Efficiency of the Clarity Generator

Discussion in 'Clarity' started by Geor99, Aug 30, 2019.

  1. Geor99

    Geor99 Active Member

    I realize that this might be a difficult question to accurately answer but...

    Let's suppose that I drive the Clarity in ev 100% of the time (or as close as is possible;) yet I never plug in.

    Let's suppose that while in park, I charge the battery with the HV charge feature. Then while moving I turn hv charge off and just use the battery by staying in ev as much as possible.

    What would the mpg be?

    And yes, I realize that it would be ridiculous to do this, and that it will only charge to 58% this way.

    But I wonder how it would compare to the advertised 42mpg using gas as a propelant???
  2. craze1cars

    craze1cars Well-Known Member

    Pretty easy to find out. Do it. In the interest of science....
  3. jdonalds

    jdonalds Well-Known Member

    1st I always assume that charge mode can take advantage of some residual power from the ICE. in other words if the ICE has to run to keep the car going 70 mph then it's likely more power is being generated than needed for that task.

    2nd my observation is when the car registers 40 mpg in HV mode it registers about 30 mpg in HV Charge mode.

    The least efficient use of HV Charge would be your example.
    ClarityDoc likes this.
  4. 2002

    2002 Well-Known Member

    I think that might be true for regular cars because they only have a gas engine and it has to be sized for the maximum acceleration that the manufacturer decides is necessary in order to meet customer expectation for that particular type of car. Even if the driver only needs that amount of power once a month, once a year, or maybe even never, people still want that power to be there if they ever do need it. That's one reason regular gasoline powered cars are so inefficient because they have to lug this big overpowered gas engine around everywhere, which besides the added weight it's also not spending a lot of time running at its most efficient power band. And why Cadillac and others were doing things like shutting off unused cylinders etc. when cruising.

    The much smaller, lighter Atkinson cycle engines that get put in cars like Clarity are already quite efficiency at the power levels normally needed. So using extra power to generate electricity to store in a battery, to be pulled out later, with all of the accompanying losses, just doesn't seem like it's going to be a benefit. The only reason it stores energy is because it will be needed for accelerations both large (starting from a stop) and small (slight ongoing speed adjustments). But those accelerations are usually needed only a small percentage of the total drive, so the engine usually has no problem building the charge back up in between accelerations.

    Anyway that's how I understand it, although my opinion has evolved over the years and is always subject to modification as new information becomes available. I actually wonder if car makers don't like to talk about this in detail as I assume many of the strategies and algorithms are proprietary.
  5. David Towle

    David Towle Active Member

    If you do this you should compare to an alternative; never plugging in, but instead use HV charge while you are moving when needed (say whenever it gets down to 4 bars).
  6. DaleL

    DaleL Active Member

    Basically the Clarity is an EV. In the HV mode, except during hard acceleration between 45 and 65 mph, the ICE does NOT directly engage with the drive train. It is the 181 hp (equivalent) drive motor that does all the work. The ICE powers the generator (60 hp equivalent) which puts electricity into the drive motor and/or the high voltage battery. The car runs the ICE at the most efficient RPM possible, which at low speed results in the annoying high revs. In HV mode, the car switches the ICE on and off as needed to maintain the battery charge. In the HV charge mode the ICE runs all the time with the excess electricity going into the battery. Obviously, mpg will drop in the HV charge mode, but once the stored electricity is used, the overall mileage should be the same as if the entire drive was in the regular HV mode.

    In short, the effective overall mpg should be 42 (44/40).

    The only reason to run the car in HV charge mode that I can think of is to maintain a range reserve on long trips. This is because of the tiny 7 gallon gasoline tank. On any long trip, it is useful to drive until the gasoline tank is almost empty. Knowing that the car can drive another 20 miles on electricity helps to reduce range anxiety.
  7. sniwallof

    sniwallof Active Member

    60 hp equiv. is about 45 kW if google convert is working correctly, which might be a little high. Did you see it in a spec. somewhere?

    I've been curious about how big the Clarity generator is. WAG is more like in the 25 - 35 kW range, Clarity dips into battery for more power as needed, then puts charge back later to maintain about a set level. So, the genny itself does not need to be rated for worst case power scenarios.
    Last edited: Aug 31, 2019
  8. insightman

    insightman Well-Known Member

    Actually, if you call for hard acceleration when you're traveling in Engine drive mode (some call it "gear mode" because a gear appears between the wheels in the energy-flow diagram), the Clarity drops out of Engine drive mode and reverts to Hybrid drive mode, where the traction motor drives the car using power from both the battery and the ICE.

    My eternal question about the Clarity's 3 basic underlying modes: If EV drive mode (battery-power only) produces 121 hp and Hybrid drive mode produces 181 hp, and you can't call for hard acceleration in Engine drive mode, how can Honda claim 212 hp as the Clarity's maximum horsepower? When does that happen?

    Here's the possibly incorrect Clarity mode chart that I made for last year's Drive Electric Week event. It presumes 212 hp is available in Engine drive mode only because Honda tells us how much power is available in the other two drive modes.
  9. r1ptide64

    r1ptide64 New Member

    I'm no expert, but I don't believe that this is accurate. If you've ever ridden a bike with a light powered by a generator, you know that you have to work harder with the generator running. This was humorously exaggerated in an old Simpson's episode.

    Your Clarity's engine also needs to work harder if it's powering both the wheels and the generator compared to the wheels alone. If you're cruising at 70 mph with your Clarity in engine drive mode (engine is directly providing propulsion to the wheels), and you were somehow able to disable the generator while keeping the engine's power output fixed, you'd speed up.
  10. 2002

    2002 Well-Known Member

    Yes but what is being referred to is that gas engines have an optimal range of speed which is known as the power band where they are most efficient. A lot of the time an engine is operating somewhat inefficiently because the power need at the moment is lower than the power band. That happens especially in non-hybrid cars where the engine is larger and more powerful than is needed in normal driving, because it is the only source of power and is called on occasionally to provide large amounts of power even though usually only on more rare occasions. Thus it may operate less efficiently at lower power demands. However if the engine also charges the battery at the same time, which as you said requires more power, the engine can in some cases run more efficiently and the additional unneeded for the moment power is being sent to the battery for later use.

    My supposition however is that while the above is true for larger engines used in regular cars, it may not be as true for the smaller more efficient engines used in hybrids because in those cases the engines are sized so that the engine runs in the power band most of the time even at normal loads. So I suspect that there is less benefit to increasing rpms to generate electricity for later use. The only reason it stores electricity is to allow for smoother and quieter acceleration during driving, which is what allows installing an underpowered but very efficient gas engine, one which would be unacceptably rough and noisy if it was often called upon to provide instantaneous acceleration by itself.
  11. r1ptide64

    r1ptide64 New Member

    Interesting. That was totally different from how I read it.

    "More power is being generated than needed for that task" makes it sound like you can get charging for 'free' while driving at 70 MPH. As though it requires (making up numbers here) 50 kW of engine power to cruise at 70 MPH, regardless of whether or not you're also running the generator, but 3 kW of power is 'wasted' if you're not also charging, so you might as well charge.

    jdonalds, would you mind clarifying?
  12. r1ptide64

    r1ptide64 New Member

    Also wanted to weigh in on the main question of this thread:
    My "armchair engineer's" expectation is that you'd get the advertised 44 MPG city fuel economy (or damn close), and worse-than-advertised 40 MPG fuel economy on the highway.

    HV-mode driving in the city, the Clarity is a series hybrid. You burn gas to generate electricity -> powers the traction motor -> propels the vehicle. The battery behaves as a buffer, storing energy when production exceeds demand, and providing energy when the opposite is true. When the buffer is full, the engine shuts off, and the car runs solely on the stored energy for a bit until the buffer gets low again. The Clarity gets 44 MPG when operated this way.

    The scenario you described is not really all that different, except you're using a much bigger buffer, and you're loading it up ahead of time. Whether oscillating between 5% SOC and 10% SOC as you drive in HV mode, or running continuously (without driving) until you reach 58% SOC, then driving in EV mode, it's the same engine/generator system with the same efficiency.

    I would expect this would perform a lot worse on the highway (and you'd get below the advertised 40 MPG) because you'd never be able to take advantage of engine drive mode. That mode exists for a reason -- electric motors are quite a bit less efficient at high speeds.
  13. Geor99

    Geor99 Active Member

    I'm not sure that I follow your logic. When in hv mode and no battery charge (or ending your trip with the same amount of charge as you left,) one is said to get 42 mpg.

    Using the generator to charge the batteries to run purely on ev is a totally different set of motors.

    Why would you think that they both would get 42 mpg when completely different motors are moving the car in the 2 situations?

    You described hv mode as the gas engine charging the batteries so that the electric motor can propel the car.

    I dont think that's how it works entirely. The gas motor may do this to some degree; but I believe that the gas motor directly propels the car much more often in hv mode.
  14. 2002

    2002 Well-Known Member

    True, and is another drawback to generating EV range with the gas engine, when extensive sustained use of EV is really only efficient when using grid power. I can't prove that with numbers, only by the fact that people have been driving Clarity for nearly two years now and we don't have a bunch of people saying "Hey gang, a couple of months ago I started using charge mode on part of my 100 mile daily commute and boy am I using less gas now". Most people who try it seem to find either no noticeable difference, or negative. I have heard of a couple of people who have reported better efficiency using charge mode, but it seems like it was always based on just one or two tests and no real information was given on the testing method used.

    Yes but of course the premise is hypothetical as in normal use no one will be letting their car run in car charge mode in their driveway for half an hour before driving to work, I assume it was presented as a way to maybe view the question more clearly of whether charge mode can be efficient at times. Ultimately it may not be the best way to test the competing theories, the only way to do that is through systematic testing including measurement of actual gas used on a repeated commute route along with accounting for several other variables. Just looking at mpg readings on the display like some have done to try and answer these type of questions is not going to be definitive.
    Last edited: Sep 1, 2019
  15. Geor99

    Geor99 Active Member

    You are correct in the following quote-This is absolutely why I made the ridiculous hypothetical:)

    "Yes but of course the premise is hypothetical as in normal use no one will be letting their car run in car charge mode in their driveway for half an hour before driving to work, I assume it was presented as a way to maybe view the question more clearly of whether charge mode can be efficient at times."
  16. r1ptide64

    r1ptide64 New Member

    42 MPG is the Clarity's combined city/highway mileage rating. It's not actually obtained experimentally. It's just the average of its two experimentally-obtained ratings: city (44 MPG) and highway (40 MPG).
    Have you read the drive mode description document that's been passed around this forum? I attached another copy to this post -- I think it should help clarify. HV Driving mode is described on the bottom of page 3:
    What you're talking about (gas motor directly propels the car) is called engine drive mode, and is described on pages 7-8:
    Engine drive mode is only used for high-speed cruising, and never factors into city driving under typical circumstances. That's why I'd expect you would get the same 44 MPG fuel economy for city driving whether using HV mode or using the generator to charge the batteries to run purely on EV. These two scenarios use the same set of motors in the same configuration.

    But I also mentioned: I don't think you'd be able to match the Clarity's 40 MPG rating on the highway doing this, since you'd never take advantage of engine drive mode.

    I think we're actually agreeing with each other, believe it or not :p

    Attached Files:

  17. craze1cars

    craze1cars Well-Known Member

    Geor99 I believe you may be misunderstanding how the car works. The engine has no transmission with gears, so the engine does not, and cannot couple to the wheels except at midrange highway speeds under light loads. Only in this circumstance is the engine providing a small amount of propulsion power to the car.

    In HV mode the engine is running in order to run the generator, to charge the battery and create electric current, which in turn feeds the electric motor, which in turn propels the car.

    In a Clarity, 100% of the time, engine running or not, in every mode, and at every state of charge; when you accelerate from a stop ONLY the electric traction motor is accelerating the car. There are no exceptions to this fact, ever.

    Even when you floor the go pedal and the gas engine kicks on at high rpm, it is 100% disconnected from the wheels and will never couple to them. When floored the engine is simply spinning the generator very fast to generate an extra shot of electric current, to give the electric motor maximum thrust for max acceleration while the car is floored.

    I think many don’t fully understand this.

    So the car is NOT moving under the power of a completely different set of motors in different modes, as you state in the first 2 opening paragraphs where I quoted you above. It is actually still running on the electric motor, even when the gas engine is on and running.

    This is where Clarity is not at all like a Prius or any other normal hybrid, even when battery is depleted in HV mode. A typical hybrid like Prius is indeed running largely and directly off the engine, with a battery boosted electric motor helping at times. But the Clarity is nearly the opposite in that it is almost always being pushed entirely by the electric motor, even in HV mode with depleted battery, with an occasional slight boost from the engine only at times during cruising speeds and low loads.
    Last edited: Sep 1, 2019
  18. ab13

    ab13 Active Member

    The drive system tries to avoid operating in low efficient RPM for the gas motor, so the gas RPM may be bumped up to a higher RPM range even if that generates more power than needed to drive the car, so the excess goes to charge the battery.
  19. ab13

    ab13 Active Member

    HV charge isn't that efficient due to losses in the ICE, which is about 40% thermal efficient (only Prius Prime is at 41%), generator losses, AC to DC to battery losses. However, if you won't be able to charge and are going to get gas soon anyway, then why not empty the tank.

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