[thackney]

What has a higher energy density than diesel? Acetylene?

I think that would be limited to longer chain hydrocarbons. #6 Fuel Oil (bunker fuel) will have a greater energy density.

http://www.engineeringtoolbox.com/energy-content-d_868.html

[Eric in the Ozarks]

Typical N6 is 149,000-150,000 BTUs/gallon.
Negative gravity slurry oils, some a full pound per gallon heavier, can run 159,000-160,000 BTUs/gallon.
Now, that's heat !