Miniature homogeneous charge compression ignition free-piston engine compressor

Abstract

Utilizing the high energy density of hydrocarbon fuels, an internal combustion engine based free-piston engine compressor is a promising solution as a power supply for miniature size mobile fluid power applications. A free-piston engine is a type of engine without crankshaft, and piston motion depends on force balance instead of mechanical constraints. Homogeneous charge compression ignition (HCCI) is a combustion mode in which homogeneous fuel air mixture is compressed to the point of auto-ignition. The objective of the thesis work is building a miniature size engine compressor to work as a compact fluid power supply, while combining the HCCI and free-piston engine configuration. Mathematical models for components were calibrated and verified by components testing. The overall engine compressor model correlated well with testing results of a proof-of-concept prototype. Although the prototype only achieved 0.6% overall efficiency from fuel energy to cooled compressed air because of poor compressor efficiency and high fuel short circuiting loss, the miniature free-piston engine compressor showed the potential to be a high energy density, compact fluid power supply.