Coasting, B or other?

take your foot off the pedal and/or apply the brakes if necessary to stay at the speed limit. unless your battery is full, there is no need for b, which is engine braking. you want to regen as much electricity as possible.
engine braking uses the pistons to create drag to slow the car down, useful in the rockies to save brake pad wear.

what does the prime owners manual say about b mode?

 

If the battery doesn't fill up from coasting / light (regen) braking / cruise control (downhill regen), then don't use B. B will throw away some energy that could otherwise be recovered. At merely 1 mile with reasonable slopes, even a regular liftback shouldn't fill its little battery, let alone the larger battery of a PiP or Prime.

{This presumes that I'm not overlooking some special feature of B-mode that applies to plug-ins only. I have been known to do that, and will likely do so again.}

That is, unless you start with the battery fully charged. With a Plug-In, that might mean not letting it fully charge at home. Stop it at a point less than full, so that it has some room remaining to accept the downhill regen.

More importantly, it is used to save brakes from overheating and failing. 1 mile of reasonable slopes shouldn't overheat the brakes. As others keep pointing out, mere brake pad wear is cheaper than engine wear.

 

It depends on your battery charge. If your EV battery is not fully charged, put cruise on (or coast) and the battery will charge (about .2 or so EV miles). If it is full, use HV mode and accelerate staying in HV battery mode to gain momentum going up the other end of the hill.
Oh yeah, congrats on you new Prime!

 

In my 2004, in Ev mode B just adds regen and doesn't start the engine. Others have reported the same in the Prime, as long as the battery isn't full to where it can not longer accept the regen. The brake pedal does the same thing as long as you don't push it hard enough to need the friction brakes. B mode won't use the friction brakes.

 

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I advise against N, the HV Battery cannot charge in N
In a Prime, B may be a good choice, if it never rotates the engine.
Lightly pressing the accelerator so no arrows show on the energy screen may be a true coast.

 

Isn't B behaviour different with the plug-ins too? I believe it was with first itteration PIP, and now with the Prime also?

Anyway, if the hill is just gradual downhill, enough that coasting more-or-less maintains speed, I can't see there being a need to do anything out of the usual. That case would be hypermile nirvana.

It's steep and unrelenting downhill, necessitating a lot of constant braking, that requires B mode. At least with regular Prius. Again, not sure about PIP or Prime.

 

Just coast in D if that's sufficient.
If the car is going too fast, switch to B. Switch back to D when your ready to press the accelerator pedal.
If it's still going to fast, brake.

The car is in light regenerative braking when coasting in D.
It is in moderate regen braking in B.
It is in regen braking under light braking and friction braking when more pedal pressure is applied.

"engine braking uses the pistons to create drag to slow the car down, useful in the rockies to save brake pad wear."
I somewhat disagree. The engine braking effort is from pulling a suction with closed intake valves on what would have been a power stroke. The main reason to use engine braking on any long downhill is to keep from overheating the brake pads and fluid. Overheated pads and fluid cause crashes.

 

How does the engine change the valve timing to get the suction against closed intake valves? I know large diesel trucks with Jake Brakes can do this but I didn't know the Prius could. I believe the engine just gets spun with no fuel being injected like most newer cars.

 

It is spun with no fuel and with the valves closer to Otto cycle to hold air captive

 

Like many newer cars, the Prius has variable valve timing.

 

In particular, the gasoline engine in all Prius models is a modified Atkinson Cycle engine, optimized for efficiency rather than power. Unlike many other manufacturers of hybrids, Toyota did not just drop one of their standard engines into their hybrid line. It is specially designed for hybrid operation.

 

Gen 2 valve timing.

While the head is unique to the Prius, the block is used in Otto cycle Toyotas.
Toyota NZ engine - Wikipedia Atkinson first, then Otto

Toyota ZR engine - Wikipedia Otto first, then Atkinson

 

It doesn't, I believe he described it wrong. The normal power stroke is balanced against the compression stroke, so they are a wash.

The braking effect is the result of pulling the intake stroke against a strong intake manifold vacuum, caused by a mostly closed throttle valve. The exhaust stroke then pushes against full atmospheric pressure, so this pair of strokes is very asymmetric and don't wash out.

Valve timing does change to maximize this effect, as shown in Jimbo's graphic. But the valves are never opened or closed in a stroke where they would be the opposite during normal power producing operation.

 

(I stole that graphic from Hobbit) If you minimize overlap, you should make the engine do the most work as an air pump, I think.

 

All this talk about "Otto" reminds me of the (very) old joke: 'Heard on the golf course: "that Otto fellow, looks like he's a pretty good golfer, every single hole, he does in 4 strokes! ...uncanny!"'

 

I don't really have any hills around here to try this, but what about coasting downhill with the Prius Prime in forced charge mode. Has anyone tried this as a form of braking with the benefit of charging the battery?

Posted via the PriusChat mobile app.

 

That won't work. Drive down a hill with B-mode in Ev instead.