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Energy-efficient driving

I am currently experimenting with ways how to drive my Tesla as energy-efficiently as possible, and I have found a way where preliminary results show a 10% reduction in consumption.

Story: I currently have a loaner, provided by the fantastic Denmark Tesla team, when my 12-V Battery malfunctioned for the 3rd time in 3 weeks - they actually transported a loaner 400 miles to Stockholm and brought down my car for repair - WOW!!

This loaner, though, consumes about 10% more energy than my proper car with identical trips, identical settings and, as far as I can see, identical driving. Because of an imminent long haul this concerned me very much, so I started to think of ways of bringing down the consumption. I got the advice from a Tesla man: drive it as you would drive an ICE.

OK, fine. I have developed a style of driving with ICE:s that invariably beats their own consumption figures (and we all know that they are the "optimal" ones anyway). It is quite easy - roll the car in Neutral as much as circumstances allow.

I tried this with the loaner, and it immediately yielded really good results - minus 10%, from 310 Wh/mile to 278 Wh/mile in 40 degrees F.

The Tesla is built with such a small wind and rolling resistance so that these 2 factors coupled with the huge mass makes the Tesla roll "for ever", especially in a down-slope. If there's just a small decline it doesn't even lose speed.

The gearbox is so well-made that I learned in less than 2 minutes how to change back and forth between Drive and Neutral without anyone noticing anything.

So, if you free-roll and want to lose speed, put it in Drive and engage re-gen. If you want to keep the speed or increase it, just put it in Drive and - well, drive.

The Cruise Control doesn't do this well - it constantly gives small "puffs" of energy and then brakes, for instance when you reach a small hill, and the re-gen cannot be 100% efficient, so you lose more than you gain, whereas, if the hill is small enough, you just roll over it in Neutral or you see it coming and accellerate slightly BEFORE the hill and roll down after it (never accellerate uphill, if it can be avoided).

This car has such eminent rolling features that this can be done without at all disrupting the traffic flow, if applied intelligently (mostly meaning rolling downhill, but also anticipating a traffic jam or light).

I called Tesla and asked them if this could in any way, shape or form damage the car (I am driving a loaner, and I am much more careful with other people's properties than with my own), and the answer from the Danish SC was an unequivocal NO.

My lifetime average consumption (incl. the multiple showing-off accellerations in the beginning - I am only human...) with my proper car is 289 Wh/mile, with the temp has varied between 29 F and 50 F. Applying the free-roll system, I think I will get it down to 260 or thereabouts. Will keep you posted.

Would be interesting to hear, if anyone else has tried this and their experiences.

Robert

@robert
"experimenting with ways how to drive my Tesla as energy-efficiently"

Well, I can guess you'll hear some of the following comments on this post, ad infinetum - and then there will be some outliers. ;-)

1) Great service experience!
2) 12 V issues three times in three weeks? You've got a lemon!
3) Wow, that much, really? Might be worth a try to see if I get that in my P85 too.
4) What percentage are you really going to save in a flat area, no hills?
5) Are you crazy? I bought this car to drive it and I still pay much less in electricity than gas in an ICE!

I'm partial to #3, but locked into the #5 zone right now. Grin!
:-)

And my answers would be:

1) Yes, unique, fantastic.
2) That possibility is occupying a lot of my thoughts.
4) None. However, Sweden isn't Utah, nor even Holland or Denmark.
5) I bought the car "to save the planet". Having bought it, an additional feature is the almost indecent happiness that driving a Tesla brings with it. But primarily it is all about energy consumption and CO2, and to waste energy is tantamount to killing this planet, regardless of the price. Think where most energy is derived from!!!!! Then stop the wasting of it.

Robert

I just got an answer from Jerome Guillen to my direct question if the free-roll system can harm the car:

"You are free to drive safely as you please."

I take that as a NO, it won't harm the car. Excellent.

Robert

@robert:

I am not sure Neutral can re-generate electricity back to the battery or not. The less re-generative brake you can feel (such as switch from Standard to Low) the less re-generating electricity to your battery.

So, may be the question is: Is it better to 1) put it in Neutral so the car can roll freely without using electricity from the battery or 2) keep it in Drive so it can re-generate electricity back to the battery?

I would set the car to cruise as much as possible and release it (by pushing the stalk away from you) when I want to slow down to a stop then let the regenerative brake system does its magic.

Can you put the car in neutral while driving at high speeds as well (100 miles/hour) in an ICE car?

@Benz:

There's no technical restriction to put an ICE to Neutral while rolling downhill at 100mph.

I experimented that on a Prius at about 65 mph and that would shut the gasoline engine off while the car coasted downhill but that would not regenerate any electricity to the battery so I always set it on Drive from then on.

Very interesting. I too use to coast the ICE car. Two comments:

1: Coasting also means less influx and efflux of current to and fro, and less battery wear which is good.

2: I fear accidentally engaging the Parking brake which could be fatal driving at high speed, or is there a mechanism preventing that?

@Tam
Neutral doesn't re-generate anything, but doesn't waste anything, either. The whole point is that it is more energy-saving to let the car roll freely, when topography so allows, than either driving with the foot or Cruise Control. Drive doesn't generate anything at all, as long as you don't brake (=regen). Same as in an ICE car, but Tesla has advantages that normal ICE cars don't have, i.e. extremely low rolling (no regular transmission)/air resistance coupled with a heavy mass.

@Benz
Oh yes, no problems at all, if it is a manual gear, i.e. stick shift car. Automatic cars should NOT roll in Neutral, hence my question to Tesla. Tesla can, according to the 2 answers I have got.

Robert

Robert, actually by driving the Model S in neutral you can get a higher range out of a single charge. That is great.

@Notre

Manual 4.7 If you try to shift into a gear that the current speed pr4ohibits, you hear a chime and the gear does not change.

It doesn't say so specifically, but I am very convinced that it applies to the parking brake as well. You don't have to use the coasting technique to accidentally press the Parking Brake, and I feel sure that this has been foreseen.

BUT I AM NOT GOING TO TRY!

Robert

@Notre:

If I recall correctly, at high speed for a Prius, shifting to Park would become Neutral. and the car would continue to coast.

For Model S, at low speed (such as 10's mph, pushing the shiny chrome Park button would abruptly stop your car (Make sure you wear your seat belt or you might suffer teeth/head injuries from this.)

I have not experimented with high speed on Model S but I assume it would abruptly brake the car to a stop.

@Benz

That was the whole object of the exercise. I need to go about 240 miles with no chance (or time) for recharging, in almost freezing weather. So I started thinking.

Robert

I just checked the Parking brake. Driving faster than 8 km/h (5 or 6 mph) it does not engage, so no problem. Will definitely try free rolling to extend max range.

Regenned energy returned to the battery is a % (60 - 80) of the amount used to get to that speed in the first place, less rolling resistance. Then when you use it (from the battery) there are losses (around 10-20%) involved.

A perfectly timed rolling stop with no regen suffers no losses other than rolling resistance (which eventually stops you). But the limitations on your control and time spent are rather extreme! Nothing for nothing.

@Robert

If you have the accelerator pedal between acceleration and regeneration, Does it consume a lot of energy ?

@Brian H

I honestly - after some 30 years driving this way - can't see where I lose control at all. It all depends on how intelligently one applies the system. Unless you're alone on the road, it has to do with mental forecasts on what is going to happen, the topography, speed limitations, traffic lights etc, and that applies to all driving, whether a Tesla or not. One has to look a bit further afield than just the car in front of you. The only consequence that this system MIGHT have is more wear of the normal brakes, but, it is a matter of a split second to engage Drive and lift the right foot, and then you're back to where you normally would be.

The time spent is precisely the time the journey takes, and "my" system does not slow the ride, nor does it interrupt the normal flow of the traffic. So no time loss.

So I don't understand either of your drawbacks. Had you said: you have to concentrate on your driving more than otherwise, I would agree, even if it becomes second nature after a while. At the same time I don't think that would be a bad thing, generally speaking.

And, isn't it regenerated rather that regenned? :-)

Robert

@Sofie S

I have just had the Tesla for 3 weeks. I think probably not, but I am not an authority. I personally find it very difficult indeed to keep the meter precisely there, and I don't honestly want to direct my concentration towards that aspect of my driving, since it would entail my staring at the Energy app all the time. Coasting/free rolling saves you from that - then the energy for propelling the vehicle forward definitely is off.

Robert

Oh, stupid me, Sofie S.

I should have answered:

Yes. Mental. :-)

Robert

Like with the climate system, should Tesla add a "Range Mode" setting for cruise control - which would optimize more towards energy efficiency than maintaining close to a constant speed?

It only takes minimal practice to press the accelerator just enough for neutral. Keeping my foot in contact with the side wall gives you more precise control. The car does roll for what seems an extended distance but keep in mind it weighs two and a half tons. This way when your speed overshoots you can just lift your foot slightly to add regen or wait to bleed of speed if there are no cops around. You should only use regen to come to a complete stop or when slowing down is a must for the given distance to obstacle. I have gotten the energy consumption savings you are seeing and my car is set for max regen. Amazingly engineered vehicle.

+1 bp

Robert,

There is one source of loss with coasting that you should thing about: drag. If you are going down hill in vacuum (hold your breath) then it is fine to let the car accelerate from gravity as potential energy is converted to kinetic energy and then let the process reverse as you go back up the hill on the other side - nothing lost and nothing gained. In the real world, I calculate and Tesla’s range vs. speed graphs would seem to confirm, that if your downhill speed is allowed to increase more than about 15% then the losses from increased drag will exceed the losses from regenerations and you would have been better off using regen to keep the speed down. In fact going slower will always win when it comes to efficiency.

bp +++1!

I would really like for Tesla to add an economy mode for folks who are less interested in the performance aspects of the car and more interested in conserving energy. They don't have to market this new feature (if they feel that would detract from the car) and continue to make the normal (high performing) mode the default.

@ shs ,

I hear this opinion quite a bit, but have yet to find someone who can actually prove the assertion using equations or citing someone who has made the calculations. I don't believe that regen is more efficient than coasting, assuming that you do not need to limit your coasting speed. Intuitively, kinetic energy goes through fewer conversions, hence fewer losses, than converting that kinetic energy to electricity, storing that electricity, and then withdrawing that power at a later time when it's needed.

Efficiency is described as the greatest distance traveled using the least amount of energy. By coasting, especially on a downhill, you are maximizing distance traveled while using almost zero energy. Using regen limits your speed, converting any kinetic energy that would have otherwise gone into moving your car forward into electrical energy and incurring approximately 40% loss of energy during the conversion (minimum 30% loss during regen plus efficiency loss when later removing that energy from the battery). The negative effect of wind resistance would have to be greater than the 40% energy loss incurred through the regenerative braking system in order for regen to be the more efficient process.

At least that's how I look at it.

Drag goes with the square of the speed, and so a 15 to 20 percent increase in speed should increase drag losses to be equal to the regen losses you mention. Or perhaps another way of looking at it is that drag will keep you from completely converting you potential energy into kinetic energy as you will not gain as much speed in air as you would in a vacuum. If you look at the Tesla range vs speed graphs, loss of range with higher speed is very evident - increased drag is the main reason.

i was told shifting back and forth between Drive and Neutral while going down a hill is not advisable in an ICE because it shortens the life of the transmission. Although the Model S does not have a transmission, turning the regen On and Off abruptly with a load can't be good either.

In the US at least, I believe that it is not legal to drive in Neutral.

you can't drive in neutral; only coast. :)

@tezzla,

You are right of course, but at least in CA it is coasting in neutral that is not legal!

While drag may increase with the square of the speed, what is the actual amount of energy reduction due to drag? If you have a really tiny number, you can square it many times and never come close to the loss number experienced during regen. How large is the overall number, that's my question. Just having one variable in an equation squared doesn't mean the resulting number is going to be meaningfully high.