Several times I've been going downhill in "B" and the engine suddenly revs up to a very high rpm -- loud enough to hear.

Most of the time, shifting into "D" takes care of it, but once I had to shift to "N" to slow it down.

This over-revving only happens maybe 1 in 10 times I'm going down the same hill.

Any idea as to what's going on?

 

Engine braking. Putting the vehicle in "B" causes a higher rpm's, which is what slows you down via the engine. Think of it like shifting into a lower gear, the engine will get louder because it is working harder. I rarely use "B" because it puts more wear on the engine and I'd rather wear out my brakes than my engine, brakes are cheaper to replace.

 

Tell me more about this 'hill'. What would really help is if you could use "TopoZone" to identify the specific hill you are going down. This will give us the length and altitude change(s), something to identify the grade.

It would also help to know your entry speed to the grade and what typical speed ranges you are seeing. The reason is the amount of 'braking force' required is a function of your speed. The faster you go down a hill, the greater the retardation force needed to control the speed.

There is no reason to worry about the ICE over-speeding. It is computer controlled and the controller will prevent any problems. It may seem like "sound and fury" but it signifies 'nothing' to worry about.

Given your report that it happens "1 in 10 times," I'll speculate that it is a function of your traction battery "State of Charge." It may be the higher revs are associated with the battery becoming fully charged and unable to absorb more energy. In that case, the ICE has to work, effectively trying to 'go into reverse' to slow you down. But absent other observations such as MFD display arrows or MPG changes or auxillary instrumentation, this is pure speculation.

I have tried going down some local hills in my NHW11, 03 Prius, at a pretty good clip, entry 70 mph, using "B" and so far, I don't remember hearing the engine race. Using "B" works and my NHW11 Prius reaches a near steady-state descent at 65 mph. If I enter too slow, the vehicle slows to less than 30 mph while descending. If I enter too fast, it will try to run away but a light application of the brake brings it back to a stable speed.

Bob Wilson

 

The next time it happens, I'll check the battery state.

http://www.topozone.com/map.asp?lat=41.07212&lon=-74.24627&size=l&symshow=n&u=4&datum=nad27&layer=DR G

The road is called Skyline Drive and seems to drop 300 feet in about a mile.

Thanks

 

Nice hill! It looks like 320 ft, from 920 ft. to 600 ft. What is your target speed going down?

My hill is a little taller, ~480 ft., and a little longer:

http://hiwaay.net/~bzwilson/prius/pr...r_Brindley.jpg

The "Heart of Dixie Highway" is posted at 65 mph and "B" handles that very nicely.

Bob Wilson

 

I hold it between 35 and 40. The road is twisty; only one lane in each direction. One stretch I have to brake, otherwise "B" works fine, except when it revs. Then I have to brake.

 

You don't have to brake when the engine rev's up! Bob explained the high revolutions are perfectly normal. While in "B" mode just let the Prius do its own thing unless "B" is not slowing enough.


Wayne

 

Grades steeper than 6% are rare on paved public roads, and that's about what you are looking at. The use of Prius B is apparently somewhat controversial. No doubt that it can be overdone, but I wouldn't say that engine wear is a major factor. It makes more noise, there are some unusual things going on inside, but nothing harmful to the engine.

I have driven 2001 Prius almost 100k miles, and I use B on all long descents. Whether 300 feet qualifies as 'long' might be a matter of mood. One thing that is different about the newer model Prius is that you are not allowed to use cruise control in 'B gear', but that may be unrelated to your observations.

DAS

 

From a 'human engineering' stand point, I would prefer "B" require a 'release' before engage. The problem is when you leave "P", you hold down the button and have to stop at "P". If you just ride the handle to the stop and let up, you' re in "B" and take a mileage hit. I'm convenced this is what happened to my wife when we rented a Prius in 2001.

Bob Wilson

 

I watched the Energy screen on the way down my 300 foot hill and, sure enough, as soon as all the battery lines appeared, the engine revved up.

I'm having a hard time believing that the high revs are OK for the engine; it just doesn't sound right.

I want to figure out how to get the battery as low as possible on the way up the hill. Any suggestions?