    ok, thx. Very helpful.
     
  13. ChapmanF

    ChapmanF Senior Member

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    Well, under most conditions, it isn't necessary to 'power' MG1, just to provide counter-torque for the engine to push against. I learned this really young, because my dad picked up an army-surplus handcranked dynamo for me to play with, and we would hook up lightbulbs and stuff to it.

    When there is nothing connected for it to power, it is pretty easy to crank, just freewheels around. As you hook up more lightbulbs etc., stuff that accepts more power from the generator, cranking that handle turns into more and more of a workout. This is nothing other than the TANSTAAFL principle, a/k/a conservation of energy; you don't get more (electrical) work coming out, without more (physical) work going in.

    So, when the car is stationary with the engine running, the IPMs (intelligent power modules) are basically neither powering MG1, nor accepting much power from it, so it just freewheels around. Because of that, very little mechanical force reaches MG2, and the car stays put.

    As you start to press the go pedal, the IPMs begin to accept electrical power from MG1. That's what makes MG1 start to resist rotation, so the planet gears push against it and deliver some mechanical power to the wheels. At the same time, what happens to the electrical power being accepted from MG1? Mostly, it just gets sent over to MG2. So the power reaching the wheels is getting there by two different paths: one mechanical, because of the gear train, and one electrical, power from the engine turned into electricity at MG1 and sent over to MG2 to be turned back into motion. As you can see, neither path can work without the other.

    There are three situations where the IPMs do in fact send power to MG1. The first is obvious: to start the engine. The next is when you are coasting or on a downhill, and MG2 is generating more power than the battery can store; then the excess is sent to MG1 and spins the engine, giving a braking effect that feels and sounds just like downshifting in a conventional car.

    The third case where power can be sent to MG1 is the weird one: high-speed cruising at low power, the sort of conditions where you'd want an overdrive gear in a conventional car. Here you want the wheels to be getting less torque and more speed than the engine output, instead of the other way around.

    How does the computer pull that off? By telling the IPMs to accept electrical power from MG2! Wait, doesn't that make MG2 resist rotation, as if you were braking? How can that be efficient?

    Well, making MG2 (slightly) resist rotation is another way of saying, subtract something from the torque delivered to the wheels, and what happens to the power being accepted from MG2 is it gets fed into MG1, in the proper direction so the gear train adds to the rotational speed of MG2. Voilá, torque reduced and speed increased through the transmission, just what you want from an 'overdrive' gear, and all made to happen just by letting the electric power flow in a counterintuitively backwards direction. Some of the first people to figure out what this mode was doing gave it the name "heretical mode" because it sounds all wrong, but algebraically, it does just what you want.

    So what are these IPMs that control when power is being accepted from, or delivered to, the MGs? In broad strokes, they are just sets of paired electronic switches, for each of the three coil windings of each MG. Depending on which pair of switches turns on, each coil can be connected to the high-voltage bus in either polarity (that is, the two ends can be connected to the bus either way, or left unconnected when the switches are off). The controller also knows the exact position of each magnet-carrying MG rotor, as the magnets spin past the coil windings. All of the control of whether electrical power is accepted from or delivered to each MG is done by nothing other than precise timing of which switch pair turns on and off at what instant.
    And all that switch timing (in the thousands of switches per second, depending on MG speeds, which you can sometimes hear as a high whine) is the whole story of how the "continuously variable" transmission ratios are achieved.

    -Chap
     
  14. dangrass

    dangrass Junior Member

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    Wow! This is totally fascinating. Thanks so much.
     
  15. eric1234

    eric1234 Active Member

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    This is a great explanation. Thanks Chap.

    And (to try to build upon this explanation) - please correct me if I'm wrong (I'm not 100% certain, but this is how I've started to think of it):

    The Prius is (end of day) a Gas-powered car - and the primary purpose of these MGs is to allow the gas engine to operate at it's most efficient parameters. I like to think of it as an "electronic" transmission.

    Of course, the battery offers some storage, and some limited occasions to either accept or provide an excess or shortage of electrical power - but by and large - all the electronics are really there to allow the engine to operate at its most efficient operating point.
     
  16. ChapmanF

    ChapmanF Senior Member

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    You've nailed it.

    -Chap
     
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  17. MrMischief

    MrMischief Active Member

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    There are other videos in the series but this bit here really helped me grasp how the eCVT functions. As I understand it the big difference in the 3rd Gen setup is there is a gear so MG1 spins more slowly allowing higher top speeds without the ICE turning, and we switched from a chain to a gear to drive the wheels. And in the newest versions we now have the option of a lithium battery pack. But the general function remains the same.
     
  18. ChapmanF

    ChapmanF Senior Member

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    I believe the added gearing in the Gen 3 "compound gear unit" goes between the PSD ring gear and MG2 (which used to be directly connected), allowing a smaller, lighter MG2 that achieves high power output by spinning faster, with the compound gear reducing that speed (and increasing the torque) on its way to the PSD ring gear.

    I think they did also allow higher top speeds without the engine running, but they pulled that off simply by improving the MG1 rotor design so it can spin faster before the magnets fly out.

    -Chap
     
  19. MrMischief

    MrMischief Active Member

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    You are correct about the smaller and lighter MG2, but as I understand it the limiting factor in battery only operation is the RPM of MG1. The gen 3 greatly reduced the speed that MG1 turns.

     
  20. ChapmanF

    ChapmanF Senior Member

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    I've watched that video before, and I know why you think that, because he does say, talking about the reduction gear, "... and it also slows down MG2, which was the limiting fac- or MG1 over here, which was the limiting factor in maximum vehicle speed on battery power only." And he's even wearing a lab coat. But honestly, the reduction gear operates between the PSD ring gear and MG2, the lab coat doesn't change that. It allows MG2 to spin faster, with its speed geared down so that the PSD, and wheels, and MG1, operate just as before.

    But wait, you say, doesn't he show in the video how MG1 only goes a little more than one rev around in his demo? Sure, he does. But look closely what he's turning: he's turning MG2. That's where the reduction gear is. MG2 is geared down. MG1 spins less in that demo because the whole PSD spins less in that demo, because he's turning MG2, which is geared down.

    Assuming he really wanted to show the relationship between MG1 revs and vehicle speed, he would have had his hand right on the ring gear to turn that. That's what's connected to the final drive, differential, and wheels. And if he had done that, he would have shown that the relationship between MG1 speed and ring gear speed has not changed.

    What lets them allow higher EV vehicle speeds is just that they have built MG1 to withstand higher revs before it flies apart. Even from Gen 1 to Gen 2, they had raised this limit from 6500 to 10000 rpm (2004 New Car Features Manual, page TH-2). I could look up what it is for Gen 3 but I'm out of techinfo days at the moment.

    There's a lot of great information on the internet, and much of it's reliable, but this illustrates how not all of it is, and why checking in the actual manuals sometimes isn't a bad idea.

    -Chap
     
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