The Sanderson engine is based on a highly efficient rocker-arm mechanism to convert reciprocating motion to rotational motion. Each cylinder can have two opposite combustion chambers and the compression ratio can be changed while the engine is running (using the "control" slider). The engine can have any number of pistons, space permitting.
R. A. Sanderson, A. E. Sanderson, and K. Herbert, "Piston Engine Assembly", United States Patent 6,397,794.
Wolfram Demonstrations Project
Published: April 11 2011