While my passion goes with Plug-in Hybrid Electric Vehicles (PHEVs) (hopefully with serial, not parallel, engines), there are tremendous benefits available with diesels. And, well, that particulate spewing, choking, health-risking cloud of ‘old’ diesels is transforming to something different. And, well, in this domain, the Old World is leading the way.
For example, the rather boxy Volkswago Lupo, as written up at Ecoworld:
there is another green car already here, although virtually unheard of in the United States. That car is the Lupo, a small four passenger car produced by Volkswagon that uses a high-technology ultra-clean burning diesel engine and gets 90 MPG. This car was launched throughout Europe in the fall of 1998. Volkswagen pioneered green diesel engines beginning in the early ’90s when they introduced “direct injection” technology, in which fuel and air are pumped directly into cylinders. This innovation decreased fuel consumption of the already fuel-efficient diesels by 15%. Volkswagen engineered not only eye-opening fuel economy into their diesels, but also dramatically lowered emissions.
90 mpg? And, what is meant by lowered emissions?
The black soot coming from old generation diesel engines is caused mostly when excess fuel is pumped into the engine. Direct injection technology controls the air and fuel mixture to the engine, eliminating almost all smoke. Nearly all smoke and invisible pollutants that remain are captured by fitting the diesel engine with a catalyst, making Volkswagen’s engines the cleanest burning diesels in the world. The Lupo complies with all of the European Community’s auto-emissions regulations, some of which are quite strict.
Well, this is impressive sounding.
And, well, the Lupo hits its high gas (diesel) mileage without any 
unusual styling for extreme aerodynamics. It is, as VWVortex put it, “the Lupo has a very meek appearance that is altogether feminine but somewhat plain at the same time.”
And, VW is talking of a four-passenger diesel that would near 200 miles per gallon.
Now, if we could only make that a plug-in, hybrid electric diesel that would convert this to, perhaps, a gallon of liquid fuel per 500+ miles, then all of a sudden biodiesel starts to make more reasonable sense.