Forums

Join The Community
RegisterLogin

Is coasting possible?

Because of the inefficiencies of regerative breaking, I would think there would be plenty of opportunity to slip the MS into neutral and coast instead of keeping on the accelerator and then slowing down with the regen.
Is it possible to move into neutral while moving and would it cause any damage to the MS drivetrain?
Another possibility would be feather the accelerator so it would effecively be like neutral but this I assume would require sensitive feet!

I'm pretty sure you can slip it into neutral while driving, but I haven't tried it. After years of driving a Prius, I'm used to feathering the accelerator, so that's what I do.

Feathering's pretty easy. Haven't tried slipping into N, I'll wait for someone else's report.

www.teslamodels.wordpress.com

Yup, just a tap put you in neutral and kinda feels like you'll glide for ever. Given a steep enough grade you'll go faster yet. Rolling resistance is very low. It appears as though given a run where no stopping or slowing is required ( long hill) that there seems to be a gain from neutral over modulating the the accel pedal to 0 KW.

My experience is anecdotal though. Also, pushing on the pedal slightly before re engaging Drive makes the transition a little smoother as well.

Have fun!

Yes, according to a post I read previously, where someone doing a test drive accidentally put it in neutral by unconsciously trying to do a windshield wipe (by pushing down on the right stick). Not sure if this has any detrimental effects - I would assume not.

Accidentally put it in neutral the first day. It coasted slowing since it was uphill. I initially panicked, but the slipped it into Drive - a slight jolt, but no problem. As said by @dave above, a slight press on the pedal may have prevented the jolt.

At a recent Supercharger stop, I was conversing with other owners about coasting vs regen going down a long grade. My first assumption was the coasting was 100 efficient, while regen maybe only 75% efficient so coasting was better, unless of course you got a speeding ticket. But then I realized that since drag goes up with the square of the speed, the advantages of coasting are limited. Assuming a reference speed of 65 mph, a quick back of the envelope calculation suggested that somewhere around 75 mph the loses to drag would equal the losses from regen. So regeneration does not seem such a bad strategy going downhill.

I'm wondering what happens to the brake light when feathering the accelerator to maintain constant speed going down hill when regen is working in the green. Is the brake light off because you're applying power but not enought to go yellow?

The brake light--when not on due to the brake pedal--is activated by an accelerometer, so if you are not slowing significantly due to feathering, the lit will not activate even though you're in the green!

I'd like to understand what shs is saying. 100% efficient in what sense? Coasting at a given speed downhill neither adds nor subtracts charge from the battery. Regenerating at the same speed adds charge to the battery at lower than 100% of course, but still adds charge. If you speed up by coasting, you still have no net gain in charge. If you brake to maintain constant speed, you waste kinetic energy. With regen, a steeper hill allows you to recapture kinetic energy in order to maintain a constant speed. Then there are safety issues to consider as well.

The car will coast going forward down a steep hill even if in reverse and creep off. If you push the accelerator, the forward roll will slow down. If you push the accelerator enough, it will then go in reverse up the hill.

David,

What I was suggesting is that when going downhill one is starting with potential energy, one's elevation, and by coasting downhill and letting your speed increase, you convert that potential energy to kinetic energy. Then when you go up the hill on the other side of the valley, that extra kinetic energy would be converted back to potential energy as one gains elevation, and of course slows down. If one were driving in a vacuum (no drag from air) and with no friction, when you got back to the same elevation you started, you would have the same speed with which you started and there would be no net loss of energy. Of course we do have friction in all the moving parts and tires, and we do have drag, and because drag increases with the square of the speed, the faster you go, the more energy it costs.

If regen was 100% effective, then we could regen all that potential energy going downhill rather than letting the speed increase, and then use that energy we regened going downhill when we go back up hill. Again, in a perfect world, e.g. 100% efficient regen (and no other losses), you would have exactly the same battery charge and the same speed when you get back to the same elevation as you started.

One rule I learned riding a bike is that for every hill you go down, you have to go back up sooner or later. Driving is not all downhill. Never was, never will be.

The question then is which is the more efficient, letting the speed increase and suffering the losses that come from increased drag, or keeping the speed down using less than 100% efficient regen. I was surprised that it didn’t take too much of a speed increase over 65 mph for the drag losses to exceed the regen losses.

I had the same thought as shs on my trip from Bay Area to Phoenix. Several downhills where i took it off CC and let it 'coast' at fairly high speeds. No problem here with the speed. but started to wonder if the square issue wrt drag coefficient would cross-over the regen inefficiency. Anyone work the math with an assumed regen efficiency to tell us if we're good to 80, 90 or more before drag counters?

When I did the math I found that drag likely exceeds regen losses above about 75 mph.

Simple question - do yo have less range driving 80 or 90 than 65? Why? Same thing when you go 90 downhill.

You guys are way over analyzing! Just use regen to keep a constant speed and be happy that unlike an ICE you are putting energy back into the battery. Sheesh!

I find discussion in this thread very engaging and interesting. Keep it up, guys!

I've recently been playing around with coasting and yes around 75 MPH and above it is not very beneficial. You also have to take into account if you are going to have to regen or hit the brakes during or at the end of the coast. If you are coasting downhill to a stop sign (for example) you just wasted all the potential regen if you end up on the brakes. In any traffic you always have the moron who changes lanes without looking to see if he is clear or not.

So far I haven't gained any useful range from it. Useful as in a 10% or better gain.

David Noland has been documenting his experiences with his Tesla has posted this article that discussed some of the pros/cons of coasting.

http://www.greencarreports.com/news/1082867_life-with-2013-tesla-model-s...

According to him, it is more efficient overall to coast than to capture the regen. I don't have any personal data yet to prove/disprove this.

However, I have been driving a hybrid for years now, and I have found it beneficial to gain as much speed going downhill as practical/possible rather than try to recapture energy thru regen. But this is governed by the very limited battery capacity, so best to have momentum than use (any kind of power) going up the next hill. And of course, there is always the ICE :-) So I nudge the throttle just enough to stay right on the line between regen and expended power. I am sure the dynamics with the Tesla will be different. We'll see.

Coasting is possible. Do NOT use Neutral.

All you have to do is keep the accelerator pedal at a certain point to coast. It's not that difficult at all.

Brake lights work as they should. They come on if you press the brake pedal, or if the car's decelleration passes a certain threshold.

@rd_redford

That coasting is more efficient then regen is fairly obvious. If it was not so, it would mean that we have almost invented a perpetual motion machine (albeit of the stop-and-go variety).

When you are moving, your car has kinetic energy. When you coast, you are letting that kinetic energy take you farther, and just about the only thing slowing you down (removing the kinetic energy) is friction with the road, the air, and some internal parts of the car. The key thing with coasting is that you are continuing to use energy that's already been spent, and you're not trying to convert the energy to some other form.

When you activate the regenerative braking, all of that same friction is there, but in addition to that, the electricity generated by the regenerative braking system creates an electromagnetic force which acts against the turning of the wheels. The key thing here is that you are taking kinetic energy prdouced by the electric battery energy that you already spent) and you're trying to convert it back into electricity. Like any system we make for converting energy or doing work, it's less than 100% efficient.

In a certain sense, braking (no matter what the mechanism) is ALWAYS a waste of energy.

So yes, you should always drive in a way that will allow you to coast when possible, if you desire to drive efficiently. This is true of all cars, not just EVs. This kind of driving requires anticipating when you need to slow down or hold speed so that you can take advantage of opportunities to coast.

What may fool some people is that using the regenerative brakes will show an immediate gain in efficiency, but later, when you need to accelerate again, your efficiency will drop.

For example, in a Model S, if you go down a steep hill for a decent length of time and you let go of the accelerator (allowing the regenerative brakes to come on fully), you will notice that the Wh/m for the trip will drop sharply. For example, let's say it drops to 200 Wh/m. However, when you get to the bottom of the hill, you will need to apply the accelerator again to maintain speed. After some distance, your Wh/m will increase. Let's say it increases to 325 Wh/m.

Now, if instead of dragging on the regen brakes, if you coasted down the hill, it's true, your Wh/m will not decrease anywhere near as sharply as it would with the regen brakes. Let's say it's at 250 Wh/m. However, once you get to the bottom of the hill, you will be going faster, and you won't need to apply the accelerator as much. Also, you won't have lost some energy to the inefficiency of the regen brake system (remember, every energy system we have leaks energy). So now, you'll have travelled the same distance as the previous scenario, but your overall Wh/m will be better, say for example, 275 Wh/m.

Now of course, your ability to coast will be limited by speed limits, traffic, safety considerations, etc. For example, even though it would be way more efficient, it wouldn't be safe or legal to coast down a steep hill at 100 mph.

@olanmills
Why not use Neutral?

There is no way it can be good for the transmission to drop back into gear at 70mph.

@rdalcanto - I love it; "transmission to drop back into gear", an automatic response based on instincts developed with an ICE architecture.

For it to go completely into neutral and roll, the motor has to disengage from the drive axle. Whether that really happens, I don't know. There are gears (although one ratio) with oil. Someone else said there was a clunk when they went from neutral back to drive while moving. I know I would want more information on the drive train mechanics before I started doing that to my $100K car in order to save a couple cents....

I don't know if I can explain my thoughts better. If it is a single speed transmission that never disengages the motor from the axle, and coasting still spins the motor, then going from neutral to drive while you are moving should not hurt it. If it is a single speed transmission that is able to mechanically disconnect the motor from the axle and really coast, then I would not want to re-engage with the axle spinning rapidly with the motor stopped. The fact that someone said they got a clunk makes me worry about possibility number two, although I don't know why they would design the system that way. It seems like it would be much more reliable to have a fixed gear setup that can never be disengaged.

Coasting might make sense in some scenarios but you have to factor in what happens at the bottom of the hill.

If you have to break to stay within the speed limit as olanmills said, then regen breaking is far better than being in neutral and having to use the friction brake (brake pedal).

If you have to stop at some point (stoplight, stop sign) you also want to use regen over foot brake.

So coasting only helps if you can use the stored potential energy (speed) to cover additional distance.

Thus, for example, putting in a sensor to reduce regen when going down a hill and coast more, may not make sense because the sensor can't predict what comes next - stopping or coasting after the hill.

Ever played with one of those electric motors on display at the Tesla Stores? They spin for a very long time with just a quick twist. So, there's probably not much value in disengaging from . If you are truly at 0 acceleration / 0 regen braking it should be identical to being "electronically" in nuetral. Am I missing something?

I would guess the "clunk" described is more similar to when you drop out of cruise and go full regen. But maybe even more abrubt?

I see no reason for the car to have a clutch or ever disengage the wheels from the motor. There is no transmission just a reduction gear.... kinda makes me want to go experiment with the skate board at a gallery but there isn't one for 250 miles... and I doubt they'd let me jack it up and spin the wheel to see if the motor turns.

As @stevenmaifert tried to point out, coasting is illegal in California (and probably many other states):

From CA Vehicle Code:

Coasting Prohibited

21710.  The driver of a motor vehicle when traveling on down grade upon any highway shall not coast with the gears of such vehicle in neutral.

I assume the rationale is that you have a lot less control over the speed of the car if you are coasting with no "engine drag." Power brakes won't have nearly as much power if the engine is barely spinning, right?

Also, in some emergency circumstance, you may need to suddenly accelerate to avoid a hazard, which will not be possible if you are in neutral and coasting.

In sum, just follow Tesla's recommendations to extend your range if needed (use Cruise Control and range mode). Cruise Control on a long downhill will do a good job regulating regen. "just right" to recharge the battery and maintain a nice constant speed, much better than any ICE or driver can do.


X Deutschland Site Besuchen