This is a neat site for those interested in different types of ICEs - it has animations and descriptions of various engines, including the standard "Otto" engine and the "Atkinson" - which I know is used in the FEH. (Do any other hybrids use it?) It helped me understand why/how the Atkinson engine is more fuel efficient (a picture's worth a thousand words). (It also has the Wankel engine, two-stroke - I never knew how those things worked - and various steam engines.)

www.keveney.com/Engines.html