Coasting down a hill, lightly braking, then...

Are you in "D"? If so, it's likely that the battery or inverter temperature sensor has instructed the ICE to run.

 

The engine was probably not burning any fuel during this "run".

Fair chance that what you are experiencing is the car trying to divert energy away from the battery because it is either too full or too hot for good long-term health.

Free watts on the downslope are great, but there really can be too much of a good thing.

 

The Gen 4 Prius does exactly what it is programed to do. I have a bit over 100K on my 2017 and every once in awhile I'll notice something that I haven't noticed before. It happens. It just is. What you describe sounds normal to me although I couldn't begin to tell you why.

 

There are sensors for the temperature for MG1, MG2, Inverter and Hybrid battery, as well as the battery intake air temperature.

TOYOTA have advised:

 

A hill? Kansas? Aren't they called moguls???

What is the ambient temperature at the time, and did the ICE run going UP the hill?
I agree, probably temp related.

Question about Prii hybrid: if the ICE is "running" going down a hill, is fuel being consumed? Going down a hill, my TDIs would "run", but weren't burning fuel, even if the drivetrain raised the RPMs of the engine.

 

I'd suspect if the engine is running it is burning gas, but a minor amount.

I recall on vacation last summer, protracted round-town driving on a warm day, once in a while the engine wouldn't shut down in cases where it normally would. Chalked it up to maybe the hybrid battery getting too warm, something like that, and the computers intervening.

 

Yeah...this is kind of annoying, but I've come to accept "it does what it does". I live at the top of a ~1 mile hill...every time I leave I try to keep the engine off until I get to the bottom, by going into EV mode to get out of my driveway, then ride the brake gently on the way down. But every time it hits 20mph, even riding the brake, on the way down the hill it decides it's time to warm up the engine. Oh well.

 

What you're describing is "deceleration fuel cut-off" which has been a feature of fuel injection systems for decades. The transmission can force the engine to spin. No fuel is being passed through the injectors, so the engine is just a drag on the drivetrain.

 

Yeah think you're right about the fuel cut-off. In the scenario I described car didn't have downhill momentum, was in fact stopped.

 

It's been observed that a stationary Prius will spin the engine for a while without fuel to discharge the battery when the battery is both hot and at a high SOC.

 

That's programmed into it too - think of what would happen (yes, extreme case) if it got to, say 100mph, coasting - and then an uphill rise comes, and you need full throttle to keep up speed - on a COLD ENGINE. Disaster - it's like my dog stretches when he first wakes up in the morning - then he's ready for action.

Trying to chase EV Mode is often fruitless - it often doesn't make any difference to your litres/100km - sometimes, but not always.

 

Preebee, the activity you are describing with your most recent above post sounds completely normal. I know its frustrating when Ice comes on when showing down it does it for two reasons, (avoids use of friction brakes) and #2 attempts to avoid the battery becoming overcharged and so running ICE burns some of the excess battery off. Mine will rarely go above 2 bars to max full FYI . Just figured I'd chime in and say the behavior is completely normal.

 

...and in summer heat, 95f +, ICE cycles very differently than in winter, probably to circulate necessary coolant...

 

I do this too, and I think we're wrong. The Hybrid Synergy Drive is already running, and so the ICE should not mind joining in.

It would have been nice if the system indicated what the ICE is doing though, to satisfy my curiosity.

 

Get this display up - it shows what's running and what direction power is running:

 

That display makes me nervous though: by the time I figure out what's happening I'm almost off the road or about to plow into someone.

 

In the RX you can have that in 3 displays.

And all are different sizes.

At least they give you options to figure out which one is just right.

 

You can get a fair idea with the HUD, if that worries you.