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Discussion Starter · #1 ·
I would like to know more about what occurs with the drivetrain as I drive down a 2,450 foot mountain as I do several times a week.

Ok, as I begin desending, I shift into B. This of course helps the car slow down and I don't need to use the brakes as much. Then, as I get to about 2,000 feet down the battery becomes fully charged as shown on the Energy display, and at that point I must shift back to D to prevent what sounds like an overreving situation. This, despite the Energy display showing that the ICE is not running.

My questions related to this:

What is the sound I hear as I decend the mountain in B even before it begins its fully charged battery louder and higher "overrev" and even afterwards in D, though quieter? The transmission?

If its the ICE, why would the Consumption display show that I'm getting at least 99.9 MPG and the Energy display show that the ICE is not running all the way down? Wouldn't a reving engine cut down on MPG? Related to this, I've lost a pip during this descent--another curiousity.
 

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You can't over-rev - there's a rev limiter, 4500rpm on the Classic, 5000rpm on the 2004-?

What you're hearing is the engine braking - just what you'd expect when using B. The engine spins (pumps air) to help slow you down. At some point, if you have heavily charged your battery on the descent, the engine will be spun by the electric motor just to bleed off that excess electricity.

(If you're going over 42MPH, then the gasoline engine will always spin for transmission reasons, but you will not necessarily be burning gasoline (such as coasting or downhill).
 

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downhill B

Just so. The only thing I might add to MRV's is that there are times when the MFD says that the engine is not operating, when in fact it is.

Some experienced Prius drivers see little or no use for the big "B", but on long descents, when the HV battery loses it's appetite, engine compression is a very good way to keep those wheel brakes from heating up.

Don't mind the noise, and remember to shift out of "B" when the road levels out again.

DAS
 

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Discussion Starter · #4 ·
Re: downhill B

These explanations are helpful...thanks.

tochatihu said:
Just so. The only thing I might add to MRV's is that there are times when the MFD says that the engine is not operating, when in fact it is.
This helps me understand that it's the ICE revving even though not indicated on the MFD. I'm still not sure why the MFD wouldn't show it and also that the constant 99.9+MPG reading apparently isn't accurate during engine braking as a revving engine uses fuel--or could the explanation be that you're not accelerating and thereby actually not using fuel?.
 

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There are many times that the engine may be on, but not showing up on the Energy Screen. (All that shows up on the Energy Screen is if the Gasoline engine is sending power to the wheels, power to the electric motor to power the wheels, or power to the electric motor to recharge the battery)

The engine may be running to provide you with "waste" heat for the passenger compartment. (will not show on energy monitor)
The engine may be running to warm itself up for emissions reasons (will not show on energy monitor)
The engine may be running to do some computer recalibrations, particularly for the battery charge levels (might show up on energy monitor, usually most noticed on Classic Prius when switching to Park after driving in stealth and the engine comes on...)
The engine may be running to power the AC compressor for AC or front windshield defroster use (Classic Prius only) - will not show on the energy monitor

There is what is sometimes referred to as "warp stealth" - over 42mph, the gasoline engine is forced to turn over (spun by one of the electric motors, I think), to protect the transmission. It can be spun without any gasoline sent to it (as seen in my Classic with the Coastal engine run indicator (which monitors when the fuel pump is active). You can be coasting, or going downhill, and the engine will be spinning, but not burning any fuel (electric only). B mode will also spin the gasoline engine as an air pump like scenario, to help slow you down, and may not necessarily be burning fuel. The revving that you're hearing could even be caused by the electric motor spinning the engine, so there is fuel being used (electricity from the battery to the electric motor to spin the gasoline engine), but not gasoline to be burned in the gasoline engine...
 

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Re: downhill B

wayneswhirld said:
This helps me understand that it's the ICE revving even though not indicated on the MFD. I'm still not sure why the MFD wouldn't show it and also that the constant 99.9+MPG reading apparently isn't accurate during engine braking as a revving engine uses fuel--or could the explanation be that you're not accelerating and thereby actually not using fuel?.
From my very limited knowledge and a whole week of driving the Prius. The MFD doesn't show it because the engine is not "running" and burning gas, it's just "cycling" to provide compression braking that was being provided by the MG (generator) until the batt became fully charged. At full charge the MG is taken out of the loop to prevent the batt from overcharging and the engine is put into the loop to provide compression braking.
 

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"the ICE revving even though not indicated on the MFD."

Another explanation is that it an ENERGY screen, and the ICE is not being spun to provide energy.

One could argue it is being spun to bleed energy, but there is no arrow out into the air from the ICE on the MFD display!
 
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