A miniature free-piston engine/compressor

Portable powered devices such as construction tools and orthotics require a power supply that is light and compact. Hydrocarbon fuel has exceptionally high energy density compared to batteries but must be converted to mechanical power to be useful. We designed, constructed and evaluated a tiny internal combustion air compressor that uses a free-piston configuration combined with homogeneous charge compression ignition. The device compressed air in a small tank to 6.5 bar with an overall fuel to cold compressed air efficiency of 0.6%. The engine was self-regulating with the compression ratio automatically adjusting for variations in load and speed. Unlike a traditional crankshaft engine, the piston is not constrained and expansion is rapid so that combustion continues during expansion lowering efficiency. Switching from glow plug to true homogeneous charge compression ignition, which has fast kinetics will improve system efficiency as will better sealing to prevent blow-by leakage. Our mathematical model was able to predict the essential experimental results from the prototype engine.