Hydrocarbon fuels have a much higher specific energy than electric batteries, and are thus a desirable power supply for portable devices. A reliable lightweight long lasting power supply will be an enabler for human assist devices and miniature robotics. A prototype miniature free-piston engine compressor runs for up to 30 s. New pistons and cylinder liners with coatings did not improve reliability when implemented in the engine. The engine stalled because pressure could not be held in the cylinder due to blow-by leakage caused by too large of piston/cylinder clearance. The air compressor part of the device was tested independently. It reached a pressure difference of 450 kPa and produced a maximum 11 W of compressed air power when working against a backpressure of 170 kPa. Two thermodynamic models of the air compressor were created which matched experimental results but did not capture all the details of compressor operation.