Dual motor efficiency

[Alketi]

Would there be any value in adding a steering wheel paddle that acts as a brake, but really modulates the regen? You could just about remove the disc brakes from the maintenance list, if done correctly.

Also remembering that Musk mentioned how the production steering wheel would be far more advanced than the prototype.

 

[JeffK]

Regen can only ever be on the same axle as the motor. I could be wrong but I believe the negative torque is monitored and controlled without front brake engagement. (Unless you actually hit the brake)

 

[David99]

Can the Tesla gurus to enlighten me as to the % of Regen Tesla achieves?
How can regen be anywhere near 100% w/o use of Regen on Front Wheels [AWD]?

There are different losses. The motor and inverter can be seen as one unit as they only work together. They are about 90% efficient, but driving and at regen. The battery also has about 90% efficiency. So very roughly when you regen, you get about 80% of the energy back as usable in the battery. Now of course the car still has rolling resistance and wind drag and many other losses. All of these apply when driving as much as when regen.

What wheel is connected to the motor when doing regen is not important. If all you do is use the motor in regen mode to slow down the car then all of the kinetic energy that is available gets captured back (minus the losses). Having RWD doesn't mean you loose a certain percentage of possible regen at the front wheels. They don't do anything, they are not slowing the car down.
I think some people are confused when they read about the front/rear brakes contributing 70/30% when slowing down. That's how the brakes are designed, but has nothing to do with how regen works.

 

[jerry33]

100% of what? Regardless of anything else, max regen power will always be a fraction of max acceleration power, because batteries cannot absorb power nearly as quickly as they can supply it.

Regen can only capture some part of the kinetic energy at the time the regen braking is started. (Obviously, that's far less than the amount of energy used to accelerate the car to that point as much of the power has been used to overcome air resistance and drag from the drivetrain.) V0------>V1------>R1------>R0-->V0 The car starts at V0 (stopped) and accelerates to V1 (cruising speed) it travels some distance and then needs to stop. Regeneration starts at R1 and continues to R0 where regeneration cuts out. The power used between V0 and R1 is immaterial as only the kinetic energy at R1 is available as potential regeneration.

 

[JeffK]

What wheel is connected to the motor when doing regen is not important.

Physics disagrees with that statement. Weight pushes up front during deceleration which leads to a higher normal force and more torque on the front axle, if spinning a generator, more torque = better. You'll have higher energy capture from the front vs having it only in the back.

 

[David99]

Physics disagrees with that statement. Weight pushes up front during deceleration which leads to a higher normal force and more torque on the front axle, if spinning a generator, more torque = better. You'll have higher energy capture from the front vs having it only in the back.

There is zero torque on the front wheels if you only use the back wheels to slow down or accelerate.

What you are confusing it with is when you brake all four wheels, the weight shift causes the front wheels to have more traction thus you can apply more force on the front wheel brakes. That's what the front/back brake ratio is allowing more force on the front brakes. But again, that has nothing to do with applying negative torque to only the back wheels. By your logic the front wheels would slowing down the car on it's own. Physics disagrees with that

 

[JeffK]

So you're saying if you have two equal motors one is on a FWD car and another is on a RWD car, that the FWD wouldn't capture more energy???

 

[Lunarx]

100% of what is capable.

I can get -60kW of regen (peak) on the Volt.
(Typical sustained is around -25kW +/- 10kW)
Seems like if I hit the brakes hard enough, to exceed -60kW, it goes to full friction.

That's 3.3C of Regen (at peak).
For a Tesla w/ a 60kW plus battery 3.3C would be 200kW regen.
That is double my full acceleration.

So what are you guys seeing on -kW under Regen?

 

[gregincal]

Regen can only capture some part of the kinetic energy at the time the regen braking is started. (Obviously, that's far less than the amount of energy used to accelerate the car to that point as much of the power has been used to overcome air resistance and drag from the drivetrain.) V0------>V1------>R1------>R0-->V0 The car starts at V0 (stopped) and accelerates to V1 (cruising speed) it travels some distance and then needs to stop. Regeneration starts at R1 and continues to R0 where regeneration cuts out. The power used between V0 and R1 is immaterial as only the kinetic energy at R1 is available as potential regeneration.

I'm not sure what the amount of energy has to do with it, since you can accelerate really gently but try and come to a really sudden stop. My point is that the Model S battery system can provide up to 500kW of power during acceleration, but if you tried to design a regen system that generated 500kW of braking power you would fry the battery. The Tesla regen has a max of 60kW of power. Although the supercharger can feed more power into the system at lower states of charge it's undesirable to have a regen system that always provides differing amounts of available braking power. At 60kW they only have to limit regen braking if you are at the upper end of a range charge.

In other words the battery is limiting factor for regen braking, not the physics of braking.

 

[stevezzzz]

Back in the early S days, it was explained that regen was programmed to provide a constant 0.1g deceleration, with a limit of 60kW. At roughly 45mph and slower, you get the full 0.1g of deceleration as soon as you lift off the go pedal. At higher speeds, the 60kW limit kicks in and you won't have the full 0.1g of deceleration until the car slows down (to roughly 45 mph). Things are different now, with the Model S dual motor variants and the Model X; I haven't really studied what's going on, but to my uncalibrated tush the X has significantly stronger regen compared to even the dual motor Model S variants.

 

[David99]

So you're saying if you have two equal motors one is on a FWD car and another is on a RWD car, that the FWD wouldn't capture more energy???

Correct. It makes no difference. Now, when pushing things to the extreme, the front wheels have more traction and you could apply more negative torque to them before the start to lose grip. But when talking about regen on an EV we are not anywhere near that level.

 

[JeffK]

sigh

 

[Craig9080]

I think there is some confusion about dual motors being more efficient than single. The single motor Model S has the large motor and drive unit that was originally developed. Later, Tesla designed the smaller and slightly more efficient front motor and inverter unit. It's not different gearing. It is just that the newer, smaller motor is a little more efficient.
Dual motor Model S have the small motor both in the front and in the back. The PXXD versions have the old large motor in the back and the smaller newer in the front. The PXXD can switch off the back motor when cruising and running just on the smaller motor. They call that torque sleep.
It's not gearing that makes it different, it is not running two motors instead of one. It's just the more efficient smaller motor that makes the difference. In general, the type of electric motor Tesla is using runs more efficient at higher load than partial load. Having two smaller motors has the advantage that you can accelerate fast and efficient using both motors at high load, and when cruising, you switch one motor off so the load on the other is higher making it more efficient.

Engineers doing the ride-alongs at the reveal referenced differing efficiency bands for the motors on the M3. Your references are dated and incorrect.

 

[Craig9080]

But...that first statement is not true. An EV with RWD-only will regen with the back axle until the desired braking effectiveness is insufficient and you need to use the friction brakes. Regen maxes out well below the deceleration possible with only the rear tires, under most conditions. Besides the Model S, the i3, smart ED and i-MiEV have all used RWD regen. For any given deceleration rate and under most conditions, a RWD will regen with the same efficiency as a FWD and AWD.

It is definitely true that dual axle regen will be more stable and consistent. I've read that i3's will reduce regen in wet conditions to keep the rear tires from skidding as part of stability control.

The problem is that you still have weight transfer - hence the 70% is inescapable...because physics. Go ahead and disconnect the front brakes on someones car and see how long it takes them to put it into a ditch. Without the aid of any friction brakes the effect on vehicle dynamics is essential the same as a metered application of the E-brake. I do not disagree that you CAN brake with rear tires only but simply that it is a horrid solution for a performance car and greatly upsets the vehicle dynamics. Furthermore, after weight transfer during either an abrupt braking event (cut-off/short light) or a prolonged one (coming up on traffic on the highway) the application of rear brakes becomes increasingly dangerous.

 

[gregincal]

sigh

You are correct that in some theoretical system it of course could capture more energy. The point is that none of that is the limiting factor, it's the ability to store the captured energy in the battery. With that limitation anything about where you capture it is moot. So David99 is correct that in any current or foreseeable EV it will not make any difference.

 

[LectrikPower]

What about capacitors for regen capture? Would there be less loss using them? They could be used the next time you accelerate from low speed. Once they were charged it would go to the battery. Or is the complexity not worth it?

Someone made a racing motorcycle that had regen capacitors that would give an acceleration boost after heavy breaking.

 

[Lunarx]

sigh

Actually, David99 agrees with you "when pushing things to the extreme".

He just doesn't believe EV Regen is powerful enough to necessitate being on the front axle.
If it's true that Tesla regen is only -60kW, the rear wheels could probably handle all of that, w/o too much drama (considering Tesla's long wheelbase and 4300-4900 lb curb weight).

I appreciate all the information being shared here, it was very revealing.
I still suspect Tesla is sandbagging on Regen.
In which case, AWD seems less essential again.
This is the problem with having 2Y to research your car purchase

 

[voip-ninja]

I think teslabj0rn was using a RWD tesla in Norway and didn't complain about it.

To be frank... I'm torn on the awd myself and largely due to price. I want the same upgrades you do, but I'm concerned that if I add all wheel drive, the 35 base turns more into 44, then doc and delivery brings it to 45, and then sales tax can bring it up to 48-49. That's an expensive car! If the full federal tax credit is still there, great, it'll come down a little to 41 or so.

Without AWD, I'm thinking it might be closer to 38-39k.

For me, I won't get this car without a range upgrade or autopilot. The winter package is minor in cost so I'm not concerned about that. I'm hoping the price of awd, autopilot, and range upgrade is significantly less than the premiums they charge on the S.

If it's not, there's a chance I'll cancel my order. The 215 miles of range just won't cut it for me. I need 300+.

The car is unlikely to offer 300 mile range at any price. The battery upgrade for it is likely going to be $7500-$10,000 so it seems it is already over budget for you.

 

[David99]

What about capacitors for regen capture? Would there be less loss using them? They could be used the next time you accelerate from low speed. Once they were charged it would go to the battery. Or is the complexity not worth it?

Someone made a racing motorcycle that had regen capacitors that would give an acceleration boost after heavy breaking.

The battery is not really the problem. It can be charged at a high rate and the charging efficiency is actually pretty good. In normal driving I never found the regen amount to not be enough. Capacitors would not have any advantage in terms of efficiency. Maybe in race situations where they might be able to capture energy at a higher rate. Maybe.

 

[Skione65]

I think there is some confusion about dual motors being more efficient than single. The single motor Model S has the large motor and drive unit that was originally developed. Later, Tesla designed the smaller and slightly more efficient front motor and inverter unit. It's not different gearing. It is just that the newer, smaller motor is a little more efficient.
Dual motor Model S have the small motor both in the front and in the back. The PXXD versions have the old large motor in the back and the smaller newer in the front. The PXXD can switch off the back motor when cruising and running just on the smaller motor. They call that torque sleep.
It's not gearing that makes it different, it is not running two motors instead of one. It's just the more efficient smaller motor that makes the difference. In general, the type of electric motor Tesla is using runs more efficient at higher load than partial load. Having two smaller motors has the advantage that you can accelerate fast and efficient using both motors at high load, and when cruising, you switch one motor off so the load on the other is higher making it more efficient.

@David99,

Is the "Torque Sleep, or switching one motor off so the load on the other is higher making it more efficient" an 'automatic' operation sensed by the car speed or is something you have to manually shut off -and on?

Ski