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Efficiency and driving techniques

A place to discuss some guesses as to what should be done to gain the most range, or spend the smallest amount of energy per mile. Or not?

Drive constant speed at 21 mph.

Over in a private thread http://www.teslamotors.com/forum/forums/motor-trend-performance-and-rang..., it was suggested that getting from 0-60 in 4.4 seconds uses up a mile of range. But is that inefficient? Is there any efficiency gain in getting to 60 mph more gradually?

Calculation from other post: A mass 2180 kg going 27 meters/second (4806 pounds, car and driver, going 60 mph) holds a kinetic energy of 795 kJoules or about 221 Wh. If the Model S can go a mile using something on the order of 300 Wh, then just getting up to 60 mph, regardless of how long you take to do it, will "use up" 2/3 of a mile of range. If you take 1/3 of a mile to do this, then you use a mile of range to get up to 60 mph.

EdG;
Good point. An ICE car gets very wasteful under accel like that, but elec. is same-O same-O.

Acceleration itself is not that wasteful, but you use more time in high speed using high acceleration, so you end up losing some range. You don't use same amount of Wh/mile in every speed.

Best result would be ride a wake behind big truck, but that is very risky.

Longhorn92 comment about steady 21mph is also good, that would give you around 450 mile range. Boring, but efficient.

From other EVs it would be slow acceleration, drive with load at suburban speeds and then glide to stops.

@Timo: We know that going 80 mph will use lots more energy per mile than going 50 mph. But if you want to drive at a steady speed, say 65 mph, I'm not sure you'd be able to measure the difference between getting to 65 mph in 5 seconds versus 10 or 20 seconds. And, if that's the case, then we should all be moving up to the speed of traffic as quickly as possible, especially if that's the safest thing to do.

@ItsNotAboutTheMoney: Even driving ICE cars I try to maintain the highest minimum speed while approaching a light that's about to turn green, so gliding seems a given. But why "slow acceleration" on other EVs? Anything applicable or not to the Model S?

Sadly acceleration is the enemy. (and wind resistence). If accelerate like crazy from 0 to 60 to get to the next red light, then have to accelerate to a stop because it hasn't turned green yet, we waste energy. Oh, but it's sooooo much fun though....

So we waste a few electrons, no guilt in burning the fuel otherwise!!! I will be jumping from light to light like a jack rabbit when I'm just running around town for sure! :)

How much for replacement tires again? I better get that in my budget soon too... :D

Without a clutch to pop, and with the excellent traction control, our copilot said it was very unlikley you could spin the tires. So that should help with the wear and tear.

@jbunn: Your scenario is just saying that stopping wastes energy.

If you're going to get onto a highway where there's no one else driving, it seems the only extra loss incurred by getting up to speed quickly is the distance you might have traveled at a slower speed. That is, if you linearly go from 0 to 60 over two minutes, your average speed would be 30 mph and you would have traveled that distance with lower wind resistance than if your average speed over the same distance was higher. As Longhorn pointed out, once over about 20 mph the faster you go the more the air resistance pulls energy from the car.

But that's just saying that to save energy you should as close to 20 mph as possible. The massive waste of energy inherent in fast ICE acceleration does not happen with an EV with traction control. If you're willing to go 60 mph over large distances, there seems to be little reason to take your time getting up to that speed.

@jbunn... doesn't help with the wear as much as you might think. Even without the tires spinning, they still wear pretty quickly.

@ggr: That's the only downside, it seems. How does the traction control work? Is there some micro-skidding going on which a computer detects and then backs off power PDQ? If so, is there an indicator on the dash that tells you it was used?

Perhaps there's some medium high level of acceleration that can be used without kicking in the traction control, thereby minimizing tire wear while keeping the smiles coming? With a TC indicator, one should be able to learn how much is too much for everyday driving - if you want to conserve rubber.

The things that wear out tires (not counting "burning rubber") are:

1. Heat. The hotter the tire gets, the quicker it will wear.

2. Flexing. The more the tread flexes, the quicker it will wear.

3. Alignment. Poor alignment can wear out tires very quickly.

4. Suspension geometry (different from alignment because you can't control it). Poor design choices can cause very poor tire wear.

5. Road surfaces. Different types of roads cause different tire wear rates. Generally new roads are worse than old roads.

6. Road geometry. The more curves and hills a road has, the faster the rate of tire wear.

7. Mechanical condition. Poor tire/wheel balance, sticky brakes, loose suspension parts, etc.

A high rate of acceleration contributes to both heat and flexing.

@EdG, one acceleration in long ride doesn't do much, but in the city where you do stop and go traffic it does count quite a bit.

Steady driving, anticipating needs to stop and slowing down slowly (with or without regen) and smooth acceleration gives you best result in city. Plan ahead.

On our roadster, I accelerate briskly pretty much all the time. The only time on a normal commute when I see the traction control indicator come on is when I make a tight right turn onto a particular freeway on-ramp at full throttle (because I can...). It's a very short, very steep up-hill, and it's great to be able to merge at 75, which is what everyone else is doing at the top.

We're on our fourth set of rear tires, at 28k miles. One could buy longer-wearing tires, but then I think they wouldn't grip as well.

When you think of it, 21mph is the sweet spot for range, so maybe fast acceleration to that speed is better than slow acceleration.

@Timo: I agree completely that steady speed is where you get the efficient driving. That's due to the inefficiency of slowing down either by regen (not too bad) or braking (all energy going to heat). What's different about the EV acceleration, it seems to me, is that getting up to speed quickly is not as hugely wasteful as it seems to be with an ICE. You're not burning large quantities of fuel to get the maximum torque, you're just asking for more torque at the same efficiency as less torque. The losses seem to be in flexing the tires more and driving faster than 20 mph more (because you're going faster earlier).

It's gonna be tough to go back to an ICE after driving the Model S.

jerry3;
The material of the tire is a major factor. 'Sticky' tires leave bits of themselves behind, by design.

EdG,

Well, sort of... What I'm realy saying is accelerating uses energy. That could be starting. Or stopping. Or changing directions.

I have a habit of driving with one foot on the gas, and one foot on the brake. Not good for fuel efficiencey. I need to improve on that.

On the other hand, I'm probably going to use every chance I get to floor it.... At least at first. I guess best case I try to time lights, fit in with the traffic flow, and dont punch it so hard I need to accelerate to a slower speed.

I haven't had a ticket in... I don't even remember. 20 years? But I'm pretty sure my number is coming up with this car.

The co-pilot at the drive said tickets on the Amped drive were not tickets. They were souvineers.

Never grinned through a ticket before. Wonder what that will be like?

Brian -- 'Sticky' tires leave bits of themselves behind, by design.

That's included in the flexing. I didn't make that clear enough.

"Vigorous" acceleration can waste energy, even when you can coast to the next light without regen or braking. It goes a little something like this: When you try to draw energy from the battery faster than it can do so efficiently, it generates excess heat in the battery. This is a double whammy, because you've wasted energy by creating the heat, and the battery pack management has to use more energy to cool the battery.

I can't say whether a 5.6s 0-60 exceeds that threshold but it seems more likely that a 4.4s 0-60 would.

My takeaway is this: eventually we will learn where the line between vigorous and excessive acceleration is, and adapt our driving habits accordingly.

Teoatawki,

That's my thought too. The test drive indicated that the instrumentation to figure that out is already in place.


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