High rmp in “B” mode full battery

I'm sure it sounded scary high, because that's what everyone always reports.

I would encourage you to equip yourself with a scan tool or a phone app with interface so that you can prove this for yourself.

 

I should also add that once you hit the engine compression braking mode RPM figures listed earlier, going faster down a steeper hill won't spin it up to higher RPMs as would happen in a traditional non-hybrid. Instead, the ECUs hold it at that RPM, which is no longer sufficient to hold the car to a steady speed, so the car gradually creeps up to ever higher speeds. When this happens, then you must include some friction pad braking to control speed, though considerably less of this is needed than if no engine braking is used. And the engine is protected from overspeeding.

 

The funny thing is, it kind of does please me to listen to. Same as the "CVT drone" during acceleration that so many "car enthusiast" writers can't seem to stand. It's the sound of this cool machine I'm driving doing the cool stuff the other machines don't do. I like that.

 

Yup engine braking is b mode. Using the engine as a brake when charge is too high, it typically starts at 80% soc automatically unless you manually select B mode. Like down shifting in a traditionally geared transmission will put a lot of extra drag on the car from the engine not being fueled under decel with each extra downshift to increase rpms. A lot of the times on my motorcycle or tow vehicle or etc.. i can just shift down gears and not have to use mechanical brakes until the last 5 mph to stop at the stop light.

Refering to use of mechanical brakes the car does "regen only" to a very small pedal input just in side of the charging bar and then activates the mechanical brakes once you go past it. Then the curve takes over. As braking force increases so does charging force.
You can actually see charging units and brake pressure on a scan gauge with an x code so you can see both in real time.

Basically most efficient braking is to stay in the charging bar or the car will activate mechanical brakes. Then you have to let off the brake for a second for the car to reset and you can get back to regen only braking.

I did notice if you set cruise control when coming off the interstate you can hold the down position on the stalk and it will give you stronger regen only than the brake pedal itself. Not that useful, I just am mindful of the pedal and use regen only portion with plenty of space to slow down.

Sticking to regen only or coasting is a great way to get better gas mileage and almost infinite pad life as well. lol

 

At what people commonly understand as "common highway speeds," aero+tire drag alone will give you >0.03-0.05g, which adds to the regenerative braking deceleration.

 

The car will use engine braking in any mode, if either the cruise control or driver pedal input is calling for more slowing than the battery can accept in regen.

B mode slightly adjusts the parameters of those computations; it adjusts "foot off go pedal" to mean a request for a bit more slowing than in D, and it reduces the usual bias toward capturing energy into the battery, so that a little more engine braking may be heard in comparable circumstances. But the car always has all the same tricks up its sleeve. D mode vs. B mode are merely small adjustments to the balance.

 

Might that include the ICE-spinning drag (in older generations, highway speed is well above the maximum speed threshold where the ICE can be held stationary), plus the light foot-off-gas-pedal regeneration that synthesizes a mild engine-compression-like drag, to make the Prius feel somewhat more like a traditional non-hybrid under similar circumstances?

I lack recorded direct measurements or hard test figures. But from crude seat-of-the-pants testing long ago, gliding down hills while watching speeds stay steady or creep up or down, depending on slope, was I figuring the total neutral-mode vehicle drag to be in the 0.02x g range, certainly below 0.03 g. At least at my 'highway' speeds, which are admittedly lower than in some other parts of the country. Maybe someone else has solid figures?

 

May need to do a 60-0 test with the dragy in the prius where we monitor decel g to 25mph when cruise controls kicks off vs regen only braking, watching mechanical brake pressure on both so as not to use mechanical brakes. I can watch rpm on both. Holding the down direction on the cc stalk doesn't rev the engine higher.

 

The car will use engine braking in, literally, any mode when driver input or cruise control is calling for more slowing than the battery can accept as regen. There are many ways the car can be instructed to slow down, and it will use any trick in its bag as appropriate to follow the instruction (except for the friction brakes, which in Gen 3 are only involved if the instruction is coming from the brake pedal).

I use cruise to hold speed down regularly when driving in mountains, and it'll run the engine braking clear up to 4900 RPM if that's what it takes.

My usual technique is to just get a "one-pedal driving emulation" by setting the cruise to the lowest speed it'll take, 25 MPH, then driving faster with the go pedal. That results in strong slowing with the go pedal released, and engine braking when the battery's had enough.

I very rarely rely on the hold-cruise-stalk-down input when slowing is needed, just because they don't seem to have designed it very well. It's overloaded on the same gesture you use to set the target speed, and if you hold it down while picking up speed, sometimes it seems not to know what you mean and doesn't interpret the input as a call for slowing at all. That may explain some cases where it doesn't bring in engine braking.

 

I deal with this by doing a minor brake check to bleed off a little speed at the top of the hill, then setting the cruise a few notches under my true target. Not hard to dial it in with familiar loads on familiar hills.

 

Glad it doesn't only happen to me. If the car would let me post on the "just need to vent" thread while moving, I'd have mentioned it many times before now.

 

Yes, the engine-spinning drag is a factor above 42-46 mph, as are also miscellaneous bearing drag, brake drag, oil-related drag in the transaxle, etc. So, when I said "aero+tire drag alone," I should've instead said overall drag (of which aero and tire are the biggies, at "highway speed").

If the car will coast at a constant speed down a grade of, for example, 5%, that means deceleration coasting at the same speed on level road would be 5%.

 

My Prius won't do a Neutral glide down a 5% grade at constant speed, it keeps going faster. The grade needs to be significantly more shallow for a constant speed.

At 7%, it can't hold a constant 60 mph even with full 4900 RPM engine compression braking. It keeps going faster.

 

Much like with earlier vehicles, it's a question of catching them earlier. Ours can hold 33mph down a 9% in B, but if I start the descent any faster it'll get away as you describe.

 

We shouldn't be going that fast in "N," but there's no way it will "keep going faster" if you enter a 5% downgrade at a speed near the equilibrium speed for that grade (and other relevant conditions). In "D," the "IC-spinning drag" you mentioned earlier will reduce the terminal coasting velocity slightly (assuming accelerator positioned to eliminate both regenerative braking and the opposite).

 

My understanding has been that when entered with ICE spinning, Neutral has no real speed limit. The only problem is when entering Neutral at low speed with ICE not spinning, then rolling up beyond the MG1 protection threshold speed while the ICE cannot start spinning.

I personally have no problem with playing with Prius Neutral on a downhill slope shallow enough that speed doesn't creep up above speed limit. It isn't like this is a heavy old-era vehicle with fading drum brakes and a transmission that refuses to re-engage at speed. Done right, the officer watching should not be able to have a clue that you are in Neutral. Though 5% is not shallow enough.

I'll have to explore more and take some more hard data. But checking some of my regular highway downhill sections on topo maps to compute slopes, I must wonder what your acceptable equilibrium speed is. It seems that yours must be significantly higher than what is safe or legal on my roads. Or your vehicle and tires have more drag than mine.

 

There's an interesting passage in the Gen 2 New Car Features manual explaining that while the car says "I promise" no electrical tricks with MG1 or MG2 while in Neutral, it actually crosses a couple fingers behind its back for three special cases.

One of those was if you happen to roll up past that RPM threshold. It will then, ah, forget that it's in Neutral, just long enough to kick the engine into motion, and then go back to looking all innocent-like.

As Gen 2 was already advertised to be that devious, I assume that Gen 3 is also.

 

Hopefully I don't come across as clever-clogs, but that translates to "use the brakes".