Car keeps switch to ICE On ICE descent

I don't believe the engine is actually running, i.e., burning fuel by the time it reaches the bottom of the mountain. My understanding is the traction battery is spinning the engine via the MG1/2(?) to consume the excess electric charge. But, maybe in a Prime, the engine will first go through a warm-up cycle before the fuel is shut-off in order to bring the ICE to operating temp. The OP probably shouldn't fully charge at the top of the mountain if he doesn't want the ICE to get involved in the descent.

 

There are several reasons the engine will come on going down a hill in EV mode. The battery being full is not the only one. There seems to be a certain amount of charge that's allowed to go into the battery at once, or maybe it's just the temperature of the battery. So even making sure the battery is not fully charged won't prevent the engine from coming on.

B mode seems to make the engine more likely to come on than using the brakes
Colder temperatures make it more likely
Hot temperatures probably make it more likely, if the battery gets too hot, but I haven't noticed
Cruise control might make it more likely, since the cruise control sometimes randomly turns on the engine when it's not needed.
The battery being mostly charged might make it more likely, since the last few percent of charging typically need to be slower than charging when the battery is less full.

It makes sense that the engine comes on, because if you work out how fast the battery charges during regenerative braking it is similar to or even faster than DC fast charging.

I don't think the engine burns much fuel for this engine braking, but it might use a little since my MPG usually drops to around 600 when I hear it come on for engine braking. Then it will continue the warm-up cycle when you get to the bottom and no longer need engine braking. The warm up cycle is a good idea if even a little fuel was used so you don't get condensation left in your engine or exhaust from running it while it's cold.

The only way I can think of to prevent the engine from coming on would be to drive down the hill in neutral. But that's bad for your brakes, and could be dangerous.

Having the engine on it traffic is not the worst. The warm-up cycle doesn't put much load on the engine anyway, and any excess energy will go into the battery. I think people say the warm-up cycle uses less than 1/2 gallon of gas, but in my experience it's more like 1/8 gallon or maybe even less.

 

The short answer is there is no solution nor workaround to the OP problem for engine turning on while downhill. This is, unfortunately, the design of the vehicle. It was not designed as a full-on EV to begin with.

The most practical way in the OP's situation is to use hybrid mode as soon as you hit downhill (or when you leave) and until you have reached flat surfaces then turn-on EV mode. By then, the engine would have warmed up enough so it could be turned off to EV mode immediately. Using hybrid mode also allows quicker heating and cooling.

 

It has been my experience that the only sure way to stop the engine from coming on is to put the car in neutral before the engine turns on. This causes the friction brakes to slow the car and may be illegal. I would not do this myself on a hill the size of yours.

Once the engine starts, the only way to stop it before warmup is complete is to turn the car off then on again. This may hurt the engine as it doesn’t allow the engine to properly warm up.

Forcing the engine on at the top of the hill may allow it to go through the warm up before you hit stop signs and lights at the bottom. I would test mileage for the trip this way and the “just drive it” way and use the method that gives you the best trip mpg.

 

Clarification please, I just got my Prime and have a lot to learn.

Will regenerative braking add to the traction (EV) battery or just the regular HV battery? My impression (right or wrong!) is that they are 2 separate batteries, not just extra cells in the HV battery. If regen does add to the traction battery, it seems one could refill it on steeper hills just through regen.

Does the car:
Use up the EV battery (in EV or EV Auto) then go to HV which will use the HV battery and work as it did in non plug on versions of the Prius?
If so, if the HV battery gets full, will the car then start storing power in the EV battery even though you don't switch to Charge mode? It seem like that could happen coming down a long mountain road.

I know that I have seen the OPs situation in a Gen II Prius, all green bars at the bottom of the mountain road, engine running at a red light. Sure wished I could have explained to the car that I was done going downhill and would be using that battery power in the interstate in one minute. No need to "make room" in the battery for more regen power.

Thanks

 

It's all one battery.

 

There is just one big battery. I think the state of charge display might make a switch in scale between EV and hybrid modes. When Ev range is nearly gone, the display looks empty displaying pack's total capacity. With the switch to HV, the display switches to show the lower capacity that normal hybrid operation works within, and thus a nearly empty battery display becomes a full one. Which could lead to misunderstanding.

Regen can add enough energy to charge the battery out of that virtual hybrid zone, but it normally takes coasting down a mountain to do so.

That burning of energy after a mountain descent isn't to make space for regen energy, you still got friction brakes after all , it's to get the charge level back into the safe zone for long life. It would have been nice to tell it you'd be using it up right away, but the battery had likely been heated up by the charging. Fully charged and hot is not a good thing for NiMH or Li-ion life. Just ask a Leaf owner in Arizona.

 

As long as you have even a bit over 0% charge indicated in the traction battery, you can put the car in EV only mode (toggle via the EV button on the far right). This will put the car in EV mode, and when you go down the hill in B mode, the energy will be dissipated into the battery in the form of charge. The ICE should never run in this mode unless the battery becomes fully charged.

I've gone down the long grades on I-70 in Colorado and put up to about 30% or so of charge back in the traction battery by doing this.

 

OK, so the entire single battery is 8.8 kWh. Some part of it is 'reserved' as the "EV battery" and the rest at what would be the 'normal' HV battery capacity of a non plugin?

If one were to EV until that "part" was gone and then climbed a long hill using up most of the HV "part" of the battery, will plugging it in charge more than ~6 kWh that we are allowed to fully charge the EV "part" of the battery?

And if one is driving along in EV and runs out of that "part" of the battery, the car uses the rest in HV mode and in regular driving acts as if the EV "part" doesn't exist regenning into and using only the much smaller HV "part" unless there is an extreme downhill such that regen will fill into the EV "part" or you put it in charge mode?

IIRC (which could be way wrong) the PIP would use up the EV "part" and there was no way of refilling it shy of plugging in. ASSUMING that is correct, it seems maybe the Prime is different?

So confusing!