Speed (and acceleration) Kills (Range that is)

[kindofblue]

With all the threads and discussion about Tesla drivers not getting the electric range they expected, I thought it would be helpful to put up this thread with some actual data on trips comparing different speeds. And a startling note about acceleration and kw/mi juice. Please feel free to contribute your data as well as your comments.

I live in Arroyo Grande CA and drive to San Luis Obispo 3x wk to swim at a fitness club there. It’s 12.7 mi to the club taking a back road Hwy 227 and 14 mi taking the 101. Much of the 227 route has nice curves then bigger sweepers that limit your speed to 45-55 mph. I can usually drive the 101 on the other hand at 72mph unless there’s a lot of traffic (and I don’t usually drive this route during rush hour). Oh and temps were ~60 F w/no true wi

Using the trip function in the Tesla I noted these data:
Hwy 227 45-55mph 241 Wh/mi
Hwy 101 72mph 275 Wh/mi
That’s a bit over 10% difference in power consumption. It’s my hypothesis that increasing the speed to 75-85 would increase that difference much more.

What was really eye popping was that when I left the club and was accelerating up to the speed limit (which I like to do quickly), the Wh during that acceleration was 1,100+ Yes it’s only momentary but I have to wonder how much that adds up over time.

 

[Sophias_dad]

Acceleration should NOT be a major factor in range. Unlike an ICE, efficiency of an electric motor is not horribly affected by asking it for extra power. It would be interesting(for those with the equipment) to do a comparison of a leisurely/normal/brisk/maximum acceleration in detail.

Those without the right equipment could do it as well, with a lot less precision. That leisurely acceleration might take one tenth the power of the maximum acceleration but could well take ten times as long, and where's the fun in that?

Note that for all these power measurements, you sorta gotta subtract the base(keep the computers awake) power.

 

[jcanoe]

Tesla Range Plotted Relative To Speed & Temperature (Graphs)

Range loss with the different wheel options

 

[kindofblue]

Tesla Range Plotted Relative To Speed & Temperature (Graphs)

Range loss with the different wheel options

Thanks Jcanoe. That's exactly the kind of data I was looking for! Is there a way to suggest to moderators that your post be "pinned" so that it stays "above the fold"?

 

[jcanoe]

Thanks Jcanoe. That's exactly the kind of data I was looking for! Is there a way to suggest to moderators that your post be "pinned" so that it stays "above the fold"?

I don't know. One link is to CleanTechnica, probably copyrighted. The other is an older TMC forum post. I save each as a link and then I can easily find it when needed.

 

[boulder.dude]

Acceleration should NOT be a major factor in range. Unlike an ICE, efficiency of an electric motor is not horribly affected by asking it for extra power. It would be interesting(for those with the equipment) to do a comparison of a leisurely/normal/brisk/maximum acceleration in detail.

Those without the right equipment could do it as well, with a lot less precision. That leisurely acceleration might take one tenth the power of the maximum acceleration but could well take ten times as long, and where's the fun in that?

Note that for all these power measurements, you sorta gotta subtract the base(keep the computers awake) power.

When I’ve accelerated hard, I’ve heard the cooling system running loud/hard. If hard acceleration heats the battery, then that waste heat is coming from somewhere. I suspect it is less efficient (and harder on the tires).

 

[thesmokingman]

Acceleration aka known as aggressive driving is one of the biggest efficiency killers. We all know its fun but it comes with a cost.

We Test the Tips Part II - Edmunds.com

What Gas-Saving Tips save you the most money? We tested the tips to find the truth behind Fuel Economy.

www.edmunds.com

 

[Sophias_dad]

Acceleration aka known as aggressive driving is one of the biggest efficiency killers. We all know its fun but it comes with a cost.

We Test the Tips Part II - Edmunds.com

What Gas-Saving Tips save you the most money? We tested the tips to find the truth behind Fuel Economy.

www.edmunds.com

IMHO, that article isn't really relevant to EVs from an acceleration perspective. Its also 13 years old.

 

[Sophias_dad]

When I’ve accelerated hard, I’ve heard the cooling system running loud/hard. If hard acceleration heats the battery, then that waste heat is coming from somewhere. I suspect it is less efficient (and harder on the tires).

Sure, I'll go with the fact that any energy that gets fed into the electric motors that goes to wearing off tires is wasted. I don't think I've ever heard the cooling system running as a result of acceleration.

 

[thesmokingman]

IMHO, that article isn't really relevant to EVs from an acceleration perspective. Its also 13 years old.

Your opinion... yea ok.

 

[Sophias_dad]

Ok, so I did a bit more math on this, and it isn't quite as rosy as I thought. Due to the heat output of a conductor increasing with the SQUARE of the current passing through it, the motor in the 3 is going to be dumping way more heat than usual during that acceleration run(for four seconds) than usual. To get an idea of the scale of this, here's the math I came up with for a long range REAR wheel drive model 3, since the front motor just complicates things:

1800 kg car+driver times 27^2 times 1/2 = 656100 joules of kinetic energy. The 27 is ~60mph expressed as m/s.

656100 joules divided by 3600 gives us 182.25 watt-hours to get the car up to that speed, as an absolute minimum. Its being delivered over five seconds, so at a rate of 656100/5 = 131.2 kw. If we then head over to get model 3 motor specs, we find the RWD 3's rear motor is rated at 211kw. The difference is probably due to that as the speed increases the actual remaining power left(after wind/road/gear resistance) to accelerate the car is going down a good bit as speeds increase. Its also neglecting the entirely non-negligible moment of inertia of four ~50 lb wheels spinning up to 771rpm! Maybe later I'll try to figure that one out.

Looking for a moment at the INPUT power ratings of that motor, which are apparently 800A at 370V, that comes to a whopping 296kw, meaning a full-on efficiency of 211/296 = 71.2% This is a little less than I expected, TBH.

So at WOT you are burning off 29% of the input power as heat, as compared to maybe 10% when you are accelerating as slow as possible. I'm assuming the motor is 95% efficient when just coasting along at speed. Putting this in perspective, that 182.25 watt hours OUT would be 255wh in if you were at WOT and 202.5wh in if you were accelerating slowly.

Certainly not nothing, but an extra 50wh or even a few of them isn't going to get my attention on a ten mile drive.

 

[boulder.dude]

Is there a corollary here for “hard” regen being less efficient than “gentle” regen?

And, can’t help but ask – in cold weather, can the waste heat in the battery be put to good use, I.e. warming the cabin?

 

[Sophias_dad]

Is there a corollary here for “hard” regen being less efficient than “gentle” regen?

And, can’t help but ask – in cold weather, can the waste heat in the battery be put to good use, I.e. warming the cabin?

Regen is generally viewed as less efficient overall, and yes, I'd think there's a little efficiency loss with hard regen. I'm sure that max regen is well below max acceleration. If it were similar, even I would be unhappy, in addition to the wife and kids. Imagine being thrown forward with the same deceleration as you are thrown backward with during acceleration! As a result, hard and gentle regen are probably close enough efficiency-wise that it might get lost in the noise.

I'm pretty sure that waste heat can indeed be put to use for warming the cabin. I don't know the actual temperatures the motor/coolant attain in normal or spirited driving, but I'm pretty sure its well above ~70f typical desired cabin temperatures. OTOH, I've driven the ~30 miles to work in pre-covid days with the temperature in the 20s(f) outside and not gotten the snowflake to go away until I started flogging it in the last few miles(pretty much all highway).

 

[jcanoe]

Is there a corollary here for “hard” regen being less efficient than “gentle” regen?

And, can’t help but ask – in cold weather, can the waste heat in the battery be put to good use, I.e. warming the cabin?

Those who owned, drove Chevrolet Volt vehicles had lengthy discussions on this subject. The Volt had standard regenerative braking (when the shifter was in D and increased regenerative braking when the shifter was in L (for Low, to simulate driving in somewhere between L1/L2 with an ICE vehicle with automatic transmission.) In most cases there was no measurable advantage in selecting D versus L. When driving in L you did not need to use the brake pedal very often to slow the Volt. The GM engineers at the time had been quoted as stating that free wheeling (coasting) was most efficient, followed by standard regenerative braking and finally Low (max regen.) The explanation was that any time you are converting mechanical energy into electrical energy there are conversion losses. Coasting on public roads can be dangerous and annoy other drivers.

 

[sleepydoc]

Ok, so I did a bit more math on this, and it isn't quite as rosy as I thought. Due to the heat output of a conductor increasing with the SQUARE of the current passing through it, the motor in the 3 is going to be dumping way more heat than usual during that acceleration run(for four seconds) than usual. To get an idea of the scale of this, here's the math I came up with for a long range REAR wheel drive model 3, since the front motor just complicates things:

1800 kg car+driver times 27^2 times 1/2 = 656100 joules of kinetic energy. The 27 is ~60mph expressed as m/s.

656100 joules divided by 3600 gives us 182.25 watt-hours to get the car up to that speed, as an absolute minimum. Its being delivered over five seconds, so at a rate of 656100/5 = 131.2 kw. If we then head over to get model 3 motor specs, we find the RWD 3's rear motor is rated at 211kw. The difference is probably due to that as the speed increases the actual remaining power left(after wind/road/gear resistance) to accelerate the car is going down a good bit as speeds increase. Its also neglecting the entirely non-negligible moment of inertia of four ~50 lb wheels spinning up to 771rpm! Maybe later I'll try to figure that one out.

Looking for a moment at the INPUT power ratings of that motor, which are apparently 800A at 370V, that comes to a whopping 296kw, meaning a full-on efficiency of 211/296 = 71.2% This is a little less than I expected, TBH.

So at WOT you are burning off 29% of the input power as heat, as compared to maybe 10% when you are accelerating as slow as possible. I'm assuming the motor is 95% efficient when just coasting along at speed. Putting this in perspective, that 182.25 watt hours OUT would be 255wh in if you were at WOT and 202.5wh in if you were accelerating slowly.

Certainly not nothing, but an extra 50wh or even a few of them isn't going to get my attention on a ten mile drive.

I started to reply to your earlier post with exactly what you calculated out here - good thing I kept reading!

The big point is that energy loss in the batteries and motor is not necessarily linear. Another complicating factor is the fact that the internal resistance of the battery may not act like a traditional conductor since part of it is electrolyte. Finally, the car monitors the battery temperature and actively cools when necessary. That’s not an issue in MN in January, but in the middle of the summer it may well come in to play.

Anecdotally I can say that hard acceleration gives worse efficiency. You can see the Wh/mi shoot up and it doesn’t completely return to baseline after you‘re up to speed and cruising.

 

[Sophias_dad]

I haven't actually measured my wheel/tires moment of inertia, but here are some rough estimates for my 18" wheels with aero caps and Michelin PS4S tires...
Tire diameter is 668mm. I'm gonna cut that down by an inch and ignore the fact that there are sidewalls, giving me a radius of 321mm. Tire weight is 23lbs, or 10.5 kg. I'm going to only count 80% of that, or 8.4kg to account for sidewalls absence.
Wheel diameter is 431mm. I'm gonna cut that down by an inch(giving me a radius of 203mm) and ignore the hub itself, and again only count 80% of the weight. Wheel/cover weight is another 23 lbs, so another 8.4kg for the wheel/cover after the 80%

So the moment of inertia of the tire is 0.321 squared times 8.4kg, or 0.866
and that of the wheel is 0.203 squared times 8.4kg, or 0.346

Add those up, and the moment of inertia of the assembly is 1.212kgmm

The kinetic energy of a spinning body is half the moment of inertia times the square of the angular velocity(measured in radians!). At 771 rpm, that's 12.85rps, or 80.69 radians per sec. Square that and we get to 6512, then multiply by half of 1.212 gets us to 3946 joules per wheel/tire, times four gets us just less than 16000 joules. Comparing that to the 656100 of the motion of the car itself its almost nothing. This result surprised me, and I welcome any corrections that people may have.

 

[ElectricAnt54]

With all the threads and discussion about Tesla drivers not getting the electric range they expected, I thought it would be helpful to put up this thread with some actual data on trips comparing different speeds. And a startling note about acceleration and kw/mi juice. Please feel free to contribute your data as well as your comments.

I live in Arroyo Grande CA and drive to San Luis Obispo 3x wk to swim at a fitness club there. It’s 12.7 mi to the club taking a back road Hwy 227 and 14 mi taking the 101. Much of the 227 route has nice curves then bigger sweepers that limit your speed to 45-55 mph. I can usually drive the 101 on the other hand at 72mph unless there’s a lot of traffic (and I don’t usually drive this route during rush hour). Oh and temps were ~60 F w/no true wi

Using the trip function in the Tesla I noted these data:
Hwy 227 45-55mph 241 Wh/mi
Hwy 101 72mph 275 Wh/mi
That’s a bit over 10% difference in power consumption. It’s my hypothesis that increasing the speed to 75-85 would increase that difference much more.

What was really eye popping was that when I left the club and was accelerating up to the speed limit (which I like to do quickly), the Wh during that acceleration was 1,100+ Yes it’s only momentary but I have to wonder how much that adds up over time.

Based on 10 years of driving a Volt, your conclusions seem valid. The Volt had an efficiency meter which would you could see as you drive. It was great aid for maintaining the most efficient cruising speed. Rapid acceleration sucked up more electricity and subsided when cruising speed was achieved. On the Volt, cruising speeds over 55MPH became increasingly inefficient. Not sure what ideal cruising speed is on Tesla and given the high availability of superchargers, no need to worry as much .

 

[rrolsbe]

Is there a corollary here for “hard” regen being less efficient than “gentle” regen?

And, can’t help but ask – in cold weather, can the waste heat in the battery be put to good use, I.e. warming the cabin?

It can if the car has a heat pump but AFAIK on other models the heat energy can only be used to (possibly) help pre-condition the battery for Supercharging and give better regen. If not needed for those two, it is just waste heat.

 

[D.E.]

Anecdotally I can say that hard acceleration gives worse efficiency. You can see the Wh/mi shoot up and it doesn’t completely return to baseline after you‘re up to speed and cruising.

But Wh/mi is an instant readout, and since you’ll be up to the desired speed much more quickly, that high reading will be for a much shorter interval of time. To see the total power used, you could integrate the area under the curve, that would tell you whether there’s a significant increase in total power used with a transient rapid acceleration. The car provides that graph so it shouldn’t be that difficult to photograph, plot, then calculate.

There’s more, of course, there always is. You’d be at cruising speed more quickly so would experience the increased drag from wind resistance for a little longer than with the slower acceleration to cruising speed. I think that would be negligible, and further, could be subtracted from the other end of the trip, which one would reach slightly earlier, but I speculate.

 

[mangrove79]

But Wh/mi is an instant readout, and since you’ll be up to the desired speed much more quickly, that high reading will be for a much shorter interval of time. To see the total power used, you could integrate the area under the curve, that would tell you whether there’s a significant increase in total power used with a transient rapid acceleration. The car provides that graph so it shouldn’t be that difficult to photograph, plot, then calculate.

There’s more, of course, there always is. You’d be at cruising speed more quickly so would experience the increased drag from wind resistance for a little longer than with the slower acceleration to cruising speed. I think that would be negligible, and further, could be subtracted from the other end of the trip, which one would reach slightly earlier, but I speculate.

There's more. Faster acceleration means you're smashing a few more bugs into windshield decoration whereas slower speed simply bounces them off. Conservation of momentum suggests your car is slowing down by 0.000000001 mph with each hit. But that may be negligible.