Interesting Reddit Discussion from r/Prius

I hadn't seen you ask that particular question before, but now I can see you did, in #48.

I think that's a different question than what has occupied most of this thread. I don't think anybody here, not even Paul, disputes that the engine revolves without combustion in B mode. After all, that's engine braking, and everybody seems to agree engine braking can occur in B mode.

Everybody doesn't seem to agree about whether engine braking can also occur in D mode. (Specifically, Paul still doesn't agree it can also occur in D mode.)

As for your second question, "why is the fuel economy much, much worse in the B mode than in the D mode?", it might be necessary to consider PHEV models (like Prime) separately from HEV models. The HEV owners' manuals explicitly warn that B will give worse fuel economy and should be avoided in normal driving. The PHEV owners' manuals describe B mode differently, and actually call it "regeneration boost" and offer it as a quasi-one-pedal-driving option.

The difference is the much larger battery in the PHEV. For the HEV's small battery, there would be no point offering a "regeneration boost" behavior.

Because of those differences between the HEV and PHEV, it may turn out that the economy cost of B mode in the PHEV is less significant than it is in the HEV.

 

It is not consuming gasoline. It is decreasing the electricity regeneration by inducing a drag through compression (pumping losses). In other words, less gravitational potential or inertial energy is being converted into electricity, as the engine is negating gravity or inertial energy. It is an HEV; therefore, less electricity regeneration results in less miles per gallon.

 

I thought his whole point was that the engine never revolved in the B mode with the ignition cut off.

Yes, I see much worse fuel economy in the B mode with my Gen 4 PHEV. There is no regeneration boost in Gen 4 PHEV.

So, perhaps it is possible to get more regenerative power in Gen 5 PHEV than in Gen 5 HEV. In any case, I hardly ever exceed the available regenerative power in my Gen 4 PHEV; therefore, it is mostly a moot issue.

By the way, for Gen 5 PHEV, it is also recommended to use the D mode to improve fuel efficiency (page 256 of the manual).

 

Where did “without” come from? I am talking about the B mode. I said that the fuel economy was a lot worse in the B mode than in the D mode because the engine revolved without ignition/fuel/combustion in the B mode.

 

I thought I explained it clearly. The fuel consumption is not going up. There is no fuel consumption down the hill at all. Miles per gallon is going down because you are putting less electricity regeneration in the battery. You will make up for the less electricity regeneration with higher fuel consumption later, which will lower your miles per gallon.

 

I have explained it twice already. See my two previous replies to you.

 

I am not sure why you are having problems with these concepts.

Of course we are not talking about the instantaneous mpg. The instantaneous mpg is infinite when the fuel is cut off. We are talking about the average mpg.

 

My explanation is not defying the laws of physics. You are refusing to understand concepts.

 

from prior posts it's clear you like to have the last word, but sometimes - people get a thrill out of playing against those kinds of people, like a fine cello. Let the poor guy believe what he believes and whether or not he ever figures it out doesn't really matter does it?

.

 

Internet arguments are the best

 

Agreed: it looks like the language redesignating B as "regeneration boost" first appears in the gen 5 PHEV owners' manual.

That's not the way I have understood Paul's point. I believe he does agree that engine braking is the engine revolving with fuel and ignition cut off, and he does agree that engine braking happens in B. Where Paul departs from the common understanding is that Paul does not agree engine braking ever happens except in B.

Others have understood Paul's position the same way I have:

Paul's own posts in this thread make clear that's the point he's harping on. If B mode is not selected, Paul believes that no engine braking can happen, and the car's resistance to forward motion will simply drop to zero—like running away in neutral—as soon as the battery state of charge precludes further regen:

That "should be automatic, not manually activated with the 'B' shifter. Maybe in later generations" is key to what Paul is claiming. He knows engine braking will happen in B. What he thinks is that if you're not in B, and the battery fills so that regen has to stop, the slowing effect from regen will just drop to zero and the car will careen down the hill.

Of course it's not hard to demonstrate that isn't what happens, and engine braking takes over instead, so it already is "automatic", not just "in later generations", but back as far as gen 2 at least. (I didn't say gen 1 just because I don't happen to remember if it was in gen 1, but maybe someone who does can chime in.) That's exactly why people keep coming to PriusChat—without any clue about walking into a dispute like this—just to ask "how come my engine started howling in D when I was off the go pedal on a downhill?" kinds of questions.

Paul proposed two tests he would try, last summer, when driving in hilly regions, to show there was no automatic engine braking in D and that the car would just run away once the battery was full. Neither of those worked out (as earlier discussed), simply because his PHEV's large battery never did become that full during his tests, meaning the condition to be tested never occurred, so no conclusion can be drawn.

More recently, Paul has twice mentioned conducting some test or tests he says did support his position:

However, he hasn't given any information yet on when, where, or how this testing was conducted, what instrumentation he had, or how he got around the problems that scotched the tests he did report last summer.

Again, it's not that Paul doesn't think engine braking would happen if he shifted to B. What he is claiming is that it didn't happen while the shift mode was D.


Now it seems like we've also got this side discussion too about how B mode reduces fuel economy:

Here, it looks like Paul's thinking could be getting stuck on the flip side of his belief about engine braking only happening in B. Maybe Paul also thinks that nothing but engine braking ever happens in B.

The fact is, you can shift into B and still drive the car around just fine. You can stop, start, accelerate, and turn corners. It drives just like a car, except it feels like a downshifted car when you lift off the pedal, and does some engine braking then. It does not use engine braking (even though the indicator says B!) when you are accelerating or cruising—just when you're slowing.

So the MPG cost of driving in B isn't a matter of fuel being used in the moments when engine braking happens. That's a red herring. The issue is that B mode causes those engine-braking moments to happen even when the battery isn't full, so it is leaving regen opportunities on the table. And those missed regen opportunities get paid for in gasoline the next time you accelerate or cruise and there is less charge in the battery to assist.

In fairness, Paul may not have considered what happens when driving around extensively in B mode, because Paul would only shift to B on big downhills and shift right back to D afterward—and that's how Toyota has always recommended using it. The MPG cost comes if you leave it in B for normal driving circumstances where D would be more efficient, which might be something Paul never thought of doing anyway.

 

i'm still waiting for an apology

 

I don’t think so. See his post #7. He is talking about the B mode, not the D mode. He made it clear that he thought the engine never revolved with the ignition cut off, even in the B mode.

Interesting Reddit Discussion from r/Prius | PriusChat

During our last interaction, he made it clear that he didn’t understand how a hybrid electric vehicle (HEV) worked. He didn’t know that the overall fuel economy was also a function of the regenerative braking. He kept insisting that it played no role in the fuel economy because the fuel was cut off to the engine. It is OK not to know, but it is not OK to refuse to learn. He is like a student who refuses to learn. The learning always lies on the shoulders of a student, and there is nothing you can do if the student refuses to learn and even asks for an apology when the teacher points out his/her misconceptions.

 

If he was talking about B mode in post #7, then I would have to agree with your reading. But I don't think he was.

Post #7 itself doesn't say whether it's D mode or B mode he's talking about, so you and I can't be sure just by looking at that post, and I think we are seeing the overall context differently. I think he does believe there is engine braking in B mode.

Perhaps he will appear and clarify.

 

He didn’t even have a basic understanding of how an HEV worked. He never defined what he meant by engine braking either. He probably thinks that the engine is running with the ignition on but with a special valve timing during engine braking. Therefore, when he sees that the ignition is off (EV symbol is highlighted) on the instrument gauge, he thinks that the engine braking is not happening. There is then the whole clutch/transmission argument. So, it is not an isolated misconception but a series of several misconceptions built on top of each other. Then, he blames the others for pointing out his misconceptions and trying to politely explain things to him and demands an apology.