IC engine starts on a descent with battery fully charged

Mine does the same thing. I don't know why. My IC engine comes on at odd times so I stopped trying to figure it out. I am sure someone on this site has an answer though.

 

It is perfectly normal. When the battery is fully charged and there is no room to put more electrons when regen is working, the engine starts. This tendency is more pronounced when the ambient temp (thus battery temp) is colder. In the summertime, there is probably just enough room to put the regen energy back into the battery even though it is full. In winter with a cold battery pack, the car decides much quicker to start the engine mostly to protect the battery.

Once the engine starts, it has to complete the warm-up cycle. It won't stop until then. There are some other instances when the engine starts or the engine will not stop as stated in the manual.

 

I have noticed similar behavior when descending long grades with a full battery, but noticed that the engine gets up to very high revs. I think what is going on is that the computer is trying to use engine compression as braking since the battery can not take any more energy out of the normal brake-to-battery recharging path.

 

Just a pendantic nit: I believe that with gasoline engines, it is really manifold vacuum braking being used to dissipate energy when the battery is full .

Diesels use compression braking (Jake brake).

 

Here’s what confuses me. When I’m reading battery charge is gone, going slow with the ICE, and suddenly it shuts off and gets quiet and seems to be running off the electric miles that aren’t there. Sometimes it happens when parking for a moment, sometimes when slowing down but not stopping.

 

You are driving HV mode with EV. After the EV mode depletes the battery, there is still ~10-14% SOC remaining in the traction battery. You are now driving the car as a regular hybrid Prius. Initially, the engine will cycle for warmup but after that, the car can drive in EV (not the EV Mode) and cycle between the engine and battery. If you drive gently and as long as the terrain does not require a lot of energy, you can coast and glide in "EV" for a quite while.

 

I came from a 2014 Prius and was surprised and delighted when I saw my new Prime using the battery at 65mph in HV mode (on the older model it would not operate at over ~40mph). I have a 35-mile one-way commute with most being on a highway and the last ~4 miles in an urban area. What I've discovered is that once I've depleted the traction battery the EV is running about half of the time on the highway. Then, when I've turned off the highway and into city streets I'm almost always on battery, except for inclines. This specific route gives me ~350mph recorded for the 35 mile trip.

 

If your commute did not have the highway speed, you could make the entire 35 miles in EV mode for the 999.9mpg. My commute is about 18 miles one way. During summer and warmer months without using HVAC, I can sometimes make the round trip on all EV modes. I drive on a rural road, with no stop and go, and a speed limit of 45-50mph at an average speed of ~30mph.

 

I've always had the 999.9 mpg quest per trip since driving the 14 PiP. With the 17 Prime Plus the gauge is limited to 199.9 mpg so I have to do some manual estimations ie: did I get 999.9 or somewhere between 999.9 mpg and 199.9 mpg per trip.
Still, 350 mpg for a trip, no matter how long the engine runs, is not too shabby or pretty damned good, considering how fast mpg go down from 999.9 when the engine switches on. It's about a 3x slower reduction in the mpg reading in the Prime than it was in the 14 PiP (estimated). but still, not bad for mostly highway driving.

 

30 seconds of warmup cycle is not very long and indicates the winter temps in your area are not extremely cold. Some of us have to drive a mile or 2 miles and some probably even more, before the engine will stop running.
The two easiest things to remember when the gas engine switches on are temperature and speed. When temperatures start to go lower, the Prime behaves differently and trigger points for the gas engine coming on change and continue to change subtly as temps fall lower and lower.

It gets more complex in how long it takes after the the gas engine turns off as to when the EV mode works much closer to how it behaves in summer time. That usually takes at least a few more miles of driving slowly before all the components warm up enough to provide semi normal EV behavior.

So the easy way to learn where those tipping points are during your trip down the hill is to slow down to a speed where the gas engine doesn't turn on. You'll have to find a time to experiment when there is no traffic around, because the speed you may have to slow down to ( although not likely in your situation ) can be dangerous if there is traffic around you.

It's still cold enough locally after sunset that it takes 1.5 mile (2 - 3km) for the warmup cycle to complete for me. It's probably a lot warmer than that for you locally now 3 months after posting your question, although it may help you next winter, at very least.

 

That just happened, and you are absolutely right, I avoided the highway and used secondary roads. It was right at the edge of range, but there were enough small hills that I could generate a percent or two of traction battery several times. It took a lot of thinking and active power management, but clearly I can do it on my commute with some planning.