Smoke is only the visible part of the exhaust. I don't know exactly everything that's in exhaust. Diesels are much cleaner than g@ssers in most points and g@ssers are much cleaner than diesels in some other points.

I've seen some g@ssers of the same era put out MORE smoke than ours.

Side note: If you think our diesels put out alot of thick smoke, think of every time the space shuttle has launched