The "B" shift is much improved in the Gen III Prius. I got to experiment with a 6.5% downgrade at various speeds to see how the "B" shift helped slow the car on steep downslopes. I was used to the Gen II were the "B" just helped slow the car but braking was crucial to maintaining a safe speed. The Gen III "B" astonished me with it's control of speed and flexibility. At 65 mph, I slipped into the "B" shift. The car did not accelerate past 65 even though the grade got steeper! I approached a 45 mph posted curve and braked to 48 then let off the brake and still the car held that 48 with only the engine revving away. Still in "B" shift, I accelerated to 55 and let off the gas..... the car maintained the 55 as long as the grade steepened! It seemed to be like a cruise-control, the engine revving and slowing as necessary to keep the car at the chosen speed when "B" is selected. I am really impressed with this improvement over the Gen 2 which only helped on the steep grades. Gen III with it's more powerful ICE is able to control speed effectively on serious downslopes. Lee
Yup, good observation. That's certainly nothing I'd ever use often enough to notice a pattern on. Of course, don't give the ICE all the credit. I'm sure the bigger MG2 contributes to the improvement too. .
What's actually going on in "B"? Is the ICE running (ignition and fuel)? Is there still regeneration?
There is a lot of detailed information already on this site, but in a nutshell, B mode spins the engine without fuel or spark. The engine acts as an inefficient air pump, wasting energy. Regeneration also occurs, assuming there is room in the HV battery. Tom
I had used "B" when descending from my hill climb tests and noticed it was more effective but didn't quantify the effect. Thanks! Bob Wilson
That's pretty cool, Lee, and thanks for posting your observations. Here in mountainous Chicagoland I'm sure to get a lot of use out of that.
That's good know. I think it is a cool feature... Downhill Cruise Control. I think it is the first of it's kind. It is probably one of the 1,000 patents loaded in it.
I wonder what the maximum grade/load is on the ability to maintain speed. Could you sustain a (safe) constant speed for the entire trip down Mt. Washington with a full car? I'm planning on visiting family up that way and I was looking forward to trying to hike up (I did it once when I was a kid). I seem to remember hating the down trip much more than the up. I think I could be talked into being a passenger for a trip down... Does this sound too contrived?
My parents have a very interesting story related to that: My dad and mom took a trip to visit friends in Colorado and my mom kept complaining how close my dad was getting to the edge on mountainous roads. (She in the passenger seat was closest to the edge, of course.) My dad brushed it off until the day that their friend took them in his pickup truck to the top of some mountain. My mom sat in the middle, so my dad was by the passenger door this time, and the friend was very familiar with the mountain so he took it very fast. My dad was "cured" after that. Back on topic: as I remember Mt Washington, you spend most of your time -- at least near the top -- on the drop side of the road going up, and on the mountain side of the road going down. If my recollection is correct, I'd definitely prefer to be a passenger on the way down.
To emphasize for newbies, "B" always reduces fuel economy. It should only be used to avoid riding (and possibly overheating) the brakes on a long downhill. For example, there is no place in northern Indiana where "B" is ever needed.
I don't know about that, Richard. Coming down the other side of the I-65 overpasses can be rather harrowing at times. I mean, we're talking about an elevation drop as much as thirty feet here. Just kidding, of course.
I'm not sure about Mt Washington. Out West here, the longest and most steep run I have experienced is dropping down into Death Valley from about 7,000 ft. to below sea level! On my '07 the "B" shift was unable to slow the descent without additional braking. I can't wait to try it on the 2010 to see if I could maintain speed on that run. My first thought when I saw how effective this "Downhill Cruise Control" was, "cool, I'll never have to replace these brakes!" Lee
