Thomas Borchert wrote:
Jose,

Is the fuel lighter, or does it use less fuel per mile or per hour (for
the same power)?


Diesel has more energy per weight than Avgas. And the modern diesel
engines use less fuel for the same power. Much less. But the engines, at
least the ones certificated, are not lighter. Some are heavier than
comparable Avgas engines.


The thermal energy per weight unit is roughly the same for
all hydro-carbon liquids, AVGAS and JET-A1 (used by aviation
diesels) included.

What makes for the better fuel economy of a Diesel engine (compared
to the gasoline engine Otto process) is from two sources:

- the greater compression ratio makes an ideal thermal
engine (Carnot process) work with better efficiency,

- the long combustion in a Diesel engine makes the process
approximate the ideal process better (constant pressure).

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Tauno Voipio
tauno voipio (at) iki fi