DME - No Soot Fuel for Diesel Engines...

[Rawze]

steam engines have always been by far the most efficient per BTU of the fuel source.
Diesel fired closed-loop dry-steam would be the solution to go from a 40% BTU efficient engine (current modern diesel technology) up into the 80 or 90+ % range.

Multi-cylinder ultra-high-pressure helium diesel-fired sterling engine designs would be the second choice. that would yield up in the 70 - 85% range quite easily.

just my own thought... guess no one wants to further advance well established technologies in the past to improve the future any more. Seems like no one wants to actually work on mining and burning any overall less fuel.. they just want to impose restrictions and taxes on the fuel quantities already being burned.