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An alternative to shifting in a Tesla...

Discussion in 'Model S' started by fadkar, Feb 27, 2015.

  1. fadkar

    fadkar Member

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    As I was watching some popular YouTube videos of P85Ds accelerating faster than Ferraris from 0-60mph, it got me wondering possible solutions to increase the Tesla's performance at higher speeds to keep up with Ferraris even past 60mph.

    Now, I am far from an engineer, and have almost no knowledge on the mechanics of these complex automobiles so I apologize in advance if my hypothetical solution is totally absurd.

    I think the reason why Teslas lack the same punch when accelerating at higher speeds is because they only have a single reduction gear while those Ferraris have 6 or 7 gears which allows them to get back in the powerband and ultimately beat the P85D.

    Instead of using gears which can be pretty cumbersome (I'd imagine waiting for the motor to shift down from 14,000rpm to 1000rpm would take a long time!), why not use the dual motor configuration to fire both motors in an alternating pattern to give you max power at any speed? For example, let's say a hypothetical Tesla had two 400hp motors. When you floor the car, only one of them turns on and starts spinning to the max rpms. But right as the motor is done delivering peak power, the second motor kicks in and delivers its max power as the first motor slows down to prepare to deliver its peak power once the second motor reaches its limits.

    This configuration might not be the most efficient but maybe it could allow Tesla to keep up with those supercars?

    Again, pardon my ignorance on the issue.
     
  2. SR22pilot

    SR22pilot Member

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    It's not that easy. Tesla motors are geared but with a single reduction gear unit. The problem is to reduce motor rpm to a lower range where torque and power are more optimum. CVT would be great and fit the feel of the car but they tend to tear up with high power motors. The original roadster had two gears but reliability was poor. The same torque at zero rpm that makes for great 0-30 times also tends to tear up transmissions. A single reduction gear also saves money. On a dual motor you can gear each one differently to make one better at high speed and one better in the 0-60 range of daily driving. I believe the P85D does that.
     
  3. RAW84

    RAW84 Member

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    I'd imagine in your hypothetical, the Model S would lose at lower speeds with just one motor going.
     
  4. Mookuh

    Mookuh Member

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    The reason the Tesla loses at higher speeds is not because the motors are outside their optimum curve - it plays a role, yes, but the drop in available power is not that drastic.
    From my (still somewhat limited) understanding, the Model S cannot keep up with e.g. a McLaren F1 at higher speeds because of the weight to power ratio. The F1 and the P85D have similar BHP values, however the F1 weighs about half as much as the Model S does.
    The Model S wins at lower speeds because the electric motors can deliver the full torque from stand still, as is touted around frequently. Once the ICE gets to deliver its power, the two cars have similar power "pushing" them, but the Model S is slower due to the much higher weight. Aerodynamics will start to play a role aswell, and while the Model S has an incredibly low cW value, this gets multiplied by the frontal area of the car to figure out wind resistance - and the frontal area of a McLaren F1 or Ferrari is much less than that of a Model S.
    In order to "win at higher speeds", the Model S would simply require stronger motors - which would of course draw much more power.
    If you want an electric car that can compete with super cars at any speed, you will have to purpose build one in a matching frame, not beef up a 2-ton family sedan. (Rimac Automobili Concept_One).
     
  5. eye.surgeon

    eye.surgeon Member

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    I'd rather tesla focus on extending range or other real world practical drivetrain improvements. This whole YouTube drag race thing is entertaining but doesn't help my commute.
     
  6. MassModel3

    MassModel3 Member

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    +1

    I'm pretty sure I read a tweet or article that quoted Elon Musk saying that the incredible acceleration was not the goal, but was a welcome byproduct of the Model S design. Okay, he did say that the dual motor models he was hoping to match the 0-60 of the McLaren F1, but that was never the original Model S goal. Range, reliability, comfort and convenience were. Everything else is just icing on the cake.
     
  7. AnOutsider

    AnOutsider S532 # XS27

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    Tesla uses the performance of their cars as part of its appeal to the masses (and it gets them a LOT of free publicity). This is a thread to discuss a hypothetical solution to a problem for folks who enjoy performance cars. If that's not your cup of tea, why knock it over?
     
  8. Victory

    Victory Member

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    My thoughts exactly. I personally don't care as much about the speed or acceleration as some other factors in a car but the first thing people bring up when asking me about my car is the acceleration. I suspect if Tesla wasn't so fast, it would be remembered more along the lines of a faster/longer range leaf as opposed to the world renowned car it is today.
     
  9. tonybelding

    tonybelding Active Member

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    It occurred to me that, instead of a transmission with multiple gears, it could be possible to use different gear ratios on the front and rear motors. Let's say if the front had a taller gear, then it would provide less assistance to the 0-60 times, but perhaps improve passing performance up at highway speeds.

    Then it occurred to me that, even if this worked, it would probably be a needless complication to solve a non-problem.
     
  10. Mookuh

    Mookuh Member

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    That's what the dual motor variants are doing, and the reason why they can get more range, because they can use the motor closer to it's optimum power band.
     
  11. Gizmotoy

    Gizmotoy Active Member

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    Right. The idea proposed is already being used in the AWD variants. The front motor has taller gearing, and as speed increases the front motor plays a larger role in powering the vehicle, which has the added benefit of both making the car more stable at speed while also increasing efficiency.

    As for the OP, I'm not sure how he expects that to work without variable gearing. If the first engine reaches its limit, you switch to the second motor, and the second motor then reaches ITS limit, you can't simply go back to the first. It's already well exceeded its maximum rotational speed (that's why you switched to the second motor in the first place). Your only options are a transmission or adding additional motors with staggered gearing.
     
  12. tonybelding

    tonybelding Active Member

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    Hey, maybe this could finally explain the Eliica! With eight wheels, it could have up to four different gear ratios with no shifting!
     
  13. Twiglett

    Twiglett Single pedal driver

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    I think that is what he was talking about.
    Imagine a system with two identical motors connected to a six speed transmission.
    You alternate between the two motors to shift gears
    Motor A is "first, then motor B is second.
    Third gear gets motor A again now that is has had time to drop rpm from its top range.
    Then fourth get motor B and so on.
    I'm assuming it takes a while for a motor to lose all that inertia without some kind of braking system to slow it down.
    Horribly complicated.
    I'm sure I remember reading about someone coming up with a transmission for EVs though......
     
  14. fadkar

    fadkar Member

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    That makes sense. I did not know that they use slightly different gears.

    Yeah, it likely would sacrifice the initial acceleration to gain more high speed acceleration. Seems like we can't have both!

    Hmm you do have a point. However, I've read that the Tesla's power decreases quite a bit at speeds over 70mph. This isn't the case with other large (non sports cars) like our Jaguar XJ. You can be going 70mph and punch the gas and hit 90mph really quick, which leads me to believe that it is the motors. I don't have my P85D yet to test how powerful the acceleration is at higher speeds to compare though...

    - - - Updated - - -

    I agree that there are more important things that Tesla can be working on. I am not saying that they should abandon those and focus on more power. This is just a hypothetical idea...
    However, I would not discount the importance of the P85D's acceleration. I think the P85D is a huge statement to EV critics. The fact that it can outrun so many super cars is a slap across the face to the people who thought EVs are just cars for people who care about the environment. It's also why you can't find a single review that doesn't talk about the impressive performance of the car:cool:

    It may seem bad, but we didn't get the P85D because it is environmentally friendly. It is a large plus, but we got it because it's a hell of a fun car to drive, it's safe, technologically advanced, sexy, and practical.

    - - - Updated - - -

    That is EXACTLY what I was talking about! It is complicated, but I'd be interested to see how it would pan out in the real world.
     
  15. lolachampcar

    lolachampcar Active Member

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    It is really not a gearing issue at all........ You will need to look at this from another (non-ICE) perspective.

    Energy needed goes up with the square of speed. Watch the power meter on the PD and you will see that Tesla allows 460ish KW-Hr for zero to thirty then backs it down. Ideal efficiency would yield 460K/750 or 613hp but only for zero to thirty mph. Looking at it from a battery draw standpoint, Telsa is taking a range battery and pulse discharging it (like 1.5 seconds of pulse) at 460/85 or 5.4C. That does not sound like much in the model RC LiPo world but it is a lot in the BEV world.

    Fast forward (pun intended) to higher speeds and you have a Tesla's need to limit discharge rate contrasting the need for extended periods of very high energy consumption. ICE just blast along burning gas at 150 mph just like it did off the line. We BEV types can not do this without specially designed packs. You simply can not beat the low impedance energy density of liquified dead dinosaurs (and some plant matter).

    Gearing aint it :(
     
  16. fadkar

    fadkar Member

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    Ah I see what you're saying. Maybe gearing might help a little, but essentially, there's a large difference between continually dumping gas into an engine vs strategically supplying (limited) electrical power to a motor. So no formula 1 race cars will be switching to electric power anytime soon :p. But they certainly shine on the road where 0-60mph is the most important range. I just hope our P85D doesn't feel "slow" when overtaking another car on the freeway!
    Thanks for the informative answer!
     
  17. artsci

    artsci Sponsor

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    I don't think you need be concerned about that. The the P85 overtakes very quickly and the D is better.
     
  18. jdbob

    jdbob Member

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  19. fadkar

    fadkar Member

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    Wow that's cool! But it says they hit a top speed of 140mph. Even though it is a technological marvel that electric cars can race at such speeds, it doesn't compare to the 220 top speed of Formula 1. Thankfully, that isn't as relevant for a daily driving car.

    - - - Updated - - -

    Sweet. I can't wait to get the car, put it to the test, and drive it endlessly:)
     
  20. richkae

    richkae VIN587

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    #20 richkae, Feb 28, 2015
    Last edited: Feb 28, 2015
    I disagree.

    If you are looking at sustained high speed driving, then sure you aren't going to go very far if you are exerting 280kW. ( At 280kW you are going to drive for a maximum of 18 minutes on an 85kWh battery ). The peak power for the 85D is listed as 280kW and top speed is listed as 155mph so you should travel about 47 miles in those 18 minutes.

    But ICE don't blast along burning gas at 150mph just like it did off the line... at top speed a Bugatti Veyron empties its 26.4 gallon fuel tank in 12 minutes. It will travel only 50 miles in that time.

    Formula 1 cars get about 3.1 mpg during a race.

    At a guess, a typical sports car with a 15 gallon tank probably empties its gas tank in half an hour at 150 mph and travels 75 miles.
    ( Anecdotal, a friend of mine gets about 5mpg on the race track )

    ICE cars waste most of their energy as heat ( like 70% ), and need to waste even more energy venting that heat. Venting that heat means passing air through radiators and that means that an ICE car needs to have a lot more drag than an EV.
    Since an EV wastes only around 10% of its energy as heat, it can have relatively tiny radiators and add a small amount of drag to push air through those radiators.
    The more drag the ICE has to add to get rid of the heat, the more horsepower it has to add to defeat that drag. Repeat. Then remember that wind resistance is proportional to the square of speed.
    All that means that the ICE will use a lot more energy than an EV to go the exact same speed and it gets worse fast as you go faster.

    Yes the energy density ( and power density ) of gasoline is far greater than that of lithium ion batteries, but that is not an apples to apples comparison.

    You should compare the entire drivetrain.

    20 gallons of gasoline is only 121 pounds, but you should add the weight of the fuel tank, transmission, exhaust system, and an ICE that weighs as much as 8 times what an electric motor weighs.
    20 gallons of gasoline is 668kWh and I am going to make a wild-ass guess that the system to use it in a typical sportscar is probably around 1200 pounds. ( I found a reference to a 2008 Chevy Corvette motors at 575 pounds, and references to transmissions at 200-230 pounds )
    85kWh in a Tesla Model S is 1200 pounds, add the motors and inverter and charger and maybe you have 1500 pounds ( the single Motor in the Model S supposedly weighs 70 pounds ).

    So the ICE has 668kWh stored in a system of about 1200 pounds. Thats 1.8 pounds per kWh of energy stored.
    The EV has 85kWh stored in a system of about 1500 pounds. That's 17.7 pounds per kWh of energy stored.

    So the EV is 10x worse, but I bet the energy consumption of the ICE at high speed is somewhere around 4-5 times higher than the EV. The power needed is also a lot higher, how much higher - I am not sure.

    I don't think that EVs are as far behind as most think.
    I don't know how long a Model S 85D battery can sustain exerting 280kW to go 155mph, but energy and power density improvements are progressing steadily each year.

    Improvements in energy density and power density of batteries will quickly let them pass ICE cars in high speed performance.
    I think it will only take a 2x improvement and will happen in less than 10 years.

    Edit: if you read all the way to the end, I see that lolachampcar was mostly talking about the fact that the ICE can sustain its peak horsepower until it exhausts its fuel, but the Model S can not. My point still stands that the EV will need a much lower sustained horsepower to defeat an ICE car.
     

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