Somehow I stumbled across a 2004 patent by Lycoming and Toyota containing some ideas I wouldn't have expected to see from Lycoming.
The patent in question, "Cylinder assembly for an aircraft engine" (U.S. Patent No. 6832589), describes an elegant combustion-chamber design in which a domed piston with special cutouts squishes fuel into a small pocket for rapid-swirl combustion. This general notion has been patented to death in the automotive world, of course, and wasn't new in 2004. But somehow Lycoming and Toyota got the USPTO to sign off on it one more time.
In the above diagram (taken from the patent), I've colored the piston to make it easier to see. Note the close proximity of spark plugs to piston. The "squish zone" (C) calls for a piston-to-cylinder-head clearance of just .047 +/- .016 in., or about enough for five hours of deposit buildup in an O-235, but this wouldn't be for an O-235. This is a high-compression-ratio design for ultra-lean operation on low-octane unleaded fuel, and I guess that's the real punchline.
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