It must have some serious turbo boost to make 70+ hp at 1800 rpm. I would have never guessed that to be very efficient, at least for a gas engine.
1800 is a standard gen set speed, but only because they need to spin 60hz divided by a whole number. 3600 is pretty revved up for most motors, but 1800 isn't unusual for 4 liter per cylinder sets (a rather large engine).
So why a 3-banger? If you don't rev it up, and don't have to solidly mount it, even a single (with balancers) would be plenty smooth. Less cylinders means less waste. Perhaps it only makes 20 hp at 1800, and 70+ is the peak, which makes a lot more sense.
My idea for a mileage car's power train is like the Volt's in that it would be a plug-in series hybrid, front wheel drive. But the engine would be a turbo diesel, perhaps single cylinder for a small car. It and its motor-generator would be connected (via compter-controlled clutch) to the final drive gear. Essentially a manual transmission stuck in 5th, but with no extra gears to sap power. This would give an extra cruise mode where it syncs to road speed, with essentially zero drive train loss. The main electric motor would have its own clutch between its reduction gears and the final drive, so that you wouldn't have to spin it when at cruise. Also, you could disconnect it when it runs out of revs (the EV1 ran out at 70 or so I think) and let the engine and its motor carry you up to the aero limit.
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