Other techniques besides P&G?

My locale does not have extended flat terrain where there is no reason to slow down AND always drive slower than 45 mph. Elevation changes, traffic, turns, lights and stop signs all give me reason to P&G

To answer your question, if that type of terrain was present, I wouldn't P&G because it would annoy me. I don't doubt it would be the best method for highest fuel economy. By the way, you do not need much elevation change to have a reason to P&G. If you can glide 'down' from say 45 -> 40 mph over 200+ meters, then by all means.

My point is really to match technique to terrain not just for fuel economy, but also for your driving enjoyment and some consideration of other drivers. Personally, I tend to avoid large speeds swings if I do not have a terrain reason.*

*That comment was for low speed driving. On the highway, if traffic allows I lose a lot of speed going up long/steep hills in order to avoid revving the engine and driving in the POWER range. Wayne Gerdes over at cleanmpg.com calls this 'driving with load.'

 

How much energy is used in 'D' if the engine is off and no current is leaving or entering the battery ? I thought Zero*, but in truth I do not really understand 'N' on a mechanical level in a Prius.

*In both cases I am ignoring the drain that the electronics use for the car to be 'ready.'

 

In "N" mode, all power electronics to MG1 and MG2 are off. If the engine is running, it continues to run but there is no torque . . . As if someone pushed in the clutch of a manual transmission. Also MG2 has no power, a free spinning rotor.

Bob Wilson

 

If I am understanding right, this means the gears spin from inertia ?

Interesting, but if I may go back to my original question: how much *more* power is consumed in a 'D' glide compared to 'N' ?

 

It depends upon which side of the Power Split Device. Everything from the ring gear to the wheels are fixed ratio. If the wheels are turning, so too are the gears.

The engine shaft is connected to the spider gear carrier. If the engine is running, the carrier turns at idle rpm. If the engine is off, it doesn't turn. That just leaves the spider gears free to rotate between the outer, ring gear, and the center, sun gear connected to MG1.

Since MG1 is not powered and the spider gear carrier is held by the engine, MG1 will rotate if the wheels are turning. The spider gears on the carrier transfer the motion. The risk is with the engine OFF and speeds above 42 mph, MG1 will spin very, very fast, ~10,000 rpm. As the vehicle speed increases, MG1 spins faster and faster until it mechanically 'disassembles itself.'

I have never heard of this happening in service but it would be a bad thing . . . we call it 'grenading.'

When descending in "N" with the engine off, I always shift briefly into "D" so the engine begins turning and then back into "N" if the speed is OK. With the engine turning the spider gear carrier, the spider gears can not over spin MG1 and the risk of it flying apart is reduced. For example, our 2003 Prius descending a tall hill with the engine running above 42 mph, around 55 mph, the engine begins idling faster. This keeps the 2003 Prius MG1 rpm at the 6,000 rpm redline. Later Prius have a 10,000 rpm redline, or so it is reported.

So the rules of "N" are:

• speeds under 42 mph - always safe engine OFF (desirable) or ON (idle fuel consumption)
• speeds over 42 mph - always safe with engine ON (idle fuel consumption)
• DO NOT COAST DOWNGRADE IN "N" WITH ENGINE OFF AND SPEED OVER 42 MPH
  • Good Prius Friend Hobbit did this experiment and it did not grenade, NHW20
  • I did it up to ~48-49 mph and it did not grenade, NHW11

Bob Wilson