The history of attempts to make sub-cubic-inch gasoline engines is fascinating, fraught with products that struggle to get past the proof of concept/early market phase.
The 15cc Octane is just about .92ci, and seems to run fairly well.
There's the 4-stroke Toyan S100G, with .21ci of displacement. There's also a gasoline conversion for their 2-cylinder inline, might be a fun build.
The Dynamite .31 XXL, mentioned by @WVNed
I know it's slightly beyond the scope of the discussion to mention airplane engines, but there's also the SH GF6 (.37) and GF9, (.55), notable for using gas-activated glow plugs instead of spark ignition. OS has made plugs for that purpose, too. The Turnigy TP10 is a 10cc (.61) gas engine, maybe it has modding potential for surface use.
The 15cc Octane is just about .92ci, and seems to run fairly well.
There's the 4-stroke Toyan S100G, with .21ci of displacement. There's also a gasoline conversion for their 2-cylinder inline, might be a fun build.
The Dynamite .31 XXL, mentioned by @WVNed
I know it's slightly beyond the scope of the discussion to mention airplane engines, but there's also the SH GF6 (.37) and GF9, (.55), notable for using gas-activated glow plugs instead of spark ignition. OS has made plugs for that purpose, too. The Turnigy TP10 is a 10cc (.61) gas engine, maybe it has modding potential for surface use.