I've driven the HCH a while now and had some time to watch the behavior. Would like to start a thread to reengineer how the engine and transmission works, in different situations. It is of course not a very productive exercise, since the Honda engineers already know how it works, but for some reason I think they won't tell us.

The first thing that strikes me is that when driving at constant speed around 100-110 km/h, with cruise control on, the tachometer will stay around 2000 rpm and fuel consumption seems to increase in steps. There are five steps: 1.5 L/100 km, 3 L, 4.5 L, around 6.5 L, and of course 0 L. Accuracy is limited by the resolution of the display, and the value seems to be filtered with some time constant, but it is still very apparent. As long as rpm stays below 2000 rpm, fuel consumption can never go higher than 6.5 L/100 km - which makes sense, because that would quite well correspond to max torque of the engine, at around 30% efficiency. (30% fuel efficiency is a normal value for a gasoline engine at full throttle.) When the car approaches a slope uphill and more power is needed, rpms and fuel consumption rises. Sometimes when power demand is in between two steps, i.e. 3 and 4.5 L, fuel consumption will alternate between these two values. You don't sense this in the car, the speed is so high that these small steps in power can't be felt.

The behavior has made me wonder what really happens in the engine department. I would like to think the engine is running full throttle (or almost full) constantly, but shuts off 1, 2, 3 or 4 cylinders depending on the power demand. This would be ideal from effiency point of view.

Or the engine runs four other different modes. Honda mentions the ability to ignite the spark plugs (two per cylinder) in sequence or simultaneously. There is also the ability to change valve timing and lift. This could be combined, perhaps with shutting down two cylinders, to create four different modes. But I don't think valve timing nor ignition sequence is changed at these low revs.

Or the throttle opens in four discrete steps. Not very likely.

Or the display for some reason has an internal resolution of four steps. Not likely, and also not consistent because at other speeds, other values are possible.

However, I still find it hard to believe that the engine could be running full throttle on one, two or three cylinders, without any noticeable noise or rumble. It's a small engine, road noise is relatively high at highway speed, and the IMA and CVT could even out some of the vibrations, but still.

I don't mean to say that the throttle is always fully open, in all situations. The throttle is probably used in low-load conditions, when power demand is changing rapidly, and for idling.

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