This is another twist on a compact rotary engine design:
LiquidPiston unveils quiet X Mini engine prototype
http://www.greencarcongress.com/2014...41119-lpi.html
LiquidPiston X Mini 70 cc Gasoline Engine Early Prototype Demo on Vimeo
How It Works | LiquidPiston
The rotor is the valves and the combustion chamber all in one. It is a 4 stroke engine, but there are
three combustion events per revolution of the rotor.
They have tested it at 3HP on the dynamometer For a 70cc engine.
http://liquidpiston.com/
An air cooled version and a water cooled version.
I think it would make a fabulous genset for a serial hybrid.