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Discussion Starter #1 (Edited)
People here seem to think that driving in low mode is more energy efficient than driving in normal mode. At first I was buying into that. However, as I started thinking about what the bouncing ball is trying to tell us I'm not so sure.

I suspect that the ball is staying in the center, for maximum efficiency, for the mode you're in, not necessarily for overall efficiency. In other words, when driving in low, if you don't touch the brake the car slows down pretty fast and the ball stays in the center. When driving in normal if you don't touch the brake the car will coast for a very long way before stopping and the ball stays in the center. When braking in normal mode to slow down at the same rate as low the ball drops fairly far to claim that the driving efficiency isn't good, yet the car is slowing at the same rate as low.

Question for GM engineers: if the car is slowing down at the same rate in normal (using the brake pedal) as it does in low without touching the brake pedal, is there really any difference? Is the amount of brake regen the same and the only real difference the feedback?

Or, is there a difference algorithm for brake regen vs disc brake usage when in low vs normal? Meaning that the same rate of slowing down uses more disc brake in normal that it does in low.
 

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Question for GM engineers: if the car is slowing down at the same rate in normal (using the brake pedal) as it does in low without touching the brake pedal, is there really any difference? Is the amount of brake regen the same and the only real difference the feedback?

Or, is there a difference algorithm for brake regen vs disc brake usage when in low vs normal? Meaning that the same rate of slowing down uses more disc brake in normal that it does in low.
Two goals: 1 slowing down and 2 generating x amount of energy back into the battery

Using the "brake pad" and "regenerative brake" will both do 1. However imagine that if it takes you 20 seconds to slow down to a stoplight and using D(rive)+Brake_pad cause 5 seconds of regen but using L(ow)+Less_brake_pad causes 15 seconds of regen. Simply which will put more energy back in the the battery in that 20 seconds?

Interesting video here: I Finally Understand Hybrid Regenerative Braking
http://priuschat.com/forums/gen-iii-2010-prius-technical-discussion/87992-i-finally-understand-hybrid-regenerative-braking.html
 

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Question for GM engineers: if the car is slowing down at the same rate in normal (using the brake pedal) as it does in low without touching the brake pedal, is there really any difference? Is the amount of brake regen the same and the only real difference the feedback?

Been wondering this as well. Good question.

Do you apply more break pad in "normal" breaking opposed to less break pad in "low" when decelerating at the same rate.
 

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Here's two observations I've seen which may shed some light...

1) When I'm in LOW and also applying the brake pedal, I notice that all my braking disappears once I slow to about 3 MPH, unless I apply the brakes even harer.

2) When I'm in DRIVE and slowing down at the same rate as with observation #1, I notice that my braking doesn't disappear at 3MPH, but continues to slow uniformly until stop.

Now... I've always thought that the amount of regen available in either DRIVE or LOW was the same, the only difference being how much brake pedal you had to use to achieve the same result.

However, given the observations above, it seems like using LOW would achieve more regen than DRIVE. I base this on the fact that regen stops at 3MPH, and if I don't have the feeling of regen letting up in DRIVE but I do in LOW, it suggests to me that there's not as much regen in DRIVE.

Maybe someone (read WOT) can chime in on this subtlety and possibly correct any bad assumptions on my part. ;)
 

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For the sake of argument, let's say the amount of regen energy recovered is the same for both D and L. I still prefer L because it slows me down in a shorter distance, all things being equal. In practical use, that means I can stop with little or no brake pad wear more often as lights change or other cars slow. Using D would require a longer distance to do the same. So I prefer the more responsive regen of L from that standpoint alone. But I understand that L also recovers more energy as well, which is simply another good reason to use L.
 

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Plus, there is the pure joy of driving with just one pedal 90% of the time. :)

(And yes, I am careful about people following me too closely. I've driven a stick-shift for most of my life...)
 

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I think the ball tells you what you want to know. If you slowdown in either D or L the ball stays centered and therefore you are using the system to it's max potential. As soon as you touch the brake pedal the ball heads south, with more agressive braking driving the ball further to the negative side and you are losing out on energy recovery.

So my anaylsis is that if the ball stays centered in D with brake application to generate the equivalent deceleration to L, then there is no difference in efficiency. Try it and I think you will find the answer in the ball.

BTW the EV1 didn't have the ball and there was a button on the side of the shifter the selected either "coast (no regen)" or "regen". I was really excited the first time I was driving on a long down hill in coast mode looking to see the EV1 go faster than the 80mph limit. Alas the car wouldn't go faster than 80 :-( the regen kicked in to limit it to no more than 80.

VIN 0985
 

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I don't trust the ball as much as some of you do. I base that on what seems to be the fact to me that if I hit Resume on the cruise control, the car accelerates at a pretty good clip while keeping the ball in the absolute middle. IMO, if I accelerate through the same range at the same rate, the ball moves up to scold me for using too much power.

Similarly, I observe that when I slow down with the brakes (in D) with what I feel is the same amount of deceleration as using L, the ball drops a bit. Now since I know that L does not use the mechanical brakes at all, I suspect that using the brakes in D mode starts using the mechanical brakes before it has exhausted the potential of regenerative braking (but harder application will eventually max out the regen.)

For me, even if my assumption is not correct, I like L for most driving because I feel that when I'm able to slow down just from the drag of regen braking, I'm getting the maximum energy recovery out of it, and when I have to add braking to L I suspect that I'm getting no more power regenerated. Where I think L is not the most efficient is on the highway when you're closing in on another car, and just "coasting" in D is enough to match your speed to theirs. (Usually in that scenario I was using cruise control, and then when they clear the lane I can hit Resume.) That maximizes the momentum (stored energy) of the car in motion, where if I slowed more quickly using L then there is an overall loss because regeneration is far less than 100% efficient at recovering energy.
 

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Where I think L is not the most efficient is on the highway when you're closing in on another car, and just "coasting" in D is enough to match your speed to theirs. (Usually in that scenario I was using cruise control, and then when they clear the lane I can hit Resume.) That maximizes the momentum (stored energy) of the car in motion, where if I slowed more quickly using L then there is an overall loss because regeneration is far less than 100% efficient at recovering energy.
Interesting observation and you are probably correct that for long interstate tollway type driving where there are few congestion points, using D could be preferable. Obviously highway driving during busy times when there is more likely to be "pulses" of traffic congestion would benefit from L.
 

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I've always assumed that when slowing from V1 to V2 in time T and distance D it wouldn't make any difference whether you were using "D" or "L".
 

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I don't trust the ball as much as some of you do. I base that on what seems to be the fact to me that if I hit Resume on the cruise control, the car accelerates at a pretty good clip while keeping the ball in the absolute middle. IMO, if I accelerate through the same range at the same rate, the ball moves up to scold me for using too much power.
I think you need to learn to trust the ball. My feeling is that the GM engineers have nailed this one. I'm convinced that resume on the cruise control will maximize acceleration vs. demand on the battery. I see the ball as a true indicator of efficiency. My feeling is that the ball represents current draw on the acceleration side and "maximized regen" on the deceleration side. So when the ball rises the current draw on the battery is rising as well, and when the ball drops you are giving away some of the energy that could be recovered through regen.

For me, even if my assumption is not correct, I like L for most driving because I feel that when I'm able to slow down just from the drag of regen braking, I'm getting the maximum energy recovery out of it, and when I have to add braking to L I suspect that I'm getting no more power regenerated. Where I think L is not the most efficient is on the highway when you're closing in on another car, and just "coasting" in D is enough to match your speed to theirs. (Usually in that scenario I was using cruise control, and then when they clear the lane I can hit Resume.) That maximizes the momentum (stored energy) of the car in motion, where if I slowed more quickly using L then there is an overall loss because regeneration is far less than 100% efficient at recovering energy.
I agree, this pretty much how I use L and D. However, I don't use resume much because I'm willing to accelerate at a much slower rate (probably 1 mph per 5 seconds) which definately leaves the ball in the middle with the leafs still rotating.

VIN # 0985
 

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I think GM engineers have hit the head of the nail with the Ball. Hitting resume on cruise moves the ball slightly at a very fixed rate. I drive in town with the cruise a lot in L and use the brake to disengage. I am fortunate that most of the traffic system is programmed pretty well here and 2-3 mph over the limit can create the situation where multiple miles can happen between stops. This has the double benefit of max regen and minimal acceleration load while a flash of the brake light tends to signal my intent to people behind me. I try and project my stopping distance to time my cruise disengage (getting better at that every day). I do notice that some drivers seem to act like I have no brake lamps until the last few MPH during a stop, but that brake lamp flash generally works the treat as a warning of my intent.
 

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Discussion Starter #14
@scott200. Thanks. I did try searching before posting the question. There's lots and discussion about it but no clear answer to my question:

If I'm slowing down at the same rate is there really any difference between (L)ow slowing down at it's natural rate and (D)rive using the brake pedal to slow at the same rate as low?
 

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@scott200. Thanks. I did try searching before posting the question. There's lots and discussion about it but no clear answer to my question:

If I'm slowing down at the same rate is there really any difference between (L)ow slowing down at it's natural rate and (D)rive using the brake pedal to slow at the same rate as low?
I think the simple answer is that in Low, it's easy-as-pie to be extremely efficient--just lift your foot, relax, and enjoy.

In Regular it's quite challenging and distracting (dare I say dangerous?) to feather the brake pedal just right to keep the ball green and centered.
 

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@scott200. Thanks. I did try searching before posting the question. There's lots and discussion about it but no clear answer to my question: If I'm slowing down at the same rate is there really any difference between (L)ow slowing down at it's natural rate and (D)rive using the brake pedal to slow at the same rate as low?
No worries. Sometimes past threads have a lot of very good discussions surrounding the topic. Some folks may have spent a lot of their time on research and answers to several questions. When new / repeated threads come up they may not participate so people would miss a lot of their input. Lot of folks do not know about the site: keyword so I mention it as it can be more powerful and faster than many forum searches.
 

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As a non scientific observance, I make an elementary assumption that anytime you apply the break pedal-break pad, you are creating friction and heat on the brake disc, and heat is a wasted by-product. ( and break pad wear) I like the fact that the Volt regens anytime the car is coasting with your foot off the accelerator and therefore I choose to drive in L allowing me to assimilate into typical traffic without the long dreaded coast to a stop with a line of over anxious drivers behind me, but I drive in D while on the highway for less immediate deceleration.


Steve in Boca Raton #313
 

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The main confusion that I'm seeing here (and I may be making an incorrect assumption) is that slowing down=regen braking. This isn't always true however since there are different ways to slow down.

When you press on the brake peddle, you are applying the mechanical brakes to reduce the car's momentum. This results in lost energy due to heat dissipation from the disk brakes.

Regen braking occurs when the car's momentum is fed back into the engine to turn it into a generator. This happens any time you take your foot off the accelerator. People who drive manual car's know this as engine braking; it also occurs in automatics when you take your foot off the gas, but to a lesser degree.

Using the engine brake in D vs L is a matter of motor control by the Volt's systems. In D, it acts more like a coast, so you slow gradually and still regen power. Applying the brake peddle in D will hasten your stop, but decrease the amount of power that the regen can absorb back since it's being accumulated in the mechanical brakes. In L mode, the engine will absorb more power quicker (by electrically loading the engine more) which results in a more drastic deceleration.

Ideally, if you coasted to a stop in both modes from the same speed, you will regen the same amount of power. The difference is that in D, it will take considerably more distance and time (this also allows more aerodynamic drag to effect the vehicle, but it's probably negligible).

To sum it up, if you use the brake peddle, you are wasting energy with the benefit of slowing down faster. If you plan your stops in advance, you can leverage the extra power reclaimed by the regen braking by using the mechanical brake less.
 

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Discussion Starter #19
@bdbasile. Do you know for sure that the using the brake pedal lightly in Drive to simulate the deceleration of Low uses the mechanical brakes? Or, does the volt try to use regen braking first so that slowing down at the same rate in either mode is effectively the same thing?

Basically I'm trying to figure out if the Ball is claiming "efficient driving for the (Key point) mode you're in" vs "efficient overall driving".

I gotta believe coasting in Drive for a natural slow down vs coasting in Low for a natural slow down would be more efficient so that Drive coast would cover a lot more distance.

IF the volt is always being aggressive with braking regen before bringing the mechanical brakes into the picture then there really isn't a benefit to driving in Low other than the convenience of it in stop and go traffic or when descending step hills.

IF on the other hand the volt uses a different algorithm for mixing braking regen and mechanical brakes then there is more merit to using Low.

I've read as many of the discussions on this as I can but so far no one has chimed in with "according to the GM engineers here's what's actually going on."
 

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I too was on the fence with this topic... Use D or L? I was a skeptic of L! I take the same daily communte to work and have been experimenting with both modes over the last 1000 miles or so. Plain and simple, I get greater range when I use Low. You may need to do your own form of experimenting until you convince yourself like I had to do. I "feel" like I'm using more energy but by the time I deplete the battery to 0 miles, I've gotten more range out of it than if I were in D. My commute involves in-town driving, some highway, and then some more in-town driving. The added benefit is that by using L, I do not use brakes very much. Makes me feel like I'm saving wear/tear on the vehicle in not just the brake pads but other components as well. Also, it's a different experience to drive in L from any other car you've ever driven. Takes some getting used to but you learn to really like it and then miss it if you ever get into a more traditional car.
 
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