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

Hi PC folks, I've had my 2021 Prime about 3 months but my trips are very short and I've only put 550 miles on it so far. My usual trip involves a 5 mile descent followed by sitting at traffic lights, around a few city blocks and up another 5 mile hill. I've almost exclusively had the car in EV mode with the Eco setting. I've noticed every single time I get down the first hill, even with a full charge, the ICE comes on while I'm waiting at the lights and driving through city blocks. Then I'm in full EV mode for the next up hill. Great, but I don't love that i'm burning carbon around those city blocks. I live in upstate NY so I thought maybe it's the outside temp. But yesterday it was 48 F outside and it still happened.

Is there something I can do to avoid this? It's a little frustrating because all my in-town errands usually amount to 12-15 miles round trip max. I bought this car for the capability of EV mode most of the time, with the convenience of the ICE for longer trips.

From reading the thread I'm guessing it's the long initial downhill but I would love some input. I'm still really happy with the car but burning gas for no apparent reason seems odd. In my 2007 prius the ICE does not usually come on at all til I start going back up the next hill.

 

Yap, your initial downhill is the culprit. Try not to charge your traction battery full. If you have a 100% traction battery and start your day on a downhill, there is no room in your traction battery to store the regenerated energy. The car starts the engine to brake. If you are using a charge schedule, the easiest way is to set the time of departure an hour later than you actually plan to leave. So, if you usually leave the house at 7am, then set the departure time to 8am. You may need to fine-adjust the time depending on how much regen energy you can recapture in your first 5 miles of descent. The idea is you want to have just enough but not a full traction battery so that after 5 miles of descent, you will have near 100% SOC.

There are other reasons the engine will come on during EV mode with a full or partial battery.

EV driving may be canceled automatically in the following circumstances:
●When vehicle speed is more than approximately 84 mph (135 km/h).
●When power is needed temporarily, for example when the accelerator pedal is depressed firmly or when accelerating suddenly.*
●When the temperature of the hybrid system is high. The vehicle has been left in the sun, driven on a hill, driven at high speeds, etc.
●When the temperature of the hybrid system is low.
●When the heater is switched on when the outside temperature is below about 14°F (-10°C).
●When the windshield defogger switch is pressed. (→P. 491, 501)
●When the system determines that the gasoline engine needs to be started. The gasoline engine may also operate in circumstances other than those listed above, depending on conditions.
*: When traveling in the EV auto mode only

 

Good thing it'll be a while before there are 100k of these on the roads?

 

I don't think so.

This is the copy from the 2020 PP owner's manual. The * comment has not changed from the 2017 PP Manual.

 

It is very annoying that the windshield defogger turns the gasoline engine on in the EV mode. I hate it, and it makes no sense.

 

Great advice, I was thinking this might be what I need to do. I've only used the charge schedule once and the car was not fully charged by the set time. Not sure what that's about. Is it possible the car is predicting I have a descent coming up and not charging all the way on purpose?

 

No, the car has no idea where you are going. It just means the charge did not complete on time due to several possible reasons, such as needing to heat the battery if you had a battery heater enable, or heat the cabin if you had a climate preconditioning set or clock on the schedule was set wrong.

 

I think your math is off. For one, you haven't factored in CO2 emissions at the power plant.

The only time I've had the engine come on when in EV mode is when driving up a long, steep grade. That's probably caused by the battery reaching a temperature set point. Cruise control doesn't trigger it, at least on my car.

 

The dedicated "Front windshield heater" button is really meant for for serious defrosting. Like you have 1/4 inch of ice on the windshield. So it uses the gasoline engine to get serious heat. If you just need defogging, then you can direct air to the windshield/foot if you want with the air-flow controls, but I think that even in AUTO mode that it has enough sensors to sort out that you need defogging, and engage the heat pump dehumidification feature, where it directs cabin air both over the interior cold (evaporator) coil to dehumidify AND the interior hot (condenser) coil. Defogging does not not automatically trigger the gasoline engine.

 

Years ago there was a factoid that the regular Prius had over 10 million lines of code in its programming. Accept that you are not always going to understand why. YOU THINK "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." based on what? Your in depth analysis of the code, the applicable thermodynamics, and your 30 years of experience with this system? Of course it COULD do that, but the engineers have decided that it shouldn't. You are not smarter than the engineers. I was reminded of that myself with a hastily posted protest about the silly self charge mode. It turns out that it was not so silly, it was my imagination that was limited.

It seems the others are trying to tell you to relax, and it's good advice. You are trying to reverse-engineer the car, a futile task.

 

Isn’t the explanation for the defroster turning the engine on simply that it uses the AC in reverse (to dehumidify). So it needs the engine coolant for heat. And it works really well, though it has taken me quite some time to stop hitting it on short trips.