At that point, I was already experiencing anxiety about range and had called a Tesla employee from the New Jersey Turnpike to ask how to stretch the battery. She said to shut off the cruise control to take advantage of battery regeneration from occasional braking and slowing down. Based on that advice, I was under the impression that stop-and-go driving at low speeds in the city would help, not hurt, my mileage.
> Based on that advice, I was under the impression that stop-and-go driving at low speeds in the city would help, not hurt, my mileage.
If I were hiring John Broder to be a journalist covering electric cars, that right there would be enough to reveal him as disqualified: Doesn't know even the basics of thermodynamics or can't apply it to his subject.
Doesn't know even the basics of thermodynamics or can't apply it to his subject.
Oh really? You probably also think that cold water freezes faster than hot. Taking the thimbleful of knowledge that you have and making snap judgements based on it will often mislead you.
In the case of the cars, without looking it up it is not obvious to me which is worse - the energy loss in starting and stopping at low speeds, or the quadratically scaling energy loss from wind resistance at high speeds. Certainly if you go fast enough you'll find that your driving range drops substantially - this is why the old 55 mph limit was passed during the oil crisis.
If I am not certain without looking it up, then I can't fault someone else for not knowing it either.
About the water, if you have equal amounts of hot and cold water in wooden buckets and put them out in the snow, the hot water can easily freeze faster. Several possible factors are known, but their relative importance is unclear. See http://math.ucr.edu/home/baez/physics/General/hot_water.html for a detailed discussion.
> In the case of the cars, without looking it up it is not obvious to me which is worse
I wouldn't hire you either.
> About the water
Your example digs your hole deeper. There's a fundamental difference between applying thermodynamics in this case, and the regenerative braking one. That you don't know that is what's damning.
I've operated propulsion machinery completely submerged in water.
Anyone who didn't understand that energy usage is approximately quadratic with respect to speed would be considered studderingly incompetent and would fail qualification at both ends of the boat.
Of course if you don't believe me, perhaps you should ask Tesla Motors.... or look at the Prius's rated MPG for city/hwy.
I'll guess that you never passed any fluid mechanics courses either.
See? I can return snark for snark. Now let's actually discuss some physics.
There's a fundamental difference between applying thermodynamics in this case, and the regenerative braking one. That you don't know that is what's damning.
For the same trip from New York to Boston in a Toyota Camry, traveling at 80 mph the whole way takes 2.75 times as much energy as traveling at 30 mph. See http://ocw.mit.edu/courses/physics/8-21-the-physics-of-energ... and apply scaling factors to verify.
That provides a lot of headroom to add starts and stops on the 30 mph trip without using more energy than the faster trip. In fact, even without regenerative braking, you could add in multiple stops/starts per mile and the slow trip still saves energy in a conventional car. (Well, until you consider rolling friction going up hills, I don't have a good back of the envelope for what that adds.)
I'm being deliberately unfair to make a point here. Standard "highway driving" for EPA efficiency is assumed to be 60 mph. Traveling at 80 mph is a lot, lot worse than 60 mph. Elon knows this, and calls out the journalist's speed because Elon knows what difference this efficiency makes.
If the highway trip is taken at 60 mph it is much harder for a slow trip to take less energy.
Now do you understand that speed hurts efficiency?
> See? I can return snark for snark. Now let's actually discuss some physics.
Not snark. Also, already knew about the possible non-intuitive results with water.
> If the highway trip is taken at 60 mph it is much harder for a slow trip to take less energy.
Now do you understand that speed hurts efficiency?
Hmmm, putting words in my mouth to spin it so it seems like my original position is actually yours and imply that I don't know a different basic bit of physics. (You can check my recent comment history.)
Nope, definitely wouldn't hire you as a journalist.
Go back to http://news.ycombinator.com/item?id=5218597 and look at the quote you were criticizing Broder for. Here it is. Based on that advice, I was under the impression that stop-and-go driving at low speeds in the city would help, not hurt, my mileage.
You considered this proof that he didn't know basic physics, because obviously stop-and-go driving is inefficient. My point is that your criticism is wrong because this is actually a question of empirical fact that can - and does - go either way. (In fact in published mpg figures, the Toyota Prius does better in stop and go city traffic than on the highway. If you go significantly over 55 mph on the highway, you're probably getting worse highway mileage than the published figures.)
...imply that I don't know a different basic bit of physics.
I make no claims on whether you understand how wind resistance works. I do claim that your initial comment showed no understanding for how important wind resistance is IN THIS CASE. But now I pointed it out, and then explained it with reference to actual energy loss for a popular model of car, and you have been pointed to the fact that published mpg figures for the Prius demonstrate that it leads to the very pattern that you thought Broder was an idiot for thinking possible. At this point if you believe in science and measurement, you've got to admit that your analysis missed something important, and you were wrong. If you're a mature person, you'd then apologize for some of the same things.
Alternately you can demonstrate a willful denial of basic physics by continuing to insult me for having pointed out your mistake. But if you continue that path, I won't bother to respond because I trust that interested third parties can draw the correct conclusion.
And then, you proceed to talk about an entirely different set of comments. Perhaps we've been misunderstanding referents. In my reading, it seems like you've been painting me as a rube who might think that warmer water freezing faster than cold is impossible. I find that highly annoying, because that's false.
I see your point about slower driving, regen, and range. Imagine you're the manager of a group of delivery truck drivers, and a driver comes to you after having stranded himself on a trip, and gives you that same sentence. Most people are going to look at him like he's an idiot, or have to try not to. Even knowing how regen braking works, I would still probably give him that look.
> If you're a mature person, you'd then apologize for some of the same things.
I'll apologize. The motivation for my objection was to how you seemed to be trying to paint my knowledge. From my reading, you were willfully trying to paint a picture of my knowledge or lack of it. From you POV, you were trying to make a point about the capabilities of regen braking, and saw my continued objection as a denial of that, which misconstrues of my position. (Heh, you could get the referents wrong on that last sentence too, prolonging this as well, hopefully not.)
Thank you for starting to apologize. You said a lot more that I'd appreciate apologies, but it is a good start.
Now it is my turn.
You're right that I said that you probably didn't know about the hot/cold water freezing weirdness when I had no evidence of that. I shouldn't have assumed ignorance as strongly as I did, and I apologize for having done so. A large part of the reason that I did is irritation with you for saying that anyone who could think that slow stop and go could possibly be more efficient than highway driving is an idiot who doesn't know basic physics. Since I consider that possible, and I think I know basic physics, this hit a nerve.
The reason why I brought up the water example is the following parallel. We have a situation where an obvious fact leads anyone with a basic physics education to assume that the answer goes one way. In fact it lead a lot of scientists to assume that they knew the answer, despite widespread folk knowledge and historical observations to the contrary. In fact reality is more complicated, and the obvious answer in this case isn't always right.
Now are you willing to admit that he and I weren't necessarily ignorant of basic physics when we say that slow stop and go can possibly be more efficient than highway traffic? And that furthermore the fact that I would say so is not particularly damning about my knowledge base or intelligence?
Imagine you're the manager of a group of delivery truck drivers, and a driver comes to you after having stranded himself on a trip, and gives you that same sentence. Most people are going to look at him like he's an idiot, or have to try not to.
You may not be giving truck drivers enough credit. Truck drivers make their living trying to beat operational costs while driving something with the aerodynamics of a brick. I have a couple of relatives who have been long-distance truck drivers, and they are keenly aware of the value of driving slowly, drafting off of other trucks, making no unnecessary maneuvers, etc.
> Thank you for starting to apologize...it is a good start.
It would help if you understood my position.
> I shouldn't have assumed ignorance as strongly as I did, and I apologize for having done so.
That's a good start.
> A large part of the reason that I did is irritation with you for saying that anyone who could think that slow stop and go could possibly be more efficient than highway driving is an idiot who doesn't know basic physics.
No, what I'm saying is that (despite the fact that someone who knows basic physics could figure out that it could possibly be more efficient) someone who knows basic physics who goes and makes a bet against being stranded without more info than Broder had is an idiot or is willfully acting like one.
> You may not be giving truck drivers enough credit. Truck drivers make their living trying to beat operational costs while driving something with the aerodynamics of a brick. I have a couple of relatives who have been long-distance truck drivers, and they are keenly aware of the value of driving slowly, drafting off of other trucks, making no unnecessary maneuvers, etc.
Wow, you really don't do it on purpose, do you? Feel free to read my sentence from the POV that truck drivers are very good at what they do.
I've never disputed that the "journalist" had a clear axe to grind. I've just disputed your claim about the sheer stupidity of that particular statement.
And about truck drivers, I did read your sentence from the POV that truck drivers are very good at what they do. Which is why I disagreed with you about how they would react.
Funnily enough, going further in stop-and-go urban traffic than on the highway doesn't violate the conservation of energy either. (Also, remember you can't just go arbitrarily slowly on the highway - that's unsafe and a lot of places ticket people for it.)
I don't know, consumers are told lots of wacky counter-intuitive things about "how to eke performance out of technology in practice". Think of the way that lots of people like to repeatedly run their laptop batteries down because they think that it helps preserve battery life, even though that hasn't been true for about a decade.
However I think that your comment about thermodynamics is revealing because it seems to me that the real point of this article is that driving an electric car requires consumers to learn a lot (about entropy!) and think very differently from driving a normal car. It sounds frankly stressful to drive, even if everything works perfectly. That's where I think the real damage is.
You don't need a Thermodynamics Ph.D. to understand this. Moreover the normal (gas) cars have the same behavior. So it's annoying that a car expert don't know it.
You get the best mpg at a constant speed of ~50mph, and it's better with the cruise control because it reduce the times that you need to accelerate or decelerate.
If you go too fast, the mpg drops because you need more fuel to compensate the additional resistance.
If you accelerate and brake (for example in a city), the mpg drops because you need more fuel to accelerate and when you brake the energy goes away as heat.
The advantage of the regenerations brakes is that some of the energy that is "lost" as heat braking is stored and used again to accelerate. This is not magic. It doesn't make braking lossless, it only recycles part of the energy so accelerating and braking is not so bad (it's only bad). This is well known for the electric and hybrid cars (I think that there are some tries to do something like this for gas cars, perhaps store the energy mechanically, I'm not sure.)
I think that the recommendation was (paraphrase) "If you are going to drive inside a city, it's better to turn off the cruise control."
The articles that he understood something like (paraphrase) "It's better to drive inside a city than in a highway with cruise control."
It's very surprising that a car expert can understood this so totally wrong that it look like it was a willfully misunderstandment.
Moreover the normal (gas) cars have the same behavior. ...
You get the best mpg at a constant speed of ~50mph
This isn't true of EVs.
The engine in a gasoline powered cars are around 15 to 25 percent efficient, and the hotter you put them, the more efficient they are.
However, air resistance increases as speed increases.
The peak efficiency depends on a few things, but most cars you'll find it around 40-60 mph.
EVs don't have engines, they have motors. They are about 90% efficient, regardless of your speed.
But they have the same air resistance issues.
So EVs really do perform better at lower speeds. Even the Prius will show better city mileage than highway mileage, and that's only with a little bit of battery+motor.
Without knowing the exact conversation it doesn't sound super off, regenerative braking technology has been used in F1 for a while to power the KERS system, but stop-start traffic is completely different to the occasional slow down on a motorway.
Oh, and BTW, it's not mentioned, but it would be quite interesting, and much more of a burn, if Elon is actually making some of the charges, like driving in loops, with a proof via GPS logs of the test drive.
Yes, and after a little research, I find that the Tesla log contains GPS positions, so this could be done in principle. But for a short distance like within a small parking lot (as in the account of driving in circles), the positions might not produce a useful graph (typical GPS accuracies are +- 10 meters or 32.8 feet).
GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) in an absolute sense, but in a relative sense they are much more accurate and would easily show circles being driven in a parking lot (it's just that the circle might be 10 metres away from the road).
This is also used in surverying to do something called differential GPS with two receivers: one GPS receiver fixed in a known location, and another to measure GPS locations relative to the first GPS in the known location - absolute error is completely removed by this process (http://en.wikipedia.org/wiki/Differential_GPS claims 10cm accuracy). If you think about it, two receivers separated by a little space is more or less equivalent to two receivers separated by a little time, so you'd expect a similar relative accuracy of 10cm.
Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times.
> GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres)
Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence):
Good point about changing satelites. I guess an open parking lot would be fine, but who knows.
I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
> I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate.
It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.
I suspect the probem is not the gps positional accuracy, but rather the data loggers recording frequency - for 99.9% of what you'd want from a gps log on a vehicle, recording the position and speed every 10 or 20 seconds would be more than enough. To generate useful plots of a car driving in circles round a small parking lot would need a much higher recording rate. Even slow old-school gps chipsets output 1Hz position data, but logging all that would needlessly produce 10 or 20 ties more data than youd need for anything except showing up lying journalists. I'd put good money on Musk getting 1Hz or better gps logs of the _next_ bunch of test drivers...
I use Google's "My Tracks" app every day to record my running, with a GPS tracked path imposed on a Google map, and it easily shows tiny shifts in position such as which side of the street I ran on or whether I ran on or slightly off of a dirt path. There's no doubt something equivalent in a Model S would be able to accurately record laps in a parking lot.
I don't think GPS would show anything substantial if his claim of a "100-space parking lot" where Broder drove in circles is true. I'd be impressed if the GPS was that accurate.
Regardless, I don't think that's a huge sticking point to this story. Broder was at the charging station - it's possible he wanted to see just how long it would take until the battery shut down.
Just why did Elon Musk not link to the article, for everyone to easily see Broder's embarrassment beats me.
http://www.nytimes.com/2013/02/10/automobiles/stalled-on-the...