Unexplainable occurrance

in the o/m, they mention a number of reasons the ice might fire. and they say there are other reasons as well.

 

Once the ICE is triggered; it won't go dormant until it reaches operating temperature.

 

Completely normal. I would not worry about it. I have posted this copy of the manual section many times for answering the same question as yours. Toyota in its great wisdom reserves the right to operate the engine whenever the car wants it to operate. It is clearly stated in the manual.

That being said, after owning 2017, 2020, and now 2021 Prius Prime, I have noticed that Toyota may have tweaked some settings in the car that my current 2021 PP is far more sensitive to start the engine even when the traction battery has charge and none of the other conditions listed below were met. So, it is going to happen again on your car, and likely mine. But if that happens again, then just take a deep breath, relax and enjoy the ride. Everything is going to be fine even though we can't explain why it happens, The system knows it's time to start the engine.

 

Is it possible you were descending a hill when this occurred? When the SOC is high and you brake while descending a steep hill the engine may start in order to provide engine braking since there isn't much space in the battery to store regenerated energy.

Another possible explanation, did the car sit unplugged overnight? I had this happen one time with the battery around 15% SOC, it sat unplugged overnight in the cold (30F-40F) and presumably the battery being cold was the cause. If not, do you have the traction battery heater option enabled under the vehicle settings menu?

I will say, my 2021 has never started the engine for an unexplained reason. The only things I've seen it start the engine for in EV mode with a charged battery have been the event I mentioned above^, front defrost, and exceeding 84 MPH. I may seem to care too much about something so trivial but I would personally not be happy with the engine starting weekly with no explanation. I'd be able to overlook it happening occasionally but if mine starts doing this then I'd be bothered by it.

 

As a DIY mechanic I would be bothered if my ICE never turned on as some Prime owners seem to want to do. From a mechanical perspective, getting everything lubricated and moving is always good for the engine...what's the old expression, "Moss can't grow on a rolling stone". I'd even go so far as to advise folks to run their Prime battery down very low, occasionally, to force the ICE to run for a good 50 miles or so once a month. (The old "italian tuneup" is good for cars, even hybrids/primes...it's still a combustible engine!)

 

Hopefully my reply did not trigger this response, I drive in HV mode at least 5 days a week so it is no problem for me personally when my Prime uses gas after exhausting the traction battery charge and my engine certainly gets a good workout regularly. What would bother me are the unexplained starts, to me something must be triggering the engine to start and it would be nice to understand why it is starting in these scenarios. Being a DIY mechanic I'm sure you can relate to the desire to understand how the car works/why it decides to do certain things. Some situations where the engine abruptly starts are higher stress for the engine than normal driving (ex., full acceleration with a cold engine in EV auto or briefly starting but never reaching full operating temperature when the front defrost is turned on). IMO it is better for the long term health of the engine to avoid these situations which are more stressful to the engine than normal unless necessary.

Having the engine turn on 1-2 times weekly despite the battery being over 90% SOC would be an issue to me because it triggers a warm up cycle where the car will reach the minimum operating temperature for the engine to shut off, then it'll shut off and cool down until the engine is started again once the battery reaches a low SOC. It might be trivial but I see this as unnecessary wear on the engine. This sort of use is very different from the type of gasoline powered driving that would be good for the long term health of the engine where full operating temperature is reached and sustained for a decent bit of time.

A moderately long highway drive in HV mode once a month or so is a good suggestion for someone who otherwise exclusively uses EV mode.

 

Welcome....
While your logic is solid; the car isn't equipped with AI. It's slow limited microprocessor is just doing what it was programmed to do; and we don't have a full picture of what parameters trigger an ICE fire up. While our consumer electronics equipment has an "air" of being intuitive, it's more social engineering, marketing, and skillful programming.

Hope this helps.....

 

Sure, and in 30k miles I personally have not had an unexplained case of the engine starting in my car so this isn’t an issue that has affected me. There are many posts from people who claim their engines have started for no discernible reason and it would be interesting to understand why it happens. Someone in an earlier reply said their 2021 starts the engine unexplainably more frequently than their previous Primes, meanwhile mine has never done it. We may not currently have a full picture of the parameters which trigger the engine to start, but wouldn’t it be nice to have a better understanding of it? Maybe I’m in the minority for wanting to understand how the car works in this case.

In fact, if the car had AI controlling the engine then this behavior would be more understandable. Your point that it is controlled by a slow and limited microprocessor is exactly why I’m curious about the parameters which trigger the engine, there must be clear parameters which trigger it and I’m curious to know what they are beyond what the manual lists.

Really, the point of my original reply was to try and help OP find out why this happens which is why I mentioned two driving scenarios not specifically mentioned in the manual where I have seen or heard of the engine starting. If the catch all explanation in the manual satisfies you that’s fine, I’d personally prefer to understand it a bit better.

 

Besides the ICE coming on when we don't think it needs to, has anybody been in their garage and heard electronics sounds coming from their Toyota that hasn't been used? The Evap System! And if you open the driver's door, the good old regen braking system pump will activate to get it ready. TCCN has a video on some things we may not know about our Toyota's:

 

That noisy brake system pump is for the friction brakes, not regen brakes.

 

I'll try to add some info here that might help.
First, it's not a single microprocessor controlling everything the car does.
It's many.
I've read references here at priuschat to 16 different control modules in the Prime.
Beyond the many modules in play are the different network protocols, besides the well know CAN bus protocol(s) that are used between the modules that either isolate a module form another or other module(s) -
- or provides direct communication between a module and another or other module(s). It's not rocket science, but it's not cut and dry simple to understand either.
Sure there's a set of variables in the ECU module! But there's also another set of variables in every other module too.
Beyond that, the variables change (for lack of a clearer explanation) for different conditions the car gets exposed to in the wild.
(All the variables are statically programmed into each module,
but that doesn't necessarily mean that any one set of variables from any one module gets interpreted by another module the same way all the time or for every different condition -
- since each module is connected to one set of sensors -
- that don't have access to the data from another modules sensors until the other module sends it's data to the module that requests the info)..

We can chip away at the control sequences and get a fairly large set of the typical ones. But that still in no way means there is a complete understanding of all the control sequences for all the conditions the car gets exposed to.

So, what appears to be a cut and dried set of control conditions any one driver experiences,
some times even many times, doesn't match up with what another driver will see, EVEN when the two drivers describe THE SAME or similar driving conditions.

They may be slow microprocessors. but there are enough of them to make it difficult to understand what they are always talking about to each other. without techstream it might be impossible for "normal humans" to get a close to complete picture.

 

Thanks to a link @ChapmanF posted in another thread about ODB2
There was also a description a few post down of the communication networks in the Gen 2 Prius for comparison. thanks mr_guy_mann
Gen2 OBD2 app review | PriusChat
and Chap who posted a Can bus diagram from the Prius Gen 2? Electric manual in the next post.

I gathered some of my experiences into ECU module communication stuff from the 2006 Civic Hybrid Electrical and repair manuals with it's 6 ECU modules and 5 communication protocols.