Why does ICE come on going downhill at <50 °F?

On the Prime (different from the Gen3) B mode does not spin the ICE

 

How does it drain the extra power then?
The Gen 4 spins the ICE.

 

It sure does, if the EV range is deleted unless there is an unannounced change to the 2020 which would seriously impair safety. Page 333 of my manual (emphasis added):

 

Even when the traction battery is full?

Some earlier Prime owners, living at the tops of hills and fully recharging at home, indicated that theirs did spin the ICE on B mode descents. One solution was to halt recharging before the battery was full.

 

Something fishy here. It has been my understanding from back when I had the 2012 PiP and continuing with the Prime, that if you are in EV mode, and there is a reasonable enough capacity left in the traction battery, that B mode simply engages more aggressive regeneration, and the engine braking won't engage until the battery is almost full. Has that changed with the Prime???

 

No change. When you are in B, they both regen in EV and use the engine when not in EV.

Edit to add that, if the hybrid system gets too warm from all the rapid regen, it will drop out of EV and start using the engine braking. Happened to me once in Colorado. But normally, if there is EV headroom and you're in EV mode, B will use heavy regen. Fill it up or use it up and it's only engine braking.

 

In EV mode at least, B mode does not spin the ICE under normal conditions. I go down a mountain at least once a week. The enginge does not turn over.

 

It depends on the speed and other factors. There is a limit to regen that seems to depend on duration of B mode, speed (power), temperature, and state of charge. For example at 65+ mph and a 6-7% grade, the engine seems to come on within a few miles. But at lower speeds it might not.

Also, sometimes it doesn't tell you if the engine is turning (the green EV light stays on, and there's no indication on the power screen), but if you listen closely you might hear/feel it. Hybrid Assistant confirms that it is on. If it is the first time the engine runs during the trip, the car will probably complete the warm-up cycle at the bottom of the hill. If it's already warmed up, it probably won't turn it on again.

 

Does that mean the that the printed manual is in error about B mode always using engine braking? Like, the text was copied from the Prius manual, and not updated with Prime-specific information?

 

That's one way of putting it. It's at least lazy in its explanations. For example, on page 333 in the section on shift positions of my manual it says:

There's no mention in the part I recolored to red about that depending on if it's in EV or HV mode. And it does not provide any links to other sections that might give more details. Then again, the manual is almost 800 pages, so I am not surprised that it isn't perfect.

 

Are you saying that when the battery is full, it is not possible to use regenerative braking, and therefore the car switches to the ICE because ICE does not charge the battery?

Usually, when I get to that downhill section, my battery is at about 90%, and the Prius always switches out of EV at the same spot. However, to test the theory put forward in this thread, today I arranged to get to the downhill section with a battery at 50%. The Prius switched to EV at the same spot as before.

 

No, the power from the regenerative braking runs MG1 as a motor to spin the engine as its load. No gasoline is being used, just spinning the engine.

 

Are you using the Heat and or A/C while going downhill?
Have you used the Primes PreConditioning feature yet?
To make the commute all in EV you're going to have to figure out how to make the Prime go down the hill in EV and that might take a while. Be patient and you'll figure it out.

 

Answered very well by @Prodigyplace. No gas burned. It just uses the work spent spinning the ICE as sort of a sea anchor to slow you down without smoking the brakes.

Need more info. Is it heat or air conditioning causing it as @vvillovv was wondering? Or is the hill so steep that the battery is getting warm and causing the car to switch to engine braking? Going down steep grades can charge the battery very quickly.

 

Whenever the gasoline engine turns for any reason, it has to be brought up to an operating temperature to make sure things like the oil lubrication is working right. So when the system decides that it needs to use engine pumping to dissipate energy on a downhill, it first has to run it a for a little while with a little fuel (a warmup cycle). When the engine temperature is up to what it wants, the system can turn off the fuel completely. This results in a high manifold vacuum that the engine has to work against, dissipating energy. Exploiting this behavior is why there is a B mode in conventional gasoline vehicles, since the engine will be spun faster in the lower gear and thus dissipate more energy. The Prime has the option to dump energy into the battery, so in some cases it can perform the B mode behavior by doing aggressive regeneration, which has the distinct advantage of recovering the energy for later use, rather than just generating waste heat.

 

One would think there might be a driver technique that would tell the computer to switch off MG (motor generator) from charging the traction pack, while descending the hill with a full charge already in the pack
that would keep the ICE from firing on the decent.
Keep in mind, the decent is the most efficient time for the ICE to fire, if it's going to fire at all on the trip.
The trick is going to be learning how to keep the ICE from firing when the program thinks it is the best time and the driver doesn't want the ICE to fire at all on that trip.

 

There is: shift to Neutral. For when neither legality nor safety nor preservation of the transaxle matters.

At least in those mode, when you don't want run-away speeds, braking will be purely by the mechanical friction pads. No battery charging or forced engine spin, just hot brakes.

If you want this to happen, and the slope/distance/speed combination fits the situation, then just put it in HV mode (D gear) and step on the gas pedal far enough to fire the ICE.

But I'm not following how this is the most efficient, unless it is to use some otherwise wasted descent energy to perform part of the engine warmup. (Not that it matters to me, I'll spin the ICE regardless, to prevent runaway speed.) But gravity can't do it all because the ECUs force at least the first portion of warmup to use fuel. Then the ICE will be cooling or in lower efficiency near-idling operation for a while until you reach a non-descent segment where the ICE can get enough load to get up into the more efficient zone of the BSFC chart.