Engine over-running on steady cruise

I bet you were going downhill and the battery filled up via braking. If so, the engine starts to work as an engine brake, when you speed up, you need less engine brake.

 

Yep, I'm also thinking that you left out that one little detail about the traction battery being full to 8 green bars.

 

And there is no "runaway Prius syndrome". There IS a "nut behind the wheel runaway Prius syndrome".

 

Do you remember if it was hilly terrain?

 

Do you know it is called "Energy Monitor", not "Engine Status Monitor"?

Ken@Japan

 

my guess is that it has to do with how cold it is where you are.
typically the prius will run the ICE even if its not necessary to propel the car, just to maintain ICE temperature.
at any speed above 42 mph, the ICE is always spinning anyway, even if the ignition is not actually firing. because at speeds over 42 the ICE goes on and off constantly to maintain speed, maintain battery charge, and yes, maintain ICE temperature.
so if it is cold where you are, and you happen to be driving on electric only for a short period of time, and i am guessing you are also running the heat to keep the cabin warm, the ICE will kick on, especially if you are running the heat, it will draw the ICE temperature down quicker, and cause the ICE to run more just to stay at optimum temperature.
and if you are running the heat, it draws more electric, so the ICE might just come on to charge the battery also.
so i say it is cold where you are, you dont have the grill blocked, and you are running the heat and possibly other cabin accessories
this is normal ICE activity for the prius.

 

also at 70-80 mph, when the engine does come on the transmission will sound like its revving pretty high.
the prius has a CVT, so it will automatically adjust the transmission to the optimum ratio for speed ,efficiency, and charging.
at 70 to 80 mph its going to choose a ratio pretty high up to match up the transmission with the cars speed and charging needs.
its a tiny engine, only 75 hp, to make any kind of power at 70 mph it has to rev high.
its not like a big V8 engine that can chug along at 70 mph and 2000 rpm, due to its massive internal parts.
the prius engine is more like a motorcycle engine, all its power is in the high revs.

 

prius engine is 1.5 L or 1500cc redline 5000rpm
2010 is 1.8L

 

Hi Svllee,

This sounds normal to me. The engine will rev to climb grades, and reduce in RPM down grade. There are also invisible forces, such as wind drag that occur. Come around a bend into a head wind and the engine will increase in RPM. Drive back out into the wind from under a bridge and the engine will rev to compensate.

A traditional car has fixed gearing, so, the engine stays at a constant speed and the torque to the driveshaft varies with throttle opening.

The Prius operates close to a constant torque, so when more power is needed the engine revs up. This is more efficient, because on average, the number of engine pumps per kilometer is less. And thus there is less systematic pumping losses. As the engine can run at a minimum RPM, and at approximately max unenriched torque.

Completely normal.

 

I'm pretty sure this was not normal revving to climb grades. Look at what the OP said :

Look at that and I think it's obvious that he's going downhill, and probably a significant grade. So though the OP didn't notice the SOC I think it's highly likely that it was full at 8 green bars and forcing the system to engine brake (by using MG1 to spin the engine as an air pump). Case closed in my opinion.

 

This is the most likely explanation, given the few details.

Tom

 

why would the engine management system engage engine braking, if he's not trying to slow down?
if the operator mentioned using the brake pedal, or going down a steep grade, then yes, engine braking.
but he is talking about maintaining speed with minimal throttle input, no mention of braking.
i agree with your description of full green bars and needing to slow down thus engine braking engaged.
but i am not so sure that it is what this owner is talking about.
we need more info!

 

Well yeah there wasn't a lot of info from the OP so you have to decode it to get the most likely scenario.

"I recently took a long drive in France" - Lots of big hills/mountains in France.

"and on 2 occasions, I heard the engine revving up when just simply cruising steadily at 70-80mph with little throttle input" - That's a whole lot of speed for "little throttle input" which suggests downhill. Yes?

"I checked the LCD screen drive-train diagram, and there's no indicated of the engine turning during the revving"
Here he means that there were no arrows from the ICE displayed. This situation only happens when the ICE is not supplying power. So either running on electric motor or regenerating. Note that "little throttle input" does not mean that it's not regenerating. The very fist part of accelerator pedal range gives simulated engine brake and therefore regeneration. Also note that power is the product of force and speed, so at high speed you don't necessarily need a lot of braking force to get a fair bit of regenerated power.

"The car does not accelerate during the revving. Happens say for 10 - 20 seconds each time. To stop the revving, I simply give it a bit of throttle."
Stops revving when you apply so go pedal so there's the clincher. Why does this stop it? Simple, because by applying some go pedal you're stopping the regeneration and giving it another outlet to burn off some battery energy by actually powering the car.