Engine braking without an engine

[Diehard]

I am wondering about regenerative break when you are towing a trailer down the mountain. Since Semi is ready to go, I am sure Tesla has figured this out but it is all still a big question mark to me. A V8 has a better engine breaking than V6 in an ICE. Will the Regen be different in CT3/CT2/CT1 because of different towing capacity? How do they compare in practice during max towing down the hill with ICE V8 and V6? Any experience with Model X? What are your thoughts?

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[ajdelange]

In Teslas you have two choices for regenerative braking: on and off (the middle option was taken away and never restored AFAIK but to be honest I haven't looked recently because it is pretty clear that the only option you want is full on). Stop thinking of the skinny pedal as an accelerator and start thinking of it as a torque control. By adjusting its position you can get maximum negative torqe (slow down), maximum positive torque (speed up) and anything in between including 0 torque (coast). If you are towing a trailer up a hill you can push the pedal to the metal and the motors will deliver the maximum amount of torque they can but there is a limit based on how much current the battery/inverter can supply. If you are going down hill with a trailer behind you you can let the pedal all the way back and the truck will supply the maximum slowing torque it can limited by the amount of current the battery can absorb (which is lessened or even 0 if the battery is nearly fully charged or cold).

If the motors can't supply the torque you want going up hill there isn't much you can do about it. There is no additional source of torque. You will slow down. Conversely going down hill there are, and a good thing it is too, additional sources of negative torque being the friction brakes in the truck itself and those in the trailer.

Will the different CT have different regen braking capability? Yes, because they have different size batteries. The max regen limit seems to be something like 80 - 90% of the battery capacity and the three models have different size batteries. Thus the TM with its battery about twice the size of the AWD's, will be able to decelerate at about twice the rate (2 times the number of g's).

In most of the discussions about range reduction whilst towing we tend to focus on the drag load of the trailer and assume the intertial load will be recovered by regen. That is true on level terrain. But we need to keep in mind that potential energy expended going up a hill may not be recovered if the requisite braking produces more than the battery can absorb. You will be able to monitor the situation by looking at your power meter. Musk has promised extra software goodies to help manage towing.

 

[Crissa]

Regen amounts are arbitrary to the kW capacity of the motors and the power absorption of the battery and controller systems.

I have no idea how much regen Tesla will allow, but what's available is the inverse of the power available to pushing the vehicle along. Which is vastly more than any ICE engine can absorb without coming to pieces.

-Crissa

 

[VolklKatana]

In Teslas you have two choices for regenerative braking: on and off (the middle option was taken away and never restored AFAIK but to be honest I haven't looked recently because it is pretty clear that the only option you want is full on). Stop thinking of the skinny pedal as an accelerator and start thinking of it as a torque control. By adjusting its position you can get maximum negative torqe (slow down), maximum positive torque (speed up) and anything in between including 0 torque (coast). If you are towing a trailer up a hill you can push the pedal to the metal and the motors will deliver the maximum amount of torque they can but there is a limit based on how much current the battery/inverter can supply. If you are going down hill with a trailer behind you you can let the pedal all the way back and the truck will supply the maximum slowing torque it can limited by the amount of current the battery can absorb (which is lessened or even 0 if the battery is nearly fully charged or cold).

If the motors can't supply the torque you want going up hill there isn't much you can do about it. There is no additional source of torque. You will slow down. Conversely going down hill there are, and a good thing it is too, additional sources of negative torque being the friction brakes in the truck itself and those in the trailer.

Will the different CT have different regen braking capability? Yes, because they have different size batteries. The max regen limit seems to be something like 80 - 90% of the battery capacity and the three models have different size batteries. Thus the TM with its battery about twice the size of the AWD's, will be able to decelerate at about twice the rate (2 times the number of g's).

In most of the discussions about range reduction whilst towing we tend to focus on the drag load of the trailer and assume the intertial load will be recovered by regen. That is true on level terrain. But we need to keep in mind that potential energy expended going up a hill may not be recovered if the requisite braking produces more than the battery can absorb. You will be able to monitor the situation by looking at your power meter. Musk has promised extra software goodies to help manage towing.

Great explanation @ajdelange , one thing to add is that in my Tesla today, even when not towing, while going downhill or completely releasing the accelerator pedal, many times the max charge you can put back into the battery is reached immediately and then tapers off as you slow.

I suspect with the CT, and especially while towing, that there will be a higher amount being put back into the batteries, but based on the new battery properties and size, it will actually be be more difficult to reach that max input/charge number.

 

[Diehard]

Will the different CT have different regen braking capability? Yes, because they have different size batteries. The max regen limit seems to be something like 80 - 90% of the battery capacity and the three models have different size batteries. Thus the TM with its battery about twice the size of the AWD's, will be able to decelerate at about twice the rate (2 times the number of g's).

Thanks for a great explanation.

It sounds like the limiting factor is the battery and additional motors in CT2 and CT3 does not provide additional breaking/Regen. Is that what you are saying?

p.s. Does CT2 additional range (with respect to CT1) comes from larger battery? or a more efficient motor?

 

[Crissa]

It sounds like the limiting factor is the battery and additional motors in CT2 and CT3 does not provide additional breaking/Regen. Is that what you are saying?

Yes and no.

Yes, the battery is an upper limit. But so to are the motors' ability to convert energy. And controllers to transfer it. And tires not to skip on the road.

Generally batteries allow more out than in, too. And different manufacturers have come up with different ways to deal with the excess power coming from regen. Tesla has mostly opted for the simple rule of reducing regen below the battery limit.

But which particular limit you're hitting at any moment depends upon the slope, speed, load weight, and road surface.

I've gotten my Zero to skid using its mediocre regen (which they artificially limit) on wet pavement and a light load and a decent slope. More motors/wheels would have solved that.

-Crissa

 

[ajdelange]

Great explanation @ajdelange ,

Thanks. It came out pretty tight and succinct because I've recently been trying to explain this over on the Rivian forum as we've just found out, in part, how Rivian is going to manage regen (three "levels" and off). Over there the assumption is that Elon Musk is a scoundrel who is forcing folks to accept full on when clearly others ways are better simply because that isn't the way Elon does it.

....one thing to add is that in my Tesla today, even when not towing, while going downhill or completely releasing the accelerator pedal, many times the max charge you can put back into the battery is reached immediately and then tapers off as you slow.

There's nothing wrong with that but it takes a little math to see why and I know people don't like math. The deceleration, a, in g's is

a = k*Pm/(m*g*v)

in which k is between 0 and 1 depending on where you have the pedal, Pm is the maximum power the battery can absorb (or rather the limit the computer has set), m is the mass of the vehicle, g the gravitational constant and v the speed. When the pedal is all the way back k = 1 for the maximum regen and the deceleration depends on the weight, speed and Pm. At high speed you don't get many g's which is not a problem if your situation is such that the number of g's you are getting is enough to slow you as much as you need to slow. If it isn't, you have to hit the fat pedal and we don't want to do that as whenever your foot is on the fat pedal you are wasting energy (miles).

I suspect with the CT, and especially while towing, that there will be a higher amount being put back into the batteries,

Yes, there will as you can see by solving the formula for the power:

P = m*g*v*a

If towing 15,000 pounds the mass of the rig is going to be nearly 4 times the mass of the truck alone and each unit of deceleration will deliver 4 times as much power to the battery.

...but based on the new battery properties and size, it will actually be be more difficult to reach that max input/charge number.

Not so sure about that because the first form of the formula shows that the deceleration with 4 times the mass is only going to be 1/4 that which you get with no trailer. As I noed in the earlier post Tesla seems to run about 0.8C for regen. If the new batteries are tougher and they device to go 1.2C or 1.5C that still isn't going to offset a factor of 4 in the mass.

 

[Crissa]

...Well, that math explains why adding my spouse pillion means we get back so much more regen.

On my Zero, I can set the throttle regen to a max level that's arbitrary (from 1-100% of what they allow) and an additional amount of regen on the brake that triggers with the brake light. So at any time I have three levels of brake and throttle regen available through the mode button. It's kinda like downshifting.

-Crissa

 

[ajdelange]

Thanks for a great explanation.

Thanks for the thanks.

It sounds like the limiting factor is the battery and additional motors in CT2 and CT3 does not provide additional breaking/Regen. Is that what you are saying?

The batteries and motors work together on this. The TM accelerates faster than the AWD which accelerates faster than the RWD because there are more motors to supply the torque and more battery to supply those additional motors. Deceleration is symmetric. There are more motors to absorb torque and more battery to absorb the electricity generated by the motors. Pm is the formula is bigger.

p.s. Does CT2 additional range (with respect to CT1) comes from larger battery? or a more efficient motor?

Mostly the former but a little from the motors too based on the argument that to get the same amount of power from two motors each only has to supply half which means drawing half the current which means 1/4 the resistive losses in each but as there are two the total resistive loss is half. There are other losses associated with a second motor (gears, bearings etc) and the resistive losses are pretty small in these motors so I think it's mostly the battery.

 

[Royal Fish]

Boat trailers with surge brakes will be a little more challenging If you want to regen.

 

[ajdelange]

I haven't pulled a boat trailer with s Tesla yet but I really don't think it will be any different than what you experience now. If the surge brakes grab they just take load off the car/truck brake in ICE. So with a BEV will they simply take load off the regen system.

 

[Royal Fish]

I haven't pulled a boat trailer with s Tesla yet but I really don't think it will be any different than what you experience now. If the surge brakes grab they just take load off the car/truck brake in ICE. So with a BEV will they simply take load off the regen system.

Agreed but I would rather Charge batteries then scrub off speed through the trailer brakes.

 

[ajdelange]

The after market inertial controllers would let you balange between the trailer brakes and regen but I think that's going to be built into the CT. Even so it may ooffer gain adjustment. Which is, of course, moot with surge brakes. They are adjustable too, sort of, but I don't think I'd want to reduce their effect. Watch your power meter to see how much you are losing and act acordingly if you think you really want to fiddle with this.

 

[Royal Fish]

The after market inertial controllers would let you balange between the trailer brakes and regen but I think that's going to be built into the CT. Even so it may ooffer gain adjustment. Which is, of course, moot with surge brakes. They are adjustable too, sort of, but I don't think I'd want to reduce their effect. Watch your power meter to see how much you are losing and act acordingly if you think you really want to fiddle with this.

When backing up, the reverse light power turns on a solenoid that blocks brake fluid from getting to the brakes. Energizing this circuit should be easy for them to integrate into the CT

 

[John K]

CT does not need to tow a boat, CT is the boat.

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