Most fuel efficent conventional gas (petrol) engine?

[natureboy]

I wonder if anyone knows where there is a list of most fuel efficient conventional gas engines. Not thinking of hybrids or diesel.

I'm thinking of -
"Fuel efficiency:
the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work.

Not -
fuel economy,
refers to the energy efficiency of a particular vehicle model, where its total output (range, or mileage [U.S.]) is given as a ratio of range units per unit of fuel input (gasoline, diesel, etc.), usually measured in miles per gallon (mpg)"
Wikipedia

Anyone know where I can find such a list?