Basics

A Piston Engine where the engine is "free" within the cylinder; it is not connected to a Crankshaft
Can be used as Gas Generators and/or Linear Generators
If the coils of the linear generator are also used as a Linear Motor this can be used to control the position / motion of the piston thus allowing for software tunable, infinitely variable compression ratios etc
Also Part Reduction occurs etc
Main issue is in the difficulty in control, as well as Noise and Vibration / Noise, Vibration, and Harshness

Internal Links

http://www.technologyreview.com/Energy/21442/page2/ (Dead Link)
An article by Next Big Future titled "Free-piston engine could be twice as fuel efficient as combustion engines"