This looks promising. I have always thought that crankshafts thrashing around and pistons scraping up and down, accelerating from zero to maximum piston speed before being stopped dead and repeating the sequence again seemed an inefficient way to obtain energy from petrol! http://auto.howstuffworks.com/wave-disk-generator.htm http://en.wikipedia.org/wiki/Wave_disk_engine
the tricky part is sealing the rotor so that the expanding gas can make it spin......like the rotary engine the seal is everything