Research and development activities pertaining to the development of a 10 W, homogeneous charge compression ignition free-piston engine-compressor are presented. Emphasis is placed upon the miniature engine concept and design rationale. Also, a crankcase-scavenged, two-stroke engine performance estimation method (slider-crank piston motion) is developed and used to explore the influence of engine operating conditions and geometric parameters on power density and establish plausible design conditions. The minimization of small-scale effects such as enhanced heat transfer, is also explored.