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Comment by deepsun | Hacker News Reader

That's basically a hybrid vehicle where the engine is not connected to driveshaft. The thing is -- it's more efficient to connect engine directly to the driveshaft when driving at a reasonable speed, as there's no extra conversion of gas to battery charge (generator losses + battery charging losses + battery discharging losses).

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Sure, but you need a driveshaft and a gearbox for this to work.

It's worth it if you're using it all the time, but for something only used on rare occasions I'll take the _maybe_ 10% efficiency loss to not lug around the extra weight and have more things that can break.

Take a look at the 'hybrid synergy drive' model. It's a pretty streamlined gearbox. ICE output shaft connects to motor/generator 1, MG1 connects to MG2 via planetary gear, MG2 connects to wheels more or less. There's gears in there, but there's no gear selection. Depending on how you control the ICE and the motor/generators, you can any of the three movers power the wheels individually or in concert.

It's not much more complex than an ICE, a generator, and a motor for the wheels.

Bmw argues that their model is more efficient since the engine always runs at its most efficient RPM range

That's why I added "at reasonable speeds". At start-stop busy city driving it might be worse to use ICE engine (although it might combine both ICE power with electric engine power, to keep the ICE at its optimal range anyway).

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