Unwanted gas engine charging

[JCA]

Myth busted: I was able to get the engine to start using aggressive regen this morning despite the low fuel warning.

Since you're able to recreate this -- can you experiment if you haven't yet using the brake pedal instead of regen, both with light braking (which would normally be regen, but I'm wondering if the computer is smart enough to just apply more friction braking instead) and waiting to brake more strongly (which would call for at least some friction braking in any case)? Does it come on immediately or only after braking down a hill for some distance?

I haven't had the engine come on except when I've accelerated past the blue bar -- It's flat near home I have traffic lights etc to stop for within 1/4 mile of leaving with a full charge. Perhaps the energy needing to be dissipated by a flat stop is a lot less than when the 4000 pound car is also going downhill.

 

[ExpensiveToys]

I disagree. I'll admit I don't understand the concept of "plugging", so perhaps everything I believe is wrong.

You wrote that the gas engine "comes on to stop the vehicle without using mechanical brakes." How is that possible? The engine is sometimes connected to the wheels through the Engine drive clutch only when the car is traveling at speeds greater than 45 mph (or 42 mph with my Clarity), so the engine has no way to slow the car at lower speeds when it starts running in response to regen braking with a fully charged battery.

The Clarity never allows the battery to charge to its nominal 17 kWh because fully charging a Li-Ion battery--especially for a long time--is reportedly not good for its health. So the Clarity protects its battery by not allowing either EVSE charging or regen-brake charging to charge the battery beyond its preset buffer. Members of this forum have reported the battery can accept up to a maximum of about 14.1 kWh while charging. There is also a bottom-end buffer below which the Clarity will not allow the battery to discharge, so the maximum charge is 14.1 kWh plus the size of that bottom-end buffer, but always less than 17 kWh.

So why does the engine start up in response to regen braking when the battery is fully charged? One clue is that when the battery is fully charged with Honda's lesser i-MMD hybrids, they use the excess energy coming from regen braking to power the starter motor/generator to spin the engine after shutting off the engine's fuel injection and ignition systems. Why doesn't the Clarity do that, too? Perhaps because the Clarity is a heavier car and more energy needs to be dissipated. My theory is that the starter motor generator in the Clarity is actually fighting the rotation of the powered-up engine to dissipate more energy than the "dead-engine" technique used by the lesser i-MMD hybrids. I could be wrong. I'm hoping expert sleuth @Ray B will discover a paper written by Honda engineers that explains this process.

With the mechanical clutch disengaged the engine still can be connected to the wheels but its done electrically not mechanically. This is also were people get caught up on Honda's electric CVT terminology. As the electrical connection between the ICE and the drive axle in some ways mimics functionality of a mechanical CVT (but that is a whole other topic)

Plugging is reverse current braking. Some times used in diesel locomotives, which is oddly enough were a lot of hybrid car tech was stolen from. Think of it as trying to run the electric motor in the reverse direction that it is currently rotating. By applying a voltage & current in reverse of the direction of rotation you can apply a braking force to an electric motor. The downside is that it generates heat and of course consumes electrical energy. In some hybrid drive trains plugging is used to supplement the gap between regenerative and mechanical braking.

Doubt will find a Honda paper on it as I believe its Dr. Severinsky's tech that honda license from Paice. He had a paper on using plugging where primary motor generates electricity from braking and the power is applied as a counter rotational force to the direction of rotation of the ICE. There by generating a load on the engine which is countered by the engine using gas to counter the electrical energy being fed to motor. There by giving you electromagnetic braking without involving battery and can a much larger braking force then just relying on engine compression.
Much like Toyota most of Honda's hybrid tech is licensed from Paice group so am guessing the patent on this is most likely listed there.

 

[insightman]

The answer may be here:



Engine braking with a dead engine and no CVT would only be at one level. However if the engine is running the effort needed for engine braking to turn the engine could be controlled through engine RPM. At low engine RPM there would be more resistance to turning the engine than at higher RPM. So not fighting against the running engine like in your theory, more like engine braking is fighting to turn the engine, but the engine itself can help to varying degrees, thus controlling the load.

The behavior of the engine would make it obvious if actual engine-braking was taking place. The engine's speed does not vary with the road speed when the engine starts in response to regen-braking with a fully charged battery. I believe the clutch is active only when cruising above 45 mph, and only when not significantly accelerating or decelerating.

 

[insightman]

Plugging is reverse current braking.

If I understand you correctly, the starter motor/generator is trying to slow the engine and that's exactly what I've believed is happening.

 

[ClarityMTL]

Since you're able to recreate this -- can you experiment if you haven't yet using the brake pedal instead of regen

I have tested this as well. My street is relatively flat. By this, I mean that I don't perceive any incline to the naked eye. There is a stop sign about 700ft from my driveway. If I leave with all accessories off, and accelerate conservatively (under the second line on the dashboard power meter), by the time I hit the brake pedal at the stop sign the engine will come on.

My workaround is just to waste some power before leaving (heat/AC) or accelerate quicker to that first stop. My thinking is that if I'm going to waste energy anyway, it might as well be the clean type.

I also only charge when I know I'm going to drive further than the current indication on the GOM, which could mean a few days between charges.

 

[ab13]

With the mechanical clutch disengaged the engine still can be connected to the wheels but its done electrically not mechanically. This is also were people get caught up on Honda's electric CVT terminology. As the electrical connection between the ICE and the drive axle in some ways mimics functionality of a mechanical CVT (but that is a whole other topic)

Plugging is reverse current braking. Some times used in diesel locomotives, which is oddly enough were a lot of hybrid car tech was stolen from. Think of it as trying to run the electric motor in the reverse direction that it is currently rotating. By applying a voltage & current in reverse of the direction of rotation you can apply a braking force to an electric motor. The downside is that it generates heat and of course consumes electrical energy. In some hybrid drive trains plugging is used to supplement the gap between regenerative and mechanical braking.

Doubt will find a Honda paper on it as I believe its Dr. Severinsky's tech that honda license from Paice. He had a paper on using plugging where primary motor generates electricity from braking and the power is applied as a counter rotational force to the direction of rotation of the ICE. There by generating a load on the engine which is countered by the engine using gas to counter the electrical energy being fed to motor. There by giving you electromagnetic braking without involving battery and can a much larger braking force then just relying on engine compression.
Much like Toyota most of Honda's hybrid tech is licensed from Paice group so am guessing the patent on this is most likely listed there.

https://insideevsforum.com/community/index.php?threads/solved-unexpected-ice-turns-on.3402/

https://insideevsforum.com/communit...nexpected-ice-turns-on.3402/page-4#post-34848

Honda's patent and newer patent application says they use the generator motor to burn off the excess electrical power from regen braking by operating the motor in the inefficient range to make heat. The gas engine is also a load to the generator motor in this case. Though it never states if gas needs to be consumed. The charts show this is an option if SOC is equal or above 90%, if braking and speed are low then friction brakes can be used.

 

[Mark W]

I just wished Honda had decided differently. I wish they just used friction braking as EVs do instead of starting the engine. I have never noticed any difference in braking when the battery is full in my Nissan Leaf.

 

[ab13]

I just wished Honda had decided differently. I wish they just used friction braking as EVs do instead of starting the engine. I have never noticed any difference in braking when the battery is full in my Nissan Leaf.

I had commented in another thread that the message should pop up on screen that it will engage and could be dismissed from a steering wheel button. It is essentially a safety feature, because a standard ICE vehicle is easily down shifted for engine braking when going down hill. A BEV with a full battery has no option but to use friction brakes. If you decend from a ski resort or Yosemite, etc.. You may be going downhill more than 30 minutes, or over an hour. Having no "engine braking" would likely overheat the brakes.