The Clarity: i-MMD and Engine Engagement

I think you may be misunderstanding the system a bit. The engine very rarely drives the front wheels at all, and changing modes doesn’t impact this fact.

With the Clarity, the ONLY time the running engine is directly driving the wheels is when gear mode is engaged. This is not a selectable mode. When it engages itself a clutch couples the engine with the wheels and pushes them directly in what I believe to be a 1:1 ratio. It’s non selectable, just automatic, and whatever drive mode you have selected at the time is irrelevant. Generally this only happens under low load highway cruising between speeds of 50 and 65 mph (not exact, it varies with wind and hills). There is absolutely no way to manually turn this feature on or off beyond driving on pure battery in EV mode. Beyond that constant cruising speed range, the engine acts as a generator at all times, and is fully disconnected from the front wheels. It couldn’t drive the wheels if it wanted to at any other speeds or loads because there is no transmission.

This feature makes the Clarity rather odd and confuses a lot of people because the engine rpm does not follow wheel speed or accelerator position in a linear manner like most every other car, causing some to think there is a defect. It’s just different.

 

Honda measured the difference in efficiency between running the generator and driving the wheels by electric only and by driving the wheels by ICE directly. They found a small range of highway cruising speed to be more efficient by direct ICE driving the wheels. If you use the EV motor only, then anything coming out of the ICE driven generator has to go through a AC to DC conversion, then back again from DC to AC as it goes through the accelerator pedal controlled electrical inverter to the EV drive motor. That results in losses that are less efficient than direct mechanical drive. Especially since the mechanical gear can be set for highway speeds. The single EV drive gear in nearly all EVs is a low gear, not that effective at high speed. The mechanical drive system consists of only two parts, the driven gear and the clutch to engage it.

 

I think I read somewhere that it is an overdrive ratio, although I don't know what the ratio is.

 

I’ll take issue with “very rarely”. I think it depends on the conditions you normally encounter. On rolling country roads at 50-60 mph, I see the gear icon quite regularly when in HV mode.

 

Honda’s own website has this to say:

Electronic Continuously Variable Transmission (E-CVT) with Sport Mode and Deceleration Paddle Selectors:

• Ratio:2.454 ~ 0.805
• Final Drive:3.42

A bit confusing, if you ask me.

 

Totally -- Honda is treating the entire system as a black box lumping the "E-CVT" part (motor spinning at X RPM feeds generator developing A kwH which goes to the traction motor which then spins at Y RPM, thus giving a ratio of X:Y) with the single-speed linkage (which is an unpublished fixed ratio somewhere within that range)

Actually, with an OBDII capturing engine RPM and speed it should be possible to calculate the "gear mode" ratio. Next time I'm in gear mode I'll try to capture the data. Gear mode "feels" like driving in 5th gear in a conventional car, so I suspect it is an overdrive ratio, but who knows.

 

I suppose actual ratio is irrelevant. I didn’t intend to pull the conversation that direction. Sorry to OP. Even on a conventional transmission vehicle the ratio gets skewed. Say for example 4th gear on a car is 1:1. That’s just the output shaft of the trans spinning at same speed as engine. But then that shaft goes through a differential that may have a ratio of 2.74:1 or whatever before the wheels are reached. No cars are running 1:1 or maybe overdrive of 0.89:1 or whatever, when comparing engine speed to wheel and tire speed. Those ratios we are all accustomed to are comparing engine speed to transmission output speed.

But on Clarity trans gets kicked to the curb cuz it doesn’t exist. So....to continue the off track discussion let’s refer to our good friend pi.

Clarity tires have a diameter of 26.3". The circumference is 82.7" and they make 766 revolutions per mile. I’ll spare ya the math just trust me lol.

So at 60 mph the tire is turning 766 rpm. Certainly the engine is spinning faster than that when gear icon is engaged....I’ll speculate 2200 rpm? Wild guess. So if I’m close, the ratio from engine to wheel rpm is 2200/766 =2.87 gear ratio. Those of you who can read rpm of engine with gear mode engaged feel free to insert the accurate rpm number above and fix my answer.

In any event this number is a very long ways from an overdrive or a 1:1.

So I’m sorry I threw a number at it, which I admit I don’t know. AND it was clearly wrong. Doesn’t really matter. This car doesn’t have a transmission so frankly it’s moot.

I was simply trying to explain that the engine is only coupled to the wheels in a “high gear/low load” type of cruising situation. Yes much like an overdrive. But never on normal acceleration, or even when the accelerator is floored for max power. Even then the engine is fully disconnected from the wheels, which are being 100% driven by electricity only, and I didn’t think OP was understanding this.

 

There is actually a third camp which is the one that I am in, which is that I don't doubt that in at least some situations longevity can be improved by actively managing the system instead of letting the system manage everything, and by not fully charging and not fully depleting the battery. Of course we know that the system has a buffer on top and bottom, but I don't doubt that some battery longevity can be gained by steering even further away from the edges. And while the system will use ICE as needed, including (we assume) when it thinks that the engine hasn't been run enough lately, purposely running ICE more often, especially in optimal power ranges, might benefit it. However the big question for me is how much extra longevity are we talking about? I am not ready to change my driving or charging habits just for the chance of a nebulous amount of additional longevity. And while I might be making some unknown sacrifice in terms of longevity, I wouldn't go so far as to say that I am abusing the car for the next owner by letting the car manage itself.

Other than when for my own convenience I often shift EV range to later in my drive when I know that I don't have enough EV for the entire trip. But in the end I still use all of it whenever possible, which is what the car would have done if I left things alone, it would have just used it earlier. I think I am somewhat unconventional in that my goal is to be at 0 range and in HV mode when I arrive home, I do not like leaving even a small amount of EV range on the table as that adds up to multiple miles per year of gas usage that could have been EV. Now the people who avoid dropping to 0 range do so (from what i understand) because they believe that it is better for the battery. I don't doubt that it is, but how much? How many more estimated miles of battery life will I get if I never let the battery drop below say 10 miles? Or never charge above 90%? Or avoid driving in EV above say 60 mph? I am not doubting there is some effect but until it is quantified I am not planning on changing my strategy of using as much electricity as I possible can. Until further notice

 

Honda has charted the efficiency in one of their technical reports. I can look for it later. However, this is one of the simplest designs of any vehicle out there. See the picture on Insightman's website. The system consists of 2 electric motors and 4 gears, one of the gears has the clutch under it. In comparison, a Chevy Volt has 3 clutches and over twice as many gears. A Toyota has probably 50%-75% more gears.

http://www.insightman.com/Clarity/2014_Accord_i-MMD_gears.jpg

 

I don’t pretend to know for sure but I assume yes. Here we have a car exceeding 4000 lbs, with an anemic 4 cylinder engine barely capable of propelling a car 1,000 lbs lighter than this. At one particular speed and load it is capable of doing the job efficiently. The rest of the time it’s an inefficient dog of an engine which is simply outmatched. Presumably this narrow efficient window is that 50 to 65ish mph zone. So that’s when Honda engages direct drive and feels it saves gas.

At other times Honda is relying on the battery. This is where the angry bees come in some people complain about. Since the engine in the car is so grossly underpowered for its weight, it uses stored energy from the battery to give acceptable acceleration, and then relies on regen recharging and higher than necessary “cruising” rpms from the engine during other low demand times to recharge the battery, storing energy needed for future accelerations and hill climbs. This becomes extremely evident during prolonged 85 mph drives with a good headwind, with zero regen opportunities and a battery and generator that can’t quite keep up. The engine simply screams the whole time and fuel efficiency is miserable...I’ve seen mpg as low as 25 at times on long high speed cross country trips...there are indeed circumstances where this car is not very efficient at all, though they are few.

Someone else mentioned conversion losses earlier. Obviously it is wasteful to take an engine, running on gasoline which directly spins a crankshaft, use it to generate electricity, then convert this energy to electrical power which in turns spins another axle which propels the car. Why not just take the spinning engine and use it to spin the axle directly? At most speeds in a conventional car a gasoline engine is wasteful at doing this because it is running either slower or faster than its most ideally efficient speed. But at some few times it is actually running at its most efficient speed. So during those times pull out the middle man. Honda decided this time was that 50 to 65 mph window (varies based on conditions). During these few times Honda couples the engine directly to the drive wheels and makes extremely efficient propulsion power. The rest of the time it is more efficient to have the engine rev fast and pre-store a bunch of energy in the battery, or idle slowly and do very little. No doubt the programming has flaws since this is a new model, but Honda has clearly found some efficient methods of moving this heavy beast of a car with less fuel and a much smaller engine than most similarly weighted cars. And part of this is letting the engine itself propel the car during a narrow window of easy cruising opportunity, and NOT allowing it to propel the car directly during other times, instead relying on the engine to rev up and charge the battery, allowing the electric motor to do the propulsion work instead of the engine.

 

If you are somewhat unconventional then there are at least two of us. My motivation for manually switching from EV to HV when I'm on a highway is completely selfish; I want to avoid using HV other than on the highway. It has absolutely nothing to do with battery life. For one thing I am unconvinced that it makes any significant difference; the battery is exercised continuously during normal driving as energy flows into it and then out of it; I do not believe that it is sensitive to being charged in a given range. But mostly, frankly, I don't give a damn. I run the heat in the Winter, too! I bought this car for the driving experience, and for me the driving experience is maximized by using EV during off-highway driving, so that determines how I manipulate when I use HV. If I had reliable evidence that a given behavior would extend the life of the battery, I might adopt that behavior, depending on how much it impacts the overall driving experience. But I don't.

And yeah, I want to drop to EV=0 about ten feet before I pull into the garage. That's my goal. If I don't quite meet it, that's OK.

 

I tend to think that it is never more efficient for a gas engine to generate electricity to propel the wheels, even if it is doing so at its optimal power band. I think they decided to save weight and cost by not having a "real" CVT, since they assume that most people's day to day driving will be mostly EV. The tradeoff is less efficiency in HV mode, however that is offset quite a bit by the much larger battery which allows more regeneration storage. A larger battery would make any hybrid more efficient (Accord Hybrid has 1.3 kWh) the problem is the weight and especially the cost would not be worth the improved mpg in a regular hybrid. Also having the direct drive gear for steady speed highway driving helps HV mpg (we are assuming). Presumably the direct gear mechanism is lower weight and cost than a CVT.

 

I would like to see some real data on efficiency. Obviously there are losses in the conversion process from generator to motor via battery (or even direct). But there are improvements based on the engine always running at its optimum RPM. My guess is that those optimum-RPM gains are not huge; otherwise we would see the engine always running at this speed or greater. That would really freak out folks, as there would be zero correlation between accelerator position and engine speed.

There are just so many tradeoffs in the design of this kind of vehicle. The size of the car. Battery capacity versus weight. Engine size versus weight and how to power the car in all conditions. Efficiency of direct drive versus weight/bulk of a conventional transmission. One electric motor or two ... or even four. Even nits like how big the 12 volt battery should be, or the capacity of the windshield washer reservoir. And whether to have a knob to control audio volume.

 

I also prefer to shift EV to surface streets, but I don't try and hit 0 when I arrive home because if I undershoot it wastes a warmup cycle driving the last fraction of a mile through my neighborhood at low speed. Instead I try and reach 0 at least a couple of miles out so that warmup occurs during normal 45+ mph driving so that the gas isn't wasted, or at least is less wasted.

Although I am certainly no expert, batteries do seem to be sensitive to charge range, and battery life can be shortened by charging and depleting to either extreme. However Honda (and other plug-in manufacturers) protect our batteries at least to some extent by limiting the upper charge range and lower depletion range. But I suspect that there is no exact cut-off point, and they have to also factor in the sales value of a wider range. People really do look at that 47 mile official range, and talk about it, and factor it into their purchase decision. I'm sure it would have been tempting for Honda to widen the range a bit more so they could advertise 50 mile range, but apparently that would have been too detrimental to battery life. But it's also possible that setting a narrower range than what the system allows us would have slightly improved battery life, but the official range would have been less, so they put it where it is.

 

I'm with you. It's a fun game for us PHEV drivers versus an anxious experience for BEV drivers. I was an anxious BEV driver of sorts until today.

My 2006 Insight's hybrid battery died a couple of weeks ago. As with all Honda hybrids, the hybrid battery recharges the 12-volt battery through a DC-to-DC converter--there is no alternator. So the only way to recharge the 12-volt battery was with a battery charger. It was nip-and-tuck driving it while I waited for my replacement battery (which I am happy to report I successfully installed today). It was horrible experiencing severe range anxiety!

I tempted fate once too many times and knew I had lost when my instrument panel turned blank, the engine began to stutter, and my Insight conked out less than a mile from home. My wonderful wife came to get me. I removed the battery and took it home to recharge, then returned a half-hour later with enough battery power to get me home. I was reminded of many fix-it-by-the-side-of-the-road experiences I had as a youth in the 60s when I drove unreliable British and Italian sports cars.

If our Clarity's battery weighs more than the 70-lb battery I installed today, I don't think I'll be replacing it myself. Still, I'm not going to alter my driving style to hopefully extend the life of our Clarity's battery.

 

Only very rough data, but I think if you have the bar graph up that shows instantaneous mpg, you can see the mpg jump a little when the gear icon engages. I’ll look more carefully next time I drive it under the right conditions, but I think it gains a couple mpg. Not much on an individual basis, but fleet-wide I imagine it makes a big difference for Honda.

 

I suppose no night driving of the Insight the past two weeks.

 

Well said. I am sure that there was an interesting discussion that included the engineers and the marketing department. 47 miles is 75.6 km, so I'm thinking that the Japanese discussion found a "sweet spot" in kilometers, rather than looking to get to 50 miles in the ancient Imperial system. But it's a tradeoff, not black and white but with loads of grey. Honda has a motivation to be sure that the battery lasts through the warranty period. Methinks that the parameters that they picked are based on a long debate between the folks who wanted to protect the battery at all costs and the folks who wanted to be able to quote a larger EV range and didn't care about battery life.