Regen Management - Gas vs Brake

[Skotty]

A couple of thoughts to share, and possible discussion topics...

I think all hybrids right now blend regen into the brake pedal operation, which makes sense from a traditional driver's point of view. However, I know a lot of you like to drive the Volt in L mode, where regen is also blended into the gas pedal, with strong (full?) regen when the foot is completely off the gas.

I think started thinking about a recent Tesla report that said the Model S will have an option to reduce the regen and provide more of a coasting effect that is preferred by some drivers, but that it would hurt range slightly. That is when it hit me. I think maybe Tesla is not blending regen into the brake operation, and that it is only tied to the gas pedal operation.

I am now starting to feel that the gas pedal only blending (which I suggested that Tesla might be doing) is actually a better design. It would be far simpler to implement, with no real drawback to the driver (with one exception I'll note at the end). With regen only tied to the gas, the brakes can remain as simple hydraulic brakes like on any ICE car. Meanwhile, blending regen in the gas I would think should be quite easy (much more so than trying to blend into brake).

The only drawback is the issue of the brake lights not coming on when regen is active. But that seems like it would also be pretty easy to fix -- just make the brake lights come on when the brakes are pressed OR when the regen exceeds some threshold.

Thoughts?

[AZ EV Driver]

The Volt's design is the best I have eperienced. Leave it alone. Having no regen for coasting is not taking advantage of the lost energy and there are very few situations where coasting is useable. EV1 had "coast" or "regen" and I always left it in "regen". Not a fan of having the brake lights come on in regen mode as it is no different from downshifting a manual transmission.

VIN # B0985

[Selling Volts At Sundance]

Well, here is my layman's attempt to spin the magic of the Volt's Deceleration Regeneration firmware play-

Yes, this is in JPG format, but you can print it out and carry it in your wallet! lol

https://p.twimg.com/AthxKIvCEAAN6kz.jpg:large

[volt3939]

Needs spell check.

Brake, not Break...

[Selling Volts At Sundance]

Needs spell check.

Brake, not Break...

Thunk Ya fore da speelen typs, volt3939 lol To bad it's in .JPG format, can't change it for now! he he he

[CMMPro]

We have a 2006 Honda Civic Hybrid as well that I bought as our first entry into the world of hybrids.
(235,000km so far at <5L/100km, and not one repair, not bad for one of the first hybrids.)

The regen on it is tied to the throttle the same as in the Volt. (Not just the brake as mentioned in the OP.) I was actually surprised that the Volt had so little regen in normal mode. Even in sport mode the Volt regen is less than the HCH regen "off peddle". If I want to coast in the HCH I just feather the gas peddle until the regen led's go away...doing this the car will coast for at least a km on level ground with the engine spinning and not using fuel.

You can also do the same "heavy regen" as the Volt L mode by shifting into the HCH S with the tranny selector.

Practicing these (ehm, apologies in advance for using the term) "hyper-miling" skills for the last 6 years....I can drive the crap out of the Volt.

For example I got 15km driving the Volt the other day for the cost of only 3km on the battery!

[Mormegil]

No regen on coast means all energy loss is from friction (rolling / wind resistance). If you regen while coasting, you still loose the same amount of energy from friction, but in the process of converting the mechanical energy to electrical energy, then to chemical potential energy (to store in the battery), followed by eventually converting it back to electrical energy, and to mechanical energy at the wheels, results in added energy loss. Thermodynamics shows us you cannot have a 100% efficient energy conversion. Some energy will always be lost as heat. All the extra conversions involved with regen while coasting cannot physically get you better mileage (unless you are already going down hill, as gravity will pull against friction).

Or at least that's what I understand.

[Paul Churchley]

I basically agree with Mormegil.

For me, to understand the regen vs coasting issues I just go back to basics.

There is really only one basic concept that we must grasp to understand whether it is better to use regen or to coast and that is the conservation of energy and how when energy is transformed from one form to another a bit is always lost, normally as heat.

A car that is stopped has no speed, or kinetic energy. However, it does has chemical energy stored in either the gas or the battery. When you press the go pedal you start to move forward powered by the stored chemical energy, some of it is wasted as heat but most is converted into speed, or kinetic energy.

So, once you are moving if you shift to N then you will coast and slow down by converting your speed or kinetic energy into heat and sound through friction with the road and air. That friction is always there whether you are accelerating or slowing and I call it the background friction.

If you want to slow down more than background friction allows then you have to convert more of that speed, or kinetic energy, into another form of energy. On a car without regen that is normally done with the friction brakes adding additional friction to the background friction and creating more heat in the brakes. So, on a car without regen you will go further by just allowing the car to slow naturally just using the background friction than by using the brake pedal.

When we add regen to the mix nothing much changes. To slow down we must convert the kinetic energy into another form of energy. As before, we could just put the car into N and coast to a stop thereby just using the background friction. If we want to slow down more than the background friction provides we now have two choices, regen or friction brakes. Using regen will convert the kinetic energy of our speed into chemical energy in the battery by charging the battery. This is great because instead of just using the friction brakes and converting the kinetic energy to heat in the brakes and wasting it it is stored in the battery for later use.

The key thing to note though is that it is a conversion of one type of energy to another, through generator, inverter, charging circuit etc, and in all that conversion some of the energy is lost, again as heat mostly. So not all of the energy is recovered.

So, this then answers our question as to which is best to use when slowing... friction brakes, coasting or regen?

Coasting wastes the least energy at all times. Only the minimum friction is used to slow allowing the car to travel the furthest but because you travel further to slow you must give yourself a much greater distance to do so and that normally requires driving with much more anticipation. Often, it is not even possible to do so safely.

Next is regen. It allows you to slow down much more aggressively and will save some of the extra energy needed to slow in the battery but it will waste some as heat in the conversion.

Last then, we can use the friction brakes. They will allow you to slow down as fast as tire traction will allow but all the lost kinetic energy is converted to heat in the brakes thereby wasting it.

So, if you want to get best range, always coast to slow if you can. That is not always possible so then use regen when you can and as a last resort use the friction brakes.

As regen is a software programmable item (it must be because the gas pedal regen varies between L & D) so I would like to see the level of gas pedal regen user adjustable. If on a long highway trip when I have more control over my stopping distance I might want no regen on the gas pedal at all allowing me to slow down just using the background friction. Remember, this is the way I will get the most range. However, in a town environment I might want maximum regen on the gas pedal to allow me to ensure that I can get maximum regen braking without the risk of engaging the friction brakes and thereby wasting energy further.

I would like to see a setting, say from 0 to 5, adjustable via the config menu or even via a button on the central stack or even on the steering wheel. I shall dream on

[DCFusor]

Since a large part of energy loss is wind resistance, which goes as the cube of speed, at high speeds you're better off regenning, even with the conversion losses. At low speeds, hill and dale, you're better off coasting.

Since wind resistance goes up so quick with speed, I find one of the good things about the cruise control is it keeps you from going too fast - including downhill, and prevents some excess loss to wind.

I find the point at which "the curves cross" is around 35-40 mph. Therefore, I use D, normal, and CC set around that number on hill and dale roads - coast down one hill to gain a little speed and help coast up the next further before the CC has to provide power again, as my top speed is still quite low and the wind resistance low.

When I'm going faster - 55 up - I use sport+L, along with CC, which prevents me speeding up going down hills, and puts that energy into the batteries instead (minus some losses, but otherwise it's just completely wasted to the wind - remember that cube of speed function).

[Mormegil]

Since a large part of energy loss is wind resistance, which goes as the cube of speed, at high speeds you're better off regenning, even with the conversion losses. At low speeds, hill and dale, you're better off coasting.

That's a good point. If you coast at high speed, you're loosing that energy only to friction for a longer time. If you regen brake, you gain back some of that energy, and coast after some threshold, you could maximize energy usage.

I would like to see some math on this.