Two Speed EV transmission a good idea?

feye

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Just increasing horsepower to make up the issue at high speeds is no solution. The Turbo S has been out for a while now too and I am comparing cars with similar performance stats. I don’t even know why I even have to explain this. With Tesla now only the Plaid exists and the rest is gone I guess. The over 800hp Model S after 200kph accelerated like a Golf GTI, so yes the transmission makes a lot of sense.
Wasn't the major advantage of having even a simple 2 speed gear box, that you can disengage the rear PM motor and only use the front motor for range?
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Jhenson29

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Wasn't the major advantage of having even a simple 2 speed gear box, that you can disengage the rear PM motor and only use the front motor for range?
I wonder how much they really gain from that though. They could have done the same thing with just a clutch. It didn’t require two gear box speeds.

And the only extra power you need if you leave them engaged is to keep the motor and gearbox spinning. Very small compared to keeping the car moving.

I think they likely started from a place of needing the two speed gearbox, saw FWD only as an additional option, and added it to the marketing. My $0.02.
 

Jhenson29

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Sorry, one other thought.

Multi speed gearboxes on electric motors in general isn’t novel.

It’s less common, but it is used in industry.

I thought more about specific applications where I’ve seen them used, and it’s been when the max HP requirements and motor speed limits didn’t meet torque requirements at lower speeds with a single gear ratio. In those cases, larger HP would have also sufficed, but it would have been largely unused and required a larger drive and wire just to meet the same low end torque requirements.
 

feye

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And the only extra power you need if you leave them engaged is to keep the motor and gearbox spinning.
Depends on the type of motor. The PSM (in Taycan) is most efficient particularly in low speeds, but creates a current running idle. Since you have a gear translation by adding a clutch you might as well add one more translation.

Porsche Taycan Two Speed EV transmission a good idea? 1631586950201

Source: Mahle - reaches up to 96% efficiency. Formula-E teams advertise these efficiencies for their engines

This are interesting developments, to get rid off the permanent magnets in the motors. With this you have much more options to control the motor as well as just switching it off, so it can run idle.
 

fullmetalbaal

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Depends on the type of motor. The PSM (in Taycan) is most efficient particularly in low speeds, but creates a current running idle. Since you have a gear translation by adding a clutch you might as well add one more translation.

1631586950201.png

Source: Mahle - reaches up to 96% efficiency. Formula-E teams advertise these efficiencies for their engines

This are interesting developments, to get rid off the permanent magnets in the motors. With this you have much more options to control the motor as well as just switching it off, so it can run idle.
Well, Formula-E for some reason explicitly requires there to be a transmission...
It's always a fascinating read to see what rules/regulations exist, and to really hear about the backstory. Feels like most rules have a nice "logical" front but were really a political maneuver by one party or another trying to secure their own strength or prevent another party from using theirs.
 


steen

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I think Elon mentioned, that with the Plaid, the rotation of the motor was so fast, that Tesla has to develop a new ceramic plating- or something like that to prevent the motor from simply selfdestruct due to the centrifugal forces.

I think Tesla could have opted for a 2 gear solution instead, but for various reasons chose not to- perhaps they dident have the tech. Whatefter solution will prevail, time will show.
 

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The transmission weighs probably less than 50kg. With heavy electric cars this weight is like a drop on a blazing fire. It won’t change anything. That argument is invalid in my eyes.
 

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I think Elon mentioned, that with the Plaid, the rotation of the motor was so fast, that Tesla has to develop a new ceramic plating- or something like that to prevent the motor from simply selfdestruct due to the centrifugal forces.

I think Tesla could have opted for a 2 gear solution instead, but for various reasons chose not to- perhaps they dident have the tech. Whatefter solution will prevail, time will show.
They developed their own filament winder that winds under tension, which will also help with motor efficiency as well as power curve due to the smaller air gap between stator and rotor. Honestly a way better solution from an engineering perspective than a transmission, I personally don’t think we need time to tell this story. We used similar but slightly more complex filament winding at SpaceX that’s still in use today on all of the rockets/spacecraft, no doubt this contributed to the idea and tech development at Tesla.
 


fullmetalbaal

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It is decades since I studied electric traction systems but my recollection is very much that the useable duty cycle is very much dependant on time and temperature.
Our lecturer had designed the traction system for large parts of the London underground railway and our exam questions were all about how hot things got climbing inclines and what speed could be used and for how long.
It is self evident that a tightly packed motor will be more efficient and probably lighter but heat up faster and cool more slowly.
I rather think the Tesla system is very practical for a road car because it getting hotter in short bursts isn't a problem in most real world usage and the better efficiency pays dividends every day.
The Porsche system is developed from (or may even be the actual parts on the Turbo S) the 919 hybrid Le Mans prototype. In this application it is being used continually at maximum so being able to keep it cool enough long term is more critical than the last bit of efficiency and peak performance.
Anyway the engineering balance is for the individual company to satisfy themselves with.
I am quite sure a 2-speed gearbox will give better performance over a 0 to 160 mph speed range than would be possible without, but obviously it is not essential.
919 hybrid needed a transmission anyway - so probably not a proof point wrt what is necessary/ideal for purely EV race cars.

More interestingly, the Mission R doesn't seem to have a 2 gear transmission.
 

fullmetalbaal

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If you're in Normal mode, and you didn't floor it, the transmission was in 2nd the whole time anyway?
Yes, exactly. And then when you want a bit more, it shifts back down to 1st.

A good example: coasting down a major road at around 30mph...
Then the traffic light just ahead turns yellow. You want to make that little hop a bit faster.
You punch it. And... the Taycan spends 0.5s thinking/shifting.

The only way to avoid it is to always drive in sport mode, but that has its own downsides.
 
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Yes, exactly. And then when you want a bit more, it shifts back down to 1st.

A good example: coasting down a major road at around 30mph...
Then the traffic light just ahead turns yellow. You want to make that little hop a bit faster.
You punch it. And... the Taycan spends 0.5s thinking/shifting.

The only way to avoid it is to always drive in sport mode, but that has its own downsides.
It seems this can be fixed by a software update?
 

fullmetalbaal

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It seems this can be fixed by a software update?
Assuming that it's the shifting logic that's taking too long, and not just the time the transmission takes to shift that's the dominant factor, probably a bit. But it's not going away entirely. The fundamental problem is being caught in the wrong gear.

Does anybody know how this transmission compares to PDK? Is it dual clutch in the same way?
I feel that subjectively the shifts are less smooth and slower than the PDK in my 911 was.
 

buruburu

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Does anybody know how this transmission compares to PDK? Is it dual clutch in the same way?
I feel that subjectively the shifts are less smooth and slower than the PDK in my 911 was.
The Taycan uses a dogleg clutch gear box. If the Taycan has a PDK, the instantaneous high torque will likely shred it to pieces.. Dogleg gear boxes are traditionally used in very high HP racing cars for their durability. It's also why there's a very noticeable clunk when the Taycan shifts.

Transmission in Formula E is important because there's a power cap. If you cap the power only other way to accelerate faster is with gearing. Effectively when you think about it, the Taycan 4s /Turbo has 3 gear and the base has 2 gears. Because the 4s and turbo can operate with front motor only, that's effectively where the 3rd gear comes from cause they are 3 different ratio. Same w/ the Tesla dual motors, they're effectively 2 gear transmissions.

According to Wikipedia, the gen2 Formula E cars are limited to Max (Qualifying?): 250 kW (335 hp; 340 PS) and Race: 200 kW (268 hp; 272 PS). I remember gen1 cars all had the same powertrain running the same 1-speed transmission, but gen2 cars allows the team to use what they want for transmissions. I've read somewhere that some teams are sticking with 1-speed while others are even using a 3-speed transmission.

With electric motors, they have the ability to spin at a much higher RPM, this is what's allowing EV cars to get away with a single gear. However, this doesn't mean that the power efficiency is constant thorough the entire operating range of the electrical motor. I haven't seen any graphs as to what the efficiency is, but I think it's safe to assume that it's not constant across the entire RPM band. This is where you still need a transmission in order to maximize efficiency.

Will we see more EV Transmission? Without a doubt especially with all the consumers so fixated on range.

[Corrected dogleg to dog clutch]
 
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Jhenson29

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Depends on the type of motor. The PSM (in Taycan) is most efficient particularly in low speeds, but creates a current running idle.
It shouldn’t. The rotor magnets induce a voltage in the stator windings (when rotating) but current would require a completed circuit. If the motor leads aren’t connected, there shouldn’t be any current and no additional resistive load from the motor beyond bearing friction (and rotor inertia for acceleration).
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