If it is good/very good, then I would like to have a generator-motor VOLT with no batteries.

Koz, good point and this gets us back to the reason for lift-off regen — drivability. It makes an EV drive more like an ICE car and gives people some control over decel rate without having to move their foot over to the brake pedal. I’m all in favor of that as default behavior, I’d just like to be able to turn it off. Second best option is a clear indication (detent or visual display) of the pedal position which produces gliding.

You’re also right about auxilary loads. A better example is Car B lifts off 507/4 meters after Car A, decels for 507/2 meters and then drives 25 m/s for 507/4 meters. This equalizes the elapsed times, and thus the aux loads, for the two cars. It also better equalizes the aero loads. The numbers change a little but the conclusion stays the same.

My powers of interpretation had expired.

I doubt Nasaman will disagree with the accuracy of your analysis, other than you have negelected auxiliary loads which can be assumed to be equal but gliding will have them acting over a longer period of time. Thus, there will be more auxiliary losses for the glider but these will surely be less than the inefficiency losses of the regen/repower cycle.

BUT, gliding is not a practical way to drive for most people and even for the gliders it’s not practical in most driving situations. You could use the drag, the discharge characteristics of the battery, and other energy factors to determine the most optimal accerlation and speed and gliding point. This would maximize range but wouldn’t be very good for time. For that matter, you could walk or ride a bike and use less battery energy. How often will you be able to know or even be able to practically execute gliding at the optimal moment. Yes, any amount of gliding is better than none. I would prefer that they design lift-off to allow for gliding to be easily achieved before regen forces are applied, but not to do away with lift-off regen altogether. Lift-off regen will make driving in heavy traffic a lot easier and many drivers will end up with better range if they can more effectively utilize regen this way. Ultimately, I believe it is more important to avoid as much friction breaking across the entire Volt fleet than to try to increase gliding but they aren’t mutually exclusive.

If they want to allow disabling of lift-off regen for “gliders”, I don’t have a problem with that.

No way in hades.

Here’s another Car A/B example. Except for a few minor simplifications, these numbers are representative of Chevy Volt performance. Cars A and B both travel at 30 m/s (about 67 mph) toward a “Reduced speed” sign. Car A lifts off at point A and glides, hitting 25 m/s exactly when it reaches the sign. Car B lifts off halfway between point A and the sign, but decelerates more rapidly due to decel charging and also hits 25 m/s exactly as it reaches the sign.

Aero drag, rolling resistance and driveline drag exert 450 newtons of force on Cars A/B at 30 m/s (i.e. 13.5 kW). At 1500 kg, Car A thus decels at 0.3 m/sec2. At 25 m/s speed the force is down to 360 newtons and decel rate is only 0.24 m/sec2. Total decel time is thus 18.5 seconds and distance covered during decel is 507 meters.

Car B decelerates twice as fast, 0.60 m/sec2 initially falling to 0.48 m/sec2 at the signpost, because decel charging is set to exerts a decelerating force exactly equal to aero drag, rolling resistance and driveline drag. Decel charging thus bleeds off half the delta KE, or 103,125 J. If we generously assume a round-trip regen efficiency of 80%, that’s 82,500 J of recaptured energy available to drive the wheels. But how much extra energy did it take to push Car B that first 258.5 meters, while Car A was already gliding?

450 N * 258.5 m = 116,325 J

Car B thus burned 33,825 J of net energy (116,325 – 82,500) during the 507 meters while Car A burned zero. Since decel charging burns more energy, it REDUCES your range vs. gliding. This is logically consistent, since decel charging involves losses which gliding does not experience, and also matches Prius hypermiler results in which best MPG is achieved by taking great pains to avoid the built-in decel charging.

If you disagree with this conclusion, I’ll need to see your math.

You’re right if your saying that deceleration alone won’t increase range —but deceleration CHARGING WILL! That’s one reason EVs operate over typically only 50% of their battery’s SOC range of 0-100%: to allow deceleration CHARGING (think “engine braking” in ICE-powered cars), which captures & restores a substantial portion of the vehicle’s kinetic energy. If you’re thinking otherwise, you’re in disagreement with GM’s, Mercedes’ and BMW’s experts on this —not just me (a life-long physicist, astrophsicist & electrical engineer).

Extensive EV testing has proven the benefits of BOTH deceleration charging and regenerative braking charging —so if you disagree you’re also arguing with indisputable test data.

Deceleration charging does not improve range. You seem to think there is a free lunch: since cars decelerate ANYWAY when you lift off the accelerator pedal why not use regen to capture some of that deceleration energy? It’s this kind of thinking which leads people to think they can extend range by mounting small windmills on their EV (”all that air is just zooming by at 60mph, let’s convert some of it to electricity and recharge the batteries as we drive”).

If you want to separate regen into two modes, that’s fine. They are certainly distinct modes from the driver and control logic perspectives. But the physics are the same. On level ground without wind, lifting off causes deceleration due to aero drag, rolling resistance and driveline friction. If you add another force by applying fields to the drive motor such that it generates electricity, you will decelerate more rapidly. It does not matter ONE BIT if this extra force is triggered by control logic responding to lift off of the accelerator pedal or slight pressure on the brake pedal. The effect on range is exactly the same.

If two identical EVs going side-by-side at 40 mph both lift off at the same moment, Car #1 with “deceleration charging” will slow down and come to a stop before Car #2 without it. If both cars accelerate equally from their respective stopping points back to 40 mph the distance gap will be maintained. If Car #1 then accelerates to 41 mph, will the energy it captured during regen be enough to “catch up” to Car #2? No, because regen is less than 100% efficient. Car #1 experienced losses in the generator, power electronics and battery that Car #2 did not experience. Higher losses = less range.

In the real world, of course, you don’t coast all the way to a stop because drivers behind you may resort to firearms. And there are situations where even with associated losses regen can improve range, e.g. long descents at highway speeds. But the gain in range is exactly the same for “deceleration charging” as for “regenerative braking”.

The ONLY reason to invoke deceleration charging when the driver lifts off the pedal is because it makes an EV feel more like an ICE car to the driver. That’s fine by me — the last thing I want is drivers switching from ICE cars to freak out because their shiny new EV doesn’t act the way they expect. But the amount of deceleration charging should be minimal, to avoid associated losses, and drivers who value maximum range over an “ICE car driving experience” should be allowed to disable it.

“….when an electric drive motor is operating as a generator its function is to capture and store energy from….

1) regenerative braking (charging)

2) deceleration & coasting (charging)”

These are the two distinct modes of operation for a properly designed EV drive motor in its generating mode. “Deceleration & coasting” is an important mode but is distinct from actual brake-pedal actuation resulting in “regenerative braking”. I suggest we all adopt this terminology, already in use by industry leaders.

*GM, Daimler-Chrysler & BMW

Ewww… this is not correct. Not sure what I was thinking here. Over the same distance coasting will always be better than using regen, when only considering rolling resistances. This because of regen losses, but if auxialiary loads are sufficiently high then it could be more efficient to maintain speed and use regen-only deceleration saving transit time.

Nasaman,
I understand your distinction between lift-off regen and regen braking, but braking and deceleration as well as charging and regeneration are interchangable in my thinking. Perhaps for clarity it is best to use more distinctive terminology. I understand any decelerating force to be a braking force, whether it be induced by drag, regen motor load from lift-off, or regen motor load from brake pressure. How about the terms accelerator lift-off regen and brake pedal regen to distinguish the two modes.

For some reason Tesla has chosen to use lift-off regen only. I believe it is a variable load that starts at zero at increases to a safe (driver expectable) amount. I was assuming the Volt may use a similar approach to their lift-off regen. I was also hoping that the Volt would incorporate brake pedal regen as well. Perhaps if both modes are utilized then it doesn’t pay to use a varying regen amount for lift off, but I was kind hoping for this functinality. Does anyone know if break pedal regen has been confirmed for the Volt? I thought I read somewhere that Toyota has patents on break pedal regen, but didn’t EV1 use it?

I believe doggydogworld was concerned that the Volt would have any lift-off regen whatsoever because he wants to be able to coast completely inihibited from any regen load.