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gearbox

Discussion in 'Model S' started by Mitrovic, Feb 20, 2011.

  1. Mitrovic

    Mitrovic Member

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    Sorry if this was answered elsewhere, I was not able to find it?

    Will model S have the same system as the roadster? or will it have a gearbox with - lets say - 2 speeds?

    Any information?

    I'm no engineer but my roadster is at speed on the highway ( Europe )

    - noisy
    - Uses a lot of energy

    both things the model S would like to omit. As to be successfull in Europe it needs to be a miles cruncher at Autobahn-speed.
    So may be a gearbox would help there?
     
  2. richkae

    richkae VIN587

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    I am quite sure the Model S will also use a single speed and this is a good thing.

    The highway noise of the Roadster is entirely due to road and wind noise. The noise of the motor revving is almost unnoticeable.
    The car was designed with very little attention to road and wind noise. The Lotus Elise it is based on is a no-compromise, no-frills race car. There is no significant sound proofing until the 2.5 models and they have tried to improve it as best they can without a redesign.

    I am very confident that the Model S is being designed from the ground up as a luxury car and they will seal the noise out.

    The energy use of the motor is very efficient across the entire RPM range ( probably better than 90% efficient of turning electrical energy into motive force across the usable range )
    Energy use increases with the square of speed, there is no getting around that. The faster you want to go the more energy you will use.

    I am not an engineer either so someone correct me if this is wrong:

    The reason you believe that changing the gear ratio and lowering the RPM of the electric motor will be more efficient is because ICEs are have wildly varying efficiency across their RPM range. At any certain speed you can pick a gear to select the RPM you want the motor to turn.
    This choice gets you a certain efficiency and available torque.

    With the electric motor choosing a different gear would have a very minimal effect on efficiency, but may give you different available torque.
     
  3. Mitrovic

    Mitrovic Member

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    I do not bother with the noise in my roadster, that is a sports car and that is absolutely o.k.
    But the Model S has to be a lot quieter. I thought anything which could help would be great. So I thought even an electric motor makes more noise at higher speeds.
    I wonder why they did want to make the roadster with two speeds at the very beginning. I thought may be it would be possible to make the motor may be more efficient? But, of course, I do not know, I'm just thinking and I'm wondering what Tesla engineers have found out...
     
  4. jtibble

    jtibble Tesla Evangelist

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    The main reason Tesla abandoned the two-speed for the Roadster was that the shift performance was not good enough. The car would go 0-60 in just a few seconds, then you'd have to pause for an entire second while the motor spun down from 11,000 to 1,000 RPM. There is a tremendous amount of inertia in the motor when it's spinning at that speed and it just wasn't feasible to put that much of a shift delay in a high-performance car. I imagine the Model S would have the same issue (you accelerate smoothly up to 60mph, then wait a second, then continue accelerating....). It doesn't really appeal to me, especially when a single-speed motor can be geared to be maximally efficient when at highway speeds.
     
  5. Cobos

    Cobos S60 owner since 2013

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    They did also have another option for the gearbox that managed shifts at full rpm or almost full. Though those lasted a few thousand miles, so slightly expensive for Tesla to replace...

    Cobos
     
  6. SByer

    SByer '08 #383

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    From what I remember: early versions of the PEM couldn't handle the amount of current they needed for a single gearbox configuration to hit both the wanted 0-60 time and the top speed requirement, thus the need for a transmission. One was reliable but too slow in switching gears. Another was fast enough but wasn't reliable enough (I got to drive that one!). While trying to get a transmission that met both requirements, some components used in the PEM had been upgraded by the manufacturer to handle higher current, which opened the window to a fixed gearbox that met the 0-60 and top speed requirements (well, close enough ;-).

    As for autobahn speeds, can't get past cav^3, just can't - so even if Tesla gears things for a higher top speed, your 300mi big battery would only get you 120mi or so diving that fast. I suppose that should be driver choice, though ;-).
     
  7. jkirkebo

    jkirkebo Model S P85+ VIN 14420 EU

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    Hmm, maybe they should offer the Model S with different final drives ? A customer in Germany might want a 150mph top speed while here in Norway I would be perfectly happy with 80mph top speed (the national limit is 63mph and I rarely go even that fast, mostly 55mph). So you could choose yourself, higher top speed or better acceleration. Three different choices would be more than enough, maybe even just two.
     
  8. Adm

    Adm Active Member

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    Offering more final ratios would make the production more complex and would diminish the already limited economy of scale. I hope Tesla will keep it simple to begin with. It´s most important to get production going with as little delay as possible, while making a profit will be essential in the survival of the company.
     
  9. widodh

    widodh Model S R231 EU

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    I suggest you take a look at: Roadster Efficiency and Range | Blog | Tesla Motors

    There is no getting around those numbers, that is simple physics! The more aerodynamic a car is, the less energy it will use. But the faster you drive, the more energy you will use.

    The "problem" with autobahn speeds is that current ICE's get more and more efficient, the faster you drive. Since a EV is almost always efficient, you loose a lot of range when you start driving faster and faster.

    I think we can forget driving at 180km/h for a longer period with a EV. How much I love "flying" over the Autobahn, it will be a completely different game with a EV.
     
  10. Nik

    Nik Dreaming no more :-(

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    How/why does an ICE get 'more and more efficient' with speed? Yes, up to a point some energy usage is constant, and is thus spread over more miles at higher speed, but aerodynamic and rolling resistance increase with the square of speed, so will always win out.

    See:
    http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Speed_and_fuel_economy_studies
     
  11. widodh

    widodh Model S R231 EU

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    That is true, but a ICE starts to get more and more efficiënt the closer you get to it's max power. The faster you start to drive, the more power the ICE has to produce.

    Yes, your consumption goes up, but it doesn't scale the same as the power output does.
     
  12. Mitrovic

    Mitrovic Member

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    This seems not to answer the question. Of course the roadster uses more energy when driving faster.
    BUT how is it about the electric motor only? How much more energy does it use for 14 000 RPM then for 7000 RPM. This raises the question, whether sort of an Overdrive ( I'm an oldtimer guy, sorry ( I mean cars, not my age ) ). Would this help save energy if you are e.g. driving at constant speed at about 120 Km/h and it kicks in? So this would not cause any problem with acceleration, as it is used only at constant higher speed. It would be also easier to engineer I guess, as it has not to switch gears when accelerating but only when at constant speed.
    Any thoughts?
     
  13. widodh

    widodh Model S R231 EU

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    An ICE has a overdrive to get the RPM down, thus the consumption.

    A electric motor has a very high efficiency all over it's RPM range: http://zooi.widodh.nl/ev/tech/torque-curves.jpg

    But a gearbox will put in extra friction and cause the drivetrain to consume more energy. That's why all EV's a a single-fixed gear, there is no need for a gearbox.
     
  14. Mitrovic

    Mitrovic Member

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    Again, this seems not to be the answer for me. I know, of course that the electric motor is great and has high efficiency and high torque nearly at all RPM.
    BUT again: more RPM, more energy. So, if you could get the RPM's down, couldn't you put the consumption, down, too?
    A conventional gearbox has it disadvantages. But an "overdrive"?
    Why did they want to make the roadster with a 2 speed gearbox at the beginning?

    Of course, I do not say that this would be a good idea, just thinking loudly...
     
  15. JimmWilks

    JimmWilks Member

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    Personally I doubt that a two-speed gearbox would improve the efficiency that much, perhaps a bit but I'm not sure that would offset the losses from additional weight and friction. Maybe advantageous in terms of motor wear-and-tear? I don't know anything about Tesla's motor but I imagine there are some pretty immense forces associated with the circular motion of a lump of metal spinning at 13000 rpm. If gearing could reduce this maybe the motor will survive longer.
     
  16. Doug_G

    Doug_G Lead Moderator

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    The Roadster's top speed is limited by the redline for the motor. At 14,000 RPM the Roadster is going 200 kph (125 mph). A second gear would allow for higher top speed. Not that you could sustain it for very long.
     
  17. aviators99

    aviators99 Model S - R140

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    What would a second gear's impact on engine braking be? I understand that if there's less engine braking, there's less regen (thus less range). I haven't test driven the Roadster (mostly because I'm sure I would end up buying one, and I really don't want to do that), but engine braking concerns me. I imagine it feeling like an ICE car in first gear, all the way up to redline. But I gather that this is not the case?
     
  18. strider

    strider Active Member

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    Having driven a large-displacement manual transmission ICE car for a number of years (C6 Corvette) I can tell you that regen in the Roadster is both stronger and much smoother than 1st gear engine braking in my Vette. For one thing it's much more linear and easier to vary the amount of deceleration - no jerky driveline lash nor multiple gears to drop through with the accompanying jerk in each one. It's also a lot quieter so feels softer. You really should just go and drive one but you are correct to be worried - I went in for a Model S and walked out w/ a Roadster.

    On the second gear subject, aside from a few places (and getter fewer every year) on the autobahn and race tracks, one never needs more than 125mph. People always ask me, "how fast does it go?" Sure, it's top speed is slower than my Corvette but the 0-60 time is quicker which means it's more fun to drive every day. Less complexity means fewer things to break and less energy lost to friction. I for one haven't missed the top speed but love the acceleration.
     
  19. bolosky

    bolosky Member

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    A good way to think about where the energy in a car goes is to break it down in to three catagories:
    1) Engine losses (i.e., how much of the energy that goes into the engine/motor doesn't come back out as mechanical energy).
    2) Aerodynamic drag. This depends on the speed and the shape of the car (and how dense the air is), but not the engine/motor.
    3) Overhead. This accounts for stuff like the lighting, HVAC, turning the flywheel in an ICE, the radio, etc. It doesn't depend on speed.

    ICE cars have pretty high overhead, so as they speed up they become more efficient for a while, because they're amortizing the overhead over more miles. Electric cars, on the other hand, have very low overhead (unless you're running the HVAC, especially the heat), so as you speed up they don't get much more efficient.

    Losses to aerodynamic drag per unit distance go up with the square of speed. Because of that, as you get moving they grow really quickly and start to dominate everything else. In an ICE car, this effect is competing with the increased efficiency of amortizing the overhead, so you're going faster before you hit the tipping point. In an electric, there's much less overhead so the peak of the efficiency curve is much slower.

    There is some additional engine loss at very high RPMs for electric motors, but it's not that huge. Changing the gearing would help that, but since it's a tiny component of the overall energy budget, it won't help much. You'd do better to spend your money trying to reduce the coefficient of drag, say by lobbying to make it legal to replace the side mirrors with cameras, or adding fairings to the wheels.

    To be clear here, the discussion above might make it sound like it's better to have high overhead because it makes it more efficient to go faster. It's not. What's going on is that the electrics have to work with a much lower overall energy budget. A gallon of gasoline has well over 30 kWh of energy, so a full battery pack in the Roadster has only 1.5-ish gallons of gas worth of energy. Because the electric powertrain is so much more efficient, this is results in decent range even with pretty low overall energy.
     
  20. widodh

    widodh Model S R231 EU

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    @ bolosky: You said what I tried to say :)

    For us Europeans: 1 Liter of gasoline has about 10kWh of energy, where the Roadster has a battery-pack of 53kWh.

    Funny fact, I have an electric scooter with a 1.8kWh battery-pack, but I can drive about 70km before it is empty. That is efficient transportation!
     

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