Still waiting for my Volt to deliver but have a question about the regenerative braking. Can someone put into simple terms a description of how this system works?

When is the system actually recovering energy? Only when braking? When coasting? Does the Volt even "coast" or does it function in a different way than I'm thinking?

I've seen discussion of driving in "L" with the Volt. Not having my car yet I am not sure what "L" is. I would assume that means "Low" but I'm not sure why you would drive in Low all of the time for efficiency sake.

It seems like all of these different things are probably interrelated in some way I just don't get without having the car to live with yet.

Thanks!

Regenerative braking is somewhat similar to "engine braking" or "down-shifting" in a standard transmission, except that instead of making heat from friction in the engine and transmission, regenerative braking makes electricity - with an electric motor - that is stored in the battery.

The Volt will regenerate power when braking and coasting. Coasting in "L" will produce a lot more regenerative braking than coasting in "D". When you brake harder than the capacity of the regenerative system, the friction brakes will provide the remainder of the braking power.

The Volt has two electric motors. The second smaller motor spins backwards when you let off the accelerator and/or hit the brakes, providing resistance. The energy from the second motor spinning is fed to the battery. If more deceleration is needed and you hit the brakes harder, the disc brakes will also kick in....

Using L simply changes the amount of resistance/regenerative braking you get when you let off the gas only. Basically it glides less than when it is in D. You can feel the extra drag. It can allow you to get more regenerative braking, but it also gives you the ability to accelerate and decelerate without touching the brakes. Some call this "one pedal driving". The Tesla roadster operates this way all the time...

By-the-way, when you get used to driving in low, it is almost hard to go back to a regular car. I've been driving my volt about a month and had to go pick up a load of hay with my truck. What a shock. I did not realize how used to the Volt I had gotten and one peddle driving. Darn truck just kept on going... :)

The Volt has two electric motors. The second smaller motor spins backwards when you let off the accelerator and/or hit the brakes, providing resistance. The energy from the second motor spinning is fed to the battery. If more deceleration is needed and you hit the brakes harder, the disc brakes will also kick in....

Using L simply changes the amount of resistance/regenerative braking you get when you let off the gas only. Basically it glides less than when it is in D. You can feel the extra drag. It can allow you to get more regenerative braking, but it also gives you the ability to accelerate and decelerate without touching the brakes. Some call this "one pedal driving". The Tesla roadster operates this way all the time...

FALSE. The main (larger) drive motor (the one that provides the majority of forward motion) is what is used as a generator during regenerative braking. The smaller motor generator is only used for high-speed EV driving or ICE-powered change sustaining or both. Well, perhaps it is also a generator during high-speed regen, but I don't know if that is the case.

The majority (if not all) of regen comes from the large traction motor being used as a generator.

Regen happens a little bit while coasting in D mode, and more when you lift in L mode, and more during brake-pedal braking up to a point and for higher deceleration requests (harder braking) the friction brakes engage. The friction brakes also engage below 4 mph even during gentle braking, so when you're parked at a light you are being held there using friction brakes. It all done pretty seamlessly.

Ok, thanks for helping me understand. So let me follow with this. Driving ICE vehicles all my life I've just grown accustomed to pulling my foot off the throttle on a downhill slope or as soon as I realize I've got all the momentum I need to get to the red light ahead (or stop sign or what ever). It's like (almost) free mileage in an ICE vehicle to have the engine at idle instead of driving the vehicle to the point where you must brake to safely stop. In other words, once at speed use the momentum in the vehicle (stored energy) for as much forward progress as possible. It's not hypermiling but touches on the idea.

That said and from what you describe, it almost sounds like coasting in "L" (Does that stand for "low" and "D" for "drive"?) could actually slow the car while regenerating, even on a downhill slope. So do you sort of lose what I think of as free mileage if you are coasting in "L"?

What's your sense of the trade-off? Is the gain from regeneration greater than the loss of momentum to capture that energy?

After driving in "L" (= Low) for a few minutes, literally, you get accustomed to "feathering" the accelerator to control gliding, slowing down, or speeding up. It becomes quite natural, quite relaxing, and far superior to worrying about switching your foot over to the brake pedal. Furthermore, the efficiency is just as good, if not better, because you minimize use of the brake calipers (and all the dust that thay generate). Just wait until you get your Volt, and try it! :-)

In ICE, D=Drive and L=Low. Where you would shift in L for low RPM and more power. In the Volt, I would suggest that D=Drive and L="leaf" (but everyone calls it "low"--even the owner's manual). The Volt's L is the "environmental mode" allow for the driver to control the regen better without confusing with standard "friction" braking. My opinion is that GM used "L" because there is an existing "L" in cars and they didn't want anyone arguing that they got confused on the user-layout in the event of a lawsuit.

I describe "L" as those amusement park rides or go-karts. There is only one pedal to accelerate to go and stop. You press down to go, you release to engage the brakes, and somewhere in the middle, there is a mix so you can slowly accelerate / decelerate. Think of that as Volt's regen braking under "L" except the slowest it goes is about 4mph and it does not decelerate too suddenly. When you want to stop completely or more aggressively, step on the regular brakes.

I understand from this forum that the regen braking is about 60% effective / efficient. So, if you had enough "free miles" to travel 200 feet and you capture all of that energy in regen, you would be able to get about 120 feet later after the stop.

In your free miles questions going downhill, the Volt does have a D mode that will behave like what you are use to; however, in "L" mode, you will have to know how much more speed you expected to gain and press further--otherwise, the excess will be collected in regen (without you really knowing, but you can see it on one of the many screens). For instance, in an ICE, say you are driving 60 mph. When you start going down the hill, and if you leave your pedal at the same exact position, you could possibly go 65-66 mph at the bottom of the hill (of course, with your foot in the exact pedal location, you use a some gas, just less resistance; for the hypermiler's, they would shift it in N, like a manual transmission, and they may get around 63-64 mph and using less gas). In "L", with the same exact foot position, you will have 60mph at the bottom of the hill, but you would have collected about 60% of 5-6mph in the battery.

In the bigger picture of driving, these free miles are very little--definately not worth a speeding ticket. With the volt, you don't have to actively do anything, the car automatically takes care of it. So, an expert hypermiler can probably get 1000 feet; any grandmother driving the volt can get at least 600feet without effort.

In your second free miles question, it sounds like you drive conservatively when you stop accelerating when you see the light turn red. An ICE still use fuel to keep the engine running when you release the gas pedal, and where you press the brakes to a complete stop, you have just lost all of that forward energy after your desired stopping point. Maybe your car can roll another 10 feet after that 5mph, maybe another 20 feet after 10mph. With regen braking, you store 60% of that energy for you to use after you stop. The "L" mode helps you try to get your current speed down to 3mph among the distance you have--your job is to decelerate as slow as possible from 50mph, 30mph, or 10mph. The regen will capture the unnecessary energy. Then, finally, you have to press the regular brakes for a complete stop or more aggressive braking.

Hypothetically speaking, if you release the gas after seeing the red light, most car would probably hit the car in front of them or illegally / dangerous run the stop light if they do not apply the brakes. In the Volt, the distance that they would make after the stopping point and the actual stopping is reclaimed in the regen braking--at least 60% of it. So, under D or L, if you can decelerate at the slowest rate possible to your stopping point...you would still get the "free miles" that you would in an ICE, but you get the bonus of the 60% of the miles after the stopping point. D does the same reclaim as L, but it is hard for the driver to determine where the regen ends and friction break begins; with "L", the driver can engage only the regen if they do not need to brake aggressively.

-KyleH


Ok, thanks for helping me understand. So let me follow with this. Driving ICE vehicles all my life I've just grown accustomed to pulling my foot off the throttle on a downhill slope or as soon as I realize I've got all the momentum I need to get to the red light ahead (or stop sign or what ever). It's like (almost) free mileage in an ICE vehicle to have the engine at idle instead of driving the vehicle to the point where you must brake to safely stop. In other words, once at speed use the momentum in the vehicle (stored energy) for as much forward progress as possible. It's not hypermiling but touches on the idea.

That said and from what you describe, it almost sounds like coasting in "L" (Does that stand for "low" and "D" for "drive"?) could actually slow the car while regenerating, even on a downhill slope. So do you sort of lose what I think of as free mileage if you are coasting in "L"?

What's your sense of the trade-off? Is the gain from regeneration greater than the loss of momentum to capture that energy?

The smaller motor generator is only used for high-speed EV driving or ICE-powered change sustaining or both. Well, perhaps it is also a generator during high-speed regen, but I don't know if that is the case.

The majority (if not all) of regen comes from the large traction motor being used as a generator.


The car will pull power from both MGs during regen in 2 motor CD or parallel CS; however, the "off the pedal" level in L is enough to push the car back into single motor (also enough to turn the engine off most of the time, depending on the exact circumstances.)

For the OP: the most efficient way to drive is to coast to a stop with no regen - but that's not generally compatible with driving on real roads. Regen recaptures most of the energy you would otherwise lose when you stop the car (with the brakes in a typical car, or with some combination of brake and L/D off pedal in the Volt,) meaning that you spend less net power accelerating after the stop (still not as efficient as not stopping, but again it's the living on real roads thing.)