Toyota create ‘free piston’ engine with no crankshaft and electrical output - See more at: Toyota create 'free piston' engine with no crankshaft and electrical output - Interesting Engineering They have even gone ahead and produced a 2 foot long prototype, 8 inches in diameter, that can produce 15 hp. The design is theoretically scalable to something that can produce a larger power output but we’re still a long way away from that time, but we are eager to see where Toyota go with this.
As commented in the article, it's an old idea. But look at the weird intake and exhaust. It's sort of a combination of two-stroke port induction and 4-stroke overhead poppet-valve exhaust--pnuematically operated. The exhaust valves would be tough to cool.