Engine over-reving in Cruise Control

Normal when the HV battery gets too full.

Many threads on this like Unusual problem while going downhill with cruise control | PriusChat and Fully Charged HV Battery==No Regenerative Braking? | PriusChat.

I've seen it on local road when I lived in WA and on the highway going down long downhill grades before. It has nothing to do w/cruise control. It will repro w/cc off.

 

Engine braking with a full battery, to keep you from cooking the brake pads and rotors.

 

The ECU will protect you engine from "over-revving" so there is no possibility of damaging the engine.. It is just that you are not used to the fact that there is little sound insulation in a Prius and you can hear when it revs up (but well below Red-Line). The other difference is that a normal automatic transmission is obvious when it down shifts (My wife thought her Jetta was broken when it made a jarring downshift going up a steep incline) whereas our eCVT smoothly allows the engine to rev up as needed to supply more power.

One note however, if this revving up becomes excessive it is a symptom of a failing HV battery. This means that the electric MGs cannot supply sufficient energy from the HV battery for the requested acceleration.

JeffD

 

Absolutely normal. You are on a long descent, your battery has fully charged so braking won't be by regen, and the car's computer has decided to protect the brakes by using the mechanical resistance of the engine (no fuel being used) to slow down the car. You could cause the same thing yourself by shifting into "B" on another decline. It won't be as dramatic as what you've experienced, but the same activity will occur.

 

If I understand the situation, you were on a 6% slope going a steady 70mph, and you weren't applying the brake. The engine is what was keeping you from accelerating, so it was doing the braking for you.