Regenerative brake settings and lead feet

[albert mathews]

This is how I see it. When driving, an ICE powered vehicle has 5 modes of operation for the powertrain in terms of applying toque to the wheels:
1] applying accelerating torque
2] coasting (no engine or brake torque applied) (a.k.a. decelerating due to drag of all kinds)
3] decelerating due to engine braking torque
4] decelerating due to both engine braking and friction brake torque
5] decelerating due to friction brake torque
For this type a vehicle, modes 3,4,5 all results in converting the kinetic energy (or potential energy if constant speed downhill) n to heat (either friction brakes, of heating air via engine pumping loses). Mode 2 also converts the kinetic energy to heat, but not through applying torque to the wheels, so we dont need to consider this here, as this energy cannot be recovered through regen.

An EV can have the same 5 modes, but with one very important difference, "engine braking torque" is replaced by regenerative braking torque. The impact of this is that modes 3 and 4 have reduced the amount of energy converted to heat, as they can store it in the battery for later use. The topic of this thread is whether "coast regen", i.e. applying regenerative torque below a certain "go pedal threshold" is more or less efficient than only applying regeneration torque when the brake pedal is pressed.
The short answer is:
One can develop driving styles for each regen setup such that the same maximum efficiency can achieve with both setups, but developing the driving style which uses coast regen will be easier for the average driver than developing the style which does not use coast regen. i.e. not all of us can drive like the best hyper mileagers.
The long answer is:
Assuming no hills, the car must be accelerated using battery energy, but it can be decelerated by either storing energy back in the battery or by generating heat with the friction brakes. If there is no coast regen, this typically means that regenerative torque is blended with friction brake torque, which means that there is no mode 3, which means the driver uses primarily mode 4 (and to some extent mode 2) to decelerate the vehicle. This means that in order to store energy, the car must also throw some away by creating heat. On the other hand, aggressive coast regen allows the driver to use primarily mode 3 (and to some extent mode 2) to decelerate the vehicle and increase the fraction of kinetic energy recaptured. i.e. coast regen provides drivers with a natural way to increaase recaptured energy without forcing them to massively change how they naturally like to accelerate and decelerate a car.

 

[WannabeOwner]

I'm surprised by how easy I have found it to "judge" when to lift-off (the accelerator) such that full-Regen will slow the car enough before the next bend / junction / etc. But I was an avid hypermiler previously in ICE, so that might be contributing; for example, I maintain a large follow-distance where there is no possibility of overtaking, so I have plenty of time to "bleed speed" if the car in front slows down

One thing that is different about one-pedal / Regen, compared to hypermiling when I used to Coast + some-brake, is the amount of slowing down that Regen provides in the last little bit - i.e. the point at which, in ICE, I would have finished my braking and would be switching to accelerator intending to provide first some steady throttle and then (presumably) some acceleration. Now, on full Regen, if [as is often the case] I find myself just slightly too fast at that point I continue full-regen to reduce speed further AS I am starting to make the manoeuvre, such that once I am actually in the manoeuvre I am at the correct speed. Of course if I come into the manoeuvre far too fast then I do need to use friction brakes , but I find that is very rare.

Puts me in mind of a conversation I had with a Porsche-driving friend, after giving him a demo.

"I find that using Regen I very rarely use the brakes at all"

He looked disappointed and said:

"I hope you DO use the brakes SOME of the time"

 

[scottm]

Of course, the real sport of it comes into play when the battery is cold, that's when you have varying amounts of max regen according to the yellow line limiter, or heaven forbid, the snowflake. So you have to constantly adjust your coast-to-stop-sign distance estimating as the battery warms. When regen is temperature limited to 'none' you get to experience the pure glider capability of the car ("mode 2" described above). Most seasoned S drivers hate mode 2 in town because you have to lift your foot over to the brakes... which is so ICE. And a waste of energy for the car too.

On the highway, mode 2 is great when you keep a lot of distance ahead and you know the momentum (kinetic energy) of the car is going to be used in the upcoming mile.. to get over that hump, or just ride out the distance. Best possible use of energy is to keep up the kinetic if you'll need it.

I've often wished for a regen OFF setting, that I'd use for highway only. Instead of trying to emulate OFF by means of hypermiler driving techniques (keeping the power bar close to net zero) using only foot feathering on the go pedal. Which is tiresome on the foot to keep up for long trips. But very engaging style of driving.

Another way of emulating regen OFF is to flick the car into N neutral as you're driving. Glide glide glide... then pop it back into D drive when your done. But this is too tedious to use as a highway control. And it's illegal for some interpretations of the law. (Cars can't be coasted in Neutral on the highway, must always remain in gear. You can argue the Tesla is always in gear, and that's true. But selecting N emulates "no gear connection". Who has ever gotten a ticket for coasting in Neutral for an ICE car manual or automatic? Nobody I know.)

I think the Volt has paddle controls for variable regen, or regen defeat.