# No new petrol or diesel cars by 2040

I think bas is suggesting some energy would be lost due to not being able to store it in the battery quickly enough
I thought he was saying that you'd need to recover energy at a faster rate than that at which it had been used.

I thought he was saying that you'd need to recover energy at a faster rate than that at which it had been used.
He said that indeed, more than once, so presumably he had some underlying point about whether the battery could accept the charge that quickly

So that would be if you stopped instantly, so any braking force including infinity could be stored. The other problem would be having a motor/generator that could handle that.
That would be the least of your problems. If you had an infinite current you would need conductors of infinite CSA, otherwise you would get an infinite voltage drop

Kind Regards, John

He said that indeed, more than once, so presumably he had some underlying point about whether the battery could accept the charge that quickly
Indeed he did, and more than once I told him that I didn't understand what he was saying.

Kind Regards, John

I don't really understand that.
If the only way you have to slow down is regenerative braking and you want to slow down at a greater rate than you can speed up, you have to be able to charge your battery at a greater rate than you can discharge it.

That would be the least of your problems. If you had an infinite current you would need conductors of infinite CSA, otherwise you would get an infinite voltage drop
IMO the biggest problem I would have would be what would happen to my body when it was subjected to an infinite force.

Indeed he did, and more than once I told him that I didn't understand what he was saying.
Construct a theoretical vehicle, and assume a mass and a m/s² acceleration. Work out how much energy is needed to accelerate the vehicle to x m/s, and call it j.

Then assume that at maximum performance it can achieve x m/s in y s. i.e. it can discharge the battery at j/y joules/s.

If by purely regenerative braking you want to bring the vehicle to rest in z seconds, where z < y, then you have to put your j joules back into the battery in z seconds.

If z < y, then j/z > j/y.

I thought he was saying that you'd need to recover energy at a faster rate than that at which it had been used.
Indeed.

Because if you want to be able to stop in a shorter time than it took you to get to speed (which you do) then you have to convert your energy from kinetic to potential in a shorter time than you converted it from potential to kinetic.

(and if the journey finished at the same altitude as it started)
In one of Colin Kapp's Unorthodox Engineers short stories they had discovered a planet which had internal satellites made of collapsed matter. They wanted a surface transport system, but as the planet did not have a long life ahead of it (being, as it was, turned into swiss cheese by its satellites) they didn't want to invest in a proper infrastructure.

Solution? Simple unpowered wheeled carts with a brake. You waited until where you wanted to go was downhill, then released the brake.

have to put your j joules back into the battery
not necessarily back into the battery, you just have to store them somewhere. hence my capacitor suggestion, but other energy stores are available. You could lift a heavy weight up (some underground lines have the tunnels lower than the platforms for conservation reasons) or spin a heavy flywheel (I heard that used in buses once)

If the only way you have to slow down is regenerative braking and you want to slow down at a greater rate than you can speed up, you have to be able to charge your battery at a greater rate than you can discharge it.
Oh, I see.

I presume that, in reality, one would not have a vehicle in which regenerative braking was the only way of slowing it down (particularly in the 'emergency stop' scenario you mentioned), but is it necessarily the case that "you want to slow down at a greater rate than you can speed up"? ....

.... even in the 'emergency stop' situation, the 'official' (per driving test!) stopping distance at 60mph is 240 feet, which I reckon (assuming uniform deceleration, which probably is not exactly true) equates to about 5.45 seconds - but I don't think your average car can do "0 to 60 in 5.45 seconds". Given that the vast majority of braking is nothing like 'emergency stopping', I would imagine that nearly all 'ordinary braking' would be at a rate considerably slower than "the rate at which you can speed up". For those rare occasions when this ceased to be the case (e.g. 'emergency stops') one could (and would) presumably have some alternative strategy built in.

Kind Regards, John

not necessarily back into the battery, you just have to store them somewhere. hence my capacitor suggestion,
That may be a possibility.

but other energy stores are available. You could lift a heavy weight up (some underground lines have the tunnels lower than the platforms for conservation reasons) or spin a heavy flywheel
I thought we were trying to make cars lighter?

Bosch and PSA Peugeot Citroën have developed a hybrid system that use hydraulics as a way to transfer energy to and from a compressed nitrogen tank. An up to 45% reduction in fuel consumption is claimed, corresponding to 2.9l/100 km (81 mpg, 69 g CO2/km) on the NEDC cycle for a compact frame like Peugeot 208. The system is claimed to be much more affordable than competing electric and flywheel systems and is expected on road cars by 2016.

(I heard that used in buses once)
https://en.wikipedia.org/wiki/Gyrobus

I presume that, in reality, one would not have a vehicle in which regenerative braking was the only way of slowing it down (particularly in the 'emergency stop' scenario you mentioned), but is it necessarily the case that "you want to slow down at a greater rate than you can speed up"? ....
In an emergency stop you need to be able to. In an emergency stop you need to be able to convert your kinetic energy back into potential energy at a faster rate than you are able to convert it from potential to kinetic.

.... even in the 'emergency stop' situation, the 'official' (per driving test!) stopping distance at 60mph is 240 feet, which I reckon (assuming uniform deceleration, which probably is not exactly true) equates to about 5.45 seconds - but I don't think your average car can do "0 to 60 in 5.45 seconds".
Precisely.

You can stop quicker than you can start.

Given that the vast majority of braking is nothing like 'emergency stopping', I would imagine that nearly all 'ordinary braking' would be at a rate considerably slower than "the rate at which you can speed up".
It would indeed.

For those rare occasions when this ceased to be the case (e.g. 'emergency stops') one could (and would) presumably have some alternative strategy built in.
Either that, or you recharge the battery at a faster rate.

The goal should be to never waste energy if at all possible.

TBF - I had not realised (should have thought, as it's hardly surprising) how much R&D is being done by car makers into different systems. I suspect the right extra mass:energy recovered balance might be tricky to find.

Given that the vast majority of braking is nothing like 'emergency stopping', I would imagine that nearly all 'ordinary braking' would be at a rate considerably slower than "the rate at which you can speed up".
It would indeed.
That being the case, the issue you were raising would seem to be very trivial in terms of the big picture.

If one felt compelled to even recover the energy due to 'emergency stop' braking (a very rare event), one would have to explore and research the various possibilities, but I'm not sure that such provision would be cost-effective.

Kind Regards, John

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