On a Toyota Knowledge Base (http://www.kbtoyota.com/priusmileagetips.html) I came across a statement that said it is necessary to keep the speed of a Prius below 42MPH because the motor-generators (MGs) reach their maximum RPM at about 42MPH. That (according to the statement) forces the engine to run to prevent damage to the MGs.

I am not a Prius owner - but I've borrowed a friend's Prius on several occasions and did not notice any drag when I was "gliding" above 42MPH. The LCD display screen showed the engine was not operating. I can understand if you are going down a hill above 42MPH and you're not gliding (letting the engine do "normal" braking like it would in a automatic transmission vehicle) that the engine will run.

So...is it necessary for the engine to run if you are completely "gliding?" I'd appreciate your thoughts. Thanks.

 

The screen is an Energy Monitor. As such, it shows energy FLOW. If there's nothing flowing, there will be no arrows. Whether the engine is on or off makes no difference in the screen unless energy is flowing.

Above 42mph, the engine comes on to spin and keep the electric motors from overspinning. So while you can technically glide above 42mph, there's an extra drag due to the engine spinning.

IOW, "for best results" (to use a cliché), stay below 42mph.

 

Actually, above 42 miles per hour the ICE is being spun by the MGs, and not by fuel, to protect MG1 from over-revving. No gasoline is used, and the ICE is spinning "open loop." This spinning of the ICE causes a drag on the wheels (i.e., "engine braking" — small in 'D' but much greater in 'B') via both the electrical path between the MGs and the mechanical path through the planetary-gear set. Since no fuel is being used, the ICE icon does not appear in the display. There is indeed power flowing from the wheels to the ICE. See the thread "Heretical Mode" in the TCH forum for much more detail.

Stan

 

It's called "stealth mode" below 42 MPH (66 km/hr) and "warp stealth mode" above. While it is easy to get into below 42, the "band gap" to get into warp stealth on the accelerator pedal above 42 is very narrow. In warp stealth the engine turns at 960 RPM according to the Scangauge II.

 

In the TCH, the car drops in and out of "warp stealth" mode very easily when coasting at speeds above 64 km/h (~40 miles per hour), when fully warmed up. It is easily detectable on ScanGauge by the fact that the ICE goes "open loop" and, as mentioned by David Beale, the ICE rpm stays almost constant (at ~1000 rpm in the TCH) as it is spun by the MGs. Indeed, if the TCH is coasting downhill in say pure-EV mode (ICE not turning), then as its speed begins to exceed 64 km/h, the ICE will start to spin at ~1000 rpm to protect MG1, but it stays open-loop using no fuel. If the speed then drops below 64 km/h as the coast continues, the FE gauge will drop back into the blue "E mode" range.

Stan