Flat towing a Rivian

Discussion in 'Rivian General Discussions' started by CappyJax, Mar 4, 2019.

  1. stank65

    stank65 Well-Known Member

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    When you flat tow a vehicle and apply the RV break it turns on the Car brakes as well to assist in slowing both vehicles.

    This is effectively “free” energy which would otherwise be burnt off as heat in the brakes.
     
  2. stank65

    stank65 Well-Known Member

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    This is why I wrote the person does not understand flat towing that wrote it would be a burden. Rengen braking would replace regular breaking while the Rivian is towed (if they do it right.
     
  3. Hmp10

    Hmp10 Well-Known Member

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    Coordinating braking force between friction brakes and regenerative braking is a tricky business and requires sophisticated software programming, even when trying to do it within a single vehicle. But it's necessary in order to create safe and predictable braking behavior in all circumstances. This can be managed with computers operating a brake-by-wire system in an EV so that braking response is seamless and predictable.

    Exactly how would the coordination occur between the friction brakes of the tow vehicle and the regenerative braking of the towed vehicle? Friction brakes and regenerative brakes have very different characteristics, especially in the linearity of their response as speed decreases. Having significant variation in braking forces at different deceleration points between the tow vehicle and the trailing vehicle can create very unstable handling situations.
     
  4. stank65

    stank65 Well-Known Member

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    Way overthinking this. Flat tow breaking is not that complicated. Often flat tow system are set up to simply put braking force to a flat x% based on the receiving a breaking signal through the connection. Others are relative to the breaking force applied to the RV.

    The math for breaking force is not complicated. It has already been figured out because they have to figure it out to translate break pedal press to an equivalent breaking force. The input from the RV to break will be the equivalent of translating a break pedal push.
     
  5. Hmp10

    Hmp10 Well-Known Member

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    It is one thing to have applying the brakes in the tow vehicle send a signal to apply the brakes in the towed vehicle when both vehicles have conventional friction brakes. The issue is what tells the towed EV to engage only its regenerative braking and not its friction braking, as the EV brake-by-wire system is designed to integrate the action of both. Is Rivian going to put programming into the system that allows a person to select regenerative braking only for the purpose of flat towing?
     
  6. stank65

    stank65 Well-Known Member

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    #26 stank65, May 25, 2019
    Last edited: May 25, 2019
    You can’t do that. You can’t regen brake a full battery. You won’t want to set it to regen only because you will want the brake force consistent and more predictable like pressing the break pedal. The only real change to put in for flat towing an EV would be whether to regen while coasting - No gas and no brake. This could be a choice - most diesel RVs give you a selection to exhaust brake which is is in effect during coast and regular breaking - it is supplemental breaking.

    But once the battery is full or full to a certain %, the regen braking will/should turn off as to not damage the battery — just
    Like during regular braking.

    I get that you are trying to maximize the amount of energy recaptured by the regen breaking, but these systems must brake consistently, and the worst thing to do would be to overcomplicate it.
     
  7. Hmp10

    Hmp10 Well-Known Member

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    Then two different things are being discussed here. Cappyjax opened the thread to discuss using flat towing a Rivian both to assist an RV in braking or even to assist its going uphill AND ALSO to recharge the Rivian batteries when going downhill.

    You then pointed out that the Rivian's motors would be decoupled for flat towing and the Rivian would be used only to assist with friction braking. Cappyjax responded with an article saying that Rivian is claiming flat towing could be used to recharge a Rivian.

    I certainly understand how a Rivian's motors could be decoupled for flat towing and its friction brakes used to augment the braking of the tow vehicle as with ICE vehicles. What I don't understand is how a Rivian's regenerative braking could be used to recharge the batteries. Setting the Rivian to regen mode would create drag on the tow vehicle when being towed on flat terrain (and create the issue of charging a near-full battery, as you pointed out), or the regen mode would have to be turned on and off as the vehicle moved between flat and inclined terrain. If the former, then towing a Rivian would increase the burden on the tow vehicle, as the author of that article stated. If the latter, I am asking HOW regen will be turned on and off with terrain changes without a driver in the Rivian.
     
  8. stank65

    stank65 Well-Known Member

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    It sounds like you haven’t driven a car with regen braking before — I apologize if that is a wrong assumption. Regen braking is dynamic. When the car is coasting or the breaks are applied the regen breaking automatically kicks in. In many vehicles you can adjust the strength of the regen breaking. A car being flat towed is connected to the RV in a way that the braking signal is transmitted from the RV to the towed vehicle. This signal is then translated by components installed in the car to apply the breaks.

    The regen system does not have to be aware of the terrain, it only needs to know the braking input from the RV, and potentially the choice from the user to engage the regen braking during coasting.

    This decision to have passive breaking while coasting is a normal decision that RV driver make all the time with Exhaust braking. It is done with a simple switch in the RV. Honestly this is unnecessary but would be cool. If the Regen turned on solely when brakes were applied, that would be fine.
     
  9. Hmp10

    Hmp10 Well-Known Member

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    #29 Hmp10, May 25, 2019
    Last edited: May 25, 2019
    I have owned a Tesla Model S P90D since 2015, and my brother has a 2018 Model 3 which I occasionally drive. Regenerative braking does not happen on its own. A motor doesn't spontaneously turn into a generator simply when load is removed from the motor. The motors switch from propulsion to regeneration based on input signals from the pedals through an electrical relay which changes the direction of current flow. That is why an EV can be programmed for regenerative braking to be triggered either by the brake pedal and/or lifting off the throttle. Some EV's, for instance, do not offer one pedal driving and only engage regenerative braking when the brake pedal is depressed, not when the accelerator is lifted.

    In the early days of experimentation with regenerative braking in cars such as the Baker Electric Runabout and the Owen Magnetic, the driver had to manually flip a switch to put the motor into regeneration mode. In modern EVs the switching is directed by the computer's software. (This is why you can select the level of regeneration you want in your Tesla.) But the motor is still being switched from one mode to the other by an external signal, not by the intrinsic behavior of the motor itself.
     
  10. stank65

    stank65 Well-Known Member

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    #30 stank65, May 25, 2019
    Last edited: May 25, 2019
    I honestly have no clue what you aren’t understanding about how regen and flat tow would work. It is incredibly simple because the problem has already been solved for when and how to break. That combined with decoupling you can cover all flat towing scenarios sufficiently.

    Accelerate, coast, brake. Full battery, charging battery.
     
  11. Hmp10

    Hmp10 Well-Known Member

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    My point is very simple. Regenerative braking will not "automatically kick in" when an EV starts to coast or has its brakes applied. EV motors will not just go into and out of regeneration mode based on whether the EV is being towed, whether it is being braked, whether it is going up or down hill, or what a towing vehicle is doing.

    You can disengage the electric motors for towing and use only the friction braking to assist with braking the tow vehicle, as you could with towing an ICE vehicle.

    However, if you're going to recharge an EV by towing it -- which is the point around which this discussion began -- the motors have to be switched into regeneration mode. Once they are, if the EV is being towed on flat terrain the regeneration drag will increase the burden on the towing vehicle, as stated in the article Cappyjax cited. It was your claim that the author was inaccurate and that a vehicle could be towed for recharge without increasing the burden on the towing vehicle as long as it "was done right" that raised this question.

    Exactly what has to be "done right" to recharge an EV when towing without increasing the burden on the towing vehicle? The only way to have the towed EV not add drag to the towing vehicle is for the EV motors to be taken out of regeneration mode. It is simply impossible for an EV to be charged by towing without adding drag to the tow vehicle. This drag may be desirable when going downhill, which is where the concept of "free energy" comes in, as you are essentially converting the force of gravity into electrical current via the motors-cum-generators. It also may be desirable when braking the tow vehicle, as you're also getting "free energy" from converting the potential energy of the two cars' forward motion into electricity. But when towing on flat terrain or uphill, that added drag of the EV being recharged is overcome not by gravity, but by the increased output of the ICE engine in the tow vehicle. And that energy is not free. If you want to pay that penalty to charge the EV because of the lack of charging infrastructure where you are going, that's fine. But there will be that penalty.

    If you want to use flat towing to recharge an EV and do so without increasing the burden on the tow vehicle, my question remains how do you do that? What sensors, devices, or programming are in the towed vehicle to take the motors out of regeneration mode on flat terrain when there is no driver in the car and you do not want the added drag of flat towing the vehicle in regeneration mode?
     
  12. stank65

    stank65 Well-Known Member

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    The key capability to be able to flat tow an EV is being able to decouple the motors. This was earlier in the thread. Being able to actively couple and decouple the motors also allows for higher efficiencies . Decoupled wheels are allowed to spin freely without the spin loss of the motors. This is why a single motor Model 3 gets greater efficiency than a dual motor -- because of the inability to decouple and avoid spin losses.

    There are 3 different scenarios of driving that need to be accounted for, two different battery situations, and if you want to get tricky you can add a choice choice to mimic exhaust braking

    Battery has room to charge and do not mimic exhaust braking
    Accel - Decouple
    Coast - Decouple
    Brake - Couple and Regen and apply friction brakes

    Battery no room
    Accel - Decouple
    Coast - Decouple
    Brake - Decouple and apply friction brakes

    Battery has room to charge and mimic exhaust braking
    Accel - Decouple
    Coast - Couple and Regen
    Brake - Couple and Regen and apply friction brakes

    You could do this simply with the existing types of connections that they have today for ICE cars and forget the third scenario

    You could also create a new device that connects to the OBD and wirelessly communicates the situation of the RV to the Rivian

    ----------------------------------------------------

    For highway driving without being towed this could greatly improve the efficiency of the Rivian also .

    Once you get over a certain speed and reduced accelerator input, two of the motors could decouple and the wheel couple spin freely.

    The is similar to how some GMC V8 engines will put 4 cylinders to sleep under certain driving conditions.
     
  13. Hmp10

    Hmp10 Well-Known Member

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    #33 Hmp10, May 25, 2019
    Last edited: May 25, 2019
    Thank you. That is the answer I was looking for. Now the question is whether the Rivian will be equipped with a means to decouple the motors based on the occurrence of the above dynamic conditions instead of inputs from a driver. (But, of course, this means you wouldn't get any recharging while towing on flat terrain, as the drive motors are decoupled from the wheels that would turn the rotator coils to generate electricity in the stators that would flow into the battery.)
     
  14. stank65

    stank65 Well-Known Member

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    All of the conditions above are driver inputs. Not dynamic conditions. Push accelerator, push nothing, push brake. This mirrors how exhaust braking works. Push accelerator and exhaust brake disengages even if the RV is not yet accelerating. No input to accelerator or brake pedal and exhaust braking engages. Exhaust brake continues upon braking.
     
  15. Hmp10

    Hmp10 Well-Known Member

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    They are inputs from the driver to the tow vehicle. Seen from the perspective of the towed vehicle, they are changes in dynamic conditions. Driver inputs to the towed vehicle would be things such as applying or lifting off brake and accelerator pedals that are connected to the systems of the towed vehicle. This is why, for instance, that the slowing of a tow vehicle cannot in and of itself switch a towed EV motor from propulsion to regeneration. That has to be done by a switch that reverses current flow. Unless the EV is equipped with some sort of sensors that react to changes in dynamic conditions and are connected to that switch, the motor would not go into or out of regeneration mode just because of something the tow driver did.

    The same applies to decoupling. There has to be some sensor or switch in the towed vehicle that activates decoupling, not just a change in dynamic conditions . . . unless, of course, the decoupling mechanism itself operates off inertial force -- something which would be very tricky to execute and require a lot more engineering than towing recharge is probably worth.
     
  16. CappyJax

    CappyJax Well-Known Member

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    You are trying to educate someone who thinks they know everything already and refuses to research how things actually work. Been there, done that. Better to just give up.
     
    stank65 likes this.
  17. Hmp10

    Hmp10 Well-Known Member

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    I did research the matter. Perhaps Page 2 of this article will help frame my question more clearly: https://auto.howstuffworks.com/auto-parts/brakes/brake-types/regenerative-braking1.htm

    It explains that a car motor spins in one direction when generating propulsion. It must be made to spin in the opposite direction to act as a generator. The forward momentum of a vehicle that is slowing can be used as the power source to spin the motor in the opposite direction, BUT a control signal must first be sent to the motor to make it spin in the opposite direction. Without that control signal, a motor that has been deprived of its current supply does not spontaneously reverse its spin but will just spin down in the original direction as the car starts coasting, until friction and aerodynamic drag bring it to a stop.

    The article then goes on:

    "Sophisticated electronic circuitry is necessary to decide when the motor should reverse, while specialized electric circuits route the electricity generated by the motor into the vehicle's batteries. In some cases, the energy produced by these types of brakes is stored in a series of capacitors for later use. In addition, since vehicles using these kinds of brakes also have a standard friction braking system, the vehicle's electronics must decide which braking system is appropriate at which time. Because so much is controlled electronically in a regenerative braking system, it's even possible for the driver to select certain presets that determine how the vehicle reacts in different situations. For instance, in some vehicles a driver can select whether regenerative braking should begin immediately whenever the driver's foot comes off the accelerator pedal and whether the braking system will take the car all the way to 0 mph (0 kilometers per hour) or will let the car coast slightly."

    My question all along has been quite simple. Simply slowing the tow vehicle will not put the towed EV motors into reverse spin. So what sends the electronic signal to the EV motors to reverse their spin when there is no driver in the EV to press its brake or accelerator pedals that function as the switches that send that signal?
     
  18. CappyJax

    CappyJax Well-Known Member

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    "It explains that a car motor spins in one direction when generating propulsion. It must be made to spin in the opposite direction to act as a generator."

    I didn't even read your full post, because I had to stop right here. This is 100% absolutely, positively, wrong. The motor does NOT need to change direction in order to act as a generator. The current reverses direction, not the motor.
     
  19. stank65

    stank65 Well-Known Member

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    Yeah I was just going to say the same thing. This is a fundamental misunderstanding of how regen and electric motors/generators work.
     
  20. Hmp10

    Hmp10 Well-Known Member

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    I have read articles both that say a controller has to send a signal to a motor, sometimes to switch its spin direction and sometimes to reverse its current flow, to start regenerative braking. I have also read articles that support your position, such as this one from the University of Tennessee Engineering School: https://www.utc.edu/college-engineering-computer-science/research/cete/electric.php.

    One the other hand, that same article also says: "Most electric vehicles do not utilize a transmission. The motors are usually single speed and acceleration is smooth, no bump or jolt like transmissions in today's vehicles."

    In fact, that statement is wrong. Almost all EVs, including Tesla and Rivian, have transmissions (https://www.roadandtrack.com/new-ca...-electric-cars-have-multi-gear-transmissions/) . . . and their motors are not single speed motors (which are something you might find in a pool pump or a well pump, but not in a car), but actually operate over such a side RPM range that a single-speed transmission is necessary to find an optimum balance between torque and speed.

    In short, it's difficult to know which article to believe, or which part of which article to believe.
     

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