Gasification

[Edward Hernandez R]

"Despite up-front speculation, things don't appear to work that way.
BMEP/BTU for propane and NG are no better than for gasoline engines,
except at very low RPM."

That's primarily because the energy content(MJ/kg) of propane and NG are nearly the same as gasoline, not because of droplet size. But since gaseous fuels DO displace more volume, propane(and especially NG) mixtures have less bang for the buck in terms of MJ/m^3. No practical fuel can beat gasoline for  this, especially when completely vaporized, which is another reason why propane and NG are no better.
   Droplet size DOES affect fuel conversion efficiency, which is pretty good for gasoline to begin with. But more importantly, smaller drops increase burn rates, which allow one to run the engine much leaner and/or consume more EGR, both of which reduce feedgas emissions and improve fuel efficiency significantly. Faster burn rates allow higher compression ratios, which increases fuel efficiency and power.