dick -

If you read carefully I said "that can be extracted." That's sort of a fuzzy concept, but the higher compression, and a few other common practices, make it a lot easier to wring more of the "chemical" content out of diesel. Neither kind of engine is really very efficient - yet - if you try to put them in terms of the enthalpy ratios.

Not too long ago, for comparable vehicle uses, diesels were commonly expected to get at least 40% more mpg, although 70% and up wasn't uncommon. Stationary engines were often close to the 2:1 ratio. for the engines available.

Older diesels were not really compatible with widely varying or rapidly changing loads, which is one of the reasons they were most commonly used mostly in long-haul vehicles that spent most of their time running long distances at fairly uniform speed. More recent changes have partially overcome that difference, although you can still get by with 3 or 4 gear ranges on a gasoline auto but the "big boys" in the diesel rigs need 18 or 21 (or more, if you include multiple transfer boxes) for efficient operation over the same range of road speeds. The more advanced gasoline engines, with induced turbulence combustion charges or stratified charge fuel injection, precise fuel metering and feed-back control of mixtures, and such, are becoming somewhat more like diesels in this respect, while the diesels with advanced Inter-cooled turbocharging are pressing quite a bit from the other end.

The VW concept of "injected additives" is interesting, but how many miles per gallon of the urea does it get, and how many fuel stops have it handy?

John