Why does Prime switch from EV mode to gas engine mode?

Welcome to PC. The same question you just asked has been asked numerous times on this forum. Do a quick search to get more in depth answers from others commenting on this. Most frequent reasons the gas engine starting on PRIME I've seen are running front defroster, cold temperate (below 14F) with heat or going downhill with a full battery charge. But in a nut shell, there are times car decides on it's own to run the engine sometimes apparently no obvious reason understandable to the operator.

Also, once the gas engine starts, it has to complete the warm-up cycle before shutting it down on it's own. During this period usually anywhere from a few minute to up to 10 min on low temperature, switching to EV mode even with enough charge left, will not shut off the gas engine.

Read the excerpt from the manual below.

 

I realize we are dealing with a Prius Prime here...but I'm going to repost an opinion I gave yesterday to a similar question.

t seems to me, some Prius owners become to obsessed about their engine running. Any time they notice their engine running when they feel it shouldn't or doesn't have to be? They worry something is wrong.

When I owned my Prius, I just really trusted in the software, Toyota, and HSD. And I evaluated my Prius's "health" by what the total returned MPG over the course of a month was,- not by whether the engine kicked on at a stop or not.
I figured if there was a real problem with the system, gas mileage would drop and/or codes and dash lights would appear. I also kept in mind that I was driving a Hybrid, not a full electric vehicle, and with this reality, there might be a lot of scenario's programmed into HSD that would find the engine operating, when "ideally" I might think it should not be running. I always made the assumption that the system was smarter than I was.

This ignorance is bliss approach? Saved me from a lot of angst. And throughout my ownership, I was never disappointed in the MPG or the efficient nature of the vehicle.

I only bring this up, because normally I would never recommend anyone ignore a symptom from their vehicle. But when it comes to individual and specific incidences where the engine is running, against the common parameters when it is stopped?
Unless it becomes repeated, leads to codes, OR you notice a tangible drop in fuel efficiency, my advice is to just keep driving and trust the HSD software and system. " -End repost

IMO, you haven't even owned this vehicle a month. Keep in mind you bought a Hybrid. It may have an EV mode, and greater EV capacity, but it is not a full electric vehicle.
I would "assume" it's operating normally.
If it's not, and it's possible it's not, but if it's not, it will become evident by simply your overall efficiency returns.
BUT in the short term? I'd keep just driving it, and assume that the vehicle knows what it is doing, until you have a let some time pass. The fact that you're finding the ICE running more than you would like? Isn't grounds enough for defining a problem exists.

 

I have only had my Prime for less than a month but in acquainting myself with it, I found that in EV mode, when I go for defrost, the ICE starts and even if I shut down the HVAC, the ICE stays running the only way to go back to EV is to stop and restart the car. This even happens when the outside temperature is 10 or 12C
The only work-around I found is go combo defrost and turn up the heat and fan.
It bothers me but is the only bothersome significant anomaly I have so far and I can work around the issue.

 

Exactly!
Toyota has 30+ years of engineering experience in the hybrid system and I trust they had a really GOOD reason why they designed in a way that "sometimes" ICE comes on for several minutes even when the traction battery is fully charged.

 

As I mentioned, and also stated in the manual, the front windshield defogger is the surest way to turn on the ICE. And again, as I mentioned and also stated in the manual, once ICE starts it does not stop until it completes the warm-up cycle, even if you turn on the defogger and HVAC switches.

 

My untested impression was that you have to be patient and it eventually goes back to EV Mode -- presumably when the cycle ends, as quoted above. I've never restarted the car and I'm sure the ICE eventually turned off, virtually every time I noticed it wouldn't go to EV.

 

My issue is that it should be able to defrost with only the electric heat pump at +10C and not have to resort to ICE for extra heat.
I am trying to figure out what the 'circumstances/conditions' are as highlighted in yellow above.
My Insight's ICE would not auto stop in the defroster mode but it didn't have an electric heat pump based HVAC system and needed the engine for the heat source.

 

If you are on EV mode and needing to defog the windshield, in most cases AUTO on the climate control will take care of it. The car has a sensor and any fog developing on the window will switch the direction of airflow to Window and turns on the compressor to dehumidify. However, the AUTO setting will use quite a bit of battery power. If you want to minimize the use of a battery, you can switch to a manual climate setting with airflow to the window and fan speed to the desired level. Windshield defogger button is set for the maximum fan with maximum compressor for quick defogging.

 

I'm not worried something is wrong, I'm irritated that the car is wasting gas when there is absolutely 100% no reason to waste gas. The frustration is compounded by the warm up cycle which lasts much longer in the Prime than the PiP (up to 5 minutes). So something triggers ICE unnecessarily - like cruise control - and now I'm stuck in HV mode for the next 5 or 6 miles when I have enough battery to complete my whole trip and then some. When it happens every day it's responsible for a couple kilograms of unnecessary CO2 emissions every week.

The math is staggering. If this happens to only 100,000 Prius owners, that's an extra 26,000 tons of unnecessary CO2 emissions every year.

EDIT - That's the CO2 equivalent of leveling almost 2,000 acres of forest.

 

That level of angst and attention about the ICE running makes me think what you really want is a full EV vehicle.
It's the only way to totally free yourself from the reality of an ICE burning gasoline.

 

There's no angst about burning gasoline. I regularly trigger ICE intentionally. The frustration just lies in the fact that the car could be programmed better and it isn't.

BEVs have too many limitations for my situation, like uphill range mainly. 200 miles of city range can quickly turn into 50 or 60 miles uphill.

 

Well...as I said in my original post....

Call it angst or frustration, trusting that the software IS programmed to be efficient, and that I shouldn't evaluate it by something as empirical as my noticing when the engine was turning off and on, saved me a lot of both.

 

The software is programmed to be efficient given a set of assumptions. When those assumptions aren't true the system is not efficient whether the driver chooses to notice or not. It is impossible to improve a system if one isn't willing to test it for flaws, identify them, and correct them. Some people are content with good enough, and some are driven to tweak, improve, and build upon past success. There's nothing wrong with either. The only thing that could be said to be wrong is criticizing people for not sharing the same preferences.

 

I'm not criticizing you.
In fact I find your attention to the reality admirable.
However, I stand by my opinion that NOT being a software engineer, or a auto manufacturer myself, I do not assume I could do better with the vehicles programming than then the manufacturers themselves have done.

My original post was in attempted answer to the OP's post which ended with this question:

My feeling is that it IS normal, and I wouldn't worry about preventing it.