Engine Braking

You can accuse me of misquoting a source if you like, but I copy-pasted the passage unaltered.
I have a feeling that this thread is on the way to being locked, like the last one.

 

I'll be happy to ask if the mod can verify that. The wikibooks page you cited has an edit history, showing that it last changed on 15 March 2022, and does not contain the passage you attributed to it.

 

I was under the impression the CVT can dis-engage itself from the engine and the engine isn't spinning all the time, since that would introduce extra drag and wear into the drive system? It's that extra mechanical drag that slows the car - not some convoluted heated gas to energy explanation. Gen4's to be specific....

@ChapmanF, thanks for the long-winded explanation - I wasn't going to reply, to avoid the hornets nest you've just uncovered...

 

You'll find that I haven't edited a thing there. Paranoid much?

 

The OPs question was answered in post # 4 ( kithmo, Dec 29, 2016 )
There is no reference about regen braking until your first post to this old thread ( # 11 yesterday )
If this thread gets locked # 21 ( it's on you ) and I'll do us all a favor and request it, again.

If you want to argue about B mode - do us all a favor and start your own thread, again.

 

Well, if you found a way to dissipate energy without converting it, you deserve the Nobel Prize for breaking the laws of physics.

 

It must be nice to think that what you believe is beyond discussing.

 

Yes, like any transmission, ours has a way to transmit no power from engine to wheels or vice versa, when that's appropriate. Two obvious times when that's appropriate are when the engine is idling while the car is stopped, and when the car is coasting while the engine is idling or stopped.

In a conventional car, "engine is idling while the car is stopped" happens a lot, and you can do it in a stick by shifting to N or holding the clutch down, and in an automatic it's taken care of for you.

"Car coasting while the engine is stopped" is less common in other cars, but you can certainly do it, again by shifting to N or using the clutch, though in some places the authorities frown on using N while the car moves.

The Prius transmission can handle all those cases, and also cases where it is actively powering the wheels (or braking by regen) without involving the engine. Nothing inside the Prius transmission needs to 'shift' or change position or anything to transition smoothly between any of those modes of operation.

If you are coasting, or slowing by regen, and the HV control ECU then decides some engine braking would be useful, all the transmission does is transition from "no power to/from engine" to "power from wheels to engine", and it can make that transition just as smoothly as it always does, and that transmitted power naturally spins the engine.

 

Indeed you haven't (that I can see as of now), and thank you for that. Your post #17 still stands as you originally posted it, claiming a Wikibooks page as source for a passage that is nowhere on that Wikibooks page. And the Wikibooks page itself hasn't been edited by anybody since 2022, so it's not as if you somehow found that passage there earlier today.

I'd never want to make all the hard calls a mod has to make, but they're not all hard ones.

 

If the Prius drivetrain had enough frictional drag to brake the car on a downhill slope, it wouldn't be very efficient.
When you are going down a slope, there is a place to dissipate energy besides friction; it's called "regeneration" which is way to regain some of the energy that went into acceleration or climbing a hill. But when the battery is at full charge regeneration is no longer an option, and neither is prolonged braking. The only option is to dissipate the energy in some way, which is why the engine brake was developed. Granted, when you use the engine brake, most of the time there is some battery capacity to take up regeneration, but sometimes there is not. Although the situation is rare, a runaway vehicle is a dangerous situation, and an engine brake is necessary to prevent a possible wreck. In such a case, no regenerative braking is possible, nor is there enough drag anywhere in the system to prevent a runaway situation.

Although Toyota doesn't want to raise alarm by discussing this freewheeling effect, it is nonetheless enough of a safety concern to include the engine brake.

 

The only claim I made is that I read it somewhere, and when I was accused of not doing so, I provided a link.
I don't claim to be universally right as some do, but it's only one of the several sources I have cited on this subject.
According to my best efforts to verify credibility, I stand by my sources.

 

Indeed, it would be enough of a safety concern even if they just required you to do anything special when the battery ran out of regen capacity. To a driver who wasn't obsessively watching a state-of-charge meter, if the car suddenly changed from slowing at a steady rate to freewheeling at an unexpected moment, that would be alarming and unsafe. Which is why the car's transition from regen to engine braking begins a few % early (earlier if you use B mode on the shifter) and progresses smoothly until it is complete at the programmed state-of-charge limit.

 

You provided a passage and a link (which you cited as the source) and the passage verifiably isn't in that source.

I do not personally know the mod here. But I suspect it doesn't improve your position to keep layering on new posts that misrepresent your older posts that already misrepresented stuff.

 

I was able to test my car on a long, 10+ mile downslope in the mountains last summer. I was particularly interested in learning whether my gas engine was providing any engine braking. One indication was that the energy flow monitoring screen didn't show the engine running, and I also had my windows rolled down with the audio system off, to listen for engine noise. I determined that the gas engine did not run. Apparently, this was doubted, but it was never refuted.

I don't have a tachometer, but if I had, I'm sure it would have said zero, or should have. I wasn't testing the engine brake, but if I had, I'm sure the gas engine would have had to be rotating to work, and may have shown it on a tachometer. But that's not normal engine drag per say.

I also learned recently that applying the engine brake when regeneration is possible will apply partial regenerative braking. To achieve full regenerative braking, the brake pedal must be pressed. These factors could easily mislead the casual driver to assume that the gas engine provides drag. This is not possible when the gas engine is stopped. All you have is regenerative braking, friction braking, and in rare cases, the engine brake.

 

I had said I read it somewhere, and I cited a source. This is not about arguing over the validity of that source.

 

That's like pointing out that the friction brakes produce no drag when the pads aren't pressed to the rotors.

When the gas engine is stopped, no engine braking is happening, by definition. And at whatever moment, for whatever reason, the car elects to do engine braking, the transmission begins transmitting wheel power to the engine, causing it to spin.

 

It may seem like I'm trolling, but it's just the relentless persistence of logic and evidence, not me.

 

Haven't I said that exact thing, several times?
It's beginning to look a bit like you may be starting to get it.

 

Thank you. As you can see a fully-worked example with the RPMs and torques and power paths through the transmission here (and in fact you have seen it before), it may be that I've been starting to get it since February 2023 or earlier.

Can you elaborate on why it is noteworthy to you that the engine supplies no drag before engine braking causes it to spin?

You seem to make a bigger deal of that than you do of the fact that the brake pads supply no drag before friction braking presses them to the rotors, or that there is no regen drag before the car begins accepting regen current for the battery.

All three points seem to me about equally unnecessary to harp on.

 

I may need to be reeducated, but I believe that pressing the brake actuated regenerative braking to a certain point, and pressing it further activates the friction brakes.