No new petrol or diesel cars by 2040

[JohnW2]

Quite efficient energy recovery then. That's 2.6 miles of downhill coasting giving you 'about 3 miles' flat driving.

Indeed, and in this case the amount of energy (hence miles of subsequent 'flat driving') is going to be mass-dependent, since this time one is essentially converting potential energy (definitely mass-dependent) into electrical energy. If some of that potentially energy is allowed to accelerate down the hill, then that will subtract from the amount available for conversion into electrical energy, but if the car is kept at constant speed then, one way or another, all of the initial potential energy will be available for conversion into electrical energy.

Kind Regards, John

 

[ban-all-sheds]

I think I agreed with you the whole time, I was just trying to point to BAS where he was differing.
He said mass is irrelevant (which is only true when the choice is between stop and start from a given speed as opposed to just carrying on at the original speed), however under the alternative assumption that the car would have stopped anyway:
braking energy recovered is proportional to mass
the energy to start the car again would be used with or without regenerative braking
that saved energy will actually be used to extend the range of the car or save on charging, neither of which are proportional to mass
therefore mass is relevant.

The question was:

Braking from 30mph using regenerative braking. How far would you get using the energy put back into the battery? 100yds?

No mass was specified. JW2 decided to pick a particular figure and do a calculation:

If I've done my sums right, assuming a 1,000kg car and assuming 100% efficiency in converting the car's kinetic energy into electrical energy, braking from 30mph to zero would produce about 89,780 Joules, aka about 25 Wh (i.e. 0.025 kWh). I've no idea how far the car could go with that amount of electrical energy!

If he had used a larger mass he would have got a greater figure, but a heavier car would use more energy, and not necessarily go further than a lighter car would with a lower amount.

You've got a mass which is speeding up and slowing down "all the time". A larger mass will use more energy to do the first but have more kinetic energy available to be converted and reclaimed when doing the second than a smaller one, and therefore have more reclaimed energy available to be used for further progress, during which time it will use more than a smaller one.

The mass is irrelevant.

Largely.

I said, a while ago, that this was a theoretical model, and in practice it doesn't work as perfectly as that. As well as some of the energy in the battery being used to make noise and heat, "all the time" isn't necessarily true, there will be times when the vehicle has a constant velocity. But not many. The target environment for most EVs is still the short-range urban shopping/commuting/schoolchild pampering one, not the long-distance motorway cruising one. The target environment for most EVs is still one where you're accelerating most of the time. So for that "most of the time" you're either converting a stored electrical charge into kinetic energy, or back again. And during those times the mass of the vehicle is irrelevant - if it has a mass of 2,000kg instead of 1,000 it has twice as much kinetic energy to be recovered, but it will need twice as much to get back up to speed once braking is over.

 

[ban-all-sheds]

If we wish to maximise the recovery, then regenerative braking has to recover at a faster rate or the vehicle will have unacceptable braking performance.

I don't understand what you're saying - a "faster rate" than what?

At a faster rate than the car uses it to move.

To maximise the recovery, all the braking has to be regenerative - no friction at all, as that is just waste.

Think of your car. In an emergency stop does it have greater acceleration than it does when moving off from rest?

 

[ban-all-sheds]

To the two Johns - if mass is relevant then we should be making EVs as heavy as we possibly can, in order to have more kinetic energy to recover.

 

[JohnW2]

No mass was specified. JW2 decided to pick a particular figure and do a calculation: .... If he had used a larger mass he would have got a greater figure ....

I did. However, as you've seen (and quoted), I did not fully answer the question. I merely estimated how much energy would be recovered, under idealised conditions, if a 1000kg car was braked from 30mph to rest. As the last sentence of your quote then shows, I then said: "I've no idea how far the car could go with that amount of electrical energy!"

... but a heavier car would use more energy, and not necessarily go further than a lighter car would with a lower amount.

Indeed - but, as I said, I did not address that question.

You've got a mass which is speeding up and slowing down "all the time". A larger mass will use more energy to do the first but have more kinetic energy available to be converted and reclaimed when doing the second than a smaller one, and therefore have more reclaimed energy available to be used for further progress, during which time it will use more than a smaller one. ... The mass is irrelevant.

Yes, if per what you go on to suggest, the vehicle spends nearly all its time accelerating and decelerating, that's all true, and mass would indeed be irrelevant. However, I had rather assumed (maybe wrongly) that we were being asked how far the car could travel at constant speed, things are rather different. Although that will be to some extent mass-dependent, I would think that would be far less the case than during acceleration, which means that mass would affect 'the answer'.

Indeed, if we were considering a totally idealised situation, with 100% efficiencies and no losses (and if the journey finished at the same altitude as it started), then vehicle would use no net energy at all.

However, returning to topic, the comparison of interest is obviously "how far a vehicle can travel" (with whatever mass and whatever pattern of driving) with and without regenerative braking - and the qualitative answer to that is a no-brainer.

Kind Regards, John

 

[JohnW2]

Think of your car. In an emergency stop does it have greater acceleration than it does when moving off from rest?

Obviously a greater rate of change of speed. However, if all other things were equal (which they obviously aren't), the same amount of energy would be involved in accelerating a car to a certain speed (no matter how gradually) as in doing an emergency stop from that same speed, wouldn't it?

Kind Regards, John

 

[JohnW2]

To the two Johns - if mass is relevant then we should be making EVs as heavy as we possibly can, in order to have more kinetic energy to recover.

As I said a while back, you are really addressing a very different question (albeit one of much more practical importance) than was being discussed in this thread - i.e. the 'overall situation'.

I was addressing just one isolated issue - one episode of braking followed by a period of constant-speed driving, using the energy recovered during that braking.

Kind Regards, John

 

[SimonH2]

To the two Johns - if mass is relevant then we should be making EVs as heavy as we possibly can, in order to have more kinetic energy to recover.

No, because that would be assuming 100% efficiency. Since we know it's not 100% efficient round trip - chemical->kinetic->chemical - then if you were to (say) double the mass of the vehicle, then you'd double the losses. So for this doubling of vehicle mass, you'd need to double the amount of stored energy for the round trip losses to be the same as a proportion of the original stored energy.

Or put another way, if you can halve the mass of the vehicle, then you halve the amount of stored energy required for the given number of accelerate/decelerate cycles in a given journey (as well as keeping the same performance etc).

That's assuming all other factors remained the same - which they wouldn't, but it's OK as a first approximation.

 

[John D v2.0]

Interesting replies, but I think the main points for me are that for a conventional car, mass plays a large role in its consumption of energy. For a regenerative electrical car, the more efficient the car, the less effect the mass has. For a real car, some of the regenerated energy would be used to maintain constant speed and some for acceleration (and some for the heater and lights!)
But the ultimate thing is John w2 point earlier that may have been missed. Basically the extra range is only as relevant as the likelihood that they'd base their decision to drive on the charge in the battery (as opposed to charging the car and making the journey anyway)

 

[JohnW2]

Interesting replies, but I think the main points for me are that for a conventional car, mass plays a large role in its consumption of energy.

Indeed so. Energy used for (positive) acceleration is obviously very mass-dependent, and energy used during constant-speed driving is, as you've said, to some extent mass-dependent, whilst energy produced during deceleration (braking) is all lost as heat, and a little sound. The same would, of course, be true of a 'non-regenerative' EV, although they probably don't exist.

For a regenerative electrical car, the more efficient the car, the less effect the mass has.

Yes, in terms of the 'overall picture', I think that's probably true.

Kind Regards, John

 

[ban-all-sheds]

Obviously a greater rate of change of speed.

There you are, then. You would need to be able to recharge the batteries at a faster rate than they are discharged.

However, if all other things were equal (which they obviously aren't), the same amount of energy would be involved in accelerating a car to a certain speed (no matter how gradually) as in doing an emergency stop from that same speed, wouldn't it?

Things don't have to be equal. If you want a braking system which is entirely regenerative then you have to be able to recharge at a faster rate than you discharge, or you will not be able to slow down at a faster rate than you can speed up.

 

[John D v2.0]

Capacitors can charge pretty fast.

 

[JohnW2]

Capacitors can charge pretty fast.

They can ... 'infinitely' fast if you want, but that requires an 'infinite' current. We've been through this earlier in the thread. The person who suggested charging a giant capacitor 'in seconds' would have required as astronomical current to achieve that.

Kind Regards, John

 

[JohnW2]

If you want a braking system which is entirely regenerative then you have to be able to recharge at a faster rate than you discharge, or you will not be able to slow down at a faster rate than you can speed up.

I don't really understand that.

Kind Regards, John

 

[John D v2.0]

I think bas is suggesting some energy would be lost due to not being able to store it in the battery quickly enough:

There you are, then. You would need to be able to recharge the batteries at a faster rate than they are discharged.

Capacitors can charge pretty fast.

They can ... 'infinitely' fast if you want, but that requires an 'infinite' current.

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.