For all of you motorheads still interested in burning fuel instead of draining batteries, the Honda EXlink (Extended Expansion Linkage Engine) is worth a closer look. It's inspired by the 130 year old Atkinson cycle engine. What makes it unique? The power and exhaust strokes are 1.4 times the length of the intake and compression strokes and before you see the linkage it's difficult to figure out how that would work and unless it's in motion, you still won't get it. Another feature is that during the power stroke, the connecting rod never deviates more than 2.4 degrees from the direction of the piston compared to as much as 16 degrees in a conventional engine. The result is far less side load on the piston reducing friction to less than half of normal. Though the extra linkage adds friction, the EXlink still has the advantage of the extended power stroke.
The thermal efficiency of an engine is based on its expansion ratio. Normally, compression and expansion are identical so if you try to increase expansion you also increase compression, not always possible without inducing knock. In the EXlink, compression is 12.2:1 while expansion is 17.6:1, increasing thermal efficiency. Pretty slick.
Honda is currently using the liquid cooled, single cylinder EXlink engine in a household cogeneration unit using natural gas for fuel. The unit also recovers heat from the engine's exhaust to provide hot water. There's no mention of vehicle applications but continued development of the internal combustion engine is always good.
There are moving illustrations of the engine internals on the Honda website.
Thanks for the tip, Chris!