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Engineers work with marketing to make decisions. Sometimes, they're not "allowed" to do things the most efficient way possible.

Case in point: In my 2011 Volt, the engine runs in "Normal" mode, unless you change it to another, such as "Mountain" mode. In Mountain mode, the engine revs at a much higher rate, in order to put extra energy back into the battery, in anticipation of needing the extra battery buffer during ascent of...well...mountains. In Mountain mode, the engine noise can be quite noticeable. Some might even call it jarringly loud.

What I found was that if I played with the modes on flat ground, that the vehicle got about 10% better gas mileage if I kept it in Mountain mode while the engine was running. It required significant diligence on my part, to keep track of battery SOC, and turn on Mountain mode when the battery got down to where the GOM reported about 3 miles worth of charge. I'd then leave it in Mountain mode for about 20 minutes, while the vehicle engine (relatively loudly) added a buffer to the battery. I'd then turn it back to regular mode and run off the battery (in EV mode), until it got down to 3 mies EV range again. Repeat.

What this seems to show is that GM made a marketing decision, that drivers would prefer a quieter environment in the Volt, rather then maximizing the efficiency of the system by running the engine at a louder and higher RPM.

I suspect that the nigher RPM put the little engine more into the peak area of its power band, which can be more efficient. It's possible that the same can be said for the Clarity PHEV, but I haven't played with it to the same extent that I did the Volt.
I remember one person said they got better mpg in HV Charge mode, the theory being as you said that it causes the engine to operate in the more ideal part of the power band. But others say they didn't see much difference. Obermd says he tried it and lost about 20% total HV range compared to regular HV mode. But measuring all of this accurately is pretty difficult without repeated, controlled tests using the odometer and tracking actual gas usage.

I think one complication with the Clarity compared to the Volt is that in HV Charge mode the engine doesn't run all the time, it seems in many ways like it's just operating in regular HV mode but with a higher set point, so it will still shut off the gas engine at times.
 

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Not sure why it would need a larger traction motor, as this would imply that the car has a higher maximum speed in direct drive than in generating mode. Which in theory is possible I suppose, but I have read that in EV mode the speed is artificially limited to 100 mph. Maybe direct drive can go faster than that, but for most people that's a moot point.
Simple - a larger EV traction motor would eliminate the need for the Clarity to start the ICE under heavy load conditions such as hard acceleration. This is the main reason I prefer driving my Volt over my wife's Clarity. I can floor the Volt to get on the freeway (uphill approach) and get to highway speed without having to worry about the ICE turning on.
 

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Simple - a larger EV traction motor would eliminate the need for the Clarity to start the ICE under heavy load conditions such as hard acceleration.
Well sure, but that's not what was being discussed. The discussion, at least as I understood it, was about driving in HV mode, and whether ICE was more efficient in direct drive mode or in electric generator mode. rogerdodger was giving a reason why he believes direct drive is more efficient when he said:

"first is to note that if it was more efficient to use a small ICE to generate electricity and then move the car using that electricity, there would be no need for a mechanical connection (transmission/differential). You could simply eliminate all that cost and weight"

That in fact is the main argument, lacking any actual data, for why direct drive is presumably more efficient, for the simple fact that Honda wouldn't have added the cost and weight of direct drive if it didn't provide increased efficiency in certain driving conditions.

But then he added:

"and just slightly increase the size of the electric traction motors."

which I didn't quite see how that fit into the discussion. Yes when operating in EV mode it would be nice to have a larger electric motor for the reason that you gave, but it doesn't seem like that affects the question of the efficiency of HV direct drive vs. HV electric generator modes.
 

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Thermodynamics (and GM's experience with the Volt) gives the answer to HV direct drive vs. HV as a generator to then turn the electric motors. It is always more efficient to use ICE power to direct drive the drive axle. The reason for this is that this has a single energy conversion where using the ICE as a generator to power the electric motors has two energy conversions. Of course, because of the way ICE works you want to keep the gas engine at the bottom end of the power band sweet spot, so in some cases you'll have extra power to turn the electric motor to put electricity back into the battery for later use.

When GM released the information on the 2016 Volt changes they flat out stated that their experience with the Gen 1 Volt was that using ICE to directly turn the drive axle was the most efficient use of gasoline. They put a second clutch in to allow extra power to be converted to electricity.
 

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It is always more efficient to use ICE power to direct drive the drive axle.
Yep. Hopefully I am not coming across as argumentative. I just have a simple question, a rhetorical question really because I don't think anyone here can answer it, as only the Honda engineers have the answer. The question - how much more efficient is it? For some reason no matter how I word that question, it seems like it's being interpreted that I am questioning whether or not it is more efficient. I have no doubt that it is more efficient. I would just like to know how much more efficient.

Why do I care? Similar I suppose to why I like to know how much more efficient the OEM LRR tires are. I don't doubt that they are probably more efficient than most other tires out there, but knowing how much more efficient is helpful knowledge to have when shopping for tires, since there are other attributes about tires besides just fuel efficiency, some of which affect driving enjoyment, and having that specific information beyond just that the LRR's are more efficient helps when deciding on a tire purchase.

How much less efficient is HV Charge? (I know you had a bad experience). HV Charge has some benefits for drive enjoyment in some situations, and many people use it for that reason. But if it causes a big hit in efficiency then some people might decide to go ahead and put up with a noisy engine, especially with current gas prices. But if it's only a small hit on efficiency as many others have reported, then similar to the tire question, some may think it's worth a small loss of efficiency if it will make the driving more enjoyable.

In the current discussion, it's a question of whether it's okay to just drive it and let the car make all of the decisions, or is it really important to do some managing and button pushing. Probably most of us don't mind pressing buttons, some of us even think it's fun. But that's not everyone, and they question why they would need to do this. It would be nice if we could tell them that by just doing some simple management they can likely get an X increase in mpg. That would help them to decide. Instead all we can say is that it's more efficient. When they ask, "How much more efficient is it, because I really would prefer driving the way I already do unless it makes a big difference." Unfortunately all we can say is, "I don't know, but you know thermodynamics and all of that says it's more efficient".
 

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Yep. Hopefully I am not coming across as argumentative.
The questions you ask are excellent questions.

I just have a simple question, a rhetorical question really because I don't think anyone here can answer it, as only the Honda engineers have the answer. The question - how much more efficient is it? For some reason no matter how I word that question, it seems like it's being interpreted that I am questioning whether or not it is more efficient. I have no doubt that it is more efficient. I would just like to know how much more efficient.
I don't know. I do know that the engineers at Chevrolet decided it was sufficiently more efficient to make the direct drive a primary drive mode when not running in charge sustaining (HV) mode or when battery of the Volt was exhausted.

Why do I care? Similar I suppose to why I like to know how much more efficient the OEM LRR tires are. I don't doubt that they are probably more efficient than most other tires out there, but knowing how much more efficient is helpful knowledge to have when shopping for tires, since there are other attributes about tires besides just fuel efficiency, some of which affect driving enjoyment, and having that specific information beyond just that the LRR's are more efficient helps when deciding on a tire purchase.
I've seen tire manufacturers claim up to 5% lower rolling resistance for LRR tires over their touring tires. Over the life of the tire this adds up to quite a bit of fuel/energy. It used to be even higher, but the newer touring and grand touring all season tires have had the LRR technologies incorporated into them. I won't be putting LRR tires on the Clarity when its time to replace the OEM tires. I'll be putting Bridgestone Turanza tires on the car for improved traction and some LRR benefits.

How much less efficient is HV Charge? (I know you had a bad experience). HV Charge has some benefits for drive enjoyment in some situations, and many people use it for that reason. But if it causes a big hit in efficiency then some people might decide to go ahead and put up with a noisy engine, especially with current gas prices. But if it's only a small hit on efficiency as many others have reported, then similar to the tire question, some may think it's worth a small loss of efficiency if it will make the driving more enjoyable.
Significantly, to the point that I don't recommend it unless you're heading into serious climbs.

In the current discussion, it's a question of whether it's okay to just drive it and let the car make all of the decisions, or is it really important to do some managing and button pushing. Probably most of us don't mind pressing buttons, some of us even think it's fun. But that's not everyone, and they question why they would need to do this. It would be nice if we could tell them that by just doing some simple management they can likely get an X increase in mpg. That would help them to decide. Instead all we can say is that it's more efficient. When they ask, "How much more efficient is it, because I really would prefer driving the way I already do unless it makes a big difference." Unfortunately all we can say is, "I don't know, but you know thermodynamics and all of that says it's more efficient".
Based on my experience, I recommend you let the car manage the powerplants unless you're heading into a high load scenario such as long steep high speed climbs.
 
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(2002 said: "How much less efficient is HV Charge?")

Significantly, to the point that I don't recommend it unless you're heading into serious climbs.
I have found HV Charge to be useful any time that I want to add some EV miles back for various reasons. Doesn't happen that often, but it's a nice feature to have available. I haven't worried about using it since people who have measured it say they didn't notice a big difference in efficiency. It does make for a better driving experience in some situations, it would be a shame to have to stop using it except in extreme situations like long steep hill climbs. I really wonder if something else was going on when you tried it. Hard to imagine Honda would put in a feature like HV Charge if it has such a drastic negative effect on fuel economy as you are describing.
 

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(2002 said: "In the current discussion, it's a question of whether it's okay to just drive it and let the car make all of the decisions, or is it really important to do some managing and button pushing.")

Based on my experience, I recommend you let the car manage the powerplants unless you're heading into a high load scenario such as long steep high speed climbs.
The context of my statement that you are replying to was about direct drive vs. electric generator mode. All of us seem to agree that direct drive is more efficient than electric generator mode. And we suspect (we just can't prove) that the difference is likely enough to make it worthwhile to push the HV button a few times during a trip in order to maximize the amount of direct drive, by shifting as many HV miles to freeway/highway driving as possible.

A second reason is that for quite a number of people, myself included, we prefer the improved driving experience that results from using EV on surface streets and HV on the freeway. For me personally it's a win/win to control this aspect of the drive manually and not let the car manage it, especially since it's so easy to control using the HV button.

I think hp79 said it best in post #39, "Engineers know what's best for each condition but there's no way to predict and accommodate all those cases."

The fact is we know what type of driving is coming up and how many miles we will be driving, the system doesn't. So there are situations where there can be some benefits to managing things. It's why they gave us the HV button.

But as I said in previous posts in this thread, if someone finds it a bother and doesn't want to fool with it and just wants to drive, then that's what they should do as that is what is best for them.
 

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A note on what's most efficient - EVs are more efficient at slow speeds and in stop and go driving. ICEVs are more efficient on the highway. This provides some guidance if you want to manually switch to HV mode on the highway, and in fact I do this if I'm going to be driving past the electric range. If I'm staying in the electric range I'll stay in EV as this is more efficient in this case.
 

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Add me in there as using HV Charge gave me a loss of at least 20 percent loss of efficiency/HV Range (we talked extensively about gas use, looking at the numbers, yadda yadda) All I know is resetting the system with the dealer fixed the issue, and after getting it resetted twice, I have not and probably will not use the HV Charge button while I drive the Clarity.

HV mode was a lot better for me when I utilized it, without the drastic loss of range (perceived or not, regardless), in my experience.
 
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A note on what's most efficient - EVs are more efficient at slow speeds and in stop and go driving. ICEVs are more efficient on the highway. This provides some guidance if you want to manually switch to HV mode on the highway, and in fact I do this if I'm going to be driving past the electric range. If I'm staying in the electric range I'll stay in EV as this is more efficient in this case.
So you said ICEVs are more efficient on the highway then you said that staying in EV is more efficient in the case of where you are staying within electric range. Did you mean to say that an ICE vehicle is more efficient than a hybrid vehicle at highway speeds?

I’m pretty sure that ICE vehicles are never more efficient anywhere since they waste about 2/3 of the energy that is burned.
 

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So you said ICEVs are more efficient on the highway then you said that staying in EV is more efficient in the case of where you are staying within electric range. Did you mean to say that an ICE vehicle is more efficient than a hybrid vehicle at highway speeds?

I’m pretty sure that ICE vehicles are never more efficient anywhere since they waste about 2/3 of the energy that is burned.
Hybrids, especially PHEVs like the Volt and Clarity, are more efficient at highway speeds because of regenerative braking, which converts gravitational energy on the downhill stretches into usable power for the next flat or uphill stretch. Idaho National Labs did a study about 8 years ago now that showed that EVs (at the time Tesla, Leaf, Volt) were up to 80% efficient overall and it was due in large part to regenerative engine braking.
 

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So you said ICEVs are more efficient on the highway then you said that staying in EV is more efficient in the case of where you are staying within electric range. Did you mean to say that an ICE vehicle is more efficient than a hybrid vehicle at highway speeds?

I’m pretty sure that ICE vehicles are never more efficient anywhere since they waste about 2/3 of the energy that is burned.
Anything that I have read says that EV is always more efficient than gasoline, in any driving conditions. If the same amount of fossil fuel energy used to power a gasoline car is used to create electricity to power an electric car, the electric car will go many more miles on that same amount of fossil fuel energy. I am not aware of any driving conditions where gasoline would be more efficient than electricity.

Also with the exception of a few parts of the country where electricity is priced artificially high due to surcharges for electricity produced by coal, for most people driving EV it is also less expensive than driving on gas. And with current gas prices I suspect that is now true also in the states that have coal surcharges.

So the bottom line is that any drive that can be done all EV, it's more efficient and cost effective to drive all EV and don't use any gas, regardless of the driving conditions.

What these discussions are about is a different topic, it has to do with the situation where you don't have enough EV miles to complete the trip, so you will have to burn some gasoline. In that case there are decisions that can be made about what part of the trip should be driven EV and which part HV, based on our knowledge (or at least what we think we know) about the efficiency of each mode in different situations. The goal would be to use all of the EV miles during the trip, since EV is always more efficient than gas, it's just a question of when best to use them.
 

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Add me in there as using HV Charge gave me a loss of at least 20 percent loss of efficiency/HV Range (we talked extensively about gas use, looking at the numbers, yadda yadda) All I know is resetting the system with the dealer fixed the issue, and after getting it resetted twice, I have not and probably will not use the HV Charge button while I drive the Clarity.

HV mode was a lot better for me when I utilized it, without the drastic loss of range (perceived or not, regardless), in my experience.
Can you provide some details on the tests that you did where you observed the 20% loss of HV range when using HV Charge as compared to regular HV. That way we could try the same tests and see if we get similar results.
 

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Hybrids, especially PHEVs like the Volt and Clarity, are more efficient at highway speeds because of regenerative braking, which converts gravitational energy on the downhill stretches into usable power for the next flat or uphill stretch. Idaho National Labs did a study about 8 years ago now that showed that EVs (at the time Tesla, Leaf, Volt) were up to 80% efficient overall and it was due in large part to regenerative engine braking.
While PHEVs are likely more efficient at highway speeds, it’s not the regenerative braking which is responsible for that. Regenerative braking is actually less efficient than allowing a vehicle to coast to a stop. So on the highway, when you rarely have to stop, regenerative braking doesn’t play much of a part in efficiency. If you are using regenerative braking on a hill the only advantage you would get is if you’re using that to keep the vehicle below the speed limit. In any case, regenerating electricity and putting it back into the battery is less efficient than just letting the vehicle coast. If you didn’t care about the speed of the vehicle, then coasting down a hill is always more efficient than using regenerative braking.
 

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While PHEVs are likely more efficient at highway speeds, it’s not the regenerative braking which is responsible for that. Regenerative braking is actually less efficient than allowing a vehicle to coast to a stop. So on the highway, when you rarely have to stop, regenerative braking doesn’t play much of a part in efficiency. If you are using regenerative braking on a hill the only advantage you would get is if you’re using that to keep the vehicle below the speed limit. In any case, regenerating electricity and putting it back into the battery is less efficient than just letting the vehicle coast. If you didn’t care about the speed of the vehicle, then coasting down a hill is always more efficient than using regenerative braking.
Fewer energy conversions with coasting. Then again, with higher speeds comes more energy needed to overcome additional drag (by the cube of v!), so there's an "optimal" coasting speed for the speed limit vs regen factor in there, too. The number of variables involved in vehicle efficiencies is mind-boggling.
 

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While PHEVs are likely more efficient at highway speeds, it’s not the regenerative braking which is responsible for that. Regenerative braking is actually less efficient than allowing a vehicle to coast to a stop. So on the highway, when you rarely have to stop, regenerative braking doesn’t play much of a part in efficiency. If you are using regenerative braking on a hill the only advantage you would get is if you’re using that to keep the vehicle below the speed limit. In any case, regenerating electricity and putting it back into the battery is less efficient than just letting the vehicle coast. If you didn’t care about the speed of the vehicle, then coasting down a hill is always more efficient than using regenerative braking.
Watch the power display that shows where the power is coming from, or even the main power indicator for the needle going into the green area. The Clarity (and Volt) will both put power back into the battery from the drive wheels on even shallow downhills. This helps the car maintain speed while reclaiming that power. When the car needs the power it will attempt to pull from the battery first. You can also see this in some cases by monitoring the power meter - blue will creep up to half the meter when running in HV or EV depleted mode (no mode indicator). While the power draw remains in the blue, the car will switch to EV on the display.

The ability to put power back into the battery when it's not needed for propulsion is the beauty of regenerative braking. Yes, it also applies when slowing down but even at steady speed driving you get a lot of regenerative braking, which then provides power before gas is called for again. During this scenario the fuel injectors are turned off and gas engine turning via the transmission . Chevrolet calls this mode Deceleration Fuel Cut Off (DFCO) and all modern vehicles implement some form of this. During DFCO PHEVs go into regenerative braking to convert any forward momentum and gravitational energy that isn't needed to overcome friction into electricity for battery storage.
 

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The ability to put power back into the battery when it's not needed for propulsion is the beauty of regenerative braking.
The key expression here is “when it’s not needed”. The only time power is not needed is when you absolutely must stop or slow down faster than coasting will allow. So the real beauty of regenerative braking is that it puts power back into the battery that otherwise would have been wasted (usually as heat energy from friction brakes). On a flat highway or an incline regen braking generally has no benefit. On a descent some benefit may be had if the slope of the hill is such that it will cause you to exceed the speed limit and you absolutely don’t want that to happen.
 

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During DFCO PHEVs go into regenerative braking to convert any forward momentum and gravitational energy that isn't needed to overcome friction into electricity for battery storage.
During DFCO (if you want to call it that) a PHEV should try to coast as much as possible unless the deceleration is necessary. That is, if you lift your foot off the accelerator, unless your intent is to slow down or stop, you should just be coasting because that’s the most efficient use of the energy you’ve expended in getting the vehicle up to speed and maintaining speed.
 

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The key expression here is “when it’s not needed”. The only time power is not needed is when you absolutely must stop or slow down faster than coasting will allow. So the real beauty of regenerative braking is that it puts power back into the battery that otherwise would have been wasted (usually as heat energy from friction brakes). On a flat highway or an incline regen braking generally has no benefit. On a descent some benefit may be had if the slope of the hill is such that it will cause you to exceed the speed limit and you absolutely don’t want that to happen.
During DFCO (if you want to call it that) a PHEV should try to coast as much as possible unless the deceleration is necessary. That is, if you lift your foot off the accelerator, unless your intent is to slow down or stop, you should just be coasting because that’s the most efficient use of the energy you’ve expended in getting the vehicle up to speed and maintaining speed.
You're missing a use case for all vehicles and a specific use case for EVs. On downhills all internal combustion engines have drag from the compression combustion chambers (cylinders), which causes the car to slow down faster than just coasting. The specific use case for EVs is to imitate this behavior by applying resistance to one or more of the electric motors, which causes power to flow backwards towards the battery. The only way to avoid this is to disconnect the transmission from the engine by putting in the clutch or shifting to neutral.

In my Volt this results in the car decelerating at about the same rate as any other automatic transmission ICEV while in Drive. But if I put it in L the computer increases the the regenerative braking to slow down fast enough the car is required by the FMVSS to turn on the brake lights. On a long descent on cruise control my Volt in D will slowly accelerate (it doesn't have ACC), but L will maintain speed by modulating the regenerative braking system.

When you let off the throttle in the Clarity the same thing happens - the car is programmed to slow down like it's a regular automatic transmission ICEV. The regen paddles increase and decrease the regenerative resistance. When using ACC, the Clarity adjusts the regeneration level to maintain downhill speed. (I've seen reports the 2020s don't do this.)

Regenerative braking is somewhat of a misnomer - it can occur anytime the car is slowing down relative to the Earth's gravity well or road speed. Like all new ICEVs, PHEVs first turn off the fuel injectors to save gas. Power consumption is zero at this point. Unlike ICEVs PHEVs can then apply resistance to the electric motor for power regeneration to slow down relative to the road or the gravity well. As an example I set the ACC in my wife's Clarity to the posted 65 MPH coming down the south side of the Grand Mesa in Colorado. By the time I reached Delta, CO, which is 3,600 ft lower elevation, the car's regenerative braking system had recharged almost half the battery capacity. This was at a steady state road speed.

Be aware that at speeds above 60-70 MPH the power needed to overcome air resistance reduces the amount of power that can be fed back into the battery so at high speeds you may not see regenerative braking during descents through rolling hills, but you will see it coming down the high passes on I-70 in Colorado and the long steep grades through Pennsylvania and West Virginia.
 
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