Engine started when I activated cruise control?

There are a couple of threads that talk about the engine running at various times to keep the gas fresh and the engine lubed, etc.

It happened to me the first time as I entered a drive through car wash. I thought that something was broken at first! All was fine when the engine shut down a few minutes later.

 

Volt would give a message if start up was for maintenance. Don't know if any past Toyota PHEVs did.

 

As far as I know, no one has ever proven it's that.

There are lots of reasons the engine starts. Some of them are logical, but the cruise control one is likely a bug.

Off the top of my head:

• Using cruise control with >80-90% battery
• Pressing the front defrost button
• If the available power from the battery is limited (due to cold weather or low charge), particularly if you demand acceleration that's close to or exceeds the available power
• Too much regen into the battery, for example
  • Using B mode with a nearly fully charged battery.
  • Extended braking with a cold and/or nearly full battery
  • Driving down a long mountain grade, regardless of SOC.
• Using the heat below 12F
• Driving above a certain speed (84mph?) I don't think I've ever driven the Prius that fast.

You can use the Hybrid Assistant app to watch the DCL and CCL. They are basically the charge and discharge limits the car determines to protect the battery. If either starts dropping rapidly, the engine is likely to start soon if the condition causing the drop persists.

If the battery temperature is around or below freezing it's extremely difficult to drive in EV mode. If it's plugged in the battery heaters will keep it warm for a few days, but after a few days or if it's unplugged for an extended time the battery will get cold. I think I've seen down into the 20's F after leaving it parked a few days ona ski trip.

And if the outside temperature is below 12F it's extremely hard to drive in EV mode. My personal best is around 9F with the heat off (seat heater on of course). I had to open the windows to keep the humidity down. Not particularly pleasant.

 

The Toyota engineers are just screwing with us.
This happened to me (again) last week. No message, no LED, no nothing, The engine just came on.
Personally I stopped loosing sleep over it.

 

As your owners manual explains, there a multitude of reasons the ICE will run and it lists SOME of them. The vehicle monitors so many parameters and protects itself. One of the very crucial parameters is the hybrid battery temperature and it anticipates what may happen, then takes action to prevent it. The hybrid battery temperature will increase due to current flowing in or out. It could be the system is anticipating a temperature increase (or a multitude of other parameters that are trending in a direction it feels it needs to mitigate) and it just so happens that it is occurring at the time you are engaging the cruise control.

 

Another one I have heard of, which I haven't verified, is coming to a full stop in B mode (could cause the engine to start). I always switch back to D before coming to a stop to avoid that situation. I think I have accidentally stopped in B mode once or twice without the engine coming on, but under some conditions it might.

The cruise control one is surely a bug, which is why it isn't documented. They haven't fixed it because they aren't Tesla. They will only fix things that are a major problem (if there's a recall or enough people complain). It's pretty harmless as far as bugs go, but it also gives me concern about their software quality. This car was developed around the same time as the whole unintended acceleration fiasco.

Using the Hybrid Assistant app, the temperature, at least reported, doesn't change quickly. That makes sense because it's a big (250 pound?) mass. But the DCL and CCL limits do change, for example too much regen for too long will eventually cause the CCL to drop, and if it drops close to how much regen you're demanding then the engine will start. I assume they have some kind of thermal/electrochemical model that models what's happening at a microscopic scale inside the battery using known parameters like current over time, battery temperature, etc. It's interesting to watch Hybrid Assistant, and what the car is doing makes more sense when you have a little more info. I assume the battery protection algorithm is extremely proprietary, so we will probably never know exactly how it determines the safe limits.

Finally, I will add an observation that the charge and discharge limits seem to depend on the "freshness" of the charge. If you let the car sit for a few days without charging (even in mild weather), it will be easier to reach a condition where the engine starts next time you drive it. That is a reason to try to charge it as close to when you plan to drive it (in addition to being better for battery longevity and allowing the battery heater to work outside of Canada in the winter).

 

I've had a few random ICE starts. Been told if you are in EV mode and smash the throttle, it will sometimes start ICE a few seconds later even after no longer smashing. Maybe it thinks the smash and current hwy speeds means it thinks you will need full ICE+EV power soon. I wouldn't worry, seems semi-common and harmless.