Can you shift too much?

i guess the question is that does being in B mode, increase the amount of regenerated power? as opposed to normal coasting?

if the answer is yes, then it could regenerate more power under many conditions when braking would be necessary to slow the car enough.

if the answer is no, then it would then obviously be taking some regeneration potential in most cases away when braking. the ideology being that B mode slows the car faster resulting in less braking necessary. we also have to keep in mind that if rapid slowing of the car is required, then actual friction brakes would be employed reducing the amount of regenerative power captured.

i had been experimenting with using B mode as frequently as possible (although each instance was of duration lasting 20 sec or less) to see if more power was generated as i look at B mode as a gentle braking that is a bit more effective than just straight coasting and it lends to a better flow of traffic as i can go into B mode much later than if i were coasting.

my initial results did give the impression that more power was regenerated since i got my first ever 150 wH regen and then a 125 wH regen on the next 5 min segment. and i did see an increase in mileage. this was done on surface streets at speeds ranging from 30-45 mph.

but now that i've been doing it longer, it now appears that the increase in mileage is occuring for all types of city driving. so the increase could just be attributed to my straightening of the learning curve for hybrid operation.

reviewing the manual doesnt give any kind of indication that a higher level of regeneration is going on in B mode and so i called my salesman and he says he thought that B mode does regen at a higher level, but when i pressed him, he admitted that he wasnt 100% sure. he also said that the level can be achieved through the brake pedal if you have a steady enough foot.

so i guess phase 2 of my experiment will involve seeing if i can reproduce the 125 or 150 wH regen levels without using B mode.

 

Richard Schumacher said:

"No. "B" adds engine braking, that is, it uses the engine without fuel or spark as a big air compressor. To the extent that this happens as you slow down you have lost energy that might otherwise have gone into the traction battery, and thus your economy will be reduced."

Response:

Richard, what you say about engine braking makes sense (see Graham's PSD link below): Asssuming a counter force is placed on MG1 to keep it from spinning, or at least to reduce the rate at which it's spinning, then the planet carrier will be forced to turn, turning the engine, an engine working in a sort of jake brake mode (as you say, no spark, no fuel and maybe quite possibly opening only the exhaust valves at the end of each stroke - as done in trucks - the air compressor analogy).

But consider this, going down a hill you wouldn't be using MG1 as a charging source anyway, you'd be using the drive motor, MG2 in regen mode. So really, there's every reason to believe that MPG would not suffer.

Graham's link

http://home.earthlink.net/~graham1/MyToyot...tm#EngineStarts

 

Note that the Prius has no transmission. When you "shift" you are merely telling the computer to apply different conditions to the ICE and the MGs, so that different stuff happens. With no transmission or clutch, and with the brakes getting much less use as most braking is regenerative, there's a lot less to wear out on the Prius than there is on a conventional car.

Should we start calling conventional cars "dinosaur cars" or "obsolete cars"? Or maybe "landfill cars"?

 

Going down a long hill will quickly charge the traction battery in either mode, in which case "B" should not reduce MPGs (as you say) because the battery is going to get all of the energy it can hold anyway. But in more average driving, since "B" forces the use of the engine brake to achieve a more-rapid deceleration during coast, some of the energy that could have gone into the battery will instead be used to heat air. It's the same effect as if one pressed the brake pedal a little harder and engaged the friction brakes. MPGs have to decline, which is exactly what the manual says will happen. (Is all this explicitly documented somewhere? Not that I know of; this is inference based on what I know of the car's construction and what the manual and other sources say about it's operation.)

Personally I'd prefer the car to decelerate at a slightly greater rate than it now does while coasting in "D". That deceleration rate could be limited by the most efficient charging rate of the traction battery (in which case a larger traction battery would be needed), or, in contrast, it could have been set as Toyota's estimate of the most satisfying/familiar behaviour for people who are accustomed to driving a conventional automatic transmission (in which case there could in principle be a control to adjust the rate). At any rate :_> I'm not going to use "B" and waste some energy merely to increase the deceleration rate.

 

well if the B mode doesnt initiate a greater regen rate than normal coasting then Toyota dropped the ball here.

as i said before initially, it appeared that B mode did charge more as inidicated by the cars on the consumption screen.

as of yet, i still have not been able to reproduce the 150 wH of regen that i did when using B mode at every opportunity. but i need to experiment a lot more as i do not have near enough data yet to make any real conclusions.

and i still believe that when they mentioned that B mode hurt gas mileage that was only if trying to accelerate in B mode. which makes me wonder how B mode would operate during acceleration if it is using the ICE as a drag??

 

True enough. Maybe "B" *does* increase the regen rate. The ideal overall average regen rate (presumably close to what's used in "D") is probably less than the maximum battery charge rate. One doesn't want to incur conversion losses needlessly in the hope of avoiding friction losses in the future that might or might not occur (there's no way to be sure how much braking you will have needed; that is, are you going to make it through the next traffic light or not?).

I can't take the stress of uncertainty! I'm gonna sell it and take the train.

 

i know of a few buyers for your 2004!!

 

I haven't seen anything to indicate that "B" provides additional regeneration, and doubt that it does. After reviewing all the "How it Works" documents, it appears to me that "B" is simply a Jake Brake. There's no more or less regeneration in "B" than in "D" for a given state of charge.

 

First let me make it clear that I don't know what I'm talking about--this is just speculation.

My understanding is that the B setting puts you into a mode where it uses engine braking and/or additional motor regen to slow the car down. Putting on the brakes softly (in D) also produces additional motor regen, so the regen is pretty much a wash between B and D. I can envision times where you're doing so much braking (is regular braking called 'degenerative'?) that the brakes could overheat and possibly fail if they didn't have some assistance.

If this is all true then I would say that B mode is not something that should be used for everyday driving. It probably doesn't do any harm, and may make you brakes last longer, but that engine braking will probably reduce your mileage.

 

how B mode works is still a wonder. but its safe to say that excessive use of B puts undo strain on the system and i think that if used it shouldnt be used for long periods of time.

as for me, i was using it in an attempt to see if it provided better regen numbers than braking. also i used it when regular coasting wouldnt slow me down in time to avoid hitting traffic in areas where i knew that lights slow progress to a crawl.

regen slows the vehicle faster is might be preferable than coasting in traffic since coasting a very low speeds would irritate anyone stuck behind you. so in my case, B mode was never engaged for longer than say 15-20 secs at most.

im not sure i would recommend it for someone in heavy traffic since that would be one more thing that would take away from your concentration on the traffic around you. i was only doing it to get a better idea of how it works.