For those of you that have not seen the delta wing:
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Adding in flywheel regenerative storage seems like this would be a hybrid model pursuing (which VW/Audi is doing, IIRC).
In Europe, diesel is usually cheaper than petrol though the reverse is true in the US. From what I saw last December, price differences of $0.50/gallon weren't very unusual. If the price comes down, I think diesel would quickly become a no-brainer decision for car buyers.
Current diesels, having owned a Golf and now a Beetle convertible with one, are very clean, still efficient, and even fairly quiet. You could sit behind my car when I start it or its running and not detect a scent of diesel. In fact it can be less offensive than some gasoline cars.
Still I drive in a world of public buses and commercial dump trucks that just pump plumes of black stuff into the air. Makes it hard to sell to the average consumer.
That is changing, besides VW, Audi, BMW, Mercedes, and Porsche (see the pattern), Jeep will have a Grand Cherokee with a diesel, Chevrolet has a diesel version of the Cruze, and Mazda will have a 6 with one as well.
In my case I wanted a fun to drive car, its a convertible, that got very good mileage. All the hybrids to me are penalty boxes. So when I saw VW was going to offer a convertible with the efficiency that a diesel belongs I jumped at it.
I don't think the price of diesel is coming down in the USA relative to gasoline. The USA already uses lots of diesel in its trucking networks; switch the entire public to diesel and it's only getting more expensive.
Most diesel cars now have a separate urea tank which functions as after treatment. Others (primarily trucks) use exhaust gas recirculation instead so that they don't have to maintain two fluid tanks.
I'm quite a car nerd, and generally when people come to me asking whether they should get a Prius, I typically ask them about their driving habits. I live in San Diego, where there are lots and lots of freeways, so diesel cars do really well here and I often recommend them. Hell, the newer Audi diesels are quite good performance-wise as well (torque for days! woohoo!!)
About 20% more efficient. If you look at L/100km or mpg you might say 40%, but diesel fuel has more energy by volume.
Extra maintenance is certainly present.
Or, in a pinch, two rocks
The exciting thing is gasoline engines are starting to incorporate the best aspects of diesel engines. For example, direct injection and combustion shaping are allowing higher and higher compression ratios and leaner burns.