TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Could the Model S in the future be equiped with an automatic gearbox, for more range?

Discussion in 'Model S: Driving Dynamics' started by Benz, Sep 14, 2013.

  1. Benz

    Benz Active Member

    Joined:
    Nov 15, 2012
    Messages:
    1,907
    Location:
    Netherlands
    Could the Model S in the future be equiped with an automatic gearbox, for more range?

    The Model S is currently equiped with a 1 gear gearbox (which works fine).

    First of all, would it make sense to engineer an EV with a automatic gearbox?

    But suppose that it technically indeed would make sense to do that, would it effect the range of the EV? As in ICE cars driving in a higher gear leads to a lower fuel consumption, would that also be the case with EV's (more miles per kWh)?

    Somebody with a technical background could answer this question.

    When I read about the new Audi A3 E-Tron. This new Plug-In Hybrid has a 1.4 liter TFSI engine and a DSG S-Tronic automatic gearbox. And inbetween those there is the electric motor. When driving in electric mode the transmission goes via the DSG S-Tronic automatic gearbox. And that made me wonder if that could also be done with a pure electric car like the Tesla Model S?
     
  2. jerry33

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

    Joined:
    Mar 8, 2012
    Messages:
    12,753
    Location:
    Texas
    I doubt you'd get more range because there is a lot of friction in a transmission (especially when cold). Also Tesla originally tried a two speed transmission in the Roadster and basically the motor's torque just tore it apart. The main thing a multi-speed transmission would do would give you higher speeds. Prius power flow. There are some test results from Aragorn Labs, but I couldn't find the link.
     
  3. K5ING

    K5ING MegaMiler

    Joined:
    Feb 6, 2013
    Messages:
    290
    Location:
    Denton, Texas
    How about a different, slightly lower (number) gear ratio? Acceleration would take a hit, I know, but in the real world I wouldn't need 4 second 0-60 times. I could very well live with 8 second 0-60 times if I could get another 50 miles out of the battery. The motor would be turning slower and the torque may have to be software limited to avoid tearing up the gears. Does a slower motor RPM result in more range? I really don't know.

    Slightly narrower tires with the new Areo covers (yeah, I know they're ugly) would increase range too. It wouldn't handle quite as well, but again, in the real world, it wouldn't matter that much. It all depends on what your driving priorities are...performance or efficiency.
     
  4. richkae

    richkae VIN587

    Joined:
    Jan 15, 2008
    Messages:
    1,917
    The efficiency curve on an electric motor is incredibly flat across most of the rpm range. There is not much gain - if any - to be made changing the gear ratios.
    If Tesla has targeted 60mph for the peak efficiency, but you would prefer it to happen at 50mph or 70mph instead you are probably talking about tenths of a percent.

    There are 3 ways to make it more efficient:
    1) Make it lighter, this allows smaller tires and less rolling resistance. Reducing tire width without reducing weight may be a false economy.
    2) Make it smaller in height or width to reduce the frontal area - reducing drag.
    3) Make it more aerodynamic. The car is already very aerodynamic with the biggest exception being the giant wheels and wheel wells. Covering the wheel wells or aero wheels will work. Getting rid of the side mirrors for cameras will work. Making it uglier by removing some accent curves will work.
     
  5. MarkR

    MarkR Member

    Joined:
    Mar 24, 2012
    Messages:
    893
    Location:
    N. Scottsdale, AZ
    I would rather see some software development that would facilitate some basic hypermiling techniques (limited acceleration, windows up/ventilation, limit energy used, chage driving algorithm based upon terrain (per map & GPS), etc.
     
  6. Benz

    Benz Active Member

    Joined:
    Nov 15, 2012
    Messages:
    1,907
    Location:
    Netherlands
    How about a CVT automatic gearbox ("Constant Variable Transmission")?
    This has a unlimited number of gears (so to speak).
    It has been used in the second generation of the Mercedes A-Class, in the Honda Civic IMA Hybrid, and a few Nissan models as well.
    There must be a solution to make it work in an EV.
    They can do it in a Plug-In Hybrid (Audi A3 E-Tron), so why can Tesla not use it as well?
     
  7. trils0n

    trils0n 2013 P85

    Joined:
    Feb 12, 2013
    Messages:
    1,291
    Location:
    SF Bay Area
    I think the answer is Tesla doesn't want to use a multi speed gearbox. The single speed is a wonderful driving experience and plenty efficient. ICE engine cars have transmissions with multiple gears to make up for their limited usable torque/rpm/power ranges.
     
  8. XrstalLens

    XrstalLens Model S P1327 VIN P01867

    Joined:
    Dec 30, 2011
    Messages:
    115
    Location:
    Kirkland, WA
    A transmission is going to be less efficient than a straight gearbox due to the extra gearing. Automatics are even more so because they need to generate hydraulic pressure.

    The whole reason CVTs (and transmissions in general) were invented was to allow the gasoline engine to operate at its peak power output a higher percentage of the time. CVTs are especially good at that because they can keep the engine running at a constant RPM while the car is accelerating. However, CVTs are notoriously poor at handling high torque, something electric motors have in abundance. A CVT would be a very poor match for an electric motor.

    As you know, a transmission optimizes power output of an ICE by adjusting the ratio of engine RPM to wheel RPM. This is used during acceleration to ensure that the engine is in an optimal power band so the car accelerates faster. While cruising, it is used to save on gasoline consumption by reducing engine RPMs since power demands are not very high. This works on an ICE because the rate of consumption of gasoline is tied to how often the pistons fire. Both of these are necessary and worth the energy cost of the transmission.

    With an electric drivetrain, neither of these concerns matter. The electric motor has plenty of torque across a wide RPM range so that acceleration can pretty much be done as quickly as power can be supplied from the battery. While cruising, only the actual energy needed to keep the car going will be demanded and consumed. As a result, with a transmission you would be paying the energy cost of a transmission without any need of the benefits. This would actually reduce range, not increase it.

    The only real reason to put a transmission in an EV would be to increase the top speed. That's because the electric motor does lose torque as it approaches its maximum RPM, so with a single speed gearbox the top speed is limited by the motor. However, in a consumer vehicle for legal street driving this would not be worth it as you end up paying the efficiency cost of the transmission (not to mention the reliability problems) for a feature than only a small fraction of owners would ever utilize. The Model S has a single speed gearbox and it maxes out at 130 mph, which is plenty for a street car. I think Tesla realizes that 99% of customers would rather have an extra 30-50 miles of range than a higher top speed.

    - - - Updated - - -

    I'm not an expert on motors, but I'm quite certain the answer is no, at least for all intents and purposes. In order for a slower RPM to result in more range that would have to mean that the motor operates notably more efficiently at lower RPMs. I'm am quite certain this is not the case, at least not in any significant way. The Model S gets its maximum range at about 20 mph (see this blog post: http://www.teslamotors.com/blog/model-s-efficiency-and-range), but if you read the text this is not due to the drivetrain getting less efficient but primarily wind resistance. If anything in the drivetrain were to operate less efficiently at higher RPMs I would expect it to be the gearbox itself (pushing the oil around), but the output shaft will be turning the same speed no matter the gear ratio and I can't imagine the efficiency difference to be significant.

    It is clear from that blog post that external factors (wind resistance, cabin heating/cooling) are WAY more significant in affecting range at highway speeds than internal factors like drivetrain losses.
     
  9. Benz

    Benz Active Member

    Joined:
    Nov 15, 2012
    Messages:
    1,907
    Location:
    Netherlands
    Wow, you really seem to know what you are talking about.
    Thanks for your input in this thread.
    Cheers mate.
     
  10. K5ING

    K5ING MegaMiler

    Joined:
    Feb 6, 2013
    Messages:
    290
    Location:
    Denton, Texas
    #10 K5ING, Sep 15, 2013
    Last edited: Sep 15, 2013
    I'm no expert on motors either (obviously). I just didn't know if the motor would use less electricity turning slower or if it didn't make any difference what the RPM was. I had a suspicion it was more a matter of the torque requirements than RPM. In other words, it may be turning slower, but it's working harder. It never hurts to ask though, right?

    As for the tires, narrower tires (within reason) should result in lighter weight, less frontal area and less rolling resistance. As a VW diesel owner, we're always looking for ways to increase our MPGs, and the difference between narrow and fat tires can be pretty significant.

    The other things mentioned earlier to decrease drag are good, but I was thinking of little things that an owner can do at home to eek out more miles per charge. However, at our TDI forum board we have an expression that the best modification you can do is to your right foot.
     
  11. Crispix

    Crispix Member

    Joined:
    Jun 24, 2013
    Messages:
    231
    Location:
    San Diego
    I started to freak out but then I saw in your signature that you "don't own a Tesla (yet)". You'll change your mind as soon as you drive one, so I withdraw my accusation of blasphemy!
     
  12. nolngrgrsngslde

    Joined:
    Aug 14, 2012
    Messages:
    151
    The job of a transmission is to vary the Torque for a given RPM. For an Electric motor, that is done by varying the Amps fed to the motor. So you can say that the Tesla cars do have a "transmission", just not a mechanical one but a software based one.

    This is one of Tesla's innovations, mapping the characteristics of clunky ICE and transmissions into software that controls an electric motor. BTW, an A/C motor consumes Amps in a way that lends itself better to that kind of mapping (vs a DC motor). I.E., it can produce the right amount of Torque in a more easily controlled fashion. One of the reasons Tesla chose an A/C motor and one of the reason Tesla is called Tesla.

    For years we were bound to the mechanical transmission and the ICE engine and effectively kept away from the simplicity and versatility of Electronics and software. It took multiple innovations on the part of Tesla to free us up from those "Bindings". That is what makes Tesla so special.
     
  13. ElSupreme

    ElSupreme Model S 03182

    Joined:
    Jan 13, 2012
    Messages:
    4,279
    Location:
    Atlanta, GA
    XrstalLens gave a good answer. I'll add a bit more.

    As XrstalLens said ICE engines only produce peak torque in a narrow band of engine speeds. In order to give proper torque over the entire speed range of a vehicle (0-110) you need a transmission. An electric motor does not have this problem, in that it produces maximum torque until it produces maximum power then constant power until it gets close to redline. Due to this nature adding a transmission serves little function, unless your vehicle isn't going fast enough when power starts to cut out. As Tesla and all other EV manufacturers have displayed that speed is sufficiently high where multiple gears is not necessary.

    But add on to this fact efficiency. AC induction motors (and well most electric motors) are virtually equally efficient across their entire range. ICEs are not. Their peak efficiency is extremely speed dependent, and very variable across its range. So you want to cruise at low motors speeds (as this in general is where you get peak efficiency in an ICE). An electric motor is very efficient (high 90% range everywhere) so energy out is energy in. If wont matter if you are spinning at 1,000 rpm or 10,000 for the electric motor. Your efficiency is going to be sufficiently close (less loss than adding a mechanical gearbox) that it won't give you appreciably different energy use.

    Due to the complexity of managing an ICE, different fuel rates give different power, different engine speeds give different power, you end up wanting to be low on engine speed (really a sweet spot normally ~2000 rpm), but delivering high torque(thus high, but not max fuel mix ). In order to manage these two variables you need a transmission.

    Not to mention CVTs and Automatics (torque converters) tend to have fairly bad efficiencies. Your induction motor efficiency losses will be much less than adding a mechanical gear reduction. Not to mention the added simplicity, weight reduction, not having a transmission allows for.
     
  14. K5ING

    K5ING MegaMiler

    Joined:
    Feb 6, 2013
    Messages:
    290
    Location:
    Denton, Texas
    I fully expected to catch some flack for that remark, but coming from a world of VW TDIs, we're braggarts not of 0-60 times, but of longevity (see my avatar) and fuel mileage/efficiency. The first thing we think of is "how can I make this even more efficient?".

    Car enthusiasts come in more than one flavor, but I have no doubt that I would be seduced by a Tesla if I drove one. That's the one reason I've avoided doing so. I'm afraid I'll start selling body parts and my first born to get one if I did.
     
  15. dsm363

    dsm363 Roadster + Sig Model S

    Joined:
    May 17, 2009
    Messages:
    18,235
    Location:
    Las Vegas, NV
    If you drive gently and slowly like you would any other car for fuel efficiency you would get much closer to the 300 miles Tesla claims.
     
  16. K5ING

    K5ING MegaMiler

    Joined:
    Feb 6, 2013
    Messages:
    290
    Location:
    Denton, Texas
    I'd never drive so slowly that it would be impeding traffic. I just do the speed limit or +5, accelerate with the traffic, use my cruise even in 40mph boulevard driving, and coast up to lights and stop signs in gear. I still get 50mpg out of my Golf. I'll admit to going a bit crazy on deserted twisty back roads, however. :biggrin:

    It all goes back to the best modification of all.... your right foot.
     
  17. mnx

    mnx 2013 P85

    Joined:
    May 6, 2009
    Messages:
    2,281
    Location:
    Ancaster, Canada
    Speaking of transmissions, if the Model S had a 2nd gear designed for top speed what do you think the top speed could be? Could it go >300km/h?
     
  18. PaceyWhitter

    PaceyWhitter Member

    Joined:
    Dec 27, 2012
    Messages:
    75
    Location:
    Columbus, OH
  19. richkae

    richkae VIN587

    Joined:
    Jan 15, 2008
    Messages:
    1,917
    #19 richkae, Sep 16, 2013
    Last edited: Sep 16, 2013
    I think it absolutely could. It's just a question of drag vs power.
    I went to this website http://www.wallaceracing.com/top-speed-hp.php ( which could easily be wrong - it's the internet ) and punched in 186mph, 0.24 Cd and 28.6 sq ft frontal area and got 294.7 hp required.
    Using 33.8 sq ft ( which is likely overestimating and is certainly an upper bound ) yielded 348.28 hp required.
    Is my 0.24 Cd number the same as what Tesla has been claiming lately? ( My frontal area numbers are guesses, I'm not sure how to properly calculate that )
    The numbers I've seen for BMW M5s are 0.29 and 0.30, so the Model S beats the hell out of those.

    However you may run into a problem of not enough downforce at high speeds and the car might do this without some aerodynamic changes: Le Mans 1999 Mercedes Flip Crash - YouTube
     
  20. AnOutsider

    AnOutsider S532 # XS27

    Joined:
    Apr 3, 2009
    Messages:
    11,923

Share This Page