Steve Webre wrote:

6.1 lb/gal and 19,500 BTU/LB for Diesel, 6.05 lb/gal and 20,380 BTU/LB for
Gasoline (average values)

That works out to 118,950 BTU/gal for diesel and 123,299 BTU/gal for gasoline.
That doesn't jive with my understanding that Diesel packs more energy per gallon
than gasoline, but perhaps that understanding is based on usable energy (diesels
extract more of the chemical energy as work than gasoline engines do).  Upon
searching the web for references, I found:

http://www.hawaii.gov/dbedt/ert/fueltax-act143.html

The energy content of a gallon of gasoline ranges from about 109,000 to 125,000
Btu. The average is about 114,000 Btu. The energy content of diesel fuel is
between about 128,000 and 130,000 Btu per gallon (lower heating value).

Chevron says:
http://www.chevron.com/prodserv/fuels/bulletin/oxy-fuel/enrgycon.shtml
Conventional Gasoline: 115,000 BTU/gal
Conventional Gasoline + 10 Vol % Ethanol: 111,100 BTU/gal
Conventional Gasoline + 15 Vol % MTBE: 111,800 BTU/gal

Chevron's really good primer on Diesel engines says:
http://www.chevron.com/prodserv/fuels/bulletin/diesel/Diesel%20Fuel%20Rev.pdf
Regular gasoline: 114,200 BTU/gal
Premium gasoline: 116,200 BTU/gal
Jet fuel: 122,200 BTU/gal
Diesel fuel: 130,000 BTU/gal

So I'd say that Steve's 123,299 BTU/gal figure for gasoline is a bit on the high
side, while the 118,950 BTU/gal for Diesel is too low.

Steve continues:

Higher octane numbers have NOTHING to do with the energy content.

Chevron disagrees, but the difference is only 1.75%.  By contrast, oxygenating
gasoline reduces the energy per gallon by 3.5%.  So for peak performance of your
airplane when running mogas, get the non-oxygenated premium gasoline if you can.

Fortunately for the gas station owners, most folks don't believe that and run
the higher priced gas thinking they are doing good things for their car.

Um, some car's owner's manuals actually call out mid-grade or premium grade
gasoline.

Russell Kent

 
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