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range penalty from fast starts?

Discussion in 'Model S: Driving Dynamics' started by brucet999, Apr 23, 2015.

  1. brucet999

    brucet999 Active Member

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    As everyone knows, fast starts from stop sign or traffic light with ICE vehicles noticeably affects fuel economy. Since MS electric motors operate at high efficiency and develop max torque immediately, does that fast start penalty not apply? (I'm thinking of 70D, 85 or 85D, not P85D launches, which probably waste power leaving significant amounts of rubber on the pavement.)
     
  2. DaveVa

    DaveVa Sig Perf #236 VIN #484

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    I have found that fast starts impact range, but not as much as ICE environments. In normal day-to-day driving you will find that a "slow" start from a tesla is still much faster than everyone else at a light. In temperate weather spirited driving will give me an aggregate of 350-400 wh/mile conservative drivin closer to the 310-320.
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    There is still an energy penalty for fast starts:

    1) Tire slippage increases wear on the tires and wastes energy.
    2) Increased power demand results in more power losses through the inverter/power electronics, as well as more thermal losses in the battery itself converting chemical energy to electrical.
    3) Greater forces on the drivetrain increase friction.
    4) Other reasons I'm not thinking of at the moment.

    Having said all that, the cool thing is that there is much less energy loss doing a fast vs. slow start in a Tesla vs. a gas-powered engine. In other words, gunning it in a Tesla wastes significantly less energy than gunning it in an ICE. This gap widens even more when you consider that at the next red light, the Tesla recovers about 80% of the kinetic energy that the ICE cars are wasting to heat in the brakes.
     
  4. mknox

    mknox Well-Known Member

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    Yep. I drive a non-Performance 85 and just accelerating at a rate that seems "normal" to me now, I still notice the other cars from the light way back behind me in the rear view mirror.
     
  5. krisg81

    krisg81 Member

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    In any Tesla thats definitely true. When leaving a light, I have to 'try' and drive like a grandma with a broken foot to go slow enough people will hang with me. The car off the line is just so easy to get moving with little throttle input and effort. Every time I take off normally people disappear in my rear view mirror. Then again, 50% of the time if I am at the front of the light in the P85D I am running to 60MPH in 3.2 seconds or so just because it puts a smile on my face. In all honesty, I didn't buy the car to save much on gas or better the environment.. the performance of the P85D's electric motors were essentially 98% of the reason I had to have the car :)
     
  6. David99

    David99 Active Member

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    electric losses quadruple when power doubles. 4 times the power means 8 times the losses and so on. It means higher power demand doesn't lead to just higher losses, it decreases the efficiency dramatically. Both in the motor but more so in the battery. How bit the real world difference is, I'm not sure. Lots of things add up. If I had time to kill I would set up a test, but I don't :)

    There is one case where the losses can actually help in a sense. If the battery is cold, the efficiency is worse and you get less range. The battery will give you a better range if you first use part if it's capacity to warm itself up. So even though you are losing some energy to heat the battery, you still come out on top in the end. Since most of the power losses when accelerating fast are in the battery, maybe that warms up the battery and helps. Maybe.
     
  7. RDoc

    RDoc S85D

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    The obvious and straightforward fix to this is to just switch over to high temp superconductor cables, inverters and motors. Problem solved, (assuming the liquid nitrogen chillers don't use too much power).
     
  8. lphe

    lphe Member

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    Fast acceleration reduces range in the following ways:

    1. Your average speed over the entire trip will be faster if you accelerate fast. The higher your average speed, the more friction and aerodynamic drag, and hence the more energy used.
    2. During acceleration, the electric motor operates more efficiently than when driving at constant speed (provided you don't accelerate too fast). With slow acceleration, you are accelerating longer and operating the motor more efficiently longer.
    3. If you accelerate quickly, more of the power from the motor is wasted as heat and in other forms of internal energy (e.g. noise) rather than be used to propel the car.
    4. You will get less energy out of the battery with greater power demands (Peukert's Law).
     
  9. AB4EJ

    AB4EJ Member

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    Efficiency (and hence range) is greatly affected by what electrical engineers call "I-squared-R" losses. That is, the watts dissipated equals the resistance times the square of the current. During jackrabbit starts, the current is very high, leading to heating in the wiring, electronic control pack, and motor (proportional to the square of the current). This heat is lost energy and can't contribute to range (second law of thermodynamics). So yes, there will be a range penalty for hot starts (although it probably won't be huge because the hot start does not last all that long).

    I'd be more concerned about the extra stress on the drive train. The hot start can only put extra wear on gears, bearings, and contribute to fatigue of the metal of the gearbox. (Maybe that's one factor as to why so many people have mentioned that Tesla replaced their motor because it started making noise).
     
  10. ghost640

    ghost640 Member

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    +1
    I think I have a pretty normal Tesla acceleration off the light, but am surprised how quickly those cars behind me get really tiny :)
     
  11. SabrToothSqrl

    SabrToothSqrl Active Member

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    After going back to my 2009 Tahoe (I use it to tow)... even normal Tesla starts feel like warp speed.

    Actually, let me rephrase that, any other car feels like AN ETERNITY to get MOVING. and don't even get me started on normal car STOPPING.

    Once you get used to the regen, normal cars are almost scary to drive... they just keep rollin!

    Of course on some roads I've been able to average almost 1200 wh/mile for up to 5 miles... whoops :) so... only 4x normal use. I should have got a pic.

    The car is too damn fun to drive 'slow'... tires be dammed, I'm having fun!
     
  12. brucet999

    brucet999 Active Member

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    I talked to local SC about all of those motor replacements. In 2013 a bearing supplier provided sub-par product and lots of bearing failures ensued, so Tesla proactively swapped motor units from that source.
     
  13. Brass Guy

    Brass Guy Member

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    It may depend on how fast you're getting up to. I notice a pretty good hit on the kW/m average on the energy graph if I boot it from stop to 50mph or above.
    But around here those opportunities are rare, so more often I'm just getting up to 30, 35 or 40mph. I can pretty much stomp on it and hardly notice a hit at all.
    I wonder if it has more to do with the power required to accelerate from higher speeds. Can I state 0-30!=30-60? (Though it feels almost the same!)
     
  14. ThosEM

    ThosEM Space Weatherman

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    Regen is the big difference. An ICE without hybrid regen capability loses all that acceleration energy when braking. Unless you overdo it and use your friction brakes, an EV recaptures much of the energy that went into vehicle kinetic energy.
     
  15. Johan

    Johan Took a TSLA bear test. Came back negative.

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    The main reason, IMO, that you will experience quite a big "range penalty" (as the OP put it) is that in real life a lot of fast starts will also mean a lot of braking and fast deceleration.

    If in fact we were only talking about fast starts and all other things being equal I'd bet the range penalty is actually quite negligible.

    It takes the same amount of energy to accelerate the 2 ton Model S to say 65 mph regardless of if you do it in 5 seconds or 15. This is basic physics.
     
  16. lphe

    lphe Member

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    That does not take into account real world effects. The energy from the battery is not transmitted as efficiently to power the wheels with faster acceleration. With fast acceleration, you are traveling more of the trip at higher speeds. Higher speeds have more aerodynamic drag and friction consuming more energy. I have measured the difference in energy consumption on my car (not Tesla) between fast and slow acceleration. There is a difference. Faster acceleration consumed about 25% more energy over a distance of 0.25 miles.
     
  17. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Well if you travel more of the trip at higher speed you'll either travel the same distance in a shorter period of time OR travel further if you keep going for a certain period of time. So yeah, not apples to apples quite.

    My point was this: imagine I have a daily commute where there's one on-ramp where I always beed to accelerate and and two red lights where I always have to start from standing still. For one month I accelerate slowly on the on-ramp and from the lights. The next week I accelerate aggressively at these points. Apart from that my driving is exactly the same (same decelerations, same top speeds, same cruising speeds). My claim is that the difference in energy use is negligible. My real world experience confirms this.
     
  18. SabrToothSqrl

    SabrToothSqrl Active Member

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    #18 SabrToothSqrl, Apr 24, 2015
    Last edited: Apr 24, 2015
    That... is not accurate at all.

    It will take WAY more energy to accelerate it quickly... while in the S the difference isn't massively apparent, the difference is there.

    With a daily commute of under 40 miles... I just punch it off the line... almost every time... don't really care lol.
     
  19. Max*

    Max* Autopilot != Autonomous

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    I thought basic physics taught that the energy is equal to the initial energy plus the work done? (where work is property of force, and force is a property of acceleration. So accelerating "hard" uses more force, hence more work, hence more energy).

    <- not a physicist
     
  20. Johan

    Johan Took a TSLA bear test. Came back negative.

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    #20 Johan, Apr 24, 2015
    Last edited: Apr 24, 2015
    I'm sorry but it is accurate.

    Work = integral(F(t)v(t) dt, 0 to T) where F(t) is Force, v(t) is velocity at time t, T is total time of acceleration. But under constant acceleration, v(t)=Ft/m where m is mass and force F is constant.
    = integral(F^2 * t/m dt, 0 to T)
    = F^2 * T^2 / 2m
    But, F=ma, so T is inversely proportional to F, T = V/a = Vm/F where V is target velocity.
    = F^2 * (Vm/F)^2/ 2m = F^2 * V^2 * m^2 / (F^2 * 2m )
    = mV^2/2
    This is independent of rate of acceleration.

    (Disclaimer: The equations written out above are stolen from ItsNotAboutTheMoney on this board, from this post: http://www.teslamotorsclub.com/showthread.php/30731-Why-does-accelerating-fast-use-more-energy-than-accelerating-slowly?p=649186&viewfull=1#post649186)


    In simpler terms: rate of acceleration is irrelevant since the work required to add a certain amount of kinetic energy to a given mass is the same (i.e. to accelerate a 2 ton Model S to 65 mph requires the same amount of energy regardless of if you do it fast or slow).

    Now in real life there are some factors to consider:

    Higher heat losses in the drive train and possibly also a bit in the tires with faster acceleration. Higher omhic losses in the battery and inverter with higher current draw. The electric motors is very efficient from 0 RPM so that doesn't matter much, and in fact electric motors are more efficient at higher loads, which in fact favors faster acceleration.

    I stand by my claim that the difference is negligible and that any experienced real world differences are due to these other factors:

    More braking, less overall efficient driving style, going faster in general.

    Please don't make the mistake of confusing accelerating quickly with going faster. Going faster obviously uses more energy, but also gets you there faster.
     

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