By on January 17, 2020

Frank writes:

So, reading about the Lithium Mustang with its completely superfluous six-speed gearbox got me and some friends thinking about when you need a transmission and when you don’t.

We all know that steam engines and electric motors torque from the git-go and never need transmissions, whereas ICE engines can’t do that. But then we realized none of us knew why. So, that’s the question — why can’t ICE engines torque like those others?

Sajeev answers:

Um, no: electric motors do indeed need a transmission. But they don’t necessarily need a multi-speed transmission. Especially at any normal price point, but I’m getting ahead of myself. 

For example, Tesla’s Model S uses a 9.73:1 step-down transmission, which cuts the mustard thanks to the electric motor’s excellent powerband and NVH controls. A single gear could imply terrible acceleration, but unlike ICE’s massive metal reciprocating assemblies that barely run below 800 rpm, electric motors make full torque at idle… upon that first shot of electricity to their internals.

And from idle to the Model S’ 18,302-ish rpm redline, there’s either a tidal wave of torque or more-than-adequate highway horsepower, and it’s available instantly thanks to the lack of a reciprocating assembly.

That said, the Model S’ acceleration tapers off as highway speeds (70 mph) turn to Go To Jail Speeds (155 mph top speed), so let’s hypothesize that power/efficiency falters above 12,000 rpm. Which suggests lead-footed owners still have 6000-ish more revs before redline: that’s nearly triple the rev range of an average ICE. That assumption might be BS, but no matter, a “tall” one-speed gearbox is acceptable here.

The extra revving doesn’t matter to most, much like going to a sock-hop in a ’50s Buick with its Dynaflow box. But Dynaflow met its maker, relegated to the dustbin while its successor is still relevant today (well, in the enthusiast space). History is likely to repeat itself, but don’t take my word for it. 

The ZF 2-speed gearbox is likely to prove that electric cars go faster, more efficiently with multi-speed gearing.  If what I heard from an owner of a Porsche Taycan Turbo (not Turbo S) in Texas is any indication, the 2-speed gearbox significantly improves the long-distance (from Houston to Austin) range vis-à-vis the single-speed Model S. And his assessment isn’t unique.

While Tesla’s electric powertrains are a noble and wholly-impressive endeavor, they’ve wanted a second cog for a while. But this isn’t a slam on Tesla: standalone departments or third party suppliers get the funding they deserve because transmissions are terribly complicated compared to other componentry. If properly funded, the next-generation Model S will likely have that ZF gearbox, or similar.

Back to the price point: Google the Porsche Taycan’s starting MSRP.  That’s a lotta stacks for two gears, Son!

I dream of a day when it’s possible to make a sub-20k electric family car (we gotta take 1/3 of the cost outta the Nissan Leaf, no biggie amirite?) with a single speed gearbox, and 2-3 speed units for buyers of 40+k machines. Who knows when battery prices shall fall in line with this dream, but a correlation between multi-speed transmissions and lofty(er) electric vehicle asking prices is likely.

[Image: Ford]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice. 

 

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28 Comments on “Piston Slap: Tranny Talk on Electrified Motors?...”


  • avatar
    redgolf

    “Is a CVT transmission much lower cost to manufacture than a regular automatic transmission, due to having so many fewer moving parts?”

    https://www.quora.com/Is-a-CVT-transmission-much-lower-cost-to-manufacture-than-a-regular-automatic-transmission-due-to-having-so-many-fewer-moving-parts

  • avatar
    indi500fan

    The lithium Mustang is sort of a typical hot rod swap where they put an electric motor in front of a more conventional trans and diff.

    A two speed gearbox that doesn’t need a torque converter is pretty inexpensive especially if designed in at the start as part of the motor housing. And it doesn’t need a control system with pump/valves/fluid. We’ll see more of these as evs continue to roll out.

  • avatar
    John R

    Ah! Found the real Mustang Mach-E!

  • avatar
    jack4x

    The answer to the actual question, why ICEs can’t make max torque at 0 rpm like an electric motor is simply because they need to intake air for combustion and this is done by vacuum pressure from the cylinders moving. If the cylinders aren’t moving, no air is coming in, no power. The faster the crank is turning, the more air can be drawn in per second, and the more fuel can be injected to take advantage.

    Forced induction engines can help with this by adding air mechanically, which is why they generally have a flatter torque curve and more power down low. But they still require the engine to be moving, either for exhaust gasses to spin a turbo, or for the accessory belt to spin the supercharger.

    • 0 avatar
      haroldhill

      If designed right an ICE can in fact produce enormous amounts of torque at very low engine speeds (see “tractor”). However an engine designed to produce low-speed torque, say at 300 rpm, would literally be out of breath at 1500 rpm. Modifying valve timing allows the engine to breathe efficiently at higher engine speeds, but then the low speed performance is lost.

      • 0 avatar
        jack4x

        Sure, but even that low speed engine will have a torque curve that ramps up from 0, plateaus, and falls off.

        That is inherent to an ICE, and different from an electric motor that produces its max power whenever current is applied.

        • 0 avatar
          Scoutdude

          No an electric motor does not produce max power whenever current is applied. It makes max power when full current is applied and it is turning at 1/2 of its free speed.

          • 0 avatar
            JimZ

            how and where a motor produces torque/hp depends on the type of motor. an induction motor generally produces both peak torque and hp near the point of 10% “slip” where “slip” is the difference in the rotating speed of the rotor and the magnetic field in the stator.

            as an example, for a garden variety 2-pole motor for 60 Hz AC, the magnetic field in the stator will be “spinning” at 3600 rpm, and the rated nameplate horsepower will be listed at ~3450 rpm rotor speed.

      • 0 avatar
        PandaBear

        You can design an ICE engine with a very wide power band, but it will either be an expensive variable valve timing system mechanically or electrically, or you have to make a turbo diesel style system that cannot meet today’s emission standard.

        electric is much simpler and cheaper on everything except the battery.

  • avatar
    haroldhill

    The question is actually about ICE’s, and the answer is that, if designed correctly an ICE can produce terrific amounts of low end torque (see “tractor”). However, the torque is dependent on engine breathing efficiency and this changes dramatically with engine speed, and an engine that produces a lot of grunt at 500 rpm will be literally out of breath at 2000 rpm. This is why so much work has been put into improved breathing, e.g. varable valve timing, turbos and superchargers, etc. But at best there is still a somewhat limited band of good power delivery and a multi-gear transmission allow us to repeat that band over and over.

  • avatar
    JimZ

    steam engines do it by both being external combustion and by separating out its fuel (coal, wood, oil, etc.) from the working fluid (water.) the working fluid is pressurized in a separate boiler and admitted to the piston/cylinder for expansion.

    a internal combustion piston engine combines the fuel and working fluid into one (air-fuel charge) and burns it in-cylinder, so the engine has to be above some minimum self-sustaining idle speed before you can even think about trying to get it to move anything.

  • avatar
    Scoutdude

    Electric motors make peak torque at zero rpm because of the Lenz effect and back EMF. At zero RPM the only resistance to current flow is the resistance of the conductors. As the motor spins it creates a magnetic field which opposes the flow of current.

    https://www.motioncontroltips.com/lenzs-law/

    The thing that no one talks about is that while a motor may make peak torque at 0 rpm it also makes 0 power at that rpm as it isn’t doing any work until it spins.

    • 0 avatar
      golden2husky

      As
      Work is force times distance. No motion no work no matter how much force

    • 0 avatar
      SPPPP

      I have to think that friction starts to butt in when you get up over 12,000 rpm. And even drag from the armatures spinning in air. I think Porsche’s use of a gearbox is a good idea. Having only two gears, it shouldn’t be very complicated or heavy (relative to the torque it’s handling). And it may be able to extend motor life by reducing RPMs.

    • 0 avatar
      RazorTM

      Scoutdude, I have noticed that too many people confuse torque (force) with power. I suppose that is one of the factors that drives the never-ending peak torque vs peak horsepower arguments: they just have no idea what the difference is.

  • avatar
    slap

    While a transmission would improve performance at high speeds it would increase cost, complexity, and weight, for a modest gain at normal driving speeds. And transmissions can be expensive to repair – almost always a big dollar fix.

  • avatar
    JMII

    I would think that a 2-speed would be the ideal electric gear box, like most battery powered tools have. You could have low range for acceleration and high range for cruising. A top speed gear would be worthless other for bragging rights or a track car. I guess the real question is at what RPM is an electric motor most efficient?

  • avatar
    dal20402

    The interesting question is whether using a multi-speed gearbox could allow for use of a less powerful electric motor and thereby save enough kWh to reduce battery size and cost enough to pay for the gearbox.

    My daily commuter is an electric bike with a 350 W (~1/2 hp) mid-mounted motor. With that amount of power plus what I generate, it has no trouble reaching 28 mph on flat ground, or going 40 miles on a charge. And the battery is a lil 0.5 kWh shrimp. But the bike’s 10-speed rear derailleur gears are essential to making the thing work.

  • avatar
    ToolGuy

    EV + CVT = WIN

    • 0 avatar
      dal20402

      No mechanical CVT necessary. Just translate the Toyota hybrid powertrain into an all-electric world.

      Planetary gearset:
      Sun gear – connected directly to a small electric motor and through a reduction gear to the wheels
      Planets – connected to the large electric motor that will provide most traction power
      Ring gear – connected to another small electric motor

    • 0 avatar
      PandaBear

      Electric motor + Electric motor = no need for CVT + double the rpm = WIN

  • avatar
    mcs

    ” If properly funded, the next-generation Model S will likely have that ZF gearbox, or similar.”

    LMFAO! That’s not true. They already have a more elegant solution. The description of the Tesla powertrain in the article is not exactly correct and misses a number of details.

    Basically, Tesla pairs an induction motor with a permanent magnet motor. The induction motor is for acceleration, and the PM motor is for high-speed cruising. That’s one reason Tesla dual motor cars are more efficient and have a better range than a multi-speed-transmission equipped Taycan.

    Here’s an article that explains it better than what I can:

    https://arstechnica.com/cars/2019/04/motor-technology-from-model-3-helps-tesla-boost-model-s-range-10/

  • avatar
    PandaBear

    You don’t need a multispeed transmission if you have to electric motors, running the way Prius run its CVT. You will need an additional set of power electronics however (but at half the power output each, so it might be cheaper instead of more expensive).

    VW likely use a dedicated transmission because that’s what they are familiar with and can be developed fast. Toyota (due to 20 years head start in Prius) and Tesla (as they are from Silicon Valley) likely know electronics better and decided to skip mechanical multispeed transmission.

  • avatar
    monkeydelmagico

    Posted to let everyone know how much I drooled at the article mustang pic. Carry on.

  • avatar
    NeilM

    Note that the original Tesla Roadster was to have had a two-speed transmission. They had trouble with gearbox reliability, a problem Tesla fixed by dropping the two-speed feature entirely, retaining only the single speed reduction gear.

    Porsche probably knows a lot more about gearbox design than Tesla.

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