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2017 CR-V AWD EX-L 1.5T, 2018 Accord Touring Hybid
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We’ve only had ours for a couple weeks. I noticed a stretch yesterday at 55mph that the engine was off for about 1/2 a mile. Probably not gonna get much longer at those speeds. It is surprising how often it switches off at 50-70 in econ mode. As far as running without a battery no. The charging of the 12v system is done from the high voltage. There is times when the engine is directly powering the drive motor. It’s more of a serial hybrid with the engine engaging at 45mph, and it only has an overdrive gear with a hydraulic clutch to activate. Weber university does a good video of the gearbox that would probably help explain a lot of the power train.
The fuel efficiency benefits of iMMD come from 1) the ICE engine running on a more thermally efficient atkinson cycle, and powering the drive motors through a generator 2) recapture of energy in the small drive battery from braking. 3) computer control of everything to insure smooth and efficient switch over of drive sources during driving
The fuel efficiency benefits come mainly from what MASTAMATT9887 described. Atkinson cycle enhances the result, but it is only practical with that strategy.

Honda even has a diagram that shows how this works:
Colorfulness Rectangle Slope Font Parallel

This is from a paper in the World Electric Vehicle Journal Vol. 6, titled "Efficiency Enhancement of a New Two-Motor Hybrid.System." But I added the number labels; they are in metric units because it is a Honda paper. This technically is for the engine in the 2014 Accord Hybrid, and there have been minor improvements since then, but it is basically our engine and hybrid system. Details:
  • BSFC is how many grams of gasoline are used to produce 1 kWh of useful energy, so it is good to operate in the red areas.
  • Engine Drive is when the clutch is engaged, so it applies to MASTAMATT9887's highway experience. Hybrid Drive is when the clutch is diesngaged, and rpm are unrelated to speed.
  • In the Accord's Engine Drive, RPM=37*MPH. The CR-V is, I believe, about RPM=37.4*MPH, but I haven't double checked that.
    • So the purple dots start right about 45 mph.
  • The black dotted line is how much torque (or power) the 2014 Accord needs to maintain speed on a flat highway. It applies only in Engine Drive, since that is where speed is related to rpm.
    • The CR-V's line would be higher.
    • The engine needs to operate above this line, since there are other losses.
The point here is that for cruising in Engine Drive, the engine only needs to operate in the lower group of yellow dots. Where BSFC is about 230 to 280 g/kWh. Instead, it moves up to the same rpm on the blue "operating line," where BSFC is less than 210 g/kWh. The excess power generated is used to charge the battery. The upper group of yellow dots represent the rare occurrence, where you need just a little more power than is efficient, but not enough to drop out of Engine Drive.

The behavior MASTAMATT9887 mentions happens because you (hopefully) spend much more time on the highway cruising, than accelerating. So this shifting strategy alone tends to build up more charge than it can use. So the car will occasionally shift to EV Drive to use it up. This video, which is an excellent companion to the Weber State one, shows this happening in the 2017 Accord. You can even compare his white-board drawings to the hardware in Weber's video. (The one compromise he took is that he shows the overdrive gear going through the motor gear to get to the countershaft gear. In his defense, that is how a schematic in the paper I mentioned draws it in two dimensions, and he does indicate the correct gear ratios in the bottom right.)

Anyway, back this diagram. The blue dots show a similar strategy in Hybrid Drive. It is just a little more complicated. With a perfect CVT in a non-hybrid (instead of Honda's eCVT, that I prefer to call a virtual CVT), the engine would operate at the intersection of the required power curve (labeled on the right) and the blue line. The left- and right-groupings of blue dots show where this would be in low- and high-power requirements, respectively. The iMMD system will, instead, operate close to 2000 rpm, 88.5 lb-ft of torque, 33.7 HP, and 208.1 g/kWh. (I changed units because this is our engine, not the one in the diagram above.)

William, I included your list because the shifting of the diagram above shows what Atkinson does. With a traditional Otto cycle, the engine will very seldom be able to operate in the reddest area. Making it as red as possible does not help with mpg. A traditional car needs the red-ish area to broader. And it can tolerate it being less red at the sweet spot, since it won't operate there. Atkinson narrows the sweet spot, and lowers (reddens) it.

All of this is summarized in that same paper:
Slope Plot Rectangle Font Parallel
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