Experimental investigation of operating characteristics and thermal balance of a miniature free-piston linear engine

A miniature free-piston linear engine fueled by butane is designed using glow plug ignition. This study investigates the engine operation characteristics and thermal balance. Experimental results show that the engine can be manually started successfully and achieve self-sustainable operating without the energy support from the glow plug. Thermodynamic analysis of the engine reveals that the energy loss resulting from scavenging process contributes the largest part to the total energy loss. The running frequency and indicated power increase with the mixture flow owing to the increased charging efficiency, and the best performance is achieved when the engine operates at a slightly fuel-rich condition. The energy loss of heat transfer and incomplete combustion are important contributors to energy loss due to the increased surface-to-volume ratio. Leakage and friction loss account for small contributor (less than 3%). The order of importance of engine loss components is different between the miniature FPLE and conventional engine. For the miniature free-piston linear engine, the energy loss components arranged by order of importance are scavenging, heat transfer, incomplete combustion, exhaust, leakage and friction. For conventional-scale spark ignition engine, the energy loss components arranged by order of importance are exhaust, heat transfer, aerodynamic, friction and incomplete combustion.