> Diesels are a different animal all to themselves. There is some truth to
> the better atomization of the fuel, which does lean itself towards better
> burning. But that's where it stops. Diesels have a pollution all their own
> and it is very difficult to eliminate it. The prime area of concern is the
> soot, the black smoke you see when a diesel is under hard acceleration. I'm
> sure there is a chemical composition of this soot but I don't know what it
> is. Vw's new diesels use a direct injection system, as opposed to the older
> swirl chamber design. This allows a more complete combustion. But also
> greatly increases the noise. Diesel trucks have used direct injection for
> years. Although there are no tune ups per se. You have to be careful with
> your fuel, especially in the colder climates or you won't go anywhere. HTH
> 
You are correct on most of this, but you've obviously not heard or 
driven the new TDI!!. It is much MUCH quieter than any previous VW 
diesel, due mostly to a progressive fuel injection system, and sound 
absorbing engine cover. You forgot to mention that diesels are more 
efficient by definition, and direct injection moreso than indirect, 
meaning less burnt fuel per mile, meaning potentially less pollution 
to begin with.
Also, since a diesel will stay in tune MUCH longer than a gas engine, 
it is more likely to remain in a state of low emissions than a 
typical gas engine. This is of course less true with the latest gas 
engines, though.