Happy?

why are you using b for acceleration?

 

The "B" is for engine brake and should be used only when descending tall hills, ~500 ft or more. It keeps the engine running nearly all the time which would adversely impact efficiency. Use "D" all the time and life will be much, much better.

Not to worry, the 2003 was easy to shift into "B" instead of stopping in "D". I'm pretty sure that was why we got poor mileage the first time we test drove one.

Bob Wilson

 

+1 don't use B, only on long downhills in Cascades, and only after the traction battery is fully charged.

If you're going through Mojave desert, keep air at higher temperatures, 72F in 110F is not necessary, and have recirculation on ON.

BTW Prius does not get good MPG in mountains, we had seen ~38MPG in Sierra Nevadas, but overall round trip from east coast to west was about 51MPG indicated.

 

When you know the hill is long enough that the traction battery will fully charge with plenty of descent remaining, then I see no reason to wait until that happens. E.g. descending Stevens Pass, I always engage B at the top. This delays filling of the battery, in turn reducing the amount of time of the highest RPM engine whine after the battery does fill. But it still fills well before I run out of hill.

The short hill near my house will just barely fill the battery, but only at the very bottom, and only some of the time. Thus, B mode is not appropriate for that particular hill because it would throw away energy that could otherwise be put into the battery.

I find this to vary considerably with the particular mountains. Shallow, gradual descents can allow the car to glide and make up for the fuel penalty of the climb, and can produce wonderful round trip MPGs. Steeper descents, requiring a lot of braking and/or B mode, don't allow this make-up or 'recovery' and produce poor MPG.

 

it eats up gas mileage. it was designed for long descents, where using the brakes would keep generating electricity, then the engine has to run to burn it off. b spins the engine without gas, so it's more efficient in that scenario. when you use it on level ground, you lose your regeneration, and that costs mpg's.

it's like using a low gear in a gasser to slow the car on a steep hill to slow down without wearing out your brakes.

 

It is not dangerous to do so, it is just wasteful.

 

I mostly agree with the rest of your reply, and dispute only the underlined portion. 'B' is not more efficient in this scenario. It is intentionally inefficient, in order to divert heat energy away from the brakes, reducing the risk of overheated brakes.

Note also that this Prius 'low gear' effect does not apply to acceleration or hill climbing. But it becomes seriously wasteful during what normally should be engine-off gliding, or flat road regenerative braking.

 

thanks, so the grade would have to be sufficiently steep for a lot of friction braking to be taking place?

 

Right, steep and long. If one can descend the hill in D without friction braking (traction battery not full), then B is pointless and just hurts MPG. (There is no mechanical damage to the car.) When light or moderate friction braking is needed, but not sustained long enough to overheat them, then 'B' or 'not-B' is a matter of driver's choice, a tradeoff between engine wear vs. brake wear. Only when the brakes are at risk of overheating does it become important to select B.

The 'all knowing computer' cannot foresee the terrain ahead, so must leave this choice to the driver. Humans do have some ability to foresee this.

 

once the battery is full, it still doesn't matter?

 

I have only seen this only once descending from high pass. When battery is fully charged (another 15% on top full) ECU will switch to B mode. But it will do it when it thinks that friction brakes are working too much, and it is not as aggressive as manual B mode.

I wish I had Torque handy to see what exactly was going on.

 

To delay the time when the battery is fully charged on the long descent ( in summer) I put the AC to max cold. It eates up ~ 1 or 2 kW of power.... By the end of the hill I'm freezing but I don't need to use "B" mode that much

Alex

 

Hah! I have a favorite hill I use just like you. Guilt free A/C cooling.

 

If I didn't use "B" mode or the brakes hard, I'd be doing 90-100 mph down the hills between here and Flagstaff, not to mention those in CO, MT, etc. Cruise control helps with some hills, but not all.

 

Once the battery is full, there is no more regenerative (i.e. battery charging) braking. The only things left are friction brakes, engine compression braking, and air drag.

In 6 years of Prius ownership, I've filled the battery on downhills well over one hundred times. It is just an inherent result both of my normal routes, and of the type of places we frequently travel.

I haven't tried (or just don't remember) switching between D and B after the battery has filled on steep slopes at highway speed, just to test how much engine braking difference there is. But on shallower slopes where the D mode synthetic drag filled the battery, forcing the engine (previously not spinning during low speed glide) to spin up for engine braking, the resulting drag has been light, roughly equivalent to the normal D regenerative drag, not at all equivalent to the stronger B mode drag. For that reason, I cannot yet agree that the ECU ever automatically switches to B mode. Torque or ScanGauge or Ultragauge should clearly show RPM differences.

Downhill cruise control can really wind up the engine for drag. But despite similar effects downhill, CC and B technically are mutually exclusive.

OK, here is the answer to my above uncertainty:

 

Yep... For me that came from older cars that would overheat on long uphill. The way to fight overheating was to turn on the heat full blast... even in summer

Alex

 

I don't remember the hills around Flagstaff as being too annoying, but I do tend to slow down as much as traffic allows as I reach the peak.

When I lived in the foothills of Albuquerque I filled up the battery to green every day. I learned how to avoid deep discharge going back up but I never figured out how to time discharges.

 

If AC can offset most of your 'B' use, then your hills are not that long and steep.

Come out west. We have plenty of hills where you could turn on full AC, rear window defroster, headlights, loud music, roll down the windows for more drag, and still make bountiful use of B mode.

On one past car with a temperature gauge that often climbed above its normal nominal level to what looked high, I could often cool the engine by turning on the AC. That had the effect of forcing on the second radiator fan, which appeared to be intended primarily for the AC condenser. Only the few times when that was insufficient did I need to switch to cabin heat and open windows to cool the engine. (All cars since have had just a single shared fan.)

I have since figured out that that car's gauge was different than my others, without the same 'customer expectation management' tweaks built into all cars since. It probably wasn't overheating at those times when the AC switch did the job. The next car always had a rock steady temperature gauge. Then I got a Scangauge, and discovered that it achieved this stability by collapsing all readings from about 150F to 210F to exactly the same point.