ICE runs after going down a hill at the beginning of my drive.

[PHEV Newbie]

There's an important difference with the Insight (subject of the video) and the Clarity. The Insight is a hybrid so the ICE is on most of the time and therefore, when this regen situation occurs, if Alex on Auto is correct, no gas is used in that moment. The engine can shut off and on as it's already warmed up. However, the problem with the Clarity is that the ICE wasn't on before the regen turns a cold ICE on. Even if no gas is used in that moment of regen, the ICE still has to warm up, and this obligatorily uses gasoline.

That's an excellent point. I suppose the best thing for folks like Bob who live atop a significant hill is to partially charge to 80-90% so they can recover the energy going uphill and avoid having the ICE turn on. I'll bet this was a tough design decision for Honda. The easy way is to program no regen at all and let the friction brakes do it all going downhill. After getting use to regen braking, that would feel really strange, perhaps odd enough to cause an accident in rare cases. For that, I'm glad Honda programmed to allow regen braking when the battery is full. The cost is the ICE going on. It sounds like the car is behaving just as designed.

 

[insightman]

I thought of an even more complicated possibility. I wonder if it's possible that on a long, long downward slope a Clarity PHEV with a fully charged battery uses the engine two ways to provide braking/regen, saving the brakes from overuse?

First, the cold engine starts up and resists the starter motor/generator (which is reversing its torque for this purpose). Second, after the engine is warmed up, it shuts off and closes its valves to resist the starter motor/generator (now operating in its starter-motor torque mode) the same way the always warmed up engines of the Accord Hybrid and Insight do when decelerating with fully charged batteries?

 

[KentuckyKen]

That's an excellent point. I suppose the best thing for folks like Bob who live atop a significant hill is to partially charge to 80-90% so they can recover the energy going uphill and avoid having the ICE turn on. I'll bet this was a tough design decision for Honda. The easy way is to program no regen at all and let the friction brakes do it all going downhill. After getting use to regen braking, that would feel really strange, perhaps odd enough to cause an accident in rare cases. For that, I'm glad Honda programmed to allow regen braking when the battery is full. The cost is the ICE going on. It sounds like the car is behaving just as designed.

I totally agree with @PHEV Newbie and @insightman on this.
I’ll add one more idea to their theories. Every car I’ve owned has said to downshift to use engine compression braking on long downhills so as not to over heat the brakes. And since the Clarity has no gearing to downshift with, the only way to avoid over taxing the physical friction brakes is regen resistance. So far so good, but when the battery pack is full, the BMS won’t allow this flow of power into the battery in order to protect the battery pack from over charging. So then, as @insightman has so eloquently theorized, somehow it uses wheel driven power regen to oppose the engine/generator which requires the engine to turn on. And perhaps on lesser downhills or a single hard braking event, the same algorithm does the same thing as in Honda only programmed one algorithm to save the friction brakes and it can’t know how long the downhill is going to last.
I’m putting my hat in the ring with @insightman on this until some one proves otherwise.

@insightman, your second theory sounds interesting but is going to be difficult to observe. If I could just get a wire through the firewall, I could bring an hour meter and rpm gauge to the dashboard. Then I could tell if the engine was running or just rotating.
Hmm.. I’ve got a borescope camera that bluetooths to my phone. Maybe an inductive rpm gauge under the hood that would let me see if engine is running or just rotating. But I don’t have a good long hill close. Have to keep thinking on this.

 

[David A]

I too experience this anomaly. The first week or so I thought it was a electronic/computer issue of some sort with the car. After watching the gauge cluster and realizing the chevrons blinking means no regen...I let it ride until the battery comes down 10% or so then everything functions normally.

Like Weave says in an earlier post...I just use the brake pedal to stop until the car uses some of the energy. Living in the Mountains of North Carolina I am used to the give and take of regen on a daily basis. The reality for me is I gain nothing when leaving the house and expend more energy going the last 2 miles up to the house. I still manage 55 to 60 summertime miles on electric though which 70% of the time is sufficient for the days adventures.

C'est la vie

This thread does help clear things up a little more though.

 

[insightman]

If I could just get a wire through the firewall, I could bring an hour meter and rpm gauge to the dashboard. Then I could tell if the engine was running or just rotating.

I wonder if the Accord Hybrid and Insight light up the engine icon when it starts rotating without running? Now if someone would just make a Bluetooth-enabled engine meter (it might cost more than $6, however).

 

[Steven B]

Fully charged and heavy on the brake is another possibility. Some 'seasoned' people don't realize they brake hard EVERY TIME THEY BRAKE.