Toyota read this - what's my prime thought?

I DON'T KNOW THIS OF MY OWN KNOWLEDGE ..........
BUT I HAVE READ THAT THE NATURE OF THE CVT PROVIDES LIMITED REVERSE POWER (LIKE BACKING UP A STEEP HILL). IN ADDITION, IT WOULD SEEM THAT SURGE BRAKES (OPERATED BY TRANSFERRING WEIGHT TO THE FRONT) WOULD PROHIBIT BACKING A TRAILER UP ANY TYPE OF INCLINE?

IT IS A MOOT ISSUE WITHE ME. I HAVE NO INTENTION OF TOWING A TRAILER IN ANY HYBRID OR EV.

 

What's up with the caps?

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European trailers use the reverse light signal to detect that the car's in reverse, and disable the surge brakes, typically. That's how you can back a trailer with them.

 

You don't even have to get that complicated; just leave the reverse shoe off the trailer drum brake. The forward shoe will still apply, but won't have the leverage to grab onto the drum lining for full braking, and can easily be over powered by the car.

 

duno about the pip, but the CVT in our Lexus suv does just fine .... with both uphill steep reverse short runs & warm elevation climbs. In addition to the ICE though, reverse has the added advantage of dual motor/awd. That's with a 3k Lb trailer. It's rated for 3.5k Lb. Whatever the vehicle running a CVT, one would think the manufacturer won't rate close to any danger zone because they have to figure in additional things such as vehucle & trailer weight & sway potential.
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The ICE may not add power to the wheels with HSD while reversing, but M/G2 alone has plenty of torque at the speeds you would be backing a trailer up at.

 

I'm not arguing the point ..... and, I really can't document my source.

Several years back (I think on PriusChat) it was mentioned that the the physical, mechanical way a CVT works, does not favor backing up. My recollection was, sure, it will back-up, but the direction of the rotation of the internal components rendered less power in reverse.

Right or wrong, I remember thinking that I had proved it when one time I was in a situation where I needed to back up a steep incline in my 2010, and I thought it struggled more than it would have going forward. Perhaps it was my imagoination?

 

In the case of a power split device, when the vehicle's operating within EV capability, reverse isn't inherently any harder than forward. In either case, you're holding the engine at 0 RPM.

It's when the ICE has to run that things get interesting - MG2 has to RESIST the ICE torque (which is in the wrong direction), in addition to providing torque to propel the vehicle. It's helped in this by MG1 generating electricity from the engine, but that's only pulling ~28% of the torque off of the ICE, the rest has to be handled by MG2.

 

Wut?

 

I don't think the Prius will use the ICE when backing up in reverse. All backup power flowing to MG2 comes from the battery. So, you are limited by the power that the small hybrid battery is able to put out. The transaxle itself is perfectly happy to spin in reverse -- at least up to the speeds where MG1's rpm maxes out and it would want to start the ICE to counteract that.

 

My understanding is that the ICE can start in reverse when the HV battery is at low SOC, and that ICE torque always encourages the vehicle to go forward. Only ~28% of the torque goes to MG1, and MG1 absorbs the ICE's power, but MG2 has to act against the ICE torque, too.

 

Yes, but I've also experienced the ice coming on in reverse even when the ice is stage4 warm, & the battery is 2 bars from the top of being full.

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That is to generate more electric power to go in reverse. The motor has higher power rating than the battery can deliver and to get more torque the motor needs some extra juice.

JeffD

 

Makes sense
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I think this would work in principle but I've never seen it happen in my gen 2 Prius. I've backed up on steep hills with the accelerator to the floor in reverse gear and the ICE did not start. Perhaps things changed in later models.

 

i keep getting email notifications from toyota about the prime, with no new information. and they keep asking me if i want to sign up for email notifications.

 

Yeah, it wouldn't ever start the ICE in reverse to increase available power in reverse, as the ICE can only reduce the efficiency of reversing with the Power Split Device. It would only start the ICE to generate electricity when the vehicle's at minimum state of charge, which it tries to avoid, and reversing isn't usually for long distances. Basically, the only way I could see to get it is to either reverse for so long that you drain the battery, or you drive in a manner that depletes the battery, then quickly stop and throw it in reverse while it's recharging.

Edit: somehow missed a few posts thanks to reading this on mobile. Struck through wrong parts.

 

I don't have easy access to my gen 2 Prius right now but I will try running down the battery and then do a reverse up a hill next week to see what happens.

 

I think that applies to traditional transmissions, including cone and belt CVTs. Reverse is a separate gear that has to, well, reverse the direction of rotation going into the transmission to the opposite way when it leaves it. As long as there is enough power to back the car out of a parking space, the design is considered a success.

I've heard of one old mini car that the engine had to be shut off to go into reverse, because it had to be restarted spinning the other way. To the gears of the manual transmission, the direction of spin didn't make a difference to the amount of power that gets through them. I mention this because that is how HSD reverse works, it just spins M/G2 backwards.

Or perhaps you've just mistaken rode differently for struggled; the car is basically a RWD one at that point. The ICE also can not directly aid reverse propulsion, so you are limited to just M/G2. Its torque output is great in the beginning, but if you starting going faster than a creep, it could have started dropping off.

Graham's old Prius page has some great basic info, with the only difference going from gen2 to gen3 being the introduction of the reduction gear between M/G2 and the transaxle.
Graham's Toyota Prius
"The Prius has no reverse gear that would allow the ICE to push the car backwards. Therefore, it can only move backwards under electric power from MG2. No direct help from the ICE is possible. In most cases, the car will stop the ICE when you put the running mode selector lever in the R position. As MG2 turns the input to the reduction gears backwards, the PSD ring gear will also move backwards. With the ICE, and therefore the planet carrier, stopped, this simply means that MG1 will turn forwards, as shown in the animation at right. It turns free, without using or generating power. This is exactly like EV mode, but backwards. The computer will not allow you to go so fast backwards that MG1 overspins.

Should the ICE continue running when the running mode selector lever is in the R position, for example if the battery state of charge is low, then MG2 still simply drives the car backwards as before. [Note to self: next time I have several hours with nothing to do, make another animation.] The only difference is that with the planet carrier running forward, the sun gear and MG1 spin faster forward and the computer must limit the backward speed of the car to a lower value to protect MG1 from over-spinning. Power can be drawn from MG1 to supply MG2 and charge the battery [Note to self: this may give rise to questions beyond the scope of this explanation]."

So the struggling you perceived can also have been the system limiting speed as described above.

 

Trollbait - Thanks for the explanation. Great job.