Experimental study on the operating characteristics of a reciprocating free-piston linear engine

A miniature reciprocating free-piston linear engine (FPLE) with glow plug ignition (GPI²) was designed and manufactured. The steady-state operation of the prototype is successfully achieved with and without load. The operating characteristics of the FPLE were investigated using a premixed methane/air mixture. The experimental results showed that the in-cylinder mixture was successfully ignited by the glow plug within five cycles and the FPLE could be successfully started under the single-cylinder self-starting strategy. Based on the comparison of the FPLE operating characteristics in the GPI and spark-ignition (SI³), it is found that the performance and operating stability of the FPLE were significantly improved in the GPI due to the flexible ignition timing. Besides, the effects of key parameters such as glow plug heating power (GPHP⁴), piston assembly mass and fuel/air flow rate on the performance of the FPLE were studied. The electric power and work conversion efficiency increased first and then decreased with increasing external load, and both of them reached maximum values of 31.6 W and 42.8%, respectively, with an external resistance of 6.3 Ω.