Regenerative Braking Setting

[irrelevant]

For those of you who drive with overrun recoup off, how do you control the speed of your car say, midway through a turn/onramp/offramp, especially at a decline? How about while approaching a turn? Riding the brake, letting off, then back on the accelerator has got be inefficient for you or perhaps even annoying for your passengers, no? I imagine you'd have more dramatic swings in vehicle weight distribution with overrun recuperation off.

This is a scenario where I would probably activate the recuperation with the button on the steering wheel, though I’m confident I could smoothly modulate the brake pedal enough to allow for the same level of deceleration. The system slowing the car in either scenario is the same, so one method shouldn’t upset the chassis any more than the other.

Our e-tron has paddle-selectable, off-pedal regeneration/recuperation, but it only remains active until the next accelerator activation. I disliked that initially, but after driving the Taycan, where it stays armed until the button is pressed again, I’m finding Audi’s implementation less objectionable.

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[WasserGKuehlt]

This is a scenario where I would probably activate the recuperation with the button on the steering wheel, though I’m confident I could smoothly modulate the brake pedal enough to allow for the same level of deceleration. The system slowing the car in either scenario is the same, so one method shouldn’t upset the chassis any more than the other.

First time I tried Sport mode on a winding road nearby I found the recuperation to be very annoying.

Having said that, on an empty alpine road with lots of turns* the recuperation makes for a much better, more dynamic driving experience. You absolutely can't coast through most of the turns, and coasting feels disengaged. I now get why it's on by default in Sport mode.

*Ok, 'alpine' is doing a lot of lifting here, but it does go through and to the mountains.

 

[magnitude]

Er, use the brakes...puts more back into the battery than overrun.

Does it? Energy in, energy out... I would hope the car puts all the energy it can back into the battery no matter which mode one prefers, and only actually engage the brakes (which is what finally definitely loses energy into heat) only when it has to decelerate so fast that recuperation isn't cutting it anymore.

Now unfortunately everything has losses, and energy that you're not putting in into motion in the first place, i.e. by not accelerating past the point where you will have to use the recuperation setting or engage the brake pedal, is better than trying to get the energy back either way.

Whether recuperation or the brake pedal are more efficient in that regard is an interesting question, and probably depends on many factors. Does anyone have a good idea?

 

[WasserGKuehlt]

Whether recuperation or the brake pedal are more efficient in that regard is an interesting question, and probably depends on many factors. Does anyone have a good idea?

I suspect it's a matter of tuning; the initial deceleration one experiences with the brake pedal (in regen mode) is far stronger than the recuperation on 'idle'/'overrun'. It's going in the same place and through the same mechanism, so both methods could be equally effective; it was just a choice on Porsche's part to set the level of recuperation so low, perhaps to match the feeling of an IC-engined car. (And I'm now really curious whether the tuning is the same for all trims, or the higher-powered Taycans feel different than the RWD/4CT, so as to simulate a 'bigger engine'.)

Efficiency-wise, coasting is better than either; recovery-wise (of energy expended), braking is more "efficient" than recuperating simply because it converts more of the kinetic energy. In principle one could brake to the hydraulic threshold, release the brakes and 'recup' from there for a no-loss-to-heat recuperation of energy (though the car won't come to a full stop unless going uphill). (And, to continue with this silliness, I assume the motors, the battery and the cabling do dissipate some of that recovered energy as heat themselves...)

 

[irrelevant]

Thermodynamics says for the same amount of energy scrubbed off, it doesn’t matter which method is used, so long as the friction brakes aren’t activated.

 

[magnitude]

Thermodynamics says for the same amount of energy scrubbed off, it doesn’t matter which method is used, so long as the friction brakes aren’t activated.

I think I went into that. Thermodynamics of the practical world also say that nothing is 100% efficient. "Slow" recuperation might have less overall loss than "fast" recuperation, or vice versa. It could also depend on temperature and many other factors.

As said, energy in, energy out. It's clear that you have the same maximum of recuperated energy with both approaches. Whether anything in the system's engineering favors one or the other in terms of efficiency, is still an interesting question I think.

 

[magnitude]

braking is more "efficient" than recuperating simply because it converts more of the kinetic energy

I'm curious about that statement. The question we are trying to answer is whether it's better to (attempt to) recuperate the same amount of kinetic energy by either pushing the brake pedal, or using recuperation mode. If it was not the same amount of energy for each method, then one of the methods would either cause you to stop earlier, or slam into the car in front of you. Am I misunderstanding?

EDIT: Are you maybe considering wind resistance, which will be less if you slow down earlier, through the brake pedal?

Now some of the energy to recuperate will invariably end up as heat loss, and which method does that more is the actually interesting question I believe.

 

[mystermykee]

Let's say you're cruising at 60mph, it'll take some distance for overrun recuperation by itself (no brake pedal) to get the vehicle to a complete stop. Within that distance you'll put X amount of energy back into the battery.

Within that same distance with recuperation off, can you get the same amount of energy back into the battery? Would it be more? Less? I'd imagine you'd coast up until the very end and slam on the brakes just under 290kw of recuperation before the mechanical brakes kick in.

I guess you could also modulate the brake pedal to simulate the level of recuperation that overrun gets you, but riding the brake pedal is annoying...even more so for the person behind you.

 

[WasserGKuehlt]

I'm curious about that statement. The question we are trying to answer is whether it's better to (attempt to) recuperate the same amount of kinetic energy by either pushing the brake pedal, or using recuperation mode. If it was not the same amount of energy for each method, then one of the methods would either cause you to stop earlier, or slam into the car in front of you. Am I misunderstanding?

EDIT: Are you maybe considering wind resistance, which will be less if you slow down earlier, through the brake pedal?

I was simply saying that the deceleration is different between the 2 'methods': recuperation is constant, and what feels like (idk) .1g or even less; recuperation actuated via the brake pedal is a lot higher (.2 to .4+g) and depends on the pedal movement (how much travel, how fast). So it's not about the efficiency of the conversion of kinetic energy into stored/potential, but rather that the choice of the manufacturer was to recuperate at different rates via the two methods - simply for drivability.

 

[W1NGE]

Does it? Energy in, energy out... I would hope the car puts all the energy it can back into the battery no matter which mode one prefers, and only actually engage the brakes (which is what finally definitely loses energy into heat) only when it has to decelerate so fast that recuperation isn't cutting it anymore.

Now unfortunately everything has losses, and energy that you're not putting in into motion in the first place, i.e. by not accelerating past the point where you will have to use the recuperation setting or engage the brake pedal, is better than trying to get the energy back either way.

Whether recuperation or the brake pedal are more efficient in that regard is an interesting question, and probably depends on many factors. Does anyone have a good idea?

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

 

[f1eng]

For those of you who drive with overrun recoup off, how do you control the speed of your car say, midway through a turn/onramp/offramp, especially at a decline? How about while approaching a turn? Riding the brake, letting off, then back on the accelerator has got be inefficient for you or perhaps even annoying for your passengers, no? I imagine you'd have more dramatic swings in vehicle weight distribution with overrun recuperation off.

If you look at the power meter when using the default coasting mode if you come to a downhill stretch a small amount of regen braking is applied to reduce the acceleration.

Braking and accelerating in such circumstances has been normal for me for most of my life and it is a question of driving style. I try to be smooth when I have passengers, legato, my wife, a musician, drives staccato with sudden and heavy throttle or brake application. Nobody I know likes being her passenger - including me.

 

[outie]

It’s all about your personal preference and driving style. You won’t get any definitive answer out of this.

I live up on the hill so there’s a lot of going down long stretch of incline so it’s natural for me to use recup all the time which provides better control and I don’t have to keep riding on the brake or it will easily exceed the speed limit. In all my life I did not like riding on the brake to annoy people behind and I always pass people who do that.

 

[f1eng]

Does it? Energy in, energy out... I would hope the car puts all the energy it can back into the battery no matter which mode one prefers, and only actually engage the brakes (which is what finally definitely loses energy into heat) only when it has to decelerate so fast that recuperation isn't cutting it anymore.

Now unfortunately everything has losses, and energy that you're not putting in into motion in the first place, i.e. by not accelerating past the point where you will have to use the recuperation setting or engage the brake pedal, is better than trying to get the energy back either way.

Whether recuperation or the brake pedal are more efficient in that regard is an interesting question, and probably depends on many factors. Does anyone have a good idea?

All slowing down in a Taycan except when nearly stationary or very heavy braking is done by back torque of the motor(s) the only difference is how the programme in the controller decides to do it.

So if you applied the brakes by the same amount as the stage 1 regen setting exactly the same energy would be recouped in exactly the same way.

Look at the power meter on yours whilst braking, the amount of power being returned to the battery is shown on the power meter so you can see when your braking is hard enough to use up all the possible regeneration and the discs and pads need to be blended in too. It is surprisingly high, it surprised me anyway.

Equally try the other settings and look at the meter, it shows the magnitude of power going back to the battery in whatever setting you have chosen.

That is what I did the day I picked up the car and whilst the brakes were being bedded and before normal regeneration had been initiated.

Frankly I am pretty sure Porsche has more knowledgeable automotive engineers than forums, particularly English language ones , and their chosen default will be the most efficient and the alternatives offered for comforting familiarity or other preference.

 

[or1]

To summarize some of this: whether you take out kinetic or positional energy from the car by using the brake pedal or by activating any regen-on-accelerator-pedal functionality does not matter to which system is used and how much of the energy is stored in the battery. That is, up to so heavy deceleration that brake discs do some of the job (which they do anyway for a new car, and for a few kilometers of the first trip each day).

The efficiency part of it comes in when there is more staccato driving than "necessary", and I think coasting for most drivers is (slightly) more efficient because we manage less staccato then.

Now another question, to complicate things even more: does the brake light behave differently for brake pedal use than for regen-on-accelerator-pedal? Is it connected to brake pedal use, or to a certain level of deceleration or energy removal?

 

[WasserGKuehlt]

Frankly I am pretty sure Porsche has more knowledgeable automotive engineers than forums, particularly English language ones

I am curious what you meant by this - is it that, as a universal language, it tends to attract a wider audience of non-native speakers? Or that knowledgeable, non-English-speaking engineers aren't as likely to post on public forums?

Now another question, to complicate things even more: does the brake light behave differently for brake pedal use than for regen-on-accelerator-pedal? Is it connected to brake pedal use, or to a certain level of deceleration or energy removal?

In the US, the behavior of brake systems is regulated by NHTSA, but the spec on brake lights/signals is surprisingly difficult to find. (That is, I searched for a full minute before giving up.) Still, the brake light behavior is a function of the deceleration rate (and only that) - though it's unclear if, say, running into a wall would also actuate it.

According to other, less authoritative sources, the breakdown of thresholds appears to be:
- deceleration <= .7m/s^2: signal shall not be generated (ie brake light stays off)
- .7 < deceleration <= 1.3: signal may be generated
- deceleration > 1.3: signal shall be generated

This doc seems to indicate that the threshold for the brake signal is .35g, so way higher than the limits above.

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