Is there a legitimate use for charge mode in the Prime?

Right, diesels generally don't use throttles, so the "pull vacuum against a closed throttle" method of engine braking universal in gasoline engines isn't an option for them.

For a diesel, you can add a Jacobs brake, which manipulates exhaust valve timing to get a similar (but much more noisy) engine braking effect.

The description of Jacobs braking that you once posted saying that it came from a web site about Prius actually came, in fact, from a Cummins diesel site about Jacobs braking.

The Prius, having a gasoline engine and a throttle, does engine braking the same way other gasoline-engine cars do. You get around 13 kW of it.

 

Since we already know that modern engines manipulate valve timing electronically, it's not much of a cost to implement engine braking.

 

It has especially little cost in a gasoline engine with a throttle, which inherently supplies engine braking when it pulls vacuum against a closed throttle.

In a diesel engine with no throttle, another approach, like Jacobs braking, is needed.

Most Prius generations lack variable-timing exhaust cams, so even if there were a reason to do Jacobs braking on a gasoline engine, they couldn't do it. They still give you about 13 kW of engine braking just fine, though, being gasoline engines with throttles.

Gen 5 uses the newer M20A engine, which in some builds has an adjustable exhaust cam too. If that is present in the gen 5 Prius, then that could become the first Prius generation physically capable of involving the exhaust cams in engine braking—again if there were any reason to do that. Whether it does anything like that would be a matter for finding out. If it does, that'd make it the first Prius generation to do so.

Also, even if it does, there is no reason to suppose it does so differently depending on whether the ECU has chosen to engine-brake because of a B shifter selection or just because of the battery state of charge.

 

If there were no engine brake, all you would need to do is downshift, as in a conventional vehicle. Since there are no gears lower than "D" there needs to be an engine brake.

 

There is an engine brake, the gasoline engine kind, where the engine pulls vacuum against the closed throttle plate. That's exactly what you're doing in a conventional gas-engine vehicle when you downshift. So what does 'downshift' mean?

It means "increase the ratio of engine RPM to wheel RPM." Equivalently, "increase the ratio of wheel torque to engine torque." That ratio can be around 14 or higher as you move off from a stop, decreasing ("upshifting") until it may be no more than 3 or so as you cruise down the road. You need a higher ratio again (downshift) if you need power for passing or climbing a hill. Once you're done passing or climbing you use a lower ratio (upshift) for cruising again. Descending a hill, you may want a higher ratio again (downshift) so that the engine braking torque is multiplied at the wheels.

Every transmission does those things. How it does them depends on the transmission. Some need you to manually change ratios. Some do that automatically. Some have fixed ratios to choose from, some have a continuous range.

You've seen other cars with automatic transmissions. You know "D" is not one ratio, it's "car, change the ratio for me as needed." You know any car in D starts out at a high ratio as you're leaving a stop, and adjusts to lower ratios (upshifts) as you approach cruising speed. That includes a Prius. You know a car in D kicks back to a higher ratio (downshift) if you goose the go pedal to pass. That includes a Prius.

And, of course, if you are rolling along at, say, 35 MPH in a Prius with the engine turning less than 1200 RPM (a ratio around 2) or even not turning at all (a ratio of 0, hey, it's a Prius), and then because you're going downhill and have run out of regen capacity, you hear the engine revs increase to 4000 at the same 35 MPH road speed (a ratio around 7.6)—that, absolutely, is a downshift. The Prius has just done exactly what you already understand would be done in a conventional car to make use of engine braking.

A ratio like 7.6 from engine to wheels is about what my last stick-shift passenger car had in second gear. Apply that ratio at a speed like 35 MPH and you can't miss the engine braking effect.

There is precious little difference between how the Prius and other gasoline-engine cars do this, even down to the ratios involved. The transmission's inner workings are different, but not in any way that changes what's happening here.

 

Just to expand - the only difference that matters for the Prius is that the "as needed" can include "downshifting for brake pedal application".

A normal car can't do that because the brake pedals directly control the brakes (aside from intervention by tweaks like ABS/VSC).

In another car, pushing the brake pedal directly causes drag by brake pad application - any automatic downshift from the engine to increase drag would be on top of the brake pad drag, and cause an unexpected response.

Therefore a conventional car has to give you engine braking control separate from brake pad control - you only get the engine braking effect by manually downshifting, so you can control and understand what's happening - one control for brake pads (pedal), one control for engine brakes (gearstick).

But the Prius has the ability to not apply brake pads when you push the pedal. They're mechanically disconnected (normally - with a failsafe). So it normally chooses how to generate drag. Its first choice in normal circumstances is regeneration, but engine braking and brake pad application are the other options.

Therefore the Prius can be smarter and do all the engine braking you need with only the brake pedal to control - it can downshift both to accelerate and to decelerate.

Other cars can downshift for engine braking to decelerate automatically, but only when the deceleration is not in response to brake pedal application - for example during cruise control.

It's really the control-by-wire for the brake pedal that's special here - as ChapmanF says, all the differences in the transmission are irrelevant.

 

The Prius uses an Atchison Cycle to save fuel. This is done with variable valve timing, which can also be employed to increase engine drag.
(a.k.a. engine braking)

 

You can call it Atkinson. You can call it Miller. You doesn't have to call it Atchison.

In any case, that's done with the intake cam timing, not the exhaust. The range in degrees of cam adjustment available is shown in the New Car Features manual. Are you thinking of an adjustment within that range that would give you an additional engine braking effect?

Keep in mind that there is very little air available to compress in that situation ... because the throttle is closed ... which is the already known, long established way to achieve engine braking in spark-ignited gasoline engines.

 

I should have checked my spelling, but it doesn't diminish my point at all.

 

The spelling part doesn't. Which I see is the one part you've chosen to respond to.

 

seriously, I hope you know my likes are true, even if I don't know the person personally posting.

This is where I got lost ....
to me it's self explanatory, to others I doubt they'd get the meaning without an adjective describing the trans, is it MG1 and 2 ( hybrid synergy drive ) or the typical auto trans.
Even more to the point in this forum, the plugin version or the regular prius version.

 

Initial conditions of your hill?
Height?
Distance traveled from the top of the hill to the end of the descent?
Mass of the car?
Speed?
Can you give the coordinates of this hill to find the unknown values?
All of this is measurable and can be solved as a school problem.

 

You think if I can't answer those details, then I'm wrong?

 

I was referring back to ChapmanF's post. He was talking about the difference between a Prius transmission (plugin-or-not) versus a normal automatic or normal manual.

In both cases engine braking is achieved by sending power from the wheels and using it to push the engine at high rpm.

The detail of how the transmission achieves that - either electrically or mechanically - doesn't change the effect produced. Whichever way you push the engine at 4000rpm with power taken from the wheels, the car will experience the same amount of drag.

 

I've been on plenty of hills, driving plenty of vehicles, where no amount of engine drag will prevent you picking up excessive speed.
If you've never experienced that, it doesn't mean it doesn't exist.

It wasn't uncommon to smell burnt brake pads from previous vehicles on the way down.

 

That's exactly the school problem MAX2 was referring to.

No matter how much engine braking your engine is capable of—and no matter whether it is a gasoline engine using a closed throttle or a diesel engine using exhaust valve timing—you can always find a downhill, and a speed, and a loading of the car, such that the engine braking won't be enough, and you'll have to add friction brakes too.

It's quite simple if you just know how much engine braking the car can give you. In a gen 3 Prius with the 1.8 L engine, that's about 13 kW. Perhaps in a gen 5 with the 2.0 you could look for a smidge more, 14 and change.

Your energy gain rate from gravity in kW is a matter of your speed, the loaded weight of the car, and the downhill grade.

If you're gaining less than 13 kW, engine braking will be able to slow you down. (Or the car will twirl the engine at a lower RPM to just match your gain rate and hold your speed.)

If you're gaining 13 kW, the engine will be spinning all out and your speed will be holding.

If you're gaining more than 13 kW, the engine will be spinning all out and you'll still have to also brake to keep your speed. But you will be putting 13 kW less heat through the friction brakes than you would be without the engine braking.

If you're in that situation and you want to reduce your energy gain rate so it's 13 kW or below and the engine braking can do the whole job, there are three ways you can do it:

1. be on a less-steep downgrade
2. chuck heavy stuff out of the car, or
3. slow down

Usually you don't have much choice about (1) once you're on the hill you're on, and you don't want to chuck stuff out of the car 'cause it's stuff you want, but you generally have the option to slow down.

Of course on a steep enough hill (or with a heavy-enough load), the speed you have to slow down to for staying under 13 kW could be slower than people behind you want to drive.

But when you reach the bottom your brake pads won't be as smelly as theirs.

 

Exactly, B mode provides a Static amount of total Braking once it's switched On !

That explains a lot about the description of B modes pre programmed ( dynamic combination ) of Engine Braking and Regen Braking, in relation to the total amount of braking observed.
How much of either Engine Braking and / or Regen Braking is the dynamic part of B mode. (see also the last comment below).

which two? I get it cause I can read between the lines.

Yes, for Engine Braking, again I understand.

I'm beginning to think that Toyota recalibrated Gen 5 Prime for a lot more Engine ON time, than the Gen 4 Prime. That might explain the assumptions I'm sensing while reading that Engine Braking is always a factor when B mode is switched On.
See, with Gen 4 in EV mode, if I get Engine Braking while in EV mode, I've either misunderstood EV mode, B mode or both, and immediately take action to switch the Engine Off again as soon as possible.
Again that is only my personal evaluation, your explanation is just as valid from your experiences as I'd hope mine are.