Just got my Clarity Plug-In a couple days ago and I have an efficiency question

That's how I understand it too! Though if my round trip can be done full EV then I usually do not turn on HV mode even if I'm on the freeway.

Not sure in terms of efficiency which would be best. However, HV Charge mode can cause the engine to surge to a pretty high volume (relatively speaking) and that noise may be much more noticeable and intrusive when driving in the city vs at highway speeds.

HV Charge will burn more gas than just HV sustain mode, and I believe it is more efficient to charge the battery by plugging in than via the ICE, so it seems that just using HV mode would be better in this situation. HV Charge is probably best for situations where you have a stretch of highway to charge the battery for more city driving that is to come.

 

There will be many right ways to think about this. Electricity isn't as efficient as using gas at freeway speed. Mostly, the system will take care of that in "normal" mode. You can force it to use more electricity in EV mode, and it'll do that. I suspect that mode just decouples the motor from ICE and burns electricity w/o replenishing it via ICE. HV does just the opposite. And in normal this is "automatically done". Sport seems to change the accelerator algorithm. It doesn't seem to add power as much as it changes how much pressure you need to apply to get at that power. As a result, it will eat up more fuel (Electric and gas). I have tried, and I get the same results by simply flooring the go pedal to the detente. OK, sometimes I fail because I overshoot it. LOL

 

Based on experience with the Chevrolet Volt I see little value in HV charge (where the car is trying to increase the battery state of charge). There are significant inefficiencies in this mode.
My inclination is to save the battery for slow, in town driving and use normal HV mode for anything close to highway use if you know you're driving farther than the expected battery range.

 

It's really interesting to get used to the quirks of PHEVs. We work from home so we run around town a lot. I stay in EV mode even on the freeway because I can get home before the charge is expended. Why? Electricity is cheap in my area (about $0.10/kWh all in) while gas is above average (about $3/gallon currently). For the cost of a gallon of gas, I get about 100 miles of EV range so even if I lose EV efficiency going on the highway, it's still cheaper than gas. From my experience, my EV efficiency seems to be about 30% less efficient on the highway (65 mph) compared to driving around town. So when I'm on a long trip, I am in HV mode on the highway and I keep a charge for when I get into town. If the battery is expended, I'll go to charge mode when it's most efficient (highway speed about 55 mph and not hilly). That way, I'll have a charge when I get to the next town.

 

It's best if you avoid "charge mode". Just wait until you get home to recharge the car. Not only is this more energy efficient, it also avoids wearing out the battery pack faster than necessary.

So far as I can see, the only good reason to use "charge mode" would be when you're driving into a zone where emissions are restricted, and you want to be sure you can operate in EV mode when inside that zone. Right now I don't think there are any such zones in the USA, but there may be in the future. I could be wrong, but so far as I know, central London is the only such zone at present. We can be sure more are coming.

I've seen some recommend switching to "charge mode" before climbing a mountain, but I don't see the advantage in this. Just use HV mode for that.

 

This is incorrect. What is correct is that gas-powered mode is less inefficient for freeway driving than for low-speed or stop-and-go driving. It's still not as efficient as using EV mode, by a long ways.

It is always much more energy-efficient to use EV mode than gas-powered mode. For a pure EV vs. a pure gasmobile, the EV is about 3.5-4 times as energy efficient. Of course, a car like the Clarity PHEV is never operating as a pure gasmobile, but the rule is still good.

The thing to remember is that the amount of energy the battery pack contains is quite limited, probably equivalent to only a couple of gallons or so of gasoline. So it's best to maximize that use of energy where and when you can. Yes, you should use EV mode wherever possible in stop-and-go driving, because a gasmobile (or a PHEV in gas-powered mode) is at its least efficient in that kind of driving.

Should you switch to HV mode when driving on the highway?

Yes -- If the car doesn't have the range to make the entire round trip (returning home) on just the energy in the battery pack.

No -- If you can make the entire trip and return home on the EV range alone.

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Bottom line: If you want to maximize your energy efficiency when driving a PHEV, then maximize the number of miles which are EV powered rather than gas-powered. That means using EV mode as much as possible for every trip. Fortunately, since you're driving a PHEV, you don't have to worry about making a mistake and running out of EV range before you return home. At worse, the car will just kick in the gas motor.

A tip: From what Clarity PHEV drivers have reported, sometimes the car doesn't want to switch back to EV mode when it's in HV mode, especially if it's cold outside. If that happens, you might be able to solve the problem by stopping the car, switching it off, and then switching it back on.
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There's a lot of discussion in this thread on "switching to HV mode" at freeway speeds. However, my understanding is that the Clarity will do that automatically for you. Just stay in ECO or NORMAL mode and don't worry about switching in and out of modes. When necessary, the ICE will kick in.

Isn't that right?

 

In Econ mode, I haven't experienced the ICE go on at highway speeds. I've only driven around the speed limit though. Don't know what will happen if you hit 80 or 90 (slowly accelerating). If you floor it, it will activate the ICE at any mode. In Normal mode, I find the ICE does go on by itself at highway speeds, whenever it's most efficient.

 

Interesting. However, my point is that there is no need for the driver to ever switch between modes to maximize efficiency. Even in ECON mode, the Clarity should do what is necessary to maximize efficiency and there is no advantage to "switching" to HV mode on the freeway.

 

For me, I generally use EV around town and on the loops (35-60mph ranges), and kick in HV at freeway speeds - reasoning - on those longer freeway trips I'd like to have EV available for in town driving. As for HV Charge Mode - it is pretty inefficient/nonexistent at in town speeds, but I will use it on the freeway occasionally to maximize that in town driving at my destination. Remember, recharge will only get you 57-60% of a full charge, then stop, and it will tell you this when you hit the max.

I always like @jdonalds advice - just drive and enjoy - the car will take care of (most) of the rest.

 

The purpose of switching manually to HV mode when on the freeway is because the car does not know what your driving needs will be later in the trip. It will be more efficient to remain in EV mode, but that drains the battery pretty quickly at highway speeds. If you know you're going to be driving around the city for awhile after this stretch of freeway, it will be more efficient to push the HV button to maintain battery SOC while you're on the freeway so that you can drive around in EV once you get off the freeway, rather than have the battery drain during the freeway portion and be stuck with driving around the city very inefficiently with the ICE.

 

Excellent insight. I hadn't thought of this. Makes complete sense. I will change my behavior from now on.

 

Just to add, it isn't that EV is more or less efficient at freeway speeds in general, but rather it has to do with gearing and the transmission in the Clarity such that using the gas engine at high speeds on the highway is more energy efficient than using the EV motor. The motor has an efficiency map of RPM vs Load vs Efficiency and so does the gas engine.

If you read the technical paper in post 7 it will show you the energy efficiency mapping of the engine/motor and where they try to run the engine vs motor.
https://insideevsforum.com/community/index.php?threads/honda-clarity-power-split.418/#post-3740

You notice at high speeds and low power demands it is actually more efficient to run the energy directly to the wheels (pure ICE mode), as it is actually more efficient than trying to charge the battery or generate voltage for the motors. They use whichever mode is best for the situation. Just make sure to use up your charge if you are going to run gas, as it still is probably cheaper to run electric even if it is less efficient at high speeds than it is at low speeds.

 

So, I have a hypothetical question for you experts.

Let's say I start a freeway trip with 100% battery and then plan to do a lot of city driving once I get to my destination 200 miles and about 3 hours later. Which of the two HV modes will work better? Will they work differently?

HV MODE POSSIBILITIES
Option 1: HV mode uses very minimal battery and tries to maximize pure ICE mode when possible at cruising speeds. You arrive at your destination with battery at about 90%.
Option 2: HV mode uses battery to supplement the ICE through the entire trip. You arrive at your destination with battery at about 40% or lower.

HV-CHARGE MODE POSSIBILITIES
Option 1: Clarity tries to maintain the charge through the whole trip and you reach your destination with the battery at around 90%.
Option 2: Clarity uses battery as needed (including perhaps pure EV mode on the freeway) and then maintains charge at 57% so you reach your destination with battery at about 57%.

Any idea which option the Clarity would use in each mode? That would influence the mode I use on the highway. Would welcome insight from the more knowledgeable and technical among you.

 

Your options are confusing to me.
If HV mode is selected with the HV button, it will operate the engine sufficiently to keep the battery at whatever charge level you started out. If your hypothetical trip starts as you enter a freeway you might as well push the HV button and save as much battery as you think you'll want for city driving. If you're doing city driving before entering the freeway then leave it in Normal or Econ mode until a mile or so before entering the freeway at which point you press the HV button. This will give the engine a minute or two to warm up under relatively low load.
I don't recommend HV charge mode for the reasons that have already been discussed above.
It doesn't probably make sense to save much more battery than you're going to use, so you might as well switch out of HV mode near the end of your trip home to use up the remainder of the battery.

I drive a Chevrolet Volt. Here's a strategy that hasn't been discussed, but is valid: If I'm embarking on a trip that's of two legs (one outbound and the other returning) and I have enough battery capacity to accomplish the first leg electrically, I'll do so rather than splitting up both legs with HV mode, because it allows my engine to do only one cold start over the course of the trip. The exception to this would be if it's very cold out and I want to take advantage of the engine waste heat to heat the cabin. In that case, I'd run the engine on the first leg in hopes that I can do the second leg electrically.
There's nothing less efficient than the first minute or two of a gasoline engine's cold start, since a good part of the fuel is going towards simply warming up the engine.

I hope I haven't made things more confusing.

- Chris

 

In general, I can't see any good reason to use the "charge" mode. Using the onboard gas engine to charge the battery pack is the least efficient way to use the gasoline in a Clarity PHEV's gas tank, so why would you want to do that?

As I've said before, I can see an "edge case" to be made if you're driving into a locale with emissions restrictions, so you have to use the no-emissions "EV mode" there to avoid a fine. But otherwise, I can't see any good reason to use it.

You can just leave the Clarity in "Normal" mode if you want to let the car choose for itself whether to get power from the battery pack or the gas engine (or both). But the subject of this discussion thread is how to maximize energy efficiency when driving the car. To do that, a driver will learn to take more control of the car, by switching from "Econ" mode (maximizing use of the battery pack) and "HV" mode (minimizing use of the battery pack), depending on driving conditions and how far he's going on that trip.

Please note I'm not suggesting there is a "right" or "wrong" here. It just depends on what your goal is, and how much time and effort you want to spend to minimize your use of gasoline. Some PHEV drivers obsess about that; you can see Volt forum discussions where drivers literally do anything they possibly can to keep the gas motor from ever switching on. Some do so even in bitterly cold conditions, where avoiding use of the ICEngine to warm up the car (at the start of a trip) is probably bad for the long-term life of the battery pack. Other PHEV drivers don't spend any time or effort on that at all, pointing out -- correctly -- that PHEVs are engineered so that the driver doesn't have to worry about whether or not the car is running on electricity or on gasoline.

I think most PHEV owners are somewhere in the middle, as people generally are on most subjects. I think most PHEV owners do want to minimize their use of gasoline. If they didn't, then they would most likely have just bought a gasmobile. But I also think most PHEV owners don't want to obsess about it, even if some do.

“Everything in moderation, including moderation.” --Oscar Wilde
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Exactly, and thank you for making that clear!
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I think this is a misunderstanding on your part. The PowerPoint presentations you pointed to were not specifically using the word "efficiency" to mean energy efficiency. (Pet peeve of mine: The word "efficiency" is used to mean too many very different things!)

In engineering terms, the term "efficiency" can mean designing and building something with a minimal use of materials or complexity. That would make it more "efficient" to an engineer, even if it's less energy efficient in operation. For example, in a PHEV, it's more "efficient" to design and build the car so that when accelerating hard (or hill-climbing) at high speeds, the car needs to combine the power of the EV motors and the gas engine to attain that strong an acceleration. Even the first well-designed PHEV, the Chevy Volt, is designed that way. (Pardon me for using the Volt as an example here, but I understand how the Volt is engineered in more detail than the Clarity PHEV.) In that respect, it's more "efficient" to design a PHEV so that the gas engine alone isn't sufficiently powerful enough to accelerate the car when it's already moving at high speed. If the car was built for that, then the gasoline engine would have to be bigger and more powerful; as a consequence, it would be less energy efficient.

There may be some edge cases where, with a PHEV, it's more energy-efficient to power the mechanical drivetrain directly from the gasoline motor, rather than to use the electric motors. But it's certainly not going to be the case that in general, it's more energy-efficient to use the gas motor to push the car down the road at highway speed using the gas motor. Going back to the Volt as an example, it only directly engages the gas engine into the powertrain when (a) the battery pack is mostly exhausted, (b) the car is running at 35 MPH or faster, and (c) the drivetrain is called upon for high power output, such as fast acceleration or hill-climbing. And please note that in this case, the Volt does not engage the clutch which allows the gas engine to provide direct mechanical power to the drivetrain because it's "more energy efficient", but because it provides more power!

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A car powered by a gasoline motor generally has no better than 25% energy efficiency, and it may well average only 15% energy efficiency... or even less, if it's an older car and not well maintained. Contrariwise, a well-designed EV has a motor with an efficiency between ~85-92%, depending on what its power torque is and what RPM it's running at. Of course, with the mechanical inefficiency of the drivetrain, that drops the overall efficiency for the EV down to something like 68-74%, but that's still a heck of a lot more energy efficient than any car powered by a gas engine!

Since PHEVs are by nature a compromise, it may be that the EV powertrain won't be as efficient as it is in a pure BEV. But to say that the Clarity PHEV is more energy efficient at highway speeds when powered by the gas engine rather than powered by the battery pack... well, in general and in the overwhelming majority of cases, that's just not correct.

Or, to put it more simply: "Design efficiency" should not be confused with "energy efficiency".
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