ajdelange

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In the situation you describe you'd also want the anti-crabbing feature enabled.
 

jimrichard

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Yeah he sure did sidestep the Tankturn answer. Lots of legal stuff will have to be considered as if this feature is really available it will undoubtably be used and abused in unbelievable youtube ready ways. Right now doing a donut in a city parking lot will get you a careless driving ticket so I am sure the same would be true for a tankturn. There would have to be a button for it too as you wouldn't want it to just come on unexpectantly by just turning the steering wheel too tight. However Geofencing is so big brother creepy that it could completely kill its attraction. I can see modified versions of these vehicles being used in action scifi movies almost immediately though. Machine guns should mount on the bed racks pretty easily I would think.
 

Babbuino

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I'm wondering if the range is with all 4 motors engaged and if there would be a way to disengage them when on the highway to save juice
 

electruck

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I'm wondering if the range is with all 4 motors engaged and if there would be a way to disengage them when on the highway to save juice
I suspect this is a question Rivian has asked and answered. If they found efficiency, they would likely do it automatically without driver intervention so I wouldn't expect manual control over that either way.
 

ajdelange

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I'm wondering if the range is with all 4 motors engaged and if there would be a way to disengage them when on the highway to save juice
How would disengaging some save any energy? If you cut off 2 then for the same load the other two would have to draw twice the current resulting in 4 times the I^2R losses. Besides that I would guess that getting approximately equal amounts of thrust from fore and aft would result in more even tire wear, add to better lateral stability etc.
 

thrill

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How would disengaging some save any energy? If you cut off 2 then for the same load the other two would have to draw twice the current resulting in 4 times the I^2R losses. Besides that I would guess that getting approximately equal amounts of thrust from fore and aft would result in more even tire wear, add to better lateral stability etc.
While four motors can allow torque vectoring, which is certainly handy in dynamic situations, in normal driving there may be a power reduction available depending on design. If the gear reductions are different front and back then there are optimizations available. https://www.hindawi.com/journals/mpe/2015/814307/
 

DucRider

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If the gear reductions are different front and back then there are optimizations available.
Tesla does (did?) this in the S and X, which is why the AWD drive variants got better range than the RWD versions (before they dropped the RWD variants from the lineup).

Audi also claims the increased range for the 2021 e-tron's is partly due to disengaging the front motor under some conditions (freeway?).
 

EVJay

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Tesla does (did?) this in the S and X, which is why the AWD drive variants got better range than the RWD versions (before they dropped the RWD variants from the lineup).

Audi also claims the increased range for the 2021 e-tron's is partly due to disengaging the front motor under some conditions (freeway?).
Yes, my Tesla Model 3 AWD has dual motors - it switches from front to rear as needed to maintain max efficiency and is not driver selectable. Of course if you want to throw efficiency out the window, then put the hammer down and hold on! I would suspect the Rivian has factored in this same concept to range calculation and efficiency, albeit 4 motors vs 2. I would think this would be managed autonomously and not by the driver manually.
 

skyote

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I'm wondering if the range is with all 4 motors engaged and if there would be a way to disengage them when on the highway to save juice
Yes, I was told that the rear motors will disengage on the highway. I believe this is automatic & don't know if there will be a manual selection.
 

ajdelange

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The X has a PM motor up front and an IM in the rear. PM motors are inherently more efficient than IM motors but IM motors produce more torque at low speed so there is an obvious strategy for cruise range improvement in the X. What puzzles me about this is that the S is the other way round. The PM motor is in the rear.

So clearly there can be cases where drawing more from one motor as opposed to the other can be advantageous. In my earlier comment I made the assumption that all 4 motors in the Rivian were the same. I really had no right to do that. I assume they are all PM motors but if they have PMs in one place and IMs in the other then it would be the same as with the X.

In any case if there is assymetry such that efficiency can be improved by taking more torque from one motor than another in a particular part of the envelope you can be sure that Rivian is well aware of this and will incorporate it into their control system just as Tesla has done. As conditions can change in milliseconds this is not something the driver would be given control over other than the ability to turn on or off - something like Tesla's "Range Mode" which optimizes range via torque distribution.
 

skyote

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The X has a PM motor up front and an IM in the rear. PM motors are inherently more efficient than IM motors but IM motors produce more torque at low speed so there is an obvious strategy for cruise range improvement in the X. What puzzles me about this is that the S is the other way round. The PM motor is in the rear.

So clearly there can be cases where drawing more from one motor as opposed to the other can be advantageous. In my earlier comment I made the assumption that all 4 motors in the Rivian were the same. I really had no right to do that. I assume they are all PM motors but if they have PMs in one place and IMs in the other then it would be the same as with the X.

In any case if there is assymetry such that efficiency can be improved by taking more torque from one motor than another in a particular part of the envelope you can be sure that Rivian is well aware of this and will incorporate it into their control system just as Tesla has done. As conditions can change in milliseconds this is not something the driver would be given control over other than the ability to turn on or off - something like Tesla's "Range Mode" which optimizes range via torque distribution.
I believe all 4 motors are the same, and the difference might be one of mechanical drivetrain losses, not strictly electrical efficiency.
 

ajdelange

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The differences might be a whole host of things depending on design. Supposing all the motors to be the same we note that while the regions of high efficiency are broad for these motors nevertheless there is a torque and speed region where the efficiency is maximized. If otherwise identical motors are geared differently the speeds at which maximum efficiency is realized will be different. One might achieve some improvement in overall efficiency by taking most of the power from the motor which is closest to its sweet spot (as was suggested in #21). IOW it is, overall, rather complicated. But these cars have computers analyzing the situation and developing the proper torque commands thousands of times a second.
 

aAlpine

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In my earlier comment I made the assumption that all 4 motors in the Rivian were the same. I really had no right to do that. I assume they are all PM motors but if they have PMs in one place and IMs in the other then it would be the same as with the X.
I'm not sure if they're all exactly the same (size, power, gear ratio, etc.), but I was told at the Denver event that they are using PM motors.
 

CappyJax

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How would disengaging some save any energy? If you cut off 2 then for the same load the other two would have to draw twice the current resulting in 4 times the I^2R losses. Besides that I would guess that getting approximately equal amounts of thrust from fore and aft would result in more even tire wear, add to better lateral stability etc.
Because the efficiency of motors mostly increases as power increases. So, using this chart and referencing the red line, if you have 4 motors each putting out 200W, you have around 79% efficiency. If you turn two off and you have the other two motors each putting out 400W, then you have 85% efficiency.


unnamed.jpg
 

thrill

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The significantly higher efficiency and flatter curve of the synchronous reluctance motor per the chart above surprised me. There's a good article about motor differences at https://www.electricmotorengineerin...re-earth-free-solution-for-electric-vehicles/ and it appears only the Renault Zoe is using this setup. Rivian is not listed, and the article seems undated, as the date at the top is always today's date. However, if you view source you'll see it was published May 20, 2020.
 
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