My understanding is that the front wheels cannot completely disengage itself from the electric motors so it will generate electricity no matter what ............ Quite frankly, I have no idea how running the engine absorbs this power because the energy flow diagram shows that the running engine is actually charging the battery (and potentially causing damage).
I'm thinking about when the engine is started in normal operation. The starter/generator motor receives electricity from the battery, and acting in starter mode it uses that electricity to spin the engine crankshaft (whether directly or through gears I don't remember). As we know the rotating engine then begins to compress the fuel mixture, spark is provided and the engine begins running. At that point the starter/generator motor switches into generator mode, and begins receiving mechanical power from the engine instead of giving it. This causes the motor to generate electricity, which it then sends either to the battery or to the traction motor. Or if the engine goes into direct drive mode I think a clutch disengages the starter/generator motor from the engine, although I'm not sure of this.
But what if the starter/generator remained in starter mode even after the engine started? It would continue to use electricity to help the engine spin, similar to how some of the early mild hybrids worked, as well as Honda's original IMA system that was used in their hybrids for several years before they switched to the current i-MMD system. Although once the engine is running, instead of starter mode they referred to it as assist mode (
@insightman do I have this right or am I out in left field about how IMA worked?)
Of course in the case of Clarity it would not be used for this purpose during normal driving, I'm just saying mechanically it could do this. So my thinking is that in the full battery regen situation, the electricity coming from the traction motor is sent to the starter/generator motor, which goes into starter mode to spin the engine, then after engine ignition it continues to provide mechanical power to the engine in assist mode. Of course all of this engine rotation would do nothing in this case other than make heat and noise, similar to a regular gasoline car engine being revved up while in neutral. The engine itself would be providing some of the mechanical power to spin itself, but probably only using a small amount of fuel similar to idling. The rest of the mechanical power to spin the engine comes from the starter/generator motor acting in assist mode, spinning the engine at whatever RPM is needed to dissipate the unwanted electricity.
One possible problem with my theory is that it doesn't take a whole lot of power to spin an engine that isn't connected to anything at that moment, so I don't know if this would be enough to deal with the excess energy that is beyond what the friction brakes are (presumably) taking care of.
But even if my theory is correct, why do they introduce fuel and spark instead of just letting the starter/generator motor continue to spin a cold engine? Oil pressure would be no problem. The only issue I can think of is that it's probably not good to spin a cold engine for too long, and the system has no idea how long the excess regen situation will last, since as far as it knows you just started down from the top of Pikes Peak. By the way that is one theory why it doesn't use friction brakes exclusively in this situation, because the brakes could start overheating if the need for braking goes on for a long time.
So proactively when braking is needed with a full battery, it spins the engine and activates fuel and spark. In some cases immediately depending on the circumstances, although you seem to have observed that it will accept a certain amount of regen into the upper unused battery buffer. But at some point it says no more and begins to spin the engine and starts it up. Unfortunately even if the regen situation ends two seconds later, once the engine is started it has to run through the full warmup cycle in order to get the catalytic converted hot enough to burn off any unburnt fuel (I think I read somewhere that this is an EPA requirement because it shortens the life of the converter when cold fuel is left in it). The same thing happens when you press the pedal to the floor even for just a second, it starts up ICE then keeps it running even though it is no longer needed so that it can get the catalytic converter hot enough.
I really wonder if this a flaw in Honda's software.
My theory may be as full of holes as swiss cheese. But if my theory is correct, then the software flaw in my opinion would be that it overreacts too quickly to the situation, instead of considering the possibility that the regen situation may not last very long.
I.e. it should start out using just friction brakes alone, which I'm sure they are capable of. Then after a predetermined time limit (based on a combination of time elapsed and brake pressure) to avoid brake overheating it would start putting electricity into the unused buffer. Then when that limit is reached it would start spinning the cold engine. Then only after some predetermined limit to the amount of RPMs on a cold engine would it start adding fuel and spark so that the engine can begin warming up. In case you really are going down Pikes Peak.
My guess is that for whatever reason they didn't feel confident in setting the required thresholds for all of this, or they couldn't come to an agreement on the thresholds. So they went the safe and easy route of starting up the engine almost immediately.