Flat towing a Rivian

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

  1. CappyJax

    CappyJax Well-Known Member

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    You are not grasping the concept. You don't have the regenerative breaking applied when you are accelerating or cruising (That would be really stupid), you only apply it when you are braking. Therefore, it IS free energy, because instead of heating up the break pads of the RV, you are putting energy back into the battery of the EV.
     
  2. Hmp10

    Hmp10 Well-Known Member

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    I do grasp the concept. I did not mean "free" in an economic sense. I meant "free" in terms of the laws of thermodynamics, which hold that energy cannot be created out of thin air. During regenerative braking, the energy of the car's forward momentum is converted from potential energy to kinetic energy to turn the motors which are switched to generator mode. It's not energy for which the driver pays anything at that point -- and therefore is "free" in that sense -- but it is energy in the system that was created when electricity flowed from the batteries to turn the motors and is recaptured to turn the generators -- and therefore not free in the thermodynamic sense.

    And I was responding to the comment that the author of the cited article was wrong in stating that charging an EV by flat towing would increase the burden on the tow vehicle. The author was, in fact, correct. When an EV's motors are switched to generator mode, they create a resistance to rotation, which is the very reason regenerative braking works. It is that resistance to rotation that brakes the vehicle. To charge an EV with flat towing, the tow vehicle has to generate the pull not only to move the weight of the towed vehicle and overcome its aerodynamic drag, but also to overcome the resistance to rotation in the towed car's motors. And that increases the burden on the tow vehicle beyond what conventional flat towing would require, as the energy to drive the generators has to come from some place. In a utility generator, the energy comes from coal, natural gas, nuclear-generated steam, or hydroelectric force. In a towed EV, it comes from the towing vehicle's pull.
     
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  3. cllc

    cllc Member

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    I thought they were referring to the energy they would be recovering when they would be going down hill and when they would be using the tow vehicle to slow down the vehicle that was doing the pulling.
     
  4. CappyJax

    CappyJax Well-Known Member

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    Exactly. They wouldn't have the motors charging the batteries in cruise, that would make zero sense. They would use the EVs regenerative braking only to slow down the tow vehicle during braking or downhill. This would recharge the batteries significantly. Instead of garnering regenerative breaking from a 6,000 pound vehicle, you are now getting it from 20,000+ pound vehicle. If you were in the mountains and using it a lot, you could recharge the batteries to full in a couple hours of driving pretty easily.
     
  5. Hmp10

    Hmp10 Well-Known Member

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    #20 Hmp10, May 23, 2019
    Last edited: May 24, 2019
    The article in question read thus:

    "Rivian says that the R1T will be capable of charging its batteries while being flat towed behind an RV or another vehicle. This is an innovative idea, but it could and would add an extra burden on the vehicle that is towing it."

    There is no reference in the article to charging only while going downhill.

    Are you saying that the Rivian being towed would switch from neutral into regenerative mode when it encountered a downhill slope and then back into neutral when the road flattened out? Exactly how would it to do so? When being driven, the motors don't just go into and out of regenerative mode on their own. They are switched into and out of regenerative mode by an electrical relay based on accelerator and brake pedal inputs from the driver.

    In order to switch between neutral and regen when those inputs from the driver are missing, the vehicle being towed would have to be able to detect when it was slowing or entering or leaving a downward incline.

    I could see Rivian having a tow recharge feature that a driver could select for towing, although that would significantly increase the fuel consumption of the tow vehicle. However, for the feature to work only when the tow vehicle is braking or going downhill, there would have to be some means for the car to detect those conditions without any driver input, such as inertia or mercury switches and associated computer programming . . . and I've never seen any mention of such a setup in a Rivian.

    I have been driving a car with regenerative braking for four years. While I really like the one-pedal driving it offers, it doesn't put as much energy back into the batteries as many people imagine. (One of the reasons Tesla is exploring adding capacitors to its battery pack is the low efficiency of regenerative energy capture using just batteries.)

    And let's get real. Regarding the benefits of a 20,000+ pound arrangement, how often are you going to be towing your Rivian (when loaded to its max gross weight of 7,650 pounds) downhill behind a 12,000+ pound vehicle? This is a fantasy exercise more than a realistic way to charge an EV in real-world situations.
     
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  6. 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.
     
  7. 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.
     
  8. 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.
     
  9. 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.
     
  10. 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?
     
  11. 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.
     
  12. 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.
     
  13. 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.
     
  14. 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.
     
  15. 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.
     

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