I listen to a number of podcasts while I'm building.
Autoline After Hours episode #63 (free on iTunes) had on Dr. Peter Hofbauer from Ecomotors talking about their new Opoc engine. They recently got some serious VC funding from Bill Gates and Vinod Khosla.
Take a look at the pictures of the engine on their web site
. It has opposed pistons in opposed cylinders.
During the show, one of the questions they took asked about future applications for aviation. Supposedly this engine is much more efficient than anything out there and it can run just about any fuel. They also talked about a suitcase sized version that can generate 5 kilowatts. Very intriguing technology!