Graham wrote:
Remove it completely and you can come off the power sooner and coast further, for the same net outcome.
I haven’t really thought about what is more efficient. In a pure lift-off system, as long as you don’t touch the brake, the car gets slowed down by a combination of air resistance, rolling resistance, and electrical braking, so while there may be interesting calculations what might be more efficient when, as long as you don’t touch the brake, you are getting the max regen available for your driving style. If you want to coast you just put the foot in a position where you coast…
But honestly, I don’t think anyone cares, beyond that it is a bit of a sport to not use the brake at all. You don’t care about instantaneous energy consumption than you care about instantaneous fuel consumption in an ICE car, unless you really want to, perhaps out of curiosity or as a personal challenge.
Graham wrote:
Does any manufacturer actually have a regenerative braking and charging system that the driver can modulate instantaneously via a brake pedal?Without that capability I’ve always felt the charge-while-slowing systems give a false impression of a free lunch. Yes you get some juice for your battery, but by slowing the car down more rapidly than it would while properly coasting. You get the same energy outcome with no charging system just by coming off the power sooner and letting it coast further.
The latest Mercedes A250e models have a system where you can, in e-mode, adjust the amount of regeneration going on, from very little – ie coasting when you take your foot off the gas right up to single pedal operation – foot off the gas, it starts recuperating energy.
Does any manufacturer actually have a regenerative braking and charging system that the driver can modulate instantaneously via a brake pedal?
Yup, my Taycan does that.
Without that capability I’ve always felt the charge-while-slowing systems give a false impression of a free lunch.
It depends on the use case. If you go a lot up and down mountains (like say up to ski resorts…) you get back some (a lot) of the excess energy burned to go up the hill. You don’t get that with an IC engine in coast or engine braking mode.
Steve6443 wrote:
The latest Mercedes A250e models have a system where you can, in e-mode, adjust the amount of regeneration going on, from very little – ie coasting when you take your foot off the gas right up to single pedal operation – foot off the gas, it starts recuperating energy.
Does it then use regen when you use the brake pedal moderately (like the Taycan), or does it convert brakepads to heat (like the Tesla)?
T28 wrote:
If you go a lot up and down mountains (like say up to ski resorts…) you get back some (a lot) of the excess energy burned to go up the hill. You don’t get that with an IC engine in coast
You do if you have the space and opportunity to coast (which of course you don’t always) – it’s just a simple potential/kinetic exchange.
Every time you accelerate to a particular speed (or climb a hill) you can coast for a bit. Any sort of regen system will decelerate the car faster than pure coasting, so what you gain in battery charge you lose in the distance you can coast before applying power again.
There’s no free lunch. Every joule of energy the regen system puts into the battery could (in theory) have been applied to forward motion via a longer coast instead. The less efficient your driving style (i.e. the more braking you do) the more beneficial a regen system is. If you don’t touch the brakes much anyhow, it can get a bit marginal.
Let me rephrase. Regen in a hilly country works exactly like pumped storage works. Energy spent pumping water (or a car) uphill is recuperated when water (or the car) flows downhill, minus efficiency losses. You can’t recuperate the thermal energy spent by an IC engine. So versus an IC engine it is a “free lunch” – obviously not a free lunch from a perpetual motion perspective…
T28 wrote:
Energy spent pumping water (or a car) uphill is recuperated when water (or the car) flows downhill, minus efficiency losses.
Nope, because every unit of energy the regen system puts into the battery reduces the distance you can freewheel because the regen system is slowing the car down more rapidly than coasting, and hence you have to apply the power for the next hill sooner.
But you now have more juice in your battery, so the net outcome is the same. A regen system is more efficient than turning brake pads into heat, but it is not more efficient than coasting the speed off. It cannot be. If it were charging the battery without sacrificing coasting distance, it would be creating energy out of thin air.
First law of thermodynamics.
There is probably a complex formula for this, because – like a plane (with no engine) does best mpg at Vbg, but Vbg is too slow to be useful in the GA context – the best mpg for a car is going to be at some very low speed. For petrol/diesel cars it is about 40-45mph in the highest gear, but that’s governed by increasing drag v. decreasing engine efficiency. So the best case for regen is likely to be going down a long hill and at some very low speed.
But even this needs to be optimised further because once the battery is fully charged (or is too cold to accept a charge) you can’t use regen anymore, so the proper way would involve knowing the length and profile of the downward part of the hill ahead of you.
I am sure this has all been worked out, in huge detail, by huge numbers of car designers around the world, and it would not surprise me that the return from regen is quite small, with some exceptions like city driving.
Regen is efficient when used to replace friction braking in scenarios where such braking is largely unavoidable, such as going downhill where you cannot exceed a certain speed.
Most braking however is avoidable, outside of stop-go city driving.
For instance on my 7 mile drive to the supermarket in Chipping Norton I don’t touch the brake pedal apart from (maybe – depends on traffic) lightly at one roundabout and one junction, then a bit more to actually bring the car to a stop in the parking space. The amount of energy turned into heat is not worth the weight, expense and complexity of the regen system that would be necessary to recover it.
Ah, you are one of the old farts holding back traffic behind you coasting down for a mile when I want to take the next turn well ahead of where you want to stop? Get out of my way! 
The beauty of regen braking is that I don’t have to be a moving obstacle like @Graham, but can drive like an aspiring F1 driver lifting my foot off the accelerator at the last possible moment and drive as efficently as he can, while arriving much earlier….. at the next traffic light watching him coast in next to me. That is, unless it is a winter morning and regen doesn’t work, in which case I will crash into the car ahead.
But honestly, guys, nobody really cares. It’s a bit different, you get used to it, and forget about it, and learn to live with its quirks the same way you learned to deal with the creep of an auto when coming from a manual, or the way Graham learned to not double-clutch when down-shifting when he had his first synchronised gear-box shortly after the war.
