Behavior change in HV mode

Don’t count me in with “all”. I like to add thought provoking ideas as much as you like to drift off topic.

I just completed a 700 mile, 10 hour trip and took some notes along the way. The first 700 mile trip was done with only a week of Clarity knowledge, this one was done with 3 months of experience.

I believe it is more efficient for the ICE to run intermittently than to run continuously.

 

I'm certain that Honda didn't add the Engine Drive mode with all its associated mechanical bits to increase the amount of gasoline the car uses. The mysterious AI that controls HV switches among Engine Drive, Hybrid Drive, and EV Drive modes to maximize efficiency while attempting to maintain a constant SOC in the battery. HV runs the ICE as infrequently as possible, but sometimes it has to abandon its SOC goals and runs the ICE continuously, for example, when climbing mountains.

 

I’ve had one single oil change and have observed no change of behavior with regard to noise, power, “gear mode”, or unexpected ICE starts/durations. Couldn’t observe any changes at all.

However, I have noticed (over all, not post oil change) what appears to be variability in when and for how long “gear mode” occurs on the same stretch of rolling hills. There must be some triggering events in the algorithm that we are not aware of. Of course it is rather difficult to keep the driving and driver conditions exactly the same and to record every one of the myriad variables like SOC, wind, average and max speed, acceleration aggressiveness, ambient and battery temps, non-motive energy use like heat, A/C, etc. ad infinitum. But I still can’t account for what seems to me to be rather variable behavior.
But since I get 49-50 MPG on my few HV trips, I’m not worried about it; just curious.
Edit
@Landshark, sorry about adding you to the masses on this. But you do realize Engine Drive Mode is anything but continuously running so it does fit your theory of greater efficiency with intermittent running over continuously running. Unless I’m misreading you, that is an agreement with us on this. And besides, we don’t even get to choose or avoid it in HV.

 

I’m certain that Honda added it out of necessity, but we’re both just speculating.

If Direct Engine Drive/Gear Mode is the most efficient, why isn’t it the default mode in HV? Why doesn’t it stay engaged all the time? Why do owners have to play games with buttons to get it to engage?

The car does what it is programmed to do based on constantly changing variables. I wouldn’t call it AI, it’s just inputs and outputs.

 

Since the ICE efficiency is highly dependent on RPM's, it only makes sense to direct-couple the ICE over a band of RPM's (speeds) where efficiency is as good (or better) than generating electricity and running the drive motor.

To easily visualize this, imaging trying to run a conventional car with a manual transmission in 4th gear at 10 MPH. It would be bogged down to the point of insanity ! Or, try to run that same car at 80 MPH in 1st gear ! There is a sweet range where direct coupling makes sense in the scheme of efficiencies.

 

I have tried to express my understanding of HV mode / gear mode before, but here are a couple more thoughts:

It seems to me that when you enter HV mode, there are two setpoints.

The first one is the ICE stop-start setpoint. This is the battery SOC when you enter HV mode. The ICE will start running when the SOC drops after entering HV mode. The engine will stop running if the SOC gets above the setpoint, the engine will restart when SOC drops about 4 percent after it stopped.

The second setpoint is the gear mode operation. This is the battery SOC about 2 percent below the original HV mode. Clarity will stay in gear mode until the battery SOC drops about 5 percent below this setpoint. If the SOC drops 5 percent from the setpoint, it will drop out of gear mode, and the ICE will increase rpm's to recover SOC. After it has left gear mode, the SOC needs to get back to the setpoint before it will go back into gear mode. If the SOC goes above the setpoint, the system will stay in gear mode, but the ICE will shut off when that setpoint is reached.

Normal operation of the Clarity will have the ICE starting and stopping in gear mode. This assumes the battery SOC stays within a 'normal' range after entering HV mode.

Gear mode produces about 5/16 of the power meter (varies with speed). If loads on the system are more than produced, the SOC will drop, and the system will drop out of gear mode to build back up the SOC. If loads are less than this, the SOC will build, and the ICE will shut off.

The heater puts a significant load on the system when it is trying to maintain SOC.

If HV is entered with a full battery, the system has trouble recovering because charging rate is limited on a full battery. SOC may drop and not recover, and gear mode cannot be maintained because SOC is out of gear mode range.

There are some confusing interactions between the setpoints, and there are ways to fool the setpoints.

The first interaction of the setpoints is when Clarity is in gear mode, but regeneration takes SOC up to shut off the ICE. When the SOC drops low enough, the ICE will restart, and it will go quickly to gear mode. If the SOC was dropping too fast (with the ICE off), it will drop below the lower gear mode limit, and not go into gear mode. It will not go back to gear mode, until the SOC has recovered to the upper gear mode limit: If SOC was rising too fast, the ICE may shut off, and miss gear mode again.

Fooling the setpoints: When the Clarity drops out of gear mode, due to low SOC, the ICE will increase rpm's to recover the battery SOC. Doing a quick reset of HV mode will reset the setpoint for gear mode, put the Clarity back in gear mode, and keep the ICE rpm's low. This reset does not reset the ICE start-stop setpoint, and the ICE will run more continuously in gear mode. I find this is a quieter drive, but it is less efficient (lower mpg).

 

I believe I've seen charging occurring in "gear mode," so there must be yet another intermediate setpoint somewhere in the system.

 

Yes, I understand using different gears to keep the engine at or near a specific rpm. I’ve also seen gear mode from 40mph up to 85mph. With one gear, that would seem to have the engine operating in a quite a wide range of rpm’s. Can it be efficient in that entire range?

To be more specific with my original question, If gear mode is the most efficient mode, why doesn’t the car try to remain in gear mode as much as possible? Left to its own devices, the car will go in and out of gear mode on a regular basis.

The description from ClarityBill above, is quite informative. I’m in agreement with him that gear mode is not the most efficient mode for fuel economy.

 

I have as well.

I’ve seen the following in gear mode:
Drive the front wheels.
Drive the front wheels and send a charge to the batteries.
Drive the front wheels in combination with the electric motor driving the front wheels.

 

Sorry for the confusion in my description...

I did not mean to imply that there would be no charging in gear mode: The ICE is putting out 5/16 of power meter, if the driving energy (and other loads) requires less than that amount, the rest will go to charging the battery - that is how the SOC rises to the point of shutting off the ICE. If the driving energy is greater than 5/16 power, the battery will help the drive, and the SOC decreases - that is how the SOC decreases to cause it to drop out of gear mode and allow the ICE to go to higher rpm and produce the additional power needed.

The ICE can vary slightly from 5/16 power, even while maintaining the synchronous speed (throttle up and down). To vary the power more, the gear needs to disengage and allow the rpm's to ramp up or down.

I do find that gear mode is the most efficient mode of operation, except that 'fooling' the setpoints can cause the ICE to run continuously, and lose some of that efficiency. I like the quieter ride of gear mode, with the ICE running at constant speed, and I am willing to give up some of that efficiency.

Obviously, just my opinion...

 

My understanding of Engine Drive, which I believe is what many here refer to as Gear Mode, is when the ICE engine is connected to the traction motor via a lock-up clutch to provide motive power to the vehicle. If that is anywhere close to being correct, the engine would need to run continuously while in Engine Drive.

Your description above, of Engine Drive, is my understanding of Hybrid Drive, where the engine cycles on and off as needed.

 

My experience with Engine drive / gear mode, is the system can be in Engine drive mode, and then the ICE will stop running (like Hybrid mode), when the ICE starts again, it can go directly into Engine drive. This appears to be the most efficient operation.

It does not appear that Engine drive, and Hybrid mode are mutually exclusive.
I guess you could say the Hybrid mode can use Engine drive.

 

My point is that the description above is technically inaccurate. The engine must run continuously in order for the gear icon to be displayed, which indicates that the car is in Engine Drive and operating as a parallel hybrid.

When the engine is off, the gear icon disappears indicating that the car has switched to either Hybrid Drive, or possibly EV, and is now operating as either a series hybrid or BEV.

With the exception of EV, the car is always operating as a hybrid in one way or another. The engine can be either on or off in Hybrid Drive, but somebody is going to have to explain it real slow how the car can be in Engine Drive when the engine is off.

 

My apology, I think we just crossed our wires since the manual is anything but forthcoming on this.

@insightman has often tried to keep us straight on the somewhat confusing terminology that Honda uses. I’ll try to stay true to what he’s pulled out of the manual.
The manual states we only get to choose 3 propulsion sources (p11) (EV, HV, and HV Charge), But then on p14, states the 3 power sources are Electric (EV icon on), Hybrid, and Engine (both with EV icon off). Also, on p12, there are 3 selectable drive modes (ECON, NORMAL, and SPORT) within each of the 3 propulsion choices. And confusingly, NORMAL has no button or icon and is just the state of not being in either ECON or SPORT.
And then throw in p132 where the engine power source is called engine(direct drive).
(My head is spinning. @insightman, have I got all that straight?)

So obviously the engine drive power source can only work when the engine is running but doesn’t necessarily work every minute of HV. (I assume that’s due to speed/rpm, torque issues, and who knows what else.) And when the gear icon goes out, it remains using HV as the propulsion source, it’s just no longer mechanically coupled to the wheels.

So I like to think of engine (direct drive) or “gear mode” (as referred to by some) as a more efficient subset of HV since the HV icon doesn’t go out when it activates, it only activates in HV, it doesn’t stay on every minute of HV, and we get no choice in the matter.

Since I’ve never used it, can anybody tell me if engine (direct drive)/“gear mode” has been seen during HV Charge? I suspect not but I have no real world experience.

 

I am pretty sure that gear mode does not work in HV Charge. I use gear mode extensively. i have started to also use HV Charge (not extensive experience, but several hundred miles).

When you go to HV Charge, it dumps out of gear mode, and rpm's jump about 600.

 

In a nutshell “gear mode” is simply the Clarity going from series hybrid mode to parallel hybrid mode.

 

No apology necessary. I just wanted to be sure that we were discussing the same functions of the vehicle.

I came across a Honda document on line that described EV, Hybrid Drive and Engine Drive quite clearly, if you can believe that. Those are Honda’s words.

The cat is out of the bag with the release of the term “gear mode”. Honda already describes this function as both Engine and Direct Drive. Why add a third term?

Now, on to the question of whether Engine Drive or Hybrid Drive is more fuel efficient.

 

People refer to it as "gear mode" for the simple reason that when engaged that is how it is pictured on the screens.

 

I maintain that when the Clarity PHEV is burning gas with the Engine Drive clutch engaged, it is using gas more efficiently than in the serial-hybrid Hybrid Drive mode. That's the only way to explain the extra complexity and weight of the Engine Drive system. The asterisk here is that it is more efficient only within the range of speeds that the single-speed gearbox provides in Engine Drive mode.

Honda could have weighed down the Clarity with a CVT instead of a single-speed gearbox, and perhaps with the CVT's additional gear ratios Engine Drive mode could have been more efficient than Hybrid drive at a wider range of road speeds. But Honda decided for all their i-MMD hybrids that a single-speed gearbox was best. I cannot guess the number of variables (including cost) Honda evaluated when designing the Clarity PHEV to maximize fuel efficiency.