[Gravityloss]

What I don't understand is why don't they put a small 20 kW 500 cc one cylinder generator in cars, at the same time they could reduce the battery size quite a lot and still most trips could be made purely electrically. Cost would be a lot lower as well.

The average power needed for driving is very low, only 10-20 kW and the battery handles the peaks like when you need to climb a hill or overtake. For example in this article they used 78.2 kWh in 5 h, meaning average 16 kW. The small generator could be called a spare energy source.

Instead, all plugin hybrids have big expensive heavy four cylinder engines which defeats the whole point of building a hybrid in the first place. For example 1.4 liters in the Chevrolet Volt / Opel Ampera.

And then Tesla has no high energy density backup source which again limits the usability of that car (and forces you to keep another car). And raises anxiety...

[maigret]

"why don't they" -> Remembers me of a recent HN discussion ;)

But... Actually you are not far from something. This is actually an approach from Audi: http://green.autoblog.com/2010/03/01/geneva-preview-audi-a1-... From what I read though this approach was not approved in the mother company, VW. Let's see who wins at the end.

[Gravityloss]

That's very interesting, what Audi was planning! The Wankel engines are quite expensive though.

[bigdubs]

What you just described is basically the system in the Fisker Karma: http://onward.fiskerautomotive.com/en-us/karma/overview/

[Gravityloss]

Nope, the Karma has a big four cylinder engine too, two liters of displacement.