Holy Wind Batman

[dunginhawk]

So Ive only been a model s owner for a couple weeks (already put on 2400 miles , work, blah) but today was the first time i encountered REAL wind. Holy schmoses.
So heading north out of des moines today I made it to my destination with 120 miles of range left. Heading home later in the day I made it home with just over 50 miles. 70 miles difference. (both times i left 100% full range 265 miles Model s 85)
Thats pretty crazy...
I knew it would be bad coming home, but i didnt expect it to be that bad. If the wind had been much worse, It would have been touch and go. The albert lea, to des moines, to kansas city stretch is a LONG way between SCs. Had it been the Des moines to KC stretch, i guarantee i wouldnt have made it.

MORE SC PLEASE

 

[BluestarE3]

Have you checked out the updated Supercharger map with the planned future locations? There was an announcement today on the expansion of the network this year.

 

[Zetopan]

Also note that the HP requirements increase with the cube of velocity. That also means that if you are fighting a 30 MPH headwind, slowing down by 30 MPH would get you back to your original expected range for travel on that route. Here's a handy chart showing the approximate range vs the speed for a couple of different Tesla models (numerous assumptions apply, as stated within the text):

Model S Efficiency and Range

 

[kort677]

in high wind situations a little drafting of a big rig or even a large SUV could be beneficial.
before the drafting wars begin, you don't need to tailgate the rig to gain some relief from head winds, employing a 2 second rule and remaining attentive will get you more miles when dealing with those headwinds.

 

[ecarfan]

New So Ive only been a model s owner for a couple weeks (already put on 2400 miles , work, blah) but today was the first time i encountered REAL wind. Holy schmoses.
So heading north out of des moines today I made it to my destination with 120 miles of range left. Heading home later in the day I made it home with just over 50 miles. 70 miles difference. (both times i left 100% full range 265 miles Model s 85)
Thats pretty crazy...

Every new EV owner goes through this learning process in regards to the effect of headwinds. It's an important lesson.

When you have a nav route set in the car, when you start driving note the estimated percent remaining charge figure that is shown for when you arrive at your destination.

During your trip, occasionally check that figure to see if it is less than when you started. Also check the Energy app on the center screen and learn how to understand and interpret what it is telling you. It's pretty easy and it is important.

If your percent remaining charge is declining, look around you and check for weather conditions: wind, rain, cold.

If it looks like you are not going to arrive at your destination with a reasonable buffer (some are okay with 5% or 10%, I prefer 20% if possible), then SLOW DOWN.

Remember, the car nav does not take weather conditions into account, only speed and elevation changes.

The issue of drafting has been debated endlessly in multiple threads. I will not do it because I consider it to be unsafe. Instead, I plan my route in advance, watch my energy usage (it takes no real effort) and slow down if needed. Decreasing speed by 10mph makes a big difference in energy usage when driving highway speeds.

 

[dunginhawk]

So I did get behind a big rig yesterday for a few miles and noticed next to no difference in energy burn. Plus its boring
So i did continually look at the screen to gauge whether or not i would make it home. I do like how the screen updates, given your current burn rate.
So for example, when I left Albert Lea and plugged in my home SC it said Id make it with 31% battery remaining. After about 15 miles it was saying 11%. It had adjusted based on the previous 15 miles in to that massive headwind. So yeah, it does account for weather, after a few minutes of being in said weather.
Its funny because Ive now been from My home to KC (service center), and back (200+ miles with no chargers in between), to Omaha 3x, LIncoln 1x, Albert lea, etc. and I trust my car completely. After a few miles when it said id make it with 10%, I knew I would, and I did, exactly 10%. Obviously if the wind picked up, or it rained, etc that would change things, but the car is crazy accurate.
So I drove 77 the entire way home, went between 9-11% remaining charge the whole way and never really got concerned.

Amazing piece of car this thing.

 

[cpa]

What ecarfan says. I crawled along westbound Interstate 70 between Hays and Goodland at 48 MPH for about 90 miles. A 35-40MPH headwind plus 1,600-foot elevation gain was too much for reasonable highway speeds. I left Hays with 96% and an alleged 22-24% buffer. That cushion dropped like a stone within the first 15 miles. I drive a 2014 S85.

 

[QUBO]

The prevailing westerly winds in Kansas can be very strong! I recently drove Hays to Goodland on the way to Colorado and saw several flipped semi's and multiple grass fires. I drove the speed limit into 30+ mph headwinds and made it with 5% remaining in an S90D.

On the plus side, if you have a strong tailwind the car is near silent at 80+ and needs less than 300 Wh/mi.

I've been hoping for a SC location that would make driving to the Black Hills and beyond from Omaha through Mitchell, SD easier. As it is, the only charging is two J1772 at a Hy-Vee in Sioux City -- either that or drive for 2 extra hours to get to the SC in Worthington.

 

[kort677]

What ecarfan says. I crawled along westbound Interstate 70 between Hays and Goodland at 48 MPH for about 90 miles. A 35-40MPH headwind plus 1,600-foot elevation gain was too much for reasonable highway speeds. I left Hays with 96% and an alleged 22-24% buffer. That cushion dropped like a stone within the first 15 miles. I drive a 2014 S85.

I did that same run last winter in similar conditions, drafting at a safe distance saved me at least 20% of wh/miles

 

[dunginhawk]

According to the SC map of 2017 Sioux city is getting an SC at some point. I really want one there (as its part of my sales territory).
Id also really want one In Lamoni (iowa/MO border along i-35) a S60 could not make that trip from Des mOInes to KC (nearest service center) without driving 55 mph). Also Clear Lake/Mason city, or Along highway 20. These stretches are just too long.

Along I-80 where i live they are spaced well. In fact I can get from Des Moines to lincoln bypassing omaha Sc easily. I can get from Des Moines to davenport (bypassing Iowa City) as well. its just north south that is harder.

 

[dunginhawk]

ahh just saw the new SC map... VERY VERY nice for me. Sioux City, Shelby, Bethany, Another Des MOines, Dows. Basically the couple needed badly are Sioux City, Dows (clear lake/Mason city area), and Bethany on the way to KC).
Very very nice

 

[tinm]

On a 4200-mile trip from NM to the east coast and back last month, we encountered two days of ferocious winds on the drive home. Sometimes they were head-on headwinds, and sometimes depending on the twists and turns of the roads and the whims of nature, they'd be crosswinds.

When we got to the New Mexico border after Texas, the winds pure 90 degree crosswinds, really fierce. But it was warm out, so occasionally I opened the driver's window and a tiny crack in the rear right passenger window. Well that was a mistake! The wind was so strong it knocked the driver's side window out of its tracks and I could not even close the window! I'd never seen anything like it in any car I have ever owned... I had to roll the window down, manually grab it while fighting the wind, force it back into its normal position, and then hold it while making the window go up all the way (if I let go it'd get blown off the track again). It was really something.

Anyone else ever encounter that in their S?

 

[Derek Kessler]

Yep, winds will do that. We like say it's a EV thing, but it's just as much an ICE thing — several years ago I drove my Honda Civic into the non-stop 60mph winds of a remnant hurricane and it cut my fuel efficiency in half.

Wind resistance is a big thing for all cars (there's a reason more and more semi trucks have skirts under the trailers and add-on tails to improve aerodynamics), we're just more sensitive to it thanks to our recharge restrictions.

 

[Half Dollar Bill]

We were on a cross country trip two years ago and drove from Lusk to Sheridan on a single charge in our S85. I think this was even before the energy projection graph was out so we had to eyeball it.
The one thing I grew to appreciate was the weathervanes scattered all throughout the countryside. I'd try to pick them out and determine how fast they were spinning to help read the wind. Fond memories.

 

[jsmay311]

Also note that the HP requirements increase with the cube of velocity. That also means that if you are fighting a 30 MPH headwind, slowing down by 30 MPH would get you back to your original expected range for travel on that route.

No. No, it wouldn't.

You'd be using the same amount of power (kW) to overcome the aerodynamic drag, but you'd also be going slower, so the amount of energy per unit of distance (kWh/mile) would be higher. Hence efficiency will be lower and range will be lower.


For illustrative purposes, try taking that example to an extreme. If you were driving 1 mph into a 80 mph headwind, do you really think that would be just as efficient as driving 81 mph with no wind?

 

[QUBO]

The new supercharger map looks much better for travel on I-90 in South Dakota and patches a long stretch in Kansas on I-70. Not to mention many other locations to explore!

 

[Zetopan]

No. No, it wouldn't.
You'd be using the same amount of power (kW) to overcome the aerodynamic drag, but you'd also be going slower, so the amount of energy per unit of distance (kWh/mile) would be higher. Hence efficiency will be lower and range will be lower.

Firstly, I originally left unstated the assumption that rolling resistance and gearbox losses remained constant with changes in speed, so that only drag was being considered significant for this discussion.

Secondly, your above statement is *obviously* incorrect since Power = Torque * RPM. Traveling with the same amount of drag requires the same amount of torque and since the vehicle speed is lower in the second case so is the required power.

For illustrative purposes, try taking that example to an extreme. If you were driving 1 mph into a 80 mph headwind, do you really think that would be just as efficient as driving 81 mph with no wind?

Ignoring non-drag related losses - YES! Let's perform a no more than high school level physics calculation to see what happens. If the aerodynamic drag at 81 MPH were X pounds of force, then the required wheel torque is X * R where R is the tire radius (in units of feet). Since Power = Torque * RPM / 5252 that means that the required power in the first case is X * R * 81 / 60 * 5280 / (2 * PI * R) / 5252, where 81 is the vehicle speed in MPH, 60 converts miles per hour to miles per minute, 5280 converts miles to feet, 2 * PI * R is the tire circumference (in units of feet), and of course 5252 is the conversion constant to get HP as the output (that is why at 5252 RPM the HP and the Torque are numerically identical in US units).

For the second case the vehicle is traveling at 1 MPH with a headwind of 80 MPH. The required torque is identical to the first case (X * R) since the air velocity over the vehicle is identical. The new amount of required power is X * R * 1 / 60 * 5280 / (2 * PI * R) / 5252. This is obviously a factor of 81 times lower power than the first case, but we also have to travel for 81 times as long to cover the same distance, thus taking 81 times more elapsed time to do so.

When I originally stated: "Also note that the HP requirements increase with the cube of velocity." the stated velocity refers to *only* the vehicle forward (or reverse if you prefer driving that way) speed relative to the ground. Any additional wind velocity induced drag increases the required torque in either a linear manner (at very low speeds), or a squared manner (in turbulent flows), not in a cubic manner. Hence at higher speeds headwinds will cause a squared increase in the required power while vehicle velocity through the air causes a cubic increase in the required power, because the motor is also performing more work in a shorter time interval.

Driving slower generally has a dramatic effect on range due to the cubic relationship of power vs ground velocity. At very low speeds the effect on range is less dramatic since the drag becomes proportional to square of speed instead of being proportional to the cube of speed. None of the above analysis depends on a linear or squared relationship of drag relative to total air velocity over the vehicle since the air speed over the vehicle is identical for both cases.

 

[jsmay311]

Firstly, I originally left unstated the assumption that rolling resistance and gearbox losses remained constant with changes in speed, so that only drag was being considered significant for this discussion.

Secondly, your above statement is *obviously* incorrect since Power = Torque * RPM. Traveling with the same amount of drag requires the same amount of torque and since the vehicle speed is lower in the second case so is the required power.




Ignoring non-drag related losses - YES! Let's perform a no more than high school level physics calculation to see what happens. If the aerodynamic drag at 81 MPH were X pounds of force, then the required wheel torque is X * R where R is the tire radius (in units of feet). Since Power = Torque * RPM / 5252 that means that the required power in the first case is X * R * 81 / 60 * 5280 / (2 * PI * R) / 5252, where 81 is the vehicle speed in MPH, 60 converts miles per hour to miles per minute, 5280 converts miles to feet, 2 * PI * R is the tire circumference (in units of feet), and of course 5252 is the conversion constant to get HP as the output (that is why at 5252 RPM the HP and the Torque are numerically identical in US units).

For the second case the vehicle is traveling at 1 MPH with a headwind of 80 MPH. The required torque is identical to the first case (X * R) since the air velocity over the vehicle is identical. The new amount of required power is X * R * 1 / 60 * 5280 / (2 * PI * R) / 5252. This is obviously a factor of 81 times lower power than the first case, but we also have to travel for 81 times as long to cover the same distance, thus taking 81 times more elapsed time to do so.

When I originally stated: "Also note that the HP requirements increase with the cube of velocity." the stated velocity refers to *only* the vehicle forward (or reverse if you prefer driving that way) speed relative to the ground. Any additional wind velocity induced drag increases the required torque in either a linear manner (at very low speeds), or a squared manner (in turbulent flows), not in a cubic manner. Hence at higher speeds headwinds will cause a squared increase in the required power while vehicle velocity through the air causes a cubic increase in the required power, because the motor is also performing more work in a shorter time interval.

Driving slower generally has a dramatic effect on range due to the cubic relationship of power vs ground velocity. At very low speeds the effect on range is less dramatic since the drag becomes proportional to square of speed instead of being proportional to the cube of speed. None of the above analysis depends on a linear or squared relationship of drag relative to total air velocity over the vehicle since the air speed over the vehicle is identical for both cases.

Whoops. You're right. My bad.

I think I got tripped up by the fact that in most vehicle aero power and efficiency equations, the air speed is assumed to equal the ground speed, which isn't the case here.


Although... if you may allow me the chance to try to salvage just a shred of self-respect here...

In the example I gave of driving very slowly into a very strong headwind, while it's true in a theoretical sense that the efficiency would be the same in either case (when using "Power = Torque x RPM" or "Power = Force x Velocity" equations), those equations essentially assume a 100% efficient motor. But in the real world that's going to break down because the actual efficiency of electric motors approaches zero as RPM approaches zero.

So while I was admittedly wrong on the underlying math and (simplified) physics equations, I think there was a least a bit of truth in that example *when also accounting for motor efficiency*. (God I hope I'm not screwing this one up too!)


(Side note: It wasn't until just now that I realized you can't down-vote your own posts. Oh well.)

 

[Zetopan]

But in the real world that's going to break down because the actual efficiency of electric motors approaches zero as RPM approaches zero.

But the induction motor efficiency does not actually reach zero with your stated test conditions, it is a much larger number than that (infinitely larger in fact). As long at the motor is moving the power efficiency is definitely non-zero.

I do not have any specific performance data for the Tesla induction motors so I cannot indicate what the efficiency for those would be under your assumed testing conditions. Driving at 81 MPH is legal in some states in the US, but finding a location with a constant 80 MPH wind is rather problematic. I also do not have any access to a sufficiently long, or dyno equipped, wind tunnel to drive at 1 MPH in an 80 MPH wind (which is all really quite sad, but true).

However, the modern internet makes numerous technical resources readily available so that I can easily provide a link to a technical paper that evaluated induction motor efficiencies over a range of speeds provided by a VFD. An IGBT Variable Frequency Drive (VFD) is what is Tesla uses to control the motor power and RPM. The bottom line is that the loss in efficiency at lower speeds is actually relatively low, at least with the motors and inverters and the testing conditions they used.

However, they only evaluated these motors and inverters over a 40% to 100% relative speed range. Fortunately with the wonders of modern computers we can trivially plot their data and even extrapolate it back to essentially zero RPM (1 MPH out of 155 MPH is close to zero, but not zero). I have attached a screenshot of the resulting plot, and as you can see the efficiency under load at near zero RPM with the motors and inverters that they used (which are obviously not optimized for the wider speed range that Tesla requires) still comes in at about 73% efficient. I would not be at all surprised if Tesla does significantly better than that the tested motors and inverters over your assumed speed range.

The Tesla induction motors are actually built in-house and they are *very* efficient since they use higher conductivity copper rotors, while common industrial induction motors use lower conductivity aluminum rotors for a lower materials cost. Copper has about 66% to 71% higher conductivity than aluminum, while the best room temperature conductor (silver) is only about 5% better than the best copper. The Tesla inverters are also surely optimized for the extremely wide speed range their vehicle requires, rather than what is usually required for most industrial induction motors.

And since we are now talking about induction motor efficiency at very low RPM, we have also entered the realm where viscous and other related losses also become vanishingly small, etc. At low speeds but with significant torque loads many frictional losses become insignificant relative to the torque being produced.

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1004&context=bae_fac

 

[cpa]

High school physics? Really? My three years in high school (1968-1971) had no such class as part of the curriculum. We had chemistry and biology for the "smarter kids," and general science for the others. And that was about it. And our school district was among the finest in Southern California. No calculus either. Our math progression for the smarter kids was 9th grade--algebra; 10th grade--geometry; 11th grade--advanced algebra and intro to trigonometry; 12th grade--trigonometry and advanced mathematical concepts.

I wish they had offered physics when I was in high school. And the calculus. [Snif.]