Possible Tesla Semi mule captured on video

[WalksOnDirt]

You keep changing the goal posts. There is a significant difference between 60 mph and 50 mph. Still, if you look at this chart, it shows the range of the Model S goes from ~412 miles at 35 mph down to ~335 miles at 50 mph. Of course, that chart is not the definitive measure of range vs speed, but it's in the right ballpark.

 

[Pushmi-Pullyu]

It definitely makes a difference, but not that big of a difference.

"most cars’ fuel efficiency peaks at speeds from 35 to 60 miles per hour. After 60, though, fuel efficiency does drop significantly"
https://www.cbsnews.com/news/6-myths-about-gas-mileage/

Really, all I am asking is this:
How much does the range of an EV change between 35mph and 50mph?

Well, as they say: Don't believe everything you read.

I note that the article itself links to a U.S. Department of Energy (DOE) website, where we find the following graph:

So, the claim in the article you linked to, Jack, is flat wrong when it says that fuel efficiency is essentially unchanged between 35 and 60 MPH; there is obviously a "knee" at ~50 MPH. That higher speeds force lower fuel economy is completely in line with what physics and common sense would predict, so shouldn't be surprising. The graph is unfortunately cut off at speeds lower than 40 MPH; it would be useful to see a full graph. Googling a bit, I see some startlingly contradictory graphs. However, I note that graphs labeled "constant speed" do tend to show a similar pattern:

(source: http://www.motortrend.com/cars/chevrolet/cruze/2012/40-mpg-compact-sedan-comparison/ )

Perhaps those graphs showing improving gasmobile fuel efficiency up to ~30 MPH, and then a plateau between 30-60 MPH, are showing fuel efficiency for an entire trip based on the average speed... not a constant speed? If so, Jack, that may explain the misleading info in that article you pointed to.

* * * * *

Anyway, let's narrow the focus to BEV energy efficiency, which seems to be the focus of Jack's interest. His question "How much does the range of an EV change between 35mph and 50mph?" is indeed an intriguing one. The efficiency curve for a BEV may well be different than it is for a gasmobile. After all, the gasmobile uses a transmission so that you can adjust gear ratios, in theory keeping the running speed of the motor within the range where it will perform the best.

Contrariwise, no production BEV has a transmission, and no doubt there are other significant differences between an EV powertrain and an ICE powertrain, when it comes to pushing a car down the road at different speeds.

Presumably the first graph above shows the efficiency vs. speed curve for gasmobiles. What does it look like for BEVs? The following graph shows that not all BEVs are created equal:

(Source: http://www.plugincars.com/tesla-model-s-range-exceeds-even-teslas-expecations-121253.html )

Based on the above graph, I would be very cautious of trying to make a blanket statement about energy efficiency vs. speed for BEVs in general. Perhaps most notably, those who know more than I do about EV engineering say that the Bolt EV has been engineered for maximum (or at least better) energy efficiency in stop-and-go in-city driving, whereas the Model S has been engineered for maximum (or at least better) energy efficiency on the highway.

The graph below might be instructive, altho my guess is it's likely to be a computer simulation rather than based on empirical data, because so little data on the Model 3 is available:

(source: http://www.roperld.com/science/chevybolt.htm )

Actually, the difference between the efficiency curves for the Bolt EV and the TM3 is far less than I expected.

 

[Pushmi-Pullyu]

Also, specifically addressing the question of speed vs. fuel efficiency in a diesel semi, a quote from "Truck Fuel Efficiency Guide":

Monitor your Speed
Fuel consumption is directly proportional to speed. Going too fast will significantly increase aerodynamic drag, thus make your vehicle consume more fuel. In addition, it can also add strain to your engine and transmission. By simply monitoring your speed you could save almost a quarter of your running costs. For example, reducing your speed from 56mph to 50mph could decrease fuel consumption by up to 22%.
​

https://www.trucklocator.co.uk/hub/truck-fuel-efficiency-guide/

 

[Jack]

You keep changing the goal posts. There is a significant difference between 60 mph and 50 mph

There is not a significant difference between 60 and 50. Weather, incline, and driving style are all more significant. I originally chose 25-60 as my example because most gas vehicles have a efficiency plateau in that range. I narrowed it to 35-50 to further focus on the plateau in comparison to my general question of how speed affects EV range.

I note that the article itself links to a U.S. Department of Energy (DOE) website

Yes, I saw that which is why I narrowed my focus to 35-50. Technically the data is irrelevant to 18 wheelers, but I'm not looking for specific metrics. I'm trying to understand how EV range is affected by speed, and comparing it to the plateau of gasmobiles.

I know you said to be cautious about making a blanket statement, but I feel confident in doing so... it appears that EVs do not have a plateau in their relationship of efficiency vs speed, and It seems that ICE vehicles have good range because of their plateau. How this lack of a plateau for EVs translates to real-world highway big rig driving- I have no clue. I'm sure Tesla has addressed all of this with their coming Semi.

Also, specifically addressing the question of speed vs. fuel efficiency in a diesel semi, a quote from "Truck Fuel Efficiency Guide":

Yes, I realize that trucks are a quite different world when it comes to efficiency, but I still expect their engines to have a plateau. My father used to be in the Trucking industry and sold products that could help get drivers from 4mpg to 6mpg which is huge! Haha

 

[WalksOnDirt]

There is a significant difference between 60 mph and 50 mph

There is not a significant difference between 60 and 50.

It all depends on your definition of "significant". We'll have to agree to disagree on this.

 

[Pushmi-Pullyu]

ICE vehicles have good range because of their plateau.

No such plateau exists, at least not for any appreciable range of speeds. Certainly not over as broad a range as 25-50 MPH, regardless of what misinformation is found at any website. For all I know, what you cited was deliberate disinformation funded by Big Oil; heaven knows there is enough of that going around.

It's possible that what you cited was based on dynamometer tests, where indeed there wouldn't be any difference from wind resistance, because that type of testing is done with a stationary vehicle.

There's no question it's wrong; the only question is how such an error was arrived at.

...I narrowed my focus to 35-50.

Jack, I don't know what your history of online debate is, but I can see why WalksOnDirt says you appear to be moving the goal posts. Perhaps you have no experience engaging in formal debate?

The proper procedure in a fact-based debate such as this is to acknowledge when someone else has made a valid point refuting one of your points. If you're going to change your argument as a result, then this should be acknowledged. If you just shift your premises and keep arguing without acknowledging any change, this comes across as being disingenuous.

My apologies if this comes across as lecturing, but I think it needed to be said.

 

[Jack]

Perhaps you have no experience engaging in formal debate?

Perhaps. My history is simply talking to people. If you would like to hire a professional moderator and institute time limits, count me out. I'm not focused on exact numbers or figures. I dont really care about 50 vs 60. None of this changes my argument. I did not include numbers at first because this in not an argument about data, this is a theoretical argument about range and speed of EVs in comparison to Gas vehicles. I'm not moving the goal posts because I did not originally set up goal posts. Here is my first question:

"higher speeds = shorter range for EVs?...Diesel engines are the opposite, higher speed = more range."
It's not difficult to understand the central idea of my questions- highway driving generally produces higher mpg for gas vehicles, leading to longer ranges. I was wondering if this was true for EVs as well.

can't see the forest for the trees.
I'm not interested in the details. I'm talking big picture.

acknowledge when someone else has made a valid point refuting one of your points.

Ok fine, you presented many valid points that higher speed requires more energy (not my argument). And yes, I was wrong for diesels, higher speed does not equal more range. Obviously if you go 200mph you will not be very energy efficient.

No such plateau exists

You can deny the presence of a plateau all you want, but I know from experience, especially with diesel vehicles, that there is not a large variation in mpg at constant speed until you are over 55-60mph. It may not be a perfectly straight line but is it as steep as the EV curves? I don't think so. I can find graphs that show "plateaus", and you can find ones that show decreasing slopes. Again, this in not about data (which can be presented in countless misleading ways). I think the most helpful point that you made was about transmissions. Variable gear ratios allow for low RPMs at high speeds.

 

[Pushmi-Pullyu]

It's not difficult to understand the central idea of my questions- highway driving generally produces higher mpg for gas vehicles, leading to longer ranges.

Well perhaps I did misunderstand the central thrust of your question, and perhaps that misunderstanding has lead to a misperception on my part that you were refusing to acknowledge valid points. If so, then my apologies.

Yes, the EPA's fuel efficiency ratings for gasmobiles show higher MPG ratings for highway driving than for in-city driving. But that's due to the pattern of driving, not because ICEVs are actually more fuel-efficient at higher speeds. In stop-and-go driving, ICEVs waste much of their fuel idling at stops, and they also lose much efficiency from loss of inertia, as well as the energy loss while shifting gears.

That is, it's not the vehicle which is more fuel-efficient, it's the pattern of driving which is more efficient for gasmobiles driven on highways. If stops during highway driving were as frequent as they are for in-city driving, then the highway MPG would be even worse than the city MPG.

Anyway, the reason I was focusing on energy efficiency at a constant speed is because it eliminates all those losses from stop-and-go driving.

Now, if you want to focus in on real-world driving, then it's correct to say that gasmobiles are more efficient when driven at highway speeds than they are for in-city driving, but that's not because there is any "plateau" of ICE engine efficiency between 25-50 MPH.

Most BEVs have a higher city driving range than highway range because BEVs don't waste fuel idling at stop lights and stop signs, and of course driving at lower "city" speeds means less loss from wind resistance. I find it interesting that some say the Tesla Model S actually has better highway range than in-city range. If so, I presume that's because Tesla has optimized the car (inverter efficiency, reduction gear ratio, etc.) to maximize highway range at the expense of city range.

 

[WalksOnDirt]

I suspect it's the AC induction motor that is hurting Tesla at low speeds. It looks like that has been fixed in the Model 3.

 

[Pushmi-Pullyu]

It's easy to find articles which say that permanent magnet electric motors are more efficient than induction motors. What's not so easy to find is just how much more efficient they are.

If the minimum efficiency for an electric motor in a modern mass produced EV is 90%, which seems to be the case**, then how much more efficient can the permanent magnet motor be, even in a best-case scenario? 3%? 5%? I seriously doubt we're going to see better than a 5% improvement in a mass-produced car. Of course, increasing your EV's range by 5% is nothing to sneeze at.

I'd love to see some data on the exact efficiency difference of permanent magnet vs. induction motor when used in the same car. I rather suspect that those who think EVs can get significant energy efficiency gains from permanent magnet motors, are going to be disappointed with the actual figures.

**Not counting some cheap Chinese make EVs, some of which reportedly have abysmal energy efficiency. Are they still using cheap, inferior DC motors?