Two Speed EV transmission a good idea?

feye

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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).
In theory you have drag losses, because the moving magnet produces a current inside the motor which shows up as a voltage on the outside. If the wire is cut, the current stops, but voltage persists.

Practically, the motor is always connected so you have to handle the power in your power electronics with more losses.

When I drive my Taycan, I can sometimes see a little green recup on the power meter, even with recup options off and no pedal action...
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Jhenson29

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In theory you have drag losses, because the moving magnet produces a current inside the motor which shows up as a voltage on the outside. If the wire is cut, the current stops, but voltage persists.

Practically, the motor is always connected so you have to handle the power in your power electronics with more losses.

When I drive my Taycan, I can sometimes see a little green recup on the power meter, even with recup options off and no pedal action...
Yeah, I see that recup too (I think it starts at around 60mph and goes up from there). I figured it was just intentional on Porsche’s part. I don’t think it’s necessary.

I have been curious about it though. Separately, recup on (not auto) also seems to vary by speed and slope. I’ve been meaning to spend more time measuring it, but haven’t gotten to it yet.

Anyway, it doesn’t have to be a cut wire; just an incomplete path, which the drive output can provide. So, while it’s always physically connected, it doesn’t have to be electrically connected.

I back-drive PM motors in other applications and I’m not aware of any additional resistance from motor back emf with a disabled drive.

I have one application in particular with a pair of 75HP IPM motors on a common frame (actually 4 motors across 2 frames) and I’ll disable one to run the other independently for testing. If the first motor had a significant impact on the torque requirement, I’d see it in my current profile, but I don’t. These particular motors drive the frame on an eccentric and have a high sinusoidal load when empty. I have a torque feed forward term for the motion profile that matches whether I’m running either or both motors.

I have other examples, but this one stands out because of the hours I spent staring at that damn current profile (for other issues). 😩

These are IGBT drives that I’m using. I think I read Porche’s inverter uses something else though (?), so I’m not 100% sure what all is different there. But if it stops the current path; I would assume it behaves the same.

Anyway…I think my main point was just that I think the torque optimization and motor speed limit Is the main reason for the two speed gearbox and I think the efficient gain from decoupling is minimal, at least not worth the gearbox on its own.

For the gearbox in general, I’m on the side of “makes sense”, but more than that, “Porsche is smarter than me.” Or at least has spent way more time and resources on it than me.

But then again…I think of that backup camera and wonder…😜
 

feye

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Anyway…I think my main point was just that I think the torque optimization and motor speed limit Is the main reason for the two speed gearbox and I think the efficient gain from decoupling is minimal, at least not worth the gearbox on its own.
You might be right, but it's hard to know exactly form the outside, how Porsche designed and optimized the whole drive train. This would be an interesting question to a Porsche engineer. :cool:
 

Jhenson29

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You might be right, but it's hard to know exactly form the outside, how Porsche designed and optimized the whole drive train. This would be an interesting question to a Porsche engineer. :cool:
I can only imagine. When I was at PEC, there was so much they told me that I had not idea about. I’m probably overly confident in my sideline assumptions.

We need an “ask a Porsche engineer” thread. 🤣 Plus…you know…a Porsche engineer…
 

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The Tesla appears traction limited to 60mph, for what it’s worth. (Power graph)
A7FEB263-7F3D-401D-896B-8AA540E1EBCB.jpeg

The taycan looks traction limited to half that speed (torque graph)

C0FD0853-A835-430F-8D9E-E0B67DE34366.png


PM motors are generally most efficient close to their torque knee, where the motor transitions from constant torque to constant power, and the taycan (in high gear) and plaid knees are at the same speed. Combine this with the plaid’s smaller air gap high speed efficiency gains means that physics would almost certainly disagree that the two speed gearbox is more efficient than spending your effort and money on a filament wound stator over a gearbox.

I’d love to see the power/weight ratio of each setup. They might actually be very close. The taycan simply won’t stand up in performance and particularly highway efficiency though.
 
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Mr.Smith

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The Tesla appears traction limited to 60mph, for what it’s worth. (Power graph)
A7FEB263-7F3D-401D-896B-8AA540E1EBCB.jpeg

The taycan looks traction limited to half that speed (torque graph)

C0FD0853-A835-430F-8D9E-E0B67DE34366.png


PM motors are generally most efficient close to their torque knee, where the motor transitions from constant torque to constant power, and the taycan (in high gear) and plaid knees are at the same speed. Combine this with the plaid’s smaller air gap high speed efficiency gains means that physics would almost certainly disagree that the two speed gearbox is more efficient than spending your effort and money on a filament wound stator over a gearbox.

I’d love to see the power/weight ratio of each setup. They might actually be very close.
What about the longevity and reliability of the filament wound stator?
 

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I’d be surprised if it’s stressed enough to have a practical cycle life limitation, even at high speed. The winding count should be easily tuned to make sure that you’re in a safe place for cycle life.
 

im85288

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There’s an interesting take on this covered in this video 15 mins in approximately)
Synopsis is it was designed to deliver more power but has been proven to not be needed due to advances in other areas. Looks like Porsche is going to drop it for the future upcoming 718 EV and who knows maybe on the Macan EV. Odds on it’ll not be present in Taycan V2
 

Kingske

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There’s an interesting take on this covered in this video 15 mins in approximately)
Synopsis is it was designed to deliver more power but has been proven to not be needed due to advances in other areas. Looks like Porsche is going to drop it for the future upcoming 718 EV and who knows maybe on the Macan EV. Odds on it’ll not be present in Taycan V2
Will make our Taycan V1’s more special and unique…. :)
 
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Mr.Smith

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There’s an interesting take on this covered in this video 15 mins in approximately)
Synopsis is it was designed to deliver more power but has been proven to not be needed due to advances in other areas. Looks like Porsche is going to drop it for the future upcoming 718 EV and who knows maybe on the Macan EV. Odds on it’ll not be present in Taycan V2
The purpose of the transmission isn't for a better 0-60.
Even Jaguar stated they can make an EV go 0-60 in 1.9sec, but the reason they don't is motor longevity.
 

TycanNewHampshire

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I like the discussion.....since the EV market is really designed to think outside of the box and before I give my answer, here are my assumptions to the problem statement:
1. you want the most torque/power from stop to 70-75mph
2. you want the motor running on the least amount of rotation/power consumption at cruising speeds for optimal range.
3. There is currently not a solution outside of gearing to get the motor to use less power/rotate slower to give cruising at highway/autobahn speeds.
4. Transmissions are heavy, take oil and have a delay in changing gears as well as wear.
5. you don't want to limit the ability to accelerate at cruising speeds if you need to quickly accelerate in a time of need.

Need: Find a solution that has better advantages of gearing without additional weight, less wear and equal/more performance.

Potential Solution (pie in the sky.....maybe, maybe not?):
- Consider a second motor ("cruise assist") for each motor of the vehicle that is automatically geared much lower to rotate less revolutions per wheel spin.
-Potential reason for success: the lower geared motors could be weight neutral to the transmission and also act in a similar concept of the variable cylinder firing of V8's that switch to V4's when cruising and act as an 'overdrive' as well as a fuel-source saver.
-If practical, this could also serve as extra power-assist to the 0-70/75mph and then the main motors have electricity 'cut' from them to allow the smaller 'cruise assist' motors to maintain the speed.
-if synced with the ECU, any throttle response that is needed would instantly excite the main motors and there would be no perceived delay or shifting feel at any speed.

At any rate, I thought I would throw in my $0.02 in on this vs. just debating current-state pro's/con's and/or trying to figure out what they were thinking at the time of development. They could have been solving for two 'needs' in development: 1. needing to get to a certain 0-60 time to match the Porsche Brand and 2. Needing to get to a certain mile-range to be desirable from a consumer standpoint. Whatever the reason, I think the future is in something novel, vs. just more of the same. I am not saying I have 'the' answer, but I do think the concept has merit that you can do more with this platform vs. the traditional solutions that were available.
 

fullmetalbaal

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I like the discussion.....since the EV market is really designed to think outside of the box and before I give my answer, here are my assumptions to the problem statement:
1. you want the most torque/power from stop to 70-75mph
2. you want the motor running on the least amount of rotation/power consumption at cruising speeds for optimal range.
3. There is currently not a solution outside of gearing to get the motor to use less power/rotate slower to give cruising at highway/autobahn speeds.
4. Transmissions are heavy, take oil and have a delay in changing gears as well as wear.
5. you don't want to limit the ability to accelerate at cruising speeds if you need to quickly accelerate in a time of need.

Need: Find a solution that has better advantages of gearing without additional weight, less wear and equal/more performance.

Potential Solution (pie in the sky.....maybe, maybe not?):
- Consider a second motor ("cruise assist") for each motor of the vehicle that is automatically geared much lower to rotate less revolutions per wheel spin.
-Potential reason for success: the lower geared motors could be weight neutral to the transmission and also act in a similar concept of the variable cylinder firing of V8's that switch to V4's when cruising and act as an 'overdrive' as well as a fuel-source saver.
-If practical, this could also serve as extra power-assist to the 0-70/75mph and then the main motors have electricity 'cut' from them to allow the smaller 'cruise assist' motors to maintain the speed.
-if synced with the ECU, any throttle response that is needed would instantly excite the main motors and there would be no perceived delay or shifting feel at any speed.

At any rate, I thought I would throw in my $0.02 in on this vs. just debating current-state pro's/con's and/or trying to figure out what they were thinking at the time of development. They could have been solving for two 'needs' in development: 1. needing to get to a certain 0-60 time to match the Porsche Brand and 2. Needing to get to a certain mile-range to be desirable from a consumer standpoint. Whatever the reason, I think the future is in something novel, vs. just more of the same. I am not saying I have 'the' answer, but I do think the concept has merit that you can do more with this platform vs. the traditional solutions that were available.
Exactly. The old rules don't apply, and it's across the whole system. Things that used to be necessary no longer are, things that used to be prohibitive/impossible are easy (e.g., multiple motors). It will take some time to explore what all is a good idea. Sometimes, smaller changes will kick off virtuous cycles that magnify the impact... and that means the full impact of a small change won't be easy to estimate until you actually try it.

I do think that so far the startups of the world have tended for novel approaches, and the traditional OEMs are trying to stick to swinging the same hammers they are used to.

Witness how most startups are going cylindrical cell vs traditional OEMs all on prismatic or pouch cells. Similarly, most of the startups also have cars that have great 0-60, and all the OEMs seem to be intent on leaving space for their ICE cars or something.
 
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