In Fact, that Toyota's SuperCar runs on gas when pushed to the limits. There's no way having batteries able feed up two electric engines consuming around 300kW (total). The point is that power from 3.2 liter V6 is transformed to wheels electronically, so there's no mechanical connection between petrol engine and wheels.
and I don't think that petrol engine being quiet really... it has very short exhaust (pipes visible behind the doors on sides) which makes exhaust sounds even higher than in regular car. but when driven with electricity only, it certainly is quiet.
Interesting point is that brand new production SUV, Lexus RX-400 is already using identical technical approach as Volta. Difference to Prius is the missing mechanical connection between the petrol engine and the wheels, which again should drop mileage even more.
about Hydrogen, as a single solution IMO it is too far away. But as another power source to Hybrid, it is soon becoming viable. for example, Toyota has already 3 home market models (2 of them are Busses though.) that are Hybrids implemtating Hydrogen technology. All major car makers have their Hydrogen prototypes, others use them in marketing more, others less. GM has used their Hydrogen based prototypes in marketing quite lot and that's why they are so famous developing it.
Also, there's have been interesting prototyping with 2-stroke engine that would ignite on every cycle. Last time I heard about the project it was known problem that control logic couldn't be fast enough to handle timings. (means logic that could be fitted on massproduction car. you know, you just can't slap few opterons on it.
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2-stroke that would ignite on every cycle would raise the efficiency greatly and also burning would be more complete which again means less non-burned exhausts.