Transmission and recuperation -- can you answer how it works?

[kwanyu]

Taycan famously integrated the first 2-speed transmission in an BEV in the name of performance -- namely faster acceleration. But has anyone ever wondered why you don't feel any brake feel difference in recuperation whether you're in 1st or 2nd gear? I emailed Porsche and got no response, and I'm dying to know how they managed to tune the recuperation / brake feel regardless of which gear the car is in.

I suspect there would be some smart things you can do with recuperation strategy via the transmission.

So, anyone knows how the transmission works in recuperation mode?

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[Jhenson29]

But has anyone ever wondered why you don't feel any brake feel difference in recuperation whether you're in 1st or 2nd gear? I emailed Porsche and got no response, and I'm dying to know how they managed to tune the recuperation / brake feel regardless of which gear the car is in.

Braking with an electric motor would just be a negative torque, rotating the magnetic field in the stator slower than the rotor. The transmission is just affecting the ratio of torque between the motor and wheels. So, set a target torque, calculate a current, and set that limit in the drive. The drive’s current control loop modulates the output frequency and voltage for the desired current.

Whether you are in first or second isn’t really that interesting. It’s just part of the calculation for the current limit.

Transitioning between gears seamlessly would be more challenging, but IIRC, Porsche doesn’t do this. They keep it in gear through deceleration, and only change gears when stopped or on a charge to acceleration (If the gear change is called for). Someone can correct me if I’m wrong.

I’ve done both speed and torque control applications for both uncoilers and decoilers. Here, there’s a constant torque ratio change as the final roll diameter is constantly changing, but it’s really no big deal. Just part of the calculation, tuning, and sometimes additional filters.

 

[kwanyu]

Braking with an electric motor would just be a negative torque, rotating the magnetic field in the stator slower than the rotor. The transmission is just affecting the ratio of torque between the motor and wheels. So, set a target torque, calculate a current, and set that limit in the drive. The drive’s current control loop modulates the output frequency and voltage for the desired current.

Whether you are in first or second isn’t really that interesting. It’s just part of the calculation for the current limit.

Agreed -- the motor applies a negative torque based on brake pedal input. And the torque is modulated by a current to wheel torque mapping of some sort? And being in 1st or second is just a DC multiplier in that equation. I get this part.

My curiosity is, theoretically, there should be ways to maximize total recuperation energy in range mode (2nd gear always), and ways to maximize motor brake torque in sports mode (1st gear) right? Looking at the energy gauge, I don't seem to notice any difference.

 

[Torv]

Somewhat off topic, but in my 10 days now with my Taycan, I'm amazed at the recuperation when applying the brake pedal. It's so pronounced, I feel like it's regular friction braking and the power meter shows green between 11 and 9 o’clock on the dial depending on how hard I brake. Am I missing something or does this thing kick ass?

Oh, and another thing, I've noticed Auto Recuperation almost acts like adaptive cruise control. When I get somewhat close to the car in front, the recuperation kicks in slowing the Taycan to keep a safe distance. To be clear, I did not opt for the ACC option.

 

[Jhenson29]

Looking at the energy gauge, I don't seem to notice any difference.

Agreed -- the motor applies a negative torque based on brake pedal input. And the torque is modulated by a current to wheel torque mapping of some sort? And being in 1st or second is just a DC multiplier in that equation. I get this part.

My curiosity is, theoretically, there should be ways to maximize total recuperation energy in range mode (2nd gear always), and ways to maximize motor brake torque in sports mode (1st gear) right? Looking at the energy gauge, I don't seem to notice any difference.

Im just guessing at this point, but I’m going to suggest that braking dynamics vs max recuperation is a factor.

The Taycan can only recup at a rate of 265kW. And that is split between the front and rear motors. And not equally. Just as front brakes are larger and provide more stopping power than rear brakes with traditional friction brakes, the front motor provides more stopping power than the rear motor in the Taycan. The front motor can recup 175kW while the rear can only recup 90kW (even though the rear is the larger motor).And where is the transmission? On the rear.

So, they’re fairly limited in what they can do here. I didn’t run any numbers, but while the motors may be able to apply more torque in first gear, the car may not be able to recup it. And if it can’t dissipate it some other way, like heat through resistors, then it can’t do it period.

Curiously, the RWD appears to be spec’ed as 265kW recup also. I would love to know if it applies more braking to the rear wheels in general or if it is just able to use recup at higher braking forces while still having a healthy dose of front friction brakes.

Anyone with RWD notice more brake dust on the front wheels? If so, RWD may be the only model PCCB makes sense on. ?

 

[Jhenson29]

Looking at the energy gauge, I don't seem to notice any difference.

Keep in mind also…this is just a display. Who knows what’s really happening.

Maybe it’s accurate. Maybe it’s just pretty. Maybe it’s something in between.

As another person posted here somewhere, it would be nice if it displayed some numbers.

 

[Jhenson29]

…continuing my RWD thoughts, I wonder if braking recup being less for a given overall braking amount would help explain similar range numbers to the 4S, even though the 4S is several hundred lbs heavier.

 

[feye]

Because in your 4S recup is done mostly on the front motor.

 

[kwanyu]

Curiously, the RWD appears to be spec’ed as 265kW recup also. I would love to know if it applies more braking to the rear wheels in general or if it is just able to use recup at higher braking forces while still having a healthy dose of front friction brakes.

Anyone with RWD notice more brake dust on the front wheels? If so, RWD may be the only model PCCB makes sense on. ?

LOL indeed. PCCB makes sense on RWD Taycans.

I am guessing the 265kW max recuperation is limited by the BMS / charging circuits, and not by the individual front/rear motors themselves (since DC fast charging is also around 270kW max). Rear motor on RWD Taycan is still ~400 hp ~= 300kW in terms of output. I don't expect 100% of this to be available during recuperation -- so I'm surprised the RWD Taycan is spec'ed at 265kW recuperation also.

 

[XLR82XS]

LOL indeed. PCCB makes sense on RWD Taycans.

I believe you mean PSCB.

 

[NC_Taycan]

...the front motor provides more stopping power than the rear motor in the Taycan. The front motor can recup 175kW while the rear can only recup 90kW (even though the rear is the larger motor).

Do you have a source for this? Not that I think or know you are wrong, this just seems somewhat counter-intuitive. The larger motor should be capable of more regen than the smaller motor. However, Porsche may choose to allow the smaller motor to do more of the regen since during deceleration, more work is done by the front tires.

All that said, what I think happens is that the car interprets force applied to the brake pedal as the intended deceleration. It calculates the needed balance of regen vs. hydraulic brakes, including the front vs. rear split, to deliver that intended deceleration while recovering the maximum amount of kinetic energy possible, yet keeping the rear transmission in the optimal gear. Note it will downshift before you come to a complete stop, where and when depends on the driving mode. All in all, it's an outstanding system that does what it needs to do under everything from mundane start-stop driving in the city to aggressive driving on the track.

Chevy tried a similar approach in the 1st gen Volt however it was nowhere near as seamless as the Taycan. Tesla takes a different approach - the brake pedal directly actuates the hydraulic brakes, while the accelerator controls regen. AFAIK, there is no brake pedal actuation of regen in Tesla. This approach also has its merits, but the Taycan system is simply outstanding.

 

[XLR82XS]

FWIW, I feel the same amount of recuperation in a 4S as my RWD. On or auto.

 

[JimBob]

Here is what Porsche says about the motors and braking.

Pulse-controlled inverters control the motors
The pulse‑controlled inverter is the most important component for controlling the electric motors. In the Taycan Turbo and Turbo S, a pulse-controlled inverter is mounted on each drive module on the front and rear axles. The pulse-controlled inverters convert the direct current supplied by the Performance Battery Plus into the alternating current required to drive the electric motors. The reverse happens during braking: Here they convert the alternating current obtained during recuperation into direct current for charging the battery. In the Taycan Turbo S, a pulse-controlled inverter with a maximum current of 600 amps is used on the front axle, which can generate even more power and torque than the 300-amp, pulse-controlled inverter of the Taycan Turbo. Both pulse-controlled inverters operate with a remarkably high efficiency of almost 98 per cent.

 

[f1eng]

All that said, what I think happens is that the car interprets force applied to the brake pedal as the intended deceleration. It calculates the needed balance of regen vs. hydraulic brakes, including the front vs. rear split, to deliver that intended deceleration while recovering the maximum amount of kinetic energy possible, yet keeping the rear transmission in the optimal gear. Note it will downshift before you come to a complete stop, where and when depends on the driving mode. All in all, it's an outstanding system that does what it needs to do under everything from mundane start-stop driving in the city to aggressive driving on the track.

Indeed you are right, the choice of balance of recuperation front to rear has nothing to do with motor size and everything to do with optimised braking requirement.
This is pretty well what happens. Also inputs into the calculation are battery state of charge and, at li it braking, feedback on grip from the ABS.

I was told it was one of the most complex parts of the system to develop (I know a senior person involved so I hope I am accurately interpreting what they said) and I was super impressed by the brake feel when I had a test drive.

265 kW of recuperation is much more than most EVs are capable of, probably only the mechanically identical Audi E-tron GT has 800V architecture and such big motors.

I am surprised about how you report Tesla do it but that is by miles the easiest way to design a system, though miles from optimised or clever!

 

[Jhenson29]

Do you have a source for this?

https://www.google.com/amp/s/inside...he-taycan-regenerative-braking-deep-dive/amp/

https://evglobe.com/2020/03/01/test-drive-porsche-taycan-turbo-s/amp/

Those were the only sources I could find. I thought I had seen one from Porsche as well, but I must be mistaken or just can’t find it anymore. Probably just mistaken.

this just seems somewhat counter-intuitive. The larger motor should be capable of more regen than the smaller motor.

In general, yes the motors should be able to regen as much as they can motor, but I don’t think it’s about making it proportional to what they’re capable of. It’s about distributing it as it makes sense within the limits of the motors.

I assume the overall limit is related to battery charge rates, but it’s just an assumption.

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