Somebody said that every possible innovation, in connection with the internal combustion engine, had been proposed by the 1920s.
Today’s invention tries, nonetheless, to provide more flexible engine performance without entailing a massive amount of extra complexity.
Each piston would contain its own internal piston. This would move axially within it, sealed by a secondary set of rings. The internal piston could be arranged so that on upstrokes or downstrokes it could engage with and lock on to the outer piston. Only the innermost piston would connect to the conrod.
This would provide the possibility of computer-controlled variation in compression ratio (and fuel consumption) whilst the engine was operating. Extra levels of pistons-within-pistons might be engineered to create a much smoother (more nearly reversible) intake and exhaust flow pattern (by gradual, relative movements of these pistons).