The N Gear, what's the mechanism?

Below 42 MPH "N" gear is basically just running with both MG1 and MG2 de-energized, and some people use "N" as an easier alternative to trying to glide (without arrows on the MFD).

At and above 42 MPH however the engine has to spin, so if you select "N" it will use fuel to idle the engine whereas if you can manage to glide in "D" it will still spin the engine but without fuel injection. Even if you can't sustain a glide, and fuel injection begins, then at least that fuel is actually being used to propel the car rather than being wasted in just idling the engine!

The one exception is, when descending a hill if you select "N" before you reach 42 MPH then you can continue a true "N - glide" (engine not idling) above 42 MPH. The problem with this is that it causes MG1 to spin at more than 6500 RPM, which may be detrimental to the health of the transmission.

 

Adding to the above information, MG1 and MG2 are always connected to the ICE; there is no way to keep them from physically interacting. In N, the control system removes the field excitation from MG1 and MG2 so they can freely spin. Because of this, the wheels no longer spin the ICE (or the other way around). MG1 and the ICE are connected through a differential just like the drive wheels on a normal car. If you lift one wheel from the ground it will freely spin while the other stands still. The same happens with MG1 and the ICE.

The problem comes from the rotational speed limit for MG1. In a costing Prius, MG1 freely spins to keep the ICE from needing to turn. The faster the car coasts, the faster MG1 must spin. There is a limit, and when it is reached the Prius starts the ICE to keep MG1's speed under control.

The Prius ICE cannot start or stop in N (by design rule). If you shift into N below 42 mph with a stationary ICE it will stay off, no matter how fast you go. In theory you could damage MG1 by coasting down a big hill.

As for the legality of coasting in N, it is illegal in most states. The law dates back to manual transmissions, where it could be difficult to regain control once coasting was started. The law makes no sense with a Prius, so you can safely ignore it, other than the risk of being cited on a technicality.

Tom

 

Nearly everything I could have said has already been posted to this thread.

For you point here, I don't believe that selecting 'N' in the above glide situation offers any measurable savings over merely pressing the accelerator pedal just lightly enough to make the HSI bar disappear (Gen3) or the power flow arrows disappear (Gen2). This will accomplish almost the same thing as selecting 'N' -- the very slight remaining power flow back and forth is negligible -- but power is instantly and instinctively available if you need it, without fumbling for the shift lever.

If your foot is completely off the pedal, the synthetic engine drag will produce some small but very real conversion loss as the battery is weakly charged. But light pedal pressure should get rid of it.

 

Practice modulating the gas pedal to achieve the same effect, and you'll be safer. No fumbling for the shift lever if you suddenly need to accelerate.