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Another gearbox discussion

Discussion in 'Technical' started by just-an-allusion, Nov 26, 2008.

  1. just-an-allusion

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    Forgive me if I'm being obtuse, but since this (Tesla/electric powered vehicles) is all about the development of "new" technology/a "new" way of doing things, then why is a "gearbox" (let alone a multi-speed gearbox) such a necessity?

    Doesn't the implementation of "old school" thinking in "new school" ideology defeat the purpose altogether? I mean, judging by all of the snags, hangups, financial losses, managerial restructuring and engineering shortfalls encountered throughout the (continuing) efforts to develop a multi-speed gearbox for the Tesla, the one seems to be canceling the other, so why bother?
     
  2. just-an-allusion

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    #2 just-an-allusion, Nov 26, 2008
    Last edited: Nov 26, 2008
    P.S. Yes, while I do understand Porsche's interest in electric vehicles, afterall, I believe that it was Porsche founder, Ferdinand Porsche, who designed the first electric powered vehicle which utilized an "in-wheel" motor (and was set to market it until Ford devised the method for mass-producing his Model T's, and history notes where that went from there) http://www.autohistory.org/feature_6.html , a technique which has been utilized in a high speed supercar designed by a Japanese professor, named Hiroshi Shimizu, and built by several of his engineering students at Keio University in Toykyo called the Eliica ("e-lee-ka") eliica.com￾@-Œcœä‹`￾m‘åŠw“d‹CŽ©“®ŽÔŒ¤‹†Žº-

    I just want to know why it is felt that a multispeed gearbox is such a necessity"?"


    p.s. Hello all, I'm new here and I've got questions, LOL!
     
  3. just-an-allusion

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    I would still just like to know ?WHY? a multi-speed gearbox is such a necessity for the new-age technology know as "Tesla"?

    Anyone??
     
  4. graham

    graham Active Member

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    Perhaps I am misunderstanding your question, but current Teslas do not have multiple gears. The current transmission has only one speed.

    The original prototype and first few cars had a two speed transmission to allow for both fast acceleration as well as high top-end speed. They replaced it with the simpler 1 speed when they came up with a way to boost motor output to allow for both quick acceleration and reasonably high top speed.
     
  5. SByer

    SByer '08 #383

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    Originally, it was to meet two competing goals - a 0-60 time in a range to support the price, and a high-end speed that was respectable. With the original controller and motor cooling design, they couldn't do both with a single gear ratio.

    In-hub motors are interesting, but provide a lot of un-sprung weight, which goes against good handling. There are some in-hub motors which are compact and lightweight, and, on their own, are only slightly heavier than a "normal" wheel's un-sprung weight - in current rolling prototypes that don't have physical brakes as well. It's probably solve-able, eventually. Just not ready for primt time, yet.
     
  6. vfx

    vfx Well-Known Member

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    #6 vfx, Nov 26, 2008
    Last edited: Nov 26, 2008
    The first gearbox was 2 speed. It would only last about 5000 miles then the high turn speed of the electric motor would burn up the gears on shifts. The second try at a gearbox was equally unsuccessful.
    By then a year had gone by and newly available electronics allowed them to boost the motor to power and speed so that no multi stage transmission was necessary at all.

    Remember though, the motor spins so fast it still needs a reduction gear (box).

    Tesla calls the setup "1.5". For a complete explanation, check out what JB Straubel, Tesla's Chief Technology Officer says here: Tesla Motors - touch

    The new "gearshift" is just a simple switch. It has just three positions for Forward, Neutral, and Reverse.
     
  7. just-an-allusion

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    While I'm aware that the Tesla is capable of both, what you're saying is that they are not capable of both at the same time/it's either one or the other?
     
  8. just-an-allusion

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    Now that's more like it!

    Thank you for the input (graham) and the linked insight (vfx) clearing the whole issue up for me...it does appear that Tesla has finally chosen both the most economical and technically applicable method of getting the cars down the road (for now anyway :wink: ).

    Again, I'm finding the need to ask the question "why?" So what if the motor "...spins so fast...", isn't that what both consumers and the automakers are clamoring for?

    Or is this "reduction gearbox" more so a practical application as a means of "de-tuning" the motor so that no one kills themselves/Tesla drivers cannot outrun the police? Just curious.
     
  9. vfx

    vfx Well-Known Member

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    1n, (can I call you "1n"?), what does this remark from JB tell you?
     
  10. just-an-allusion

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    Sure, why not. I guess you found it troublesome switching between alphanumeric and numeral keypads, huh?

    That the motors are deliberately "limited", which is what I got out of it, I was merely questioning the reasoning behind the limitation, i.e., was it over practical/technical concerns (gear carnage), or was it due to humanitarian concerns?

    I only ask because everyone across the EV spectrum seems to be clamoring for "more", be that in the form of speed, mileage, duration, longevity, or perhaps even all of the above, a sort of happy medium?

    The robust competition in the field sparks curiosity over the choices the EV makers decide to run with, so just asking....
     
  11. TEG

    TEG TMC Moderator

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    #11 TEG, Nov 26, 2008
    Last edited: Nov 26, 2008
    Huh? The technology basically dictates the behavior. Are consumers clamoring for 14K RPM engines in cars? Actually for internal combustion engines many would prefer a larger slow moving engine so that it produces less noise and lasts longer. I don't think consumers want little high revving gasoline engines in all their cars. A big Mercedes with a small displacement high horsepower, low torque 14K RPM motorcycle style engine wouldn't be luxurious. An electric motor at 14K RPM produces much less noise than a gasoline engine at 14K RPMs so it is basically apples to oranges. Also, an electric motor with 14K RPM capability can still have tremendous low end torque good for fast starts. A typical high revving motorcycle engine has poor low end torque and everything at the top end which doesn't offer driving characteristics that non-racers want for stop and go traffic driving. So, no, I don't think the average driver wants the highest possible revving engine. But for Tesla's AC motor, there is no downside at the low end, and only a minor noise and reliability issue so using a very high redline works out and avoids the need for extra gears.

    No, no, no. I don't think it is in any way an intentional "de-tuning". I think Tesla would have gladly offered 0-60 in 2.9 if they could achieve it and still have it be reliable and capable of an acceptable top speed.

    If they had no gear reduction it would likely be capable of quicker 0-60 but then the top speed would be too low. I suspect that when the original 2 speed didn't work out, they scrambled to get more low end torque out of the eMotor, and once they did that they picked a gear ratio that would deliver their previously promised 0-60 in 3.9 second time and they were forced to live with whatever top speed resulted. In this case top speed is now about 125MPH down from the original 135MPH they originally specified. I think they made the right choice to preserve the promised 0-60 time and let the top speed slip a bit.

    My position all along is that top speed is a bad metric for a non-track EV to go after. The battery pack just doesn't store enough energy to provide a meaningful range when you are operating at those very high speeds. Also Tesla's air cooled eMotor technology is not well suited to extended duration runs with the eMotor producing maximum HP due to heat buildup.

    As long as the top speed doesn't limit 1/4 mile time everything should be OK.

    Personally I think the Tesla Roadster specs are near perfect for this application with the possible exception of battery life (which is yet to be fully determined).
     
  12. dexlargo

    dexlargo Member

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    From my non-expert automotive understanding...

    I think that you have a misunderstanding of the purpose of the reduction gear. If the ratio was lower, say 1:1, then for every revolution of motor, the wheels would have one revolution. This would be great, at 10k RPM of the motor, the wheels would be turning 10k times per minute. How fast would that be? Tesla rear wheel circumference is 78.452 inches, so at 10k RPM that would be 743 miles per hour. The problem is that the motor wouldn't have the power to turn the wheels at that ratio. It would be able to impart 1/8th the power that it could to the wheels as at 8:1 reduction. Essentially, there would be approximately zero torque at the wheels.

    All cars reduce through the transmission and rear axle, with each gear giving a different ratio. Higher ratios in the lower gears, and lower ratios in the higher gears, this allows the car to have power at slow speeds, and also higher speed capability.

    So what Tesla did was increase the motor size so that with a single gear ratio they could achieve sufficient torque to get to 60 in 3.9, while still retaining the top speed of 125. No conspiracy.

    At least as long as my basic understanding of automotives and the Tesla is correct. Corrections?
     
  13. vfx

    vfx Well-Known Member

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    There was talk on these pages about adding a third gear to get a higher top speed. I believe that 180 mph top end was discussed.
     
  14. WarpedOne

    WarpedOne Supreme Premier

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    Actually 170mph, but that was before I/we realized the current aircooled motor is not capable of sustaining even 125mph. At 170mph it would be outputing constant 185kW. At 80% efficiency that means almost 50kW of heat i.e. very quick "core meltdown".
     
  15. doug

    doug Administrator / Head Moderator

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    What dexlargo had to say is basically right (I didn't check his arithmetic, though :wink:). I'd suggest that 1n5p1r4t10n (and perhaps others) go back to his high school physics (mechanics) text book and review the concepts of power, torque, force, energy, work, simple machines, and mechanical advantage. A reminder of those basic concepts will go a long way towards understanding gearboxes and a bunch of other things too.
     
  16. TEG

    TEG TMC Moderator

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    I don't think they increased the motor size, actually. They just improved the motor controller ("PEM") to be able to deliver more current at lower RPMs. They probably could deliver higher current at high RPMs too, but I don't think the air cooled motor can handle making that much horsepower.

    So, although the DT 1.5 isn't really producing more peak HP, it is now able to provide closer to peak HP over a wider RPM range, thus the improved performance.

    I also suspect they have a whole slew of upcoming improvements in DT2.0 for 'Model S'. Maybe someday they could put the 'Model S' motor in the Roadster to get more peak HP, but what they have now seems plenty sufficient for most Roadster owners.
     
  17. just-an-allusion

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    #17 just-an-allusion, Nov 27, 2008
    Last edited: Nov 27, 2008
    It is this issue that prompts me to question the professed necessity of reducing the motor's revolutions for so-called practical purposes in that, would not a motor maintaining a certain amount of operable revolutions, as a byproduct of generating those revolutions, effectively and efficiently cool itself in the process, thereby serving to both increase performance and durability at the same time by maintaining optimum operational temperature across the spectrum, not to mention 0-60 and top end specs?

    Also, a motor makes very, very little noise compared to an engine, regardless of the speed of it's operation...I doubt that anyone would notice at all, not to mention the effect that an increased rotational force would have on the car's overall operational stability (depending on it's orientation that is)...screw cornering, you could drive it upside down (maybe, almost).
     
  18. TEG

    TEG TMC Moderator

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    #18 TEG, Nov 27, 2008
    Last edited: Nov 27, 2008
    I don't profess to be an expert on this, but I think you have a lot of mis-conceptions about what is going on there.

    Here are a few things to ponder:

    #1: The internal rotation of the eMotor's rotor doesn't provide much (if anything) in the way of cooling. I think it is sealed in there. The air cooling comes from an external fan blowing air over the motor. If anything, faster rotation would cause more bearing friction and generate more heat.
    #2: There are a number of things that limit how fast you can make it rotate. The bearings can only turn so fast. The PEM ("motor controller") can modulate the magnetic fields only so quickly. Wind resistance against the rotors are probably a factor at some point as well.
    #3: (In case you didn't understand) the gear reduction in the gearbox doesn't slow the eMotor down. Actually to the contrary it allows the eMotor to spin up more quickly at slower car speeds. The gear ratio provides some "leverage" to let the car accelerate more quickly. The gear ration chosen to meet a particular 0-60 time indirectly dictates the top speed of the car because the eMotor has a practical maximum RPM so multiplying max eMotor RPM x Gear Ratio ( x Wheel diameter x SomeRotationToMPHfactor) determines how fast the car can go.
    #4: As has been mentioned in some German Roadster reviews, the Roadster can't maintain max RPM for long periods of time. The motor controller intentionally slows down the eMotor to prevent if from overheating if you try to hold top speed too long. The eMotor is very efficient (doesn't product a lot of waste heat), but it also isn't the best design to be able to dissipate any heat that does build up. Some discussion suggests that future Tesla models are likely to liquid cool the eMotor to provide better substained top speed capability.
     
  19. power

    power Member

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    #19 power, Nov 30, 2008
    Last edited: Nov 30, 2008
    Part of the confusion of the gear box discussion is a basic misunderstanding of what really accelerates a vehicle. Tesla has designed a single speed gear reduction that uses a near 8:1 ratio. Coincidentally, this is a very common final ratio for most high performance piston engine cars in 1st gear and usually reach a speed of 50-60mph. Because of its near flat motor torque curve, (which is no different than many piston engine's torque curves at near the same power levels), both a piston engine in 1st gear vs a Tesla Roadster with similar peak HP power plants will accelerate at near the same rate up. (If we are talking about comparable weights). The Tesla Roadster then still is able to accelerate up to 120mph still in the same single gear.
    What's more amazing about the Tesla Roasters power plant, is not so much the flat torque curve to 6000rpm, but the slow decay of power to 14,000rpm. As we know, this limits the top speed and acceleration of the vehicle to 120mph. If the Tesla Roadster was able to have a second gear that was selected somewhere well past its peak HP was reached, its top speed would be extended as well as increased acceleration to that top speed.

    Acceleration is proportional to power at any same vehicle speed as seen by this simple Newtonian identify:
    Acceleration=Power/(mass x velocity)

    So, the idea is to maximize power at any vehicle speed. In the end, the way it will probably be done, will be to use a IVT (Infinitely variable transmisson). This way, max power would always be used when max acceleration was required, and some other rpm could be used that was in the motor's peak efficiency range for a given cruise speed.


     
  20. TEG

    TEG TMC Moderator

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    I think Tesla should just stick with "direct drive" single ratio gear boxes for all models. The eMotor is so flexible that you can get away with that and it helps efficiency, and reduces cost and complexity. Also it eliminates one more thing to think about while driving.

    If they need a bit more top speed they could work on better cooling and perhaps even a bit more high RPM capability without having to resort to the complexity of a shifting mechanism.

    CVT/IVT is great/useful when you have a gas motor with peaky torque characteristics, but the eMotor is so flat that it doesn't seem needed.
     

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