Rectangular current commutation and open-loop control for starting of a free-piston linear engine-generator

Starting a free-piston linear engine-generator consists of reciprocating a freely moving piston-magnet-translator assembly between two oppositely placed engine cylinders to initiate combustion. To provide for the reciprocating force, the machine is operated as a brushless linear motor. A starting strategy is proposed which utilizes the air-spring quality of the engine cylinders prior to combustion: by energizing the coils with fixed DC voltage and by open-loop, rectangular commutation of injected current, sufficiently high motor force is produced to reciprocate the piston-magnet assembly in small amplitudes initially. Due to repeated compression-expansion of the engine cylinders and constant application of motoring force in the direction of natural bouncing motion, the translator’s amplitude and compression pressure is expected to grow - due to mechanical resonance - to finally reach the required parameters for combustion. This work discusses the electrical aspects of the starting strategy, considers control possibilities, builds a model of the electrical subsystem and presents simulation and validation tests of the integrated model for starting investigation.