Regenerative Braking Setting

[WasserGKuehlt]

So I tried the auto recuperation on my commute today (first time in 6500 miles) - this was in a mix of traffic conditions. It seems to work well - coasted when the road was clear, and braked quite vigorously in slowing traffic, or when approaching a stopped car. However, its mortal sins (and the reasons I'll likely never use it again) are:
- it won't bring the car to a complete stop - even when it brakes hard from a speed higher than 7mph
- it won't have any effect under ~6mph (no regen, obviously)
- (therefore) it requires 'thinking' - will it slow down enough/in time, will the hydraulic brakes bite when I take over, should I let it do the initial deceleration or just touch the brakes etc.

Maybe it's just a new experience that requires some familiarization before it becomes 'automatic', but I'm not sure I see the point. For me, driving in traffic is a 'brainless' activity - even with a manual gearbox, and this type of incomplete/partial assistance is quite the opposite. Note this is not a knock on the feature itself - I understand why it behaves this way, and I don't consider it to be 'flawed' - it's just that its usability is limited.

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[magnitude]

Still, the brake light behavior is a function of the deceleration rate (and only that)

That alone clears up a major question I had.

I'm a bit surprised though. Before going EV and assistance systems, I thought just "tapping" the brake would actuate the brake light. Apparently not?

.7g also seems massive to me. I don't know, maybe .7g horizontally is not as strong as I think? .35g sounds more realistic just at first glance...

And now I'm wondering whether e.g. the deceleration that the auto-recup typically incurs is enough to turn on the brake light.

 

[magnitude]

(therefore) it requires 'thinking' - will it slow down enough/in time, will the hydraulic brakes bite when I take over, should I let it do the initial deceleration or just touch the brakes etc.

Maybe it's just a new experience that requires some familiarization before it becomes 'automatic', but I'm not sure I see the point. For me, driving in traffic is a 'brainless' activity

I use that feature very differently.

For me, it's not about "not thinking" or avoiding having to drive in traffic at all, it's very different from the "driving assistance" features like ACC/InnoDrive. In fact, it's pretty much not about stopping at all. The stuff you mention in fact does not work like that on an ICE either (you cannot come to a full stop without actuating the brake pedal).

It's about adjusting speed in situations where I only need to decelerate a little:

I love how the Taycan efficiently coasts, it's impressive. You let your foot of the brake pedal and it will happily stay at the same speed it was for a long time, just by its own inertia and very low losses.

But while this is great when the road is clear, it poses a problem when a car appears in front of me. Not abruptly, just regularly. (Be it that I approach a slower car, or either of us did a lane change.) With my previous ICE car, I would just (subconsciously, almost) let off the throttle a little, and my brain would continue modulating the throttle pedal until speed and distance match my target.

In fact, on a low-traffic highway under normal operation, there is practically never a need to engage the brake pedal in an ICE.

The Taycan cannot do that! If you only let off the gas pedal, you will slam into the car in front of you. Without recup, you have to push the brake pedal, even for minuscule adjustments.

Auto-recup solves that for me. It engages "ICE-like" behavior only when there's a car in front of me, i.e. only when there's a reason I would not want to effortlessly coast.

The effect is that I don't have to reach for the brake pedal just because I want to slightly adjust to a new (low) traffic situation. It has nothing to do with situations where I would have to brake in an ICE as well, i.e. stop and go, traffic lights, stop signs and so on. Again, an ICE also wouldn't come to a full stop by itself if I just let off the throttle.

 

[WuffvonTrips]

That alone clears up a major question I had.

I'm a bit surprised though. Before going EV and assistance systems, I thought just "tapping" the brake would actuate the brake light. Apparently not?

.7g also seems massive to me. I don't know, maybe .7g horizontally is not as strong as I think? .35g sounds more realistic just at first glance...

And now I'm wondering whether e.g. the deceleration that the auto-recup typically incurs is enough to turn on the brake light.

You've misread the units in the original post- not g's but m/(s^2), which convert at roughly 10:1 m/(s^2) to g, so it's approx. 0.07g.
But like you, I'm wondering what the implemented trigger criteria are for the Taycan's brake lights.

 

[magnitude]

You've misread the units in the original post- not g's but m/s^2, which convert at roughly 10:1 so it's approx. 0.07g.

Duh, I'm an idiot, you're completely right. This makes much more sense now.

And yeah, doesn't help with still wondering whether the person behind me sees a brake light even when I didn't hit the brakes (or conversely, if I only did so very lightly)...

 

[WasserGKuehlt]

I'm a bit surprised though. Before going EV and assistance systems, I thought just "tapping" the brake would actuate the brake light. Apparently not?

.7g also seems massive to me. I don't know, maybe .7g horizontally is not as strong as I think? .35g sounds more realistic just at first glance...

And now I'm wondering whether e.g. the deceleration that the auto-recup typically incurs is enough to turn on the brake light.

The brake pedal actuates a switch (used to be a mechanical one, physically connected to the pedal arm) which in turn sends the signal to the light. That would be based on pedal travel, and in 'classic' brakes it can act as a safe implementation: at, say, normal pad thickness and assuming a good state of the hydraulic brake system (no air etc.), then this much travel should correspond to the minimum deceleration threshold that requires a signal. That is, you may get more 'light' than necessary, but never 'insufficient light' for the actual deceleration rate. A defective switch would cause the latter, and that usually triggers a warning light in the dashboard.

Re: the .7g - I think my post said .7m/s^2, which is .07g; .7g is ~6.9m/s^2, and that's quite an effing lot . (edited for clarity)

Re: whether auto-recup triggers the light - no idea, I would assume in the right circumstances it has to. Okfine, i'll try it again on the way home tonight, and will enable the g-force meter in the dash. Hope y'all don't read about a crashed Taycan in the news tomorrow.

 

[magnitude]

Re: the .7g - that is definitely a lot, but I don't think my post said .7m/s^2, which is .07g; .7g is ~6.9m/s^2, and that's quite an effing lot .

Yeah, again, I'm an idiot. My brain somehow saw .7×9.81㎨ instead of just .7㎨.

 

[tchavei]

So if the brake light is based on deceleration only... What if you're coasting and suddently go steep uphill which will decelerate you at over 0.07g? The brakes will come on by themselves?

That's... Interesting.

 

[magnitude]

According to Porsche it is more effective and primarily because the harder you brake the more recuperation.

With respect, I don't understand how this makes sense. Maybe I'm misunderstanding what Porsche is saying here, or we are talking about a different question. But "the harder you brake, the more recuperation" does not matter when we are talking about the end result, i.e. the area under the curve.

(Also, I was talking about efficiency, you are now saying effectiveness, maybe that's the disconnect?)

 

[WuffvonTrips]

A quick Googling turned up this, in what appears to be fairly recent UN (!!!) regs:
5.2.22. Generation of a braking signal to illuminate stop lamps.
5.2.22.1. Activation of the service braking system by the driver shall generate a signal that will be used to illuminate the stop lamps.
5.2.22.2. Activation of the service braking system by "automatically commanded braking" shall generate the signal mentioned above. However, when the retardation generated is less than 0.7 m/s2 , the signal may be suppressed.
5.2.22.3. Activation of part of the service braking system by "selective braking" shall not generate the signal mentioned above.
5.2.22.4. Electric regenerative braking systems as defined in paragraph 2.17. of this Regulation, which produce a retarding force upon release of the accelerator control, shall generate the signal mentioned above according to the following provisions:
≤ 0.7 m/s² The signal shall not be generated
> 0.7 m/s² and ≤ 1.3 m/s² The signal may be generated
> 1.3 m/s² The signal shall be generated In all cases the signal shall be de-activated at the latest when the deceleration has fallen below 0.7 m/s²
5.2.23. When a vehicle is equipped with the means to indicate emergency braking, activation and de-activation of the emergency braking signal shall only be generated by the application of the service braking system when the following conditions are fulfilled: 8 6 At the time of type approval, compliance with this requirement shall be confirmed by the vehicle manufacturer. 7 During a "selective braking" event, the function may change to "automatically commanded braking". E/ECE/324/Rev.2/Add.12H/Rev.4 E/ECE/TRANS/505/Rev.2/Add.12H/Rev.4 22
5.2.23.1. The signal shall not be activated when the vehicle deceleration is below 6 m/s² but it may be generated at any deceleration at or above this value, the actual value being defined by the vehicle manufacturer. The signal shall be de-activated at the latest when the deceleration has fallen below 2.5 m/s².
5.2.23.2. The following conditions may also be used: (a) The signal may be generated from a prediction of the vehicle deceleration resulting from the braking demand respecting the activation and de-activation thresholds defined in paragraph 5.2.23.1. above; or (b) The signal may be activated at a speed above 50 km/h when the antilock system is fully cycling (as defined in paragraph 2. of Annex 6). The signal shall be deactivated when the antilock system is no longer fully cycling.
link to doc- https://unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2018/R013hr4e.pdf

 

[tchavei]

With respect, I don't understand how this makes sense. Maybe I'm misunderstanding what Porsche is saying here, or we are talking about a different question. But "the harder you brake, the more recuperation" does not matter when we are talking about the end result, i.e. the area under the curve.

(Also, I was talking about efficiency, you are now saying effectiveness, maybe that's the disconnect?)

Given point A and B separated by distance Z, in which person C wants to maintain a constant speed of X, P says that's it's more efficient to let the car T coast then applying a small recup while C lifts its foot of the accelerator just to have to accelerate again to regain speed X


Hmm. Maybe this doesn't clear it as I had hoped when I started to write this...

 

[WasserGKuehlt]

So if the brake light is based on deceleration only... What if you're coasting and suddently go steep uphill which will decelerate you at over 0.07g? The brakes will come on by themselves?

That's... Interesting.

Note the NHTSA doc suggests a signal threshold of .35g - that's quite a lot, would represent quite the incline (I'm sure someone could help calculate what that'd be for a given speed) and I struggle to imagine how you'd transition to such an incline safely to trigger the deceleration rate.

 

[magnitude]

A quick Googling turned up this, in what appears to be fairly recent UN (!!!) regs:

Alright, that clears up every question then. By my reading:

• Tapping the brake pedal will turn on the light.
• Assistance systems actively braking turns on the light, but if it's less than .7㎨, not necessarily.
• Stuff like traction control does not.
• Recuperation may turn the light from .7㎨ on, but definitely will above 1.3㎨.

 

[magnitude]

lifts its foot of the accelerator just to have to accelerate again to regain speed X

Of course I (and I hope everybode here) was not talking about re-accelerating. It's pretty clear that you lose if you have to re-accelerate. This was about monotonously slowing down a certain amount, to reach a target speed or a target stop distance.

 

[tchavei]

Of course I (and I hope everybode here) was not talking about re-accelerating. It's pretty clear that you lose if you have to re-accelerate. This was about monotonously slowing down a certain amount, to reach a target speed or a target stop distance.

In a perfect world you'd lift your foot off the brakes 2km early and let the car coast until you reach your destination. That would be the most efficient.

With recup on, let's say it would take 500m to decelerate to reach destination. The energy recovered in those final 500m is less than the energy you saved during that extra 1.5km you coasted in the first scenario. According to Porsche.

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