[pdonis]

But that alternative usually requires driving at higher speed, which more than cancels out any gains from avoiding stop and go because of the increased loss from drag and friction. There aren't any highways where you can drive 400 miles at 25 mph cruise.

Metcalf's description does say that he accelerated and decelerated very gently; see my edit to my upthread post. So he was trying to approach the ideal of driving at a steady 25 mph as close as he could. If the car's regen system had had lower losses, regen would have saved him some energy on those unavoidable decel/accel cycles.

[lutusp]

> But that alternative usually requires driving at higher speed, which more than cancels out any gains from avoiding stop and go because of the increased loss from drag and friction.

No, this is a false choice. The choice should not be between stop-and-go driving, versus speeding along. For a proper evaluation, the tested alternatives should be (a) stop-and-go driving with an average velocity of V, versus (b) driving at a constant velocity of V. In that comparison, a constant velocity is much more efficient. The reason is that regenerative braking cannot recover more than a fraction of the energy lost to braking.

The above is in keeping with the best scientific practice, in which an experiment changes just one thing and keeps everything else the same. So we should choose an average velocity, then compare steady speed and stop-and-go driving at that velocity. In that experiment, steady speed wins.

> Metcalf's description does say that he accelerated and decelerated very gently ...

Doesn't matter. Adding a given amount of energy E to a moving object requires the same expenditure of energy regardless of how quickly or slowly it's done (although in practical examples, fast acceleration is wasteful for reasons outside the simplest explanation of the physics). It's the same with removing energy from a moving object, and it is here that the unavoidable losses in regenerative braking prevent the two cases from being equal.

> So he was trying to approach the ideal of driving at a steady 25 mph as close as he could.

That ideal is only achieved by maintaining a steady speed of 25 MPH, not by stop-and-go driving. It's not clear at this point whether Broder was actually told by someone at Tesla that stop-and-go driving was more efficient or not, but if so, that person needs an education.

> If the car's regen system had had lower losses, regen would have saved him some energy on those unavoidable decel/accel cycles.

Yes, but regenerative braking can only minimize losses, it can't recover all the energy lost to braking. Therefore a steady speed is more efficient.

[pdonis]

For a proper evaluation, the tested alternatives should be (a) stop-and-go driving with an average velocity of V, versus (b) driving at a constant velocity of V. In that comparison, a constant velocity is much more efficient.

I completely agree, if you are trying to run a scientific experiment. But if you're driving on real-world roads, you're faced with a different set of choices. As I said, you can't expect to drive 400 miles at a steady speed of 25 mph in the real world.

Adding a given amount of energy E to a moving object requires the same expenditure of energy regardless of how quickly or slowly it's done

I wasn't saying that accelerating/decelerating more gently saves energy. I was saying that it probably prevented the regen system on the car from activating at all, meaning that none of the vehicle's kinetic energy was recaptured. Since he could not avoid stopping and starting again (since you can't drive 400 miles at a steady 25 mph on real-world roads), if it had been possible to reclaim some energy through regen during deceleration, it would have increased his range compared to stopping and starting again with zero regen. That's all I was saying, and it's completely consistent with what you're saying.

It's not clear at this point whether Broder was actually told by someone at Tesla that stop-and-go driving was more efficient or not, but if so, that person needs an education.

Not necessarily, because Broder's choice was not between stop and go driving at an average speed of 25 mph, or driving at a steady 25 mph. It was between stop and go driving in Manhattan (you are not, I trust, claiming that it's possible to drive through Manhattan at a steady 25 mph without stopping), at an average speed of 25 mph or so, and driving on freeways at an average speed of, say, 60 mph. Given that choice, it's entirely possible that the stop and go driving would give more range; the exact tradeoff would depend on details like the vehicle's drag coefficient, rolling friction, efficiency of regen, etc.

[lutusp]

> I completely agree, if you are trying to run a scientific experiment. But if you're driving on real-world roads, you're faced with a different set of choices. As I said, you can't expect to drive 400 miles at a steady speed of 25 mph in the real world.

A red herring. Whatever speed seems appropriate, steady speed is more efficient than stop-and go driving. My only point is that the advice to intentionally engage in stop-and-go driving is mistaken.

[pdonis]

Whatever speed seems appropriate, steady speed is more efficient than stop-and go driving.

A red herring, because the real-world choice is usually between stop and go driving at a low average speed, and steady-state driving at a high average speed. It's irrelevant to point out that a choice that was not actually available (steady-state driving at a low average speed) would be more efficient.

My only point is that the advice to intentionally engage in stop-and-go driving is mistaken.

Not necessarily, if the actual choice is as I said above. Did you read the last part of my previous post?

[lutusp]

> A red herring, because the real-world choice is usually between stop and go driving at a low average speed, and steady-state driving at a high average speed.

That's false, breathtakingly ignorant, and you have completely abandoned the original topic, which is to establish whether the advice given to Broder by Tesla (to engage in stop-and-go driving) would help or hinder battery duration and vehicle range. In point of fact, it would hinder battery duration.

> Not necessarily, if the actual choice is as I said above.

Try to focus on something other than your wish to be "right" in spite of the facts. Stop-and-go driving decreases the range of an electric vehicle, and it was incorrect advice to give to a nontechnical journalist.

Pretend to be a scientist, as hard as you may find that. Consider variables one at a time. The driver wants to maximize distance, so he is not going to travel above a moderate speed (this is proven by the fact that Broder knew this and traveled at a moderate speed after he realized his predicament). What is in question is solely whether stop-and-go driving aids or hinders maximum range. It hinders it -- this is physics 101.

[pdonis]

That's false

You're kidding, right? The choice Broder had was between driving on a freeway and driving through Manhattan. It was not in any way a choice between stop and go driving and steady-state driving at the same average speed.

Stop-and-go driving decreases the range of an electric vehicle

Compared to steady-state driving at the same average speed, yes. Compared to steady-state driving at a significantly higher average speed, not necessarily.

this is physics 101.

Okay, let's do some physics. The energy required to move a car through a distance D is F * D, where F is the force needed to push the car. For travel at a steady speed, F is given by the following equation:

F = c0 + c1 * v + c2 * v^2

where c0, c1, and c2 are constants that are determined by vehicle and environmental characteristics. (Briefly, c0 is the coefficient of friction between the tires and the road times the weight of the car; c1 is a (usually very small) constant related to the internal friction of rotating parts in the car; c2 is 1/2 rho Cd A, where rho is the air density, Cd is the car's drag coefficient, and A is the car's cross-sectional area. The key is that all of these things can be taken to be constant for the duration of the trip.)

For stop and go driving, F is given by the above formula times a constant e, where e is determined by the efficiency of regen; if e = 1 then regen is 100% efficient and all of the the vehicle's kinetic energy is reclaimed on each decel. If e > 1 then regen only captures a portion of the vehicle's kinetic energy, the portion being 1/e.

So if we compare stop and go driving at an average speed v1 to steady-state driving at an average speed v2, we have

E1 = F1 * D = e (c0 + c1 * v1 + c2 * v1^2) * D

E2 = F2 * D = (c0 + c1 * v2 + c2 * v2^2) * D

If we take v2 = 2 * v1, which is a conservative estimate for Broder's situation (25 mph average speed in the city vs. 50 mph average speed on the freeway), we have

E2 = (c0 + 2 * c1 * v1 + 4 * c2 * v1^2) * D

Now subtract to get the net energy difference:

E2 - E1 = [(1 - e) * c0 + (2 - e) * c1 * v1 + (4 - e) * c2 * v1^2] * D

Regen typically recaptures about 80 percent of a vehicle's kinetic energy, meaning e is about 1.25. So we have

E2 - E1 = [-0.25 * c0 + 0.75 * c1 + v1 + 2.75 * c2 * v1^2] * D

This is going to be positive for any vehicle except a heavy one with a low drag coefficient; practically no vehicles have that. So stop and go driving at 25 mph is going to save energy compared to steady state driving at 50 mph. This is the sort of calculation that I suspect was in the minds of the Tesla people when they told Broder that the stop and go segment in Manhattan was going to give him better range than driving on the freeway.