Why does the engine wind out when in B mode?

Well, that was interesting. Looks like that claim was added to the Wikipedia article for some reason back in January 2008 by one Leonard G., who said it was in the 2007 Prius Owner's Manual. But what the 2007 owner's manual actually says is this (just like every other gen and year owner's manual I've happened to look at):



Not the same claimed reason at all. I've fixed the Wikipedia article to accurately quote the manual.

The "may cause decreased fuel economy" is, of course, because in normal circumstances you would be using the engine to dissipate energy that you could otherwise capture in the battery.

 

Depends on what you mean by "used". MG1 is rotating in both modes (counter to engine direction in D, generally with engine direction in B). It is producing volts but no amps in D; it's producing volts and amps in B. Are you thinking of a reason either case should be a concern?

A triangle that ramps to a peak of 82 and back down within the space of a second integrates to 41 amp-seconds. At 201.6 volts that is 8,266 watt-seconds, which is 2.296 watt-hours, or a little less than one twentieth of the little 50 Wh car icon you can earn multiples of from regenerative braking.

I get that. And you asked about it on PriusChat, which was reasonable. Then you got plenty of responses explaining that (a) it's normal, and (b) it's not taking the RPM as high as you thought (which you can also see in the graphs you linked in #115).

That's one of the chief reasons for using B, because it feeds the battery a less high charging current than under the same conditions in D mode. The whole point of B is to slow the rate of battery charging on a long descent so it will probably still be fully charged by the bottom, but not get super-rapidly charged and heated in the early part of the descent.

Right. It's not advised under the conditions where you could capture more energy and get better fuel economy in D.

That would be an argument against all engine braking in all cars, but it doesn't seem to be one that has taken hold. One reason might be that all of those forces are still a lot less than they are when the engine is producing power. And engine braking in other cars does have the risk of overspeeding, which it does not have in the Prius.

 

The suggested condition for best MPG, if you can achieve it, is when the ring gear is spinning just about 1.39 times engine speed. For example, engine doing 2,160 rpm and ring gear at 3,000.

In such a condition, MG1 is close to stationary, so the 28% of engine torque going there represents virtually no power. Meanwhile, the 72% of engine torque passing through the PSD to the ring gear, times the rpm ratio 3,000/2,160, represents about 100% of the engine power, so nearly all power is taking the mechanical path, and virtually none over the electrical path with its multiple conversion losses.

This has been covered in this thread in posts #72, #80, and #102.

 

Again, the nomographs on pages TH-11 to TH-25 of the NCF would answer so many of your questions.

Lots of things can happen in D mode. To accelerate or get engine power to the wheels, MG1 must generate. That both serves as the electrical power path to MG2 and, by its torque reaction on the PSD, completes the mechanical path.

Cruising or gliding, it may be producing little or no juice. Kind of depends on what the battery needs.

Decelerating, as mentioned in #124, it's producing volts but no amps in D. In B, it's producing volts and (in the three-phase sense) receiving amps; that is, the phase relationships are such that the I·E product is negative, making MG1 act as a motor. It is supplying 28% of the torque being used to spin the engine as a power dissipator. The other 72% is coming mechanically through the PSD.

Nothing so complicated as that. The HV ECU controls the switches. Any time MG1 is turning and producing voltage (which it can't help doing, because the magnets are permanent), but the HV ECU neither wants to power it nor get power from it, it says "meh, not interested" and leaves the switches off. The voltage is still there but no current can flow, hence no power.

How thoughtful of Toyota, then, to provide a B mode so you can reduce the charging currents to the battery.

The integral helps to answer the question "so what?". A regular 100 watt light bulb draws about .8 amps at 120 V. But for a fraction of a second when first turned on, it can be more like 11 amps. Turning on a circuit with two or three 100 W lamps on it will draw more than the current rating of a 20 amp circuit. Three or four thousand watts. But nobody notices or cares, because the total energy involved is so low, because the filament comes up to temperature so quickly and the inrush current is over and done with. You wouldn't see it if you were watching your electric meter spin.

81 amps or 16,000 watts sounds like a big number, but it isn't outside the range of numbers the car's components routinely deal with, and is over so quickly that the total energy "dumped" is peanuts.

If you had used a tach and demonstrated that the engine somehow went outside the normal rpm range for engine braking (since you seemed to be alarmed at how high it "sounded" like it was revving), that would indicate you had hit something not-normal. Absent that, the odds are overwhelming that you experienced the normal engine braking of a Prius, and were alarmed by it.

The ECU does a lot more power management than that. It will increase or decrease power to or from the battery at any time because of its monitoring of battery SoC and battery temperature. MG1 has more duties than just charging the battery. The transmission, much of the time, is a closed box where electrical power is being routed between MG1 and MG2 in one direction or another. When the HV ECU wants to charge the battery, it generally skims from that existing flow. When it wants to use battery charge to boost acceleration, it just adds to that existing flow. There are certain edge cases where one MG or the other isn't participating, but those are just the edges of the same picture.

You've made it clear that it seemed that way. A tach reading would have told you whether it was or not. (And any tach reading showing it was would be big news around here.)

Why not spend some time with Alex Hart's interactive Prius nomograph over here.

It's done in Flash, so you might have to choose to allow it if your browser is concerned for your safety. As far as I can tell, it's just a Prius PSD nomograph, like it says on the tin.

Try dragging the MG2 rpm slider up to around 3,000 and the engine rpm slider up to around 2,160.

 

When presented with multiple paths of differing resistances, it splits (unevenly) and takes all of them. The resistance figures and some algebra allow you to compute how much takes each path.

 

That'd be here.

Also, I added a link to an interactive nomograph, up in #129.