JBee

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Don't expect friction brakes to go away anytime soon.
Its a fun exercise to explore the possibility though :)

You do have a point with the TM only having one front motor, that likely won't absorb full adhesion regen, but it wouldn't be possible to have a single motor front braking anyway, you need a motor per front wheel to modulate ABS otherwise you'd be going off-road faster than you think :)

But if we're on the subject of wishful thinking, another thing I'd like to see is the brake rotors on the motor side of the axle shaft. Especially on a off-road vehicle, that will keep it out of the dirt and dust a bit more, plus protect lines from brush, rock scrapes and mud, and lowers the unsuspended mass of the wheel. The thing they need to do though is make the motor hollow and put the axle shaft through it to the middle to make the axle/suspension arms as long as possible to increase wheel articulation as well.

Then chuck a planetary gear in the hub as final ratio (adds some weight again) but gives you portal axles like a Unimog. Huge clearance right up to the wheel. Then add inboard suspension as well and a central tyre inflation system (easier on a portal hub) and four wheel steering with crab and counter steer....then we'd be rocking! :-D

(P.S. Answer to your previous question customized 200 Series Land Cruiser and a "Bushranger" Range Rover)
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ajdelange

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So overall with the ongoing trend to even more EV performance there will relatively soon be a case for deleting the brakes, or at least relegating them to a emergency backup system only. I expect the CT to have much more aggressive regen. Also helps to achieve range with a load.
What do we do when the battery is fully charged? Or when the pack is cold so the regen is limited? As energy that goes to a dump bank is lost it isn't a very appealing approach to braking.

I'm sure the regulators are going to insist on hydraulic friction brakes of sufficient strength to slow the vehicle by the required amount in the event of complete regen failure and I'm sure that they are going to continue to do this for a long time. But it is fun, and instructive, to work the numbers.
 

JBee

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What do we do when the battery is fully charged? Or when the pack is cold so the regen is limited? As energy that goes to a dump bank is lost it isn't a very appealing approach to braking.

I'm sure the regulators are going to insist on hydraulic friction brakes of sufficient strength to slow the vehicle by the required amount in the event of complete regen failure and I'm sure that they are going to continue to do this for a long time. But it is fun, and instructive, to work the numbers.
Yes it is fun!

Well technically the current EV friction brakes also converts the momentum into heat :p
At least with a load dump you could make it useful to heat up the pack so you can regen more next time? And nobody should be fully charging their battery if they're worried about cycle life. :cool:

Regulators are always sand in the gears of progress.

But there is precedent for electric breaking in trucks and busses in the form of induction retarders. The best thing about them is that they are without wearing parts, and could possibly be integrated into the motor design, and you can operate them with minimal external controller or wiring, and completely without batteries if you need to. Essentially a EMF heater that slows spinning things down all thanks to our buddy eddy.:geek:

Thelma makes them in EU:

https://www.telma.com/produits/fonctionnement

uck.ca%2Fwp-content%2Fuploads%2F2013%2F07%2Ftelma1.jpg


BTW how many g's of deceleration do we get with 660kW and 1300kW with of battery charging?
 
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ajdelange

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That depends on the mass and speed. a = P/p in which P is the power absorbed and p = m*v is the momentum. a is in m/s/s so divide by 9.8 to get g's.
 

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That's fine for light or moderate braking but to stop quickly, near the limits of tire adhesion, requires stronger magnetic fields than a drive motor can produce organically. The coils would need to be energized and the power would necessarily come from the battery. The ability to brake at the limit of adhesion is required for safety in the event of the unexpected, a deer or elk jumping onto the highway, an errant human, etc, so brakes are not going away for this reason.
If you can smoke the tires when accelerating, you can smoke the tires when decelerating.

With an EV, regenerative braking speed from 60mph should be pretty similar to the 0-60 time. Electric motors are symmetrical that way.

The catch is that braking torque will be reduced at high speeds, just like accelerating torque is reduced at high speeds.


This is why EVs (and hybrids) require a traction control system in order to be drivable on the street. All of that low end torque is available in and instant, and it needs to be derated to prevent burnouts. The folks in the video above bypassed the traction control system.

If you have a 0-60 time of 2 seconds, that implies a 60-0 time of 2 seconds. That's pretty good.

There are lots of great engineering reasons to keep friction brakes on EVs (torque at the high end of the performance envelope, redundant safety system, uses less power when stopped, etc). I am not in favor of removing friction brakes from EVs any more than I'm in favor of removing the redundant ignition system used on airplane engines. However, I disagree with your reasoning about why they should remain.
 
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Luke42

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What do we do when the battery is fully charged?
You need a dump load that turns the extra electrical power into heat.

On batteryless diesel-electric locomotives, they use the excess power to warm a pretty massive electric heating element on the locomotive, and they use a fan to evacuate the hot air. The railway folks call this "dynamic braking":


An EV without mechanical brakes would likely need a dump-load like this when driving in The Mountain West (USA) to prevent runaway vehicles on the multi-mile sections of steep downhill highway they have out there. Friction brakes can easily overheat on those sections, too, though -- anything that isn't captured by the regen system is going to end up as heat one way or another.

That said, I'd prefer to keep the friction brakes as a redundant safety system. My existing vehicle has multiple redundant braking systems which can all stop the vehicle (regen, 4-wheel hydraulic brakes, mechanical parking brake, and sometimes electronically controlled trailer brakes), and that's how I likes it. I've had to use the backup braking systems on vehicles on multiple occasions when I was younger and poorer, and so I value them quite a bit.
 
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ajdelange

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What do we do when the battery is fully charged? Or when the pack is cold so the regen is limited? As energy that goes to a dump bank is lost it isn't a very appealing approach to braking.
You need a dump load that turns the extra electrical power into heat.
You don't if you have friction brakes. Of course they turn kinetic energy into heat too. I think the main point is that the friction brakes are going to be required anyway so why complicate the vehicle further?

I also thing there are problems with getting sufficient torque at low speed that don't relate to the maximum charging rate of the battery. At low speed a = P/p is large even with small P. I think the problem is getting the motor to generate enough current to provide the torque. Back EMF is certainly low at low rpm and without EMF you can't push current. I don't understand the reluctance action of these motors well enough to know what I am talking about but I do know that if I don' slow down enough early on even backing off the pedal completely does not decellerate the vehicle fast enough and friction braking is necessary even though the power meter shows only a little going to the battery.
 

JBee

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Still, as above, just add an induction retarder to the motor setup. Actually works better at higher rpm too. But you will need quad motors (1 per wheel) to make ABS etc work.

Another thing I'm interested in is the Roadster SpaceX package for braking. Why be limited to tyre adhesion. :unsure:
 

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HaulingAss

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If you can smoke the tires when accelerating, you can smoke the tires when decelerating.

With an EV, regenerative braking speed from 60mph should be pretty similar to the 0-60 time. Electric motors are symmetrical that way.

The catch is that braking torque will be reduced at high speeds, just like accelerating torque is reduced at high speeds.


This is why EVs (and hybrids) require a traction control system in order to be drivable on the street. All of that low end torque is available in and instant, and it needs to be derated to prevent burnouts. The folks in the video above bypassed the traction control system.

If you have a 0-60 time of 2 seconds, that implies a 60-0 time of 2 seconds. That's pretty good.

There are lots of great engineering reasons to keep friction brakes on EVs (torque at the high end of the performance envelope, redundant safety system, uses less power when stopped, etc). I am not in favor of removing friction brakes from EVs any more than I'm in favor of removing the redundant ignition system used on airplane engines. However, I disagree with your reasoning about why they should remain.
It's not a matter of why they SHOULD remain, they WILL remain until technology exists that is cheaper, lighter, better than friction brakes. And, realistically speaking, that day is beyond all of our lifetimes.

I love regenerative braking for it's efficiency and lack of wear benefits. The friction brakes are really only there for emergencies and that will only become more true over the next decade. So there is not a great incentive to get rid of them unless there was something better. And you are over-estimating the benefits, a favorable cost/benefit ratio will not exist for many decades. Friction brakes will get smaller and lighter as they will need less thermal capacity but they are not going away.

I look forward to regen with higher capacity that can be diverted to warm the battery and gearbox more quickly and assist with cabin heating in cold weather. No doubt, we are still in the early days of EV's and they will continue to be tweaked and improved with integrated systems lowering the cost and increasing the functionality. But it's going to take a long time (if ever) to make the friction brakes as an emergency backup go away completely.

I've used drift mode in my Model 3. It's weak medicine compared to 4 disc brakes. Yes, big clouds of tire smoke, wheels spinning. But only at slow speeds and only if all of the battery capacity is diverted to two wheels. Braking effectively requires individual control of all four wheels. Also, the batteries used in all EV's can output more current than they can accept (without damage). Yes, you can dump the power elsewhere but that is an additional expense too. And you still need friction brakes for higher speeds and for 4 mph to 0 mph.
 
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HaulingAss

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https://insideevs.com/reviews/397503/porsche-taycan-regenerative-braking-deep-dive/

You can use resistance and capacitance to dump energy or store it for short periods.

You have already made this incorrect statement in this thread.

-Crissa
Any energy dumped cannot exceed the capacity of the power transistors. I already get reduced regen when descending long-grades, even with a battery that is not too hot or too cold or too full. The power transistors go rapidly up in price as their capacity increases. They are one of the more significant components of the cost of the powertrain.

People under-estimate the amount of power required to bring all four tires to the limit of adhesion at 80 mph, let alone 100 mph. This is the beauty of friction brakes. They can absorb huge amounts of energy for brief periods of time and are relatively inexpensive considering the amount of power they can absorb for short periods. It would cost a LOT to do that with electronics and, even then, unacceptable deficiencies would remain.
 

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...Which is already more than enough to exceed the grip of the tires.

-Crissa
Not from any practical perspective. The power transistors are specified to be a high enough power rating to not fail under hard use, no more because the price goes up rapidly with size. They are liquid cooled and monitored with thermocouples. If they get too hot the controller reduces power through them, the last thing you want if you need to stop suddenly.

I have a Performance Model 3 which is not traction limited on clean, dry pavement, even with all-season radials. Performance tires have even more ultimate grip so they would be even further from being traction limited. Even with the all-season radials, with Track Mode engaged and all the battery power directed to the rear wheels they are only traction limited to around 30 mph and it is necessary to throw the car into a tight turn to get them to break loose at any speed above around 10 mph.

In normal use, braking is required at all speeds, from the top speed of the car all the way to zero. Why do you think the power transistors can handle significantly more power than they were designed for? They have thermal limits for reliability/cost reasons. Exceed those and they start failing.

It's simply not practical to expect components designed to accelerate the car to also brake the car which would not only greatly increase their duty-cycle but would impose higher loads than peak acceleration would.
 

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If you can break the tire fre in acceleration, you can skid when decelerating.

Your friction brakes can heat up and glaze, too, Hardly an excuse not to have them.

-Crissa
 
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