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Why isn't regen integrated with the brake pedal?

Discussion in 'Model S: Driving Dynamics' started by dennis, Jan 24, 2013.

  1. dennis

    dennis P85D

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    And speaking of those door handles, my right rear door now opens itself upon auto-presentation and/or the handle stays in the extended position.

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    I took the risk of looking at the screen during my experiment in the interest of science. :tongue: I don't look at the display during normal driving, as I just brake as hard as I need to. In most situations I don't need more than .25G of deceleration.
     
  2. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

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    I have never hear a Leaf owner complain about this. It seems to be a non issue. If you need to stop fast, you use the brakes anyway. Regen is for controlled slowdowns when you have plenty of space in front of you.
     
  3. eledille

    eledille TMS 85 owner :)

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    First, I like the Tesla one-pedal driving style. I think it works very well when range is not a concern, and makes the car feel very lively and responsive. I don't think the Tesla system is perfect, however.

    There are limits to how much the car can safely brake just by lifting off the accelerator. Proof: If we take one-pedal-driving to the extreme, we simply remove the brake pedal. Lifting off the single remaining pedal now signals full panic braking. Would that be safe?

    An FWD or AWD car can engine brake much more powerfully than an RWD car and other battery chemistries are capable of much faster charging, so it's theoretically possible to regen at the full motor power. If you want to do that, you would have to put some of it on the brake.

    I disagree. Logically, cars have an accelerator pedal and a brake pedal. One is used to increase speed, the other is used to reduce it. Unfortunately, ICEs can't do this cleanly, because the engine stays connected to the wheels and produces drag. This is not the case in an EV, the motor does not produce any drag whatsoever when not energized, so I would argue that the pure EV approach would be to let the car coast if no pedal is activated. Automatic engine braking mimics ICEs.

    My Think City model 4 (around year 2000) coasts, and I love this feeling (it's way more advanced than just that, though, I have posted precisely how it works in other threads). I have driven the Roadster too, though only for an hour. I think the Tesla system mostly works very well, but I found it very hard to balance the pedal at exactly zero. I imagine that this will be seriously annoying on a long trip, which is why I entered my wish for either a narrow zero zone instead of a point or a configurable brake/accelerator regen into the Model S wishlist thread.

    You do not want to maximize regen if you're trying to conserve energy - and what other reason could possibly exist for wanting to maximize regen?

    Maximum energy efficiency is achieved when you drive like you have no brakes, because regen wastes about 50 % of the energy. Therefore, the ability to coast, and then to consciously and judiciously expend energy to accelerate or regenerate is important to maximize range. You don't want to shed kinetic energy, you want to hang on to it! Regen is just a somewhat less wasteful way of getting rid of speed, and being able to choose precisely when to use it is much more important to me than knowing exactly where the mechanical brakes engage. I very rarely need those, while I coast all the time. Coasting is difficult in a Tesla, so it does not optimize for the common case, at least for me.

    Another attempt at explaining this: I would rarely need max regen, because I plan ahead when I need to conserve energy. Therefore I would rarely accidentally use the friction brakes unless I just don't care about range that day, in which case it doesn't matter. Easy coasting is more important to me.

    All that's needed is a position sensor on the first 5 mm of brake pedal travel. Making a slight click or increased resistance just before the brakes engage should not be hard either. That would allow each driver profile to either select a behavior (all on the brake, 50/50 split, all on the accelerator) or customize it by dragging a slider. In my opinion, arguing that this would somehow be a safety risk makes no sense as long as the car will simply disable regen in the cold.

    I think that due to my sometimes spirited, sometimes very economical driving style, I would prefer all braking on the brake pedal if I could/had to choose.
     
  4. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

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    You don't want more regen than you need to drive normally, when you do controlled slowdowns. If someone pulls out in front of you and you have to really stop fast, you use the brakes. No need to put extra regen there as such occurences are rare enough that you aren't losing much by using friction brakes.

    I haven't driven the Model S but 30kW on the Leaf is too little. I'd probably like around 50kW of regen on the accelerator ideally. The Model S is heavier, so around 60-70kW maybe ?

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    You can't control regen on 5mm of brake travel. You need to be able to control it, 60kW of regen on 5mm of travel would mean you cannot slow down slowly. You'd probably need a good 1/3rd of brake travel to control 60kW of regen. And you need that part to engage friction braking instead if regen is disabled or limited. Hard to do well and definately increasing cost & complexity.

    Maybe 10kW of regen could be controlled on 5mm of travel. Then you'd be limited to 10kW max.
     
  5. jhs_7645

    jhs_7645 VIN: #3305

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    Maybe I'm missing something, ICE's with standard transmissions have been braking like this for decades. I just don't see why people are getting so wrapped around the proverbial axle about this. I find the behavior very compatible with the ICE/Standard transmission engine braking. There really isn't much difference.
     
  6. mknox

    mknox Well-Known Member

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    Yes, I believe I would be willing to pay, however I think the argument that Tesla shouldn't be offering configurable options is kind of humorous, actually. The whole point of the car is its ability to be modified over time and have features added. There are already re-gen options, steering options and so forth. In this spirit, should Tesla remove those options and just give us what they believe is right? They have come up with an arguably non-standard way of handling re-gen, and having an option to switch between modes does not seem unreasonable to me.
     
  7. smorgasbord

    smorgasbord Active Member

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    #87 smorgasbord, Jan 30, 2013
    Last edited: Jan 30, 2013
    Some questions:
    If you pop your Tesla into Neutral, it feels like it'll coast forever. Does the Karma have this kind of coasting behavior, or is there some regen happening? If the Karma coasts like an ICE with an automatic transmission, then there is some regen happening.

    If you're coasting along in your Karma and start applying the brakes, does it feather in the regen? In other words, does regen start small and increase until max, with friction braking starting somewhere before max regen and then increasing even after max regen is achieved? Note that it can be hard to know what "full regen" means if you're looking at kW going into the battery (Tesla display). That's because as the car slows down the kW generated will be less even if "amount of regen" is the same.

    Finally, note that Roadster and Model S have different "throttle off" behaviors. On Roadster, if you lift off the throttle suddenly, you're in max regen almost right away. On Model S, the regen appears to feather in. Some have said that effect is simply from the car having more mass with less regen, but I haven't seen anything authoritative.

    What kind of regen display is available on the Karma?
     
  8. aaronw

    aaronw Member

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    The Fisker's motors have magnets in them, unlike Tesla. In fact, I believe all of the other EV cars out there have magnets in their motors. Fisker and Nissan use synchronous motors instead of inductive motors.
     
  9. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

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    I think Renault uses wound rotors. If that is the case, no magnets there (but two slip rings instead).
     
  10. brianman

    brianman Burrito Founder

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    I still don't think it's worth investing R&D in changing the regen characteristics in any significant way.


    That said, if they were to change anything my request would be to change the accelerator pedal from being linear to being more of an S curve. More specifically, I would have a range of pressure (instead of a very tight perfect spot) where the Consumption(orange)/Regen(green) meter shows no bars at all. Essentially, an "easy to find and maintain" coast sweet spot.

    I don't know whether that sweet spot would be variable based on speed or what, I haven't pondered it that deeply yet.


    What I would be interested in seeing them consider investing "a little bit" in, is some user control over the range of cruise control. When I set the speed at "65mph", I don't mean a range of [64.9,65.1] (battery be damned!) but rather [62,67] (keep it about right but don't waste too much juice doing so).
     
  11. dennis

    dennis P85D

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    The regen display on the Karma consists of a bar graph (not useful) and an instantaneous digital display of kW.

    With foot off of the accelerator and some regen showing, shifting to neutral drops regen to 0, so it is coasting. Applying the brake pedal while coasting in neutral does not cause any regen, so all deceleration is from friction braking. Applying the brake pedal while "coasting" in drive adds regen. The first .25G of deceleration is all regen, anything above that adds the friction brakes. The max regen I could achieve with the brakes was 90kW.

    Smorgy, I'd be happy to demo for you if you want to drop by some time.
     
  12. huntjo

    huntjo Member

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    I assumed they had about the max amount of regen they felt was safe more max battery longevity in the S. It would of course be nice to have even more regen, but if the way they designed it allows my battery to last 7-10 years, then I'll stick with what we got.
     
  13. smorgasbord

    smorgasbord Active Member

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    90kW is huge regen! Roadster's max regen is about 40kW and Model is about 60kW I believe. With its smaller battery, I can't imagine regen could actually pour 90kW into the Karma's battery. I would imagine they're doing some kind of "braking power" extrapolation and not actually measuring what's going into the battery. More possibly, it's something I just don't understand.

    Interesting that Fisker decided to put some regen into normal coast mode on the Karma. I think they did that to prevent people from having to continuously go to the brake pedal when driving in traffic. Besides, everyone is used to some engine braking, even with auto transmissions. Can you feather the GO pedal on the Karma to achieve 0 regen?


    Back to efficiency, I'd like to question the hyper-miling concept of letting the car coast downhill to a high speed rather than capture that energy via regen. Both are lossy. According to Tesla (http://www.teslamotors.com/blog/model-s-efficiency-and-range), Model S at 50MPH needs 250 Wh to go a mile, but at 70MPH it needs 340 Wh to go the same mile. 90/250 is a 36% efficiency loss, which is about the same as Regen losses (there's a thread on this for Roadster that's 18 months old or so). Then when you factor in the safety of going 70MPH in a 50MPH zone, regen is perhaps the better way after all.


    Sure, but what we should do is meet in a deserted parking lot/private road so we can conduct our testing in safety. And, while we're at it, let's have MSP drag race a Roadster. I haven't seen a video of that yet, and it should be fun - with Roadster winning at first and then losing at around 65MPH or so.
     
  14. dennis

    dennis P85D

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    Sounds like a plan! I bet we know a few people who could do an awesome video. :biggrin:
     
  15. eledille

    eledille TMS 85 owner :)

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    #95 eledille, Feb 1, 2013
    Last edited: Feb 1, 2013
    That would remove most of my objections.

    Good suggestion, but probably hard to do well. The car does not know what is happening ahead. But there might be some room for improvement.

    Fisker is using A123 cells. It's a different chemistry, and better suited to a smaller, harder-working battery. They can charge and discharge at an incredible rate, but the capacity is so-so.

    You're quite right, coasting to a high speed downhill will waste energy. Calculating exactly where regen becomes better than coasting to higher speed than normal requires parameters that I don't have, for instance how my tires behave with respect to dynamic rolling resistance. But coasting uphill and then coasting back up to speed downhill will save energy. I often see people maintain 15% above legal speed up the hill, then step on the brakes as they approach the top and realize that visibility is limited and their speed is dangerously high. Then they emit a cloud of exhaust as soon as they can see the road ahead again. How about letting the uphill take care of that speed reduction instead, and then letting the downhill bring you back up to speed? Some planning will save a lot of energy.

    To elaborate a bit on my Jekyll/Hyde driving styles - if I have the road to myself, I love to push the car a bit. If I can't do that, I'll switch to a different game - how much energy can I save without reducing average speed. After playing that game for five years I'm now able to stretch my range quite a bit :)
     
  16. brianman

    brianman Burrito Founder

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    I think you're misunderstanding my request. Elaborating.

    You're driving on a flat road, going 65. You set cruise control (at 65). You encounter a slight incline and the speed drops to 64. The car responds "omg, gotta accelerate...floor it!" I'm saying it should instead accelerate only slightly (or not at all) and save the aggressive response until 62 or so.
     
  17. MikeK

    MikeK R#129, TSLA shareholder

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    I like the Tesla's single-pedal driving, but I wish that it was easier to let the Tesla just coast. In my RAV4-EV (original), you could coast forever on some roads.

    The RAV's blended regen was also handled beautifully and seamlessly, with the car automatically regulating the hydraulic pressure applied to the brakes so that you always used up the available regen before it started wasting kinetic energy heating up the brakes.

    Lest people go too far afield wondering about motor types, the EV-1 used a 3-phase AC induction motor, and it had blended regen on the brake pedal, in system that worked very well.
     
  18. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

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    If the Karma battery can discharge at an incredible rate, why do it need to have the ICE generator running to achieve ok acceleration ? ;)
     
  19. stevezzzz

    stevezzzz R;SigS;P85D;SigX;S90D;XP100D;3LR

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    The Roadster and Model S both use AC induction motors, right? So when regen kicks in and the motor becomes a generator, what does the electrical output look like? I ask because regen peaks at about 40/60kW (Roadster/S) and the onboard charging systems can only handle 20kW of AC wall power...

    See my puzzlement? Does the AC motor become a DC generator? Or is it generating a higher voltage AC that makes up the difference?
     
  20. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Regeneration goes through the inverter, not through the chargers.
     

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