How can the Clarity Plug-In Hybrid produce 212 hp?

insightman

insightman

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I want to understand how Honda calculates the combined horsepower ratings for i-MMD two-motor hybrid systems. Specifically, how the Clarity Plug-In Hybrid can produce Honda's claim of 212 horsepower.

Clarity forum member ab13 thankfully directed us to the very interesting 2013 paper authored by Honda engineers Naritomo Higuchi and Hiroo Shimada. It is titled "Efficiency Enhancement of a New Two-Motor Hybrid System."

This paper describes three basic drive modes for Honda's i-MMD two-motor hybrid system (these are the authors' words):
  • In "EV Drive", the traction motor physically connected to the driveshaft propels the vehicle by using electric energy stored in the battery.
  • In "Hybrid Drive", the traction motor is driven by electric energy generated by the generator using engine output. The battery behaves as a passive device to provide lack of engine output or be charged by surplus energy.
  • In "Engine Drive", the engine-drive clutch located between the engine and the wheels is engaged, and the vehicle is directly propelled by engine output.
The paper's Figure 11 graphically depicts the operation of these modes. Of special note, the figure clarifies the operation of Engine Drive mode by showing that in this mode the traction motor switches to generate power to recharge the battery, as it does when the car is decelerating (assuming the battery is not already fully charged).

WEVJ6-2-039_Figure_11.jpg


Honda has stated that the traction motor produces a maximum of 121 horsepower in EV Drive mode (battery power only) and up to 181 horsepower in Hybrid Drive mode, when the starter motor/generator is functioning as an engine-driven generator to supplement the electric power coming from the battery. Honda specifies that the Clarity's 1.5-liter engine produces 103 horsepower.

Has Honda made improvements to the i-MMD system since this paper was written to describe the 2013 Accord Hybrid? If not, how can the Clarity produce 212 horsepower?

Figure 11 does not illustrate any mode where the engine and the battery-powered traction motor combine their outputs. How can the Clarity Plug-In Hybrid produce any greater than 181 horses to turn the gears that drive the front wheels?

Furthermore, how is that the new Accord Hybrid, with its 181-horsepower motor and larger 2.0-liter, 143-horsepower engine also produces exactly 212 horsepower?
 
I don't know exactly where that value comes from in the Clarity, but at least based on the diagram of the electric drivetrain that the car displays (and the description thereof in the manual) it is indeed capable of powering the wheels with both the gasoline engine (using the mechanical linkage) and the 181HP electric motor at the same time. In this state it would make sense that the gasoline engine can't be supplying in excess of 103HP from its ICE to the combination of the generator and wheels (assuming it can indeed run both at the same time at all, which I'm not sure of), then you're dealing with some combination of the theoretical 121HP that the main motor can produce from the batteries and theoretical 103HP that the ICE can produce on its own, which (apparently) adds up to a maximum of 212 HP in practice rather than 224HP (which isn't particularly surprising).

Assuming this is in fact correct, it would only be possible to achieve the maximum horsepower output of the car at speeds above 50mph (possibly way above 50mph, since the peak power output of the ICE should be at a very high speed and I've been told that the mechanical transmission is a fixed gear ratio so it can't downshift to increase RPMs for a given speed).
 
If the two power sources can work together, then Figure 11 must be missing a second Engine Drive segment where "Drive" replaces the current Engine Drive segment's "Regenerate." What does the starter motor/generator do in this additional Engine Drive segment?

What level of driver demand triggers this "all-out" Engine Drive mode? I again call for a "verbose" display mode on the center screen that reveals the decision-making processes going on in the computers under the hood and under the trunk.

It would make sense that the maximum total horsepower produced in this proposed additional Figure 11 Engine Drive segment would be less than the sum of the motor and engine numbers because the two attain their maximums at different RPMs. If the starter motor/generator is sapping engine power to generate more current for the traction motor, that further complicates the extra Engine Drive mode calculation.
 
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That chart didn't clearly show the other option. In the paper, figure 20 shows the engine drive mode relative to the road load. The red line is the engine output. The star is when the engine output is the same as the road load. If the load is less than the engine output then the battery will charge from regeneration . If the load is greater, the right of the star, the battery will output to the ev motor to make up the difference. That is where the outputs combine.
 
The Clarity PHEV has the same 181hp traction motor as the Accord Hybrid.
https://automobiles.honda.com/clarity-plug-in-hybrid/specs-features-trim-comparison

The difference is that the Accord has a larger ICE. Perhaps Honda reduces the power output of the traction motor in the Accord when in hybrid mode such that it gets the same combined maximum power rating as the Clarity. Would make sense in order to preserve the charge in the smaller Accord Hybrid battery.
 
Thanks, ab13, that engine load/road load graph does provide a more complete view than Figure 11.

Kendalf, although Honda rates both the Clarity PHEV and Accord Hybrid traction motors at 181 horsepower, it appears from a Honda press release that they do not use the same motor. The Accord Hybrid's motors are "the first" to not require rare-earth metals (why do they group them under the moniker "drive motors," isn't only the larger traction motor a "drive motor?"):

"The engine is paired with two Honda-developed electric motors that are the first drive motors in the world to use permanent magnets containing no heavy rare-earth metals."

Why Honda would throttle the potentially more powerful Accord Hybrid powertrain to only 212 horsepower will remain a matter of conjecture unless...no, I should not hope for Honda to answer our myriad questions.

Now I'm wondering if the 1.5-liter Atkinson-cycle engine in the upcoming Honda Insight produces the same 103 horsepower as it does in the Clarity PHEV. Honda's saying the Insight produces 151 horsepower, but who knows if the engine's output had to be additionally enfeebled for the car to achieve 55 mpg in the city? I was hoping the 3rd generation Insight would increase the power by the same percentage as its increase in weight over my 1850-lb 1st gen Insight.
 
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The Accord hybrid only has a small 1.7 kWh battery, which can provide only a small number of horses from the traction motor by itself, unlike the Clarity which can get 120 horses from the battery alone. To get to 181 hp, it needs to be supplemented by the ICE. It can get to 212 peak hp by also engaging the clutch so it can't provide the total 212 hp unless going over 50 mph or so. The Accord depends mostly on the ICE to generate sufficient electrons to power the traction motor. It would have to work a lot harder than the Clarity ICE to utilize the full 181 hp of the traction motor. That's the reason the Accord has a more powerful ICE, to compensate for a battery one tenth the size of the Clarity's.
 
PHEV Newbie said:
...unlike the Clarity which can get 120 horses from the battery alone.
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From Honda's specs it appears that the electric motor in the Clarity is good for 181 HP (and 232 lb-ft torque) by itself. As @insightman clarified, it is not the exact same motor as the Accord hybrid--as the variation in the Accord that does not have rare-earth metals may not be able to sustain the temperatures that an EV primary vehicle would need--but both traction motors are rated for the same output.

The total system HP of 212 is in combination with the ICE.

I agree with you that the smaller battery in the Accord hybrid probably is a factor in how it gets the same peak HP as the Clarity even though the Accord has a larger ICE. My guess is that the ICE takes a bigger load in the Accord to reduce the drain on the smaller battery.
Clarity Electric Motor.webp
 
insightman said:
What level of driver demand triggers this "all-out" Engine Drive mode?
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I assume it's triggered by flooring the gas pedal, like any other all-out mode. The car will put out as much power as it's able at the given speed. How many actual horsepower that at the wheels is will depend on your speed (and probably other factors), just like in a pure-ICE vehicle. Being an EV, in Eco mode if you don't push past the breakover, it will be limited by the output of the battery (which may vary with temperature), and will be virtually flat. If you floor it in non-Eco modes, it will output as much power as the combination of the battery and ICE are able at whatever speed you're traveling at. At lower speeds, this will presumably be capped by the electric motor. At higher speeds it may involve the mechanical transmission as well.

One comment, it's also theoretically possible that the electric motor is capable of higher-than-rated output for brief periods before the cooling capability/thermal mass limits it. I'm assuming that's not where the 212hp number comes from, but it theoretically could be.

insightman said:
I again call for a "verbose" display mode on the center screen that reveals the decision-making processes going on in the computers under the hood and under the trunk.
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Respectfully, a "verbose" mode is kind of silly to even imagine in a consumer vehicle, let alone a 4-door family sedan--and I'm an engineer who would probably have it up as the default display.

An Accord does not have a "verbose" display that shows you what the logic behind the EFI timing is, or what torque calculations it's using to decide whether to up or downshift, or what the exact torque or horsepower output is at the moment and how it's achieving that; it never has, and it never will. You get a speedometer, a tach, and maybe an indication of what gear it's in. 99.99% of owners simply do not care, 99% would not be able to interpret the information even if they did, and some measurable fraction would bring the display up, not know what it meant, and misinterpret the information.

If you want any more information than that, as with any other car you're going to have to instrument it yourself.

There is already a display that shows whether power is coming from or going to the battery, coming from the ICE or not, and whether the mechanical transmission is engaged or not. That's really about as detailed as I would ever expect to see in a non-performance consumer vehicle. A hard kW number for the electric portions where it's easily measured would be nice (a la earlier Volts, though that was pulled from Gen2), but that's about the realistic limit.
 
I think it's more important what torque you are getting. Since torque is important for low speed acceleration and highway passing. Peak horsepower is more for top speed driving. Also, you could get one of those ODB devices and a phone app (such as the Torque app) to read the system stuff. Not sure what you can get from a Clarity, but maybe the ICE RPM, which would be interesting. Oddly enough, if one of these apps supported Android Auto/Apple Carplay, then you can have those displayed on your screen.
 
M.M. said:
Respectfully, a "verbose" mode is kind of silly to even imagine in a consumer vehicle, let alone a 4-door family sedan--and I'm an engineer who would probably have it up as the default display.
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I wouldn't expect the verbose display to be a standard menu selection. I dream of a secret method, such as plugging in a USB thumb drive containing a special file that would bring up this technically entertaining display. Perhaps Hondata, the company that makes warranty-invalidating performance chips for Honda cars could figure out how to do this, but if they could do it I imagine it would be expensive due to low demand. I'd be willing to pay $300, but $600 might be too much.

[The 30-second Clarity PHEV spot just came on for the 3rd time tonight as I was writing this post. Some people criticize it, but I think it's about the best you could do in 30 seconds.]
 
insightman said:
I dream of a secret method, such as plugging in a USB thumb drive containing a special file that would bring up this technically entertaining display.
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As ab13 mentions, this is the kind of thing that people have done in other cars with an OBD port. I find it unlikely there will ever be anything similar to the ODB-based dashboard displays people get for conventional vehicles, but I know people have managed to extract fairly detailed information about what the Volt is doing electrically via either the OBD port or hacking the CAN bus, so something similar should be possible with the Clarity.

That said, does the Clarity even have an OBD-II port? I haven't looked, but attaching a dongle would be fun if it did.
 
Kendalf said:
I would add that it certainly feels like 181HP and 232 lb-ft of torque when accelerating in EV only mode. 121HP would not be able to provide the oomph that we experience for a car this heavy.
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When I tested 0-60 in the Volt 1, the i3 Rex, and Clarity, in EV only mode I saw 12-13s for Clarity, 10s for Volt and 7.8 seconds for i3.

This is right inline with Clarity having only 120 hp in EV only mode, 150 hp for the Volt, and about 170 hp for the i3.

With the Clarity in mixed mode it was faster than the Volt around 8.5 seconds which is inline with it having 212 hp.
 
Viking79 said:
With the Clarity in mixed mode it was faster than the Volt around 8.5 seconds which is inline with it having 212 hp.
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Did the gear icon appear at any time during your 8.5-second run to 60 mph? Only when both the engine and traction motor are driving the wheels could the Clarity PHEV produce 212 horsepower.

It doesn't seem possible that the Engine Drive clutch (Overdrive clutch? Direct Drive clutch? What's the official name?) could be engaged at the start of your run because that would bog down the engine due to the high direct gear ratio. With the engine powering the starter motor/generator exclusively there would be a maximum of 181 horsepower available as your run began.

What is the lowest speed at which the clutch can be engaged? How much power can the starter motor/generator generate when the clutch is engaged and the engine is serving dual purposes? Enough to produce just 212 total horsepower instead of 181+103, I guess. So many questions.

It would be nice if there was an optional audible beep to accompany the appearance of that very tiny gear icon when the Engine Drive clutch engages (I know, I know, too verbose). I've personally never seen the gear icon, but we rarely drive on the e-way.
 
I won't go as far as to say it was closer to 212 than 180, either could probably do that 0-60 run depending on assumptions and measurement accuracy, my main differentiator here was the 120 hp battery only vs 180+ combined mode. They battery is limiting traction motor performance. I will do a test later to see if the gear comes on, but really I need better equipment to measure that like an IMU.
 
Going back and looking at my notes, the Clarity was 9 sec to 60, which would likely put it closer to 180 hp, but again, it was a few runs using Torque Pro, so nothing more than informational. Would need to dig more in depth to see if Honda's claim of 212 is valid, my guess is it is a limited test case.
 
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