Naked Science Forum
Non Life Sciences => Technology => Topic started by: Petrochemicals on 29/05/2021 23:54:12
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Given that batteries perform worst under heavy loads and high temperatures, heavy loads making high temperatures that further worsen efficiency, what are the efficiency statistics for electric car accelleration?
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Given that batteries perform worst under heavy loads and high temperatures, heavy loads making high temperatures that further worsen efficiency, what are the efficiency statistics for electric car accelleration?
Don't batteries perform worst under low temperatures? As low temperatures slow down the chemical reactions inside the battery, which are needed to produce electron flow.
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what are the efficiency statistics for electric car accelleration?
That is a fair question for a fossil of a vehicle, but not reasonable for full-electric (or hybrid) cars.
Electric car efficiency needs to be measured over a cycle of acceleration and deceleration back to 0.
- Because the most important advantage of an electric vehicle is how much energy you can reuse.
For a fossil vehicle, the energy reuse is 0%, so it doesn't matter if you measure acceleration only, or a complete acceleration/deceleration cycle.
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What does the efficiency of acceleration actually mean?
Energy is supplied by the battery, is turned into motive power and makes the car go faster.
But, at high speeds the car suffers more drag so less of the energy goes into increasing the kinetic energy of the car and more goes into pushing air out of the way.
When the car reaches its top speed, all the energy from the battery is going into pushing air (etc) and none is going into making the car go faster.
At that point, it seems reasonable to say that the efficiency for making the car go faster is zero.
The same is true of a petrol car.
At the point where the car is just setting off, the car is (instantaneously) not moving.
Since the force, at that point, moves through zero distance, zero work is done, and the efficiency is zero.
Again, that's equally true for a petrol or electric vehicle.
Somewhere between standing still and flat out, there's probably (at least one) maximum efficiency.
That calculation is left to the interested reader.
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The situation is complicated by battery heating. In order to get a useable internal impedance for accelerating, modern electric vehicles use some battery power to heat the battery, so the available power is fairly constant at the start of a cold day and increases slightly as the battery warms up under useful power drain, then decreases as it discharges, increasing the internal impedance again.
So the best estimate of useable power at any time is the "max sustained" rating of the motors. Not sure how this is stated for road vehicles but for electric aircraft there is a "5 minute" rating for takeoff and initial full power climb, derated to about 80% for fast cruise. You could probably assign a "5 second" rating for a road car, at maybe 300% of max cruise power.
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For clarity I believe that their is an ideal temperature range, above and beneath which capacity reduces, which is why devices stop working in the cold.
For a given current, limited heat increace the immediate capacity, but the lifetime of the battery and the lifetime capacity of the battery is reduced .
For the same given current, colder temperatures reduce the immediate capacity but this can be recovered by warming the battery, colder temperatures also help conserve the lifetime battery capacity.
Higher loads reduce the battery capacity and increace temperature. Lower loading keeps temperature down and capacity up.
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what are the efficiency statistics for electric car accelleration?
Electric car efficiency needs to be measured over a cycle of acceleration and deceleration back to 0.
- Because the most important advantage of an electric vehicle is how much energy you can reuse.
I suppose given that the energy from the braking of a car would need to be recovered, any motor that would be large enough to brake a car could accelerate it at a similar rate. Similarly the battery would need to be large enough to supply high current, but I should think that the charging of a battery is more difficult than the use.
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So the best estimate of useable power at any time is the "max sustained" rating of the motors.
It is well known that electric vehicles have far higher torque at low speeds than fossil engines.
- So quite ordinary electric vehicles can beat a souped-up gas-guzzler across the intersection.
- But that depends on being at the head of the queue at the lights, and being in an unreasonable hurry to get to the end of the next queue
In City driving, more realistic would be the average driver behind a queue of cars at the lights - a few seconds after the lights change to green (and hopefully, before they turn back to red), there will be a gradual acceleration up to maybe 50 km/h (or 30mph), and a gradual deceleration back to stationary at the next queue.
- There is a well-defined "City" driving course used to compare pollutants from different vehicles
- It is so well standardised that Volkswagen infamously programmed their engine control system to recognise it and cheat...
What does the efficiency of acceleration actually mean?
I guess you could say that a certain amount of chemical energy (in a battery or fuel tank) gets turned into a certain amount of kinetic energy in accelerating to a velocity v (say, 50km/h).
But the kinetic energy equation also includes the mass m. What do you use as the mass?
- The mass of a 1 ton vehicle?
- A mass of m=100kg, representing the useful payload: an 80kg human and 20kg representing the week's shopping?
- Some people drive to the café for a coffee. Should the useful payload m = 250g of coffee?
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There is a well-defined "City" driving course used to compare pollutants from different vehicles
- It is so well standardised that Volkswagen infamously programmed their engine control system to recognise it and cheat...
I've always wondered about the use of language here. If the standard test is a valid approximation to city driving, then surely optimising the vehicle to that profile is a Good Thing, not "cheating". If it isn't a valid approximation, then the manufacturer has two options: optimise to reality and fail the test, so he can't sell a really good car, or optimise to the test like everyone else aspires to (because the test is a consensus approved by the major manufacturers).
I can't think of any other case where doing exactly what is asked of you is considered cheating.
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There is a well-defined "City" driving course used to compare pollutants from different vehicles
- It is so well standardised that Volkswagen infamously programmed their engine control system to recognise it and cheat...
I've always wondered about the use of language here. If the standard test is a valid approximation to city driving, then surely optimising the vehicle to that profile is a Good Thing, not "cheating". If it isn't a valid approximation, then the manufacturer has two options: optimise to reality and fail the test, so he can't sell a really good car, or optimise to the test like everyone else aspires to (because the test is a consensus approved by the major manufacturers).
I can't think of any other case where doing exactly what is asked of you is considered cheating.
I pay my taxes.
But imagine that I didn't usually, but, if the taxman came to see me, I wrote out the cheque for that month, and photocopied it and sent it.
I could show the photocopy to the taxman as evidence that I pay my taxes.
But- here's the important part- I only pay when he is looking.
During the other months, I don't send a cheque.
I do what I'm asked- I provide evidence to the taxman of paying my dues.
But I'm still cheating.
In the same way, the VW scam involved recognising the test and tweaking the parameters.
So it was only "efficient" when the inspector was watching, but not the rest of the time.
That's cheating.
Specifically, it's exploiting the fact that the test has to be standardised, while normal driving doesn't.
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I can't think of any other case where doing exactly what is asked of you is considered cheating.
As I understand it, the standardised test was to check that tailpipe pollutants were below the regulated limits, so people in cities didn't suffer so many lung problems...
- To meet more recent NOx pollution regulations, some diesel cars inject a urea solution into the exhaust stream to react with pollutants; this is commercially known as AdBlue.
- But its a pain to remember to top up the AdBlue tank
- And injection increases fuel consumption
To reduce AdBlue (and fuel) consumption, Volkswagen software only injected AdBlue while the car was running the standardised test in a virtual city (so it always passed).
- That meant it was pumping out excess pollutants all the time while driving around real cities
- Violating air pollution regulations, and presumably causing asthma and other lung irritation
- As well as fixing the software, there were some heavy fines imposed in various jurisdictions
What was asked was to reduce city air pollution from diesel engines.
- What they did was avoid polluting the standardised virtual city, while continuing to pollute the real cities
- In what way was that doing what was requested?
As I understand, it was only discovered when a university team decided to check tailpipe emissions in real life, not in the standardised test.
See: https://en.wikipedia.org/wiki/Diesel_exhaust_fluid
https://medium.com/swlh/the-lie-of-clean-diesel-f55b75031066
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I pay my taxes.
......
But- here's the important part- I only pay when he is looking.
One of those statements is untrue. You are required to pay your taxes according to a precise formula accounting for all income and expenditure over a period determined by the taxman, not yourself.
If taxes were based on one month's accounts only, you would not be cheating if you paid exactly what was required.
A I understand it, the emissions test protocol is as precise as tax law, and was not written by VW or any other single manufacturer, so nobody can be criticised for playing to the whistle.
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- But its a pain to remember to top up the AdBlue tank
More than a pain. Most vehicles (not just cars - AdBlue is pretty much mandatory for trucks and buses) will not even start if the tank is empty, but you do get about 1500 miles' warning.
I can't see how injecting tiny amounts of urea solution into the exhaust system can significantly increase primary fuel consumption, but it makes sense to adjust the DEF rate according to actual exhaust chemistry.
If the standardised test doesn't replicate real life, wouldn't it be more sensible to change the test than to complain that some engines pass it? Do you only appoint staff who can't do the job?
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One of those statements is untrue.
Yes; obviously, but the second one is imaginary, so that's not a problem is it?
What did you think I meant by this?
But imagine that I didn't...
If taxes were based on one month's accounts only,
They aren't.
so nobody can be criticised for playing to the whistle.
The existence of phrases like "the letter of the law" and " the spirit of the law" very clearly mean that you can be criticised for it.
VW are not just being criticised, but being prosecuted.
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Quote from: alancalverd on Today at 11:44:46
If taxes were based on one month's accounts only,
They aren't.
Some are. I pay VAT quarterly, as do most small traders, but you can opt for monthly payments by agreement. The rules are just as strict.
VW are not just being criticised, but being prosecuted.
And it is essential to the maintenance of justice and the rule of law that the prosecution fails. If I claim and demonstrate compliance with a published standard, it is wrong to prosecute me because someone else thinks the standard is inappropriate to their view of reality.
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I guess that'll be a no. This is the sort of thing I was looking for, but it doesn't seem very scientific and precise. .
https://www.cleantechnica.com/2018/04/16/how-ev-range-is-affected-by-quick-acceleration/amp/
Anyway, it would be nice if the thread was left a bit longer than 2 days and 4 replies before it gets spammed with adblue, conspiracy theories and nazis!
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It might have helped if you had asked the question you wanted answered, which seems to be along the lines of "the effect of driving style on the range of an electric car". And I think the answer is pretty much the same as for a conventional car: harsh acceleration and braking are less efficient than smooth driving, and since drag increases with the square of speed, fast cruising is less energy-efficient than slow cruising. I suspect there is a difference in the cruise efficiency curve which is entirely parabolic for an ideal electric car (linear torque/speed versus the drag equation) but has an optimum range at around 50 mph for most ICE motors where the power curve meets the drag curve.
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It might have helped if you had asked the question you wanted answered, which seems to be along the lines of "the effect of driving style on the range of an electric car".
ooh the irony, and not but ten minutes prior too! , it is wrong to prosecute me because someone else thinks the standard is inappropriate to their view of reality.
And I think the answer is pretty much the same as for a conventional car: harsh acceleration and braking are less efficient than smooth driving, and since drag increases with the square of speed, fast cruising is less energy-efficient than slow cruising. I suspect there is a difference in the cruise efficiency curve which is entirely parabolic for an ideal electric car (linear torque/speed versus the drag equation) but has an optimum range at around 50 mph for most ICE motors where the power curve meets the drag curve.
Yes that is what I wanted knowledge of sort of versus normal speed maintenance. One would suspect that with a battery, higher loads reduce capacity also, but that is fairly imprecise too. Batteries suffer high load efficiency loss where as a combustion engine suffers high rpm loss, slightly different.
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gets spammed with adblue, conspiracy theories and nazis!
ok, I mentioned AdBlue and NOx emissions as a problem with Diesel engines
But as far as I can see, you are the only one mentioning conspiracy theories and nazis...
...unless you count "Petrochemicals are better than electric power" as a conspiracy theory!
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Some are.
No, you really are expected to pay taxes for every month, not just the ones when the tax man visits.
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gets spammed with adblue, conspiracy theories and nazis!
ok, I mentioned AdBlue and NOx emissions as a problem with Diesel engines
But as far as I can see, you are the only one mentioning conspiracy theories and nazis...
...unless you count "Petrochemicals are better than electric power" as a conspiracy theory!
I don't mind a bit Evan, but so far it is half a page of the inland revenue.
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Some are.
No, you really are expected to pay taxes for every month, not just the ones when the tax man visits.
Which is why we have written rules, and an individual tax inspector is not allowed to reinvent them according to what he thinks represents reality.
Come to think of it, there are precedents for variation. Astrid Lindgren famously faced demands for 102% income tax in 1976 and her response led to the first defeat of the ruling Social Democrats for 40 years, followed by a rationalisation of the tax laws.
AFAIK she never bought DEF or an electric car and Sweden was neutral during WWII.