Starting of a free-piston linear engine-generator by mechanical resonance and rectangular current commutation

Starting a free-piston linear engine-generator (LG) involves reciprocating a freely moving piston-magnettranslator assembly between two oppositely placed engine cylinders for combustion to occur. The machine is operated as a brushless linear motor to produce the required motion. However, due to the very large peak compression force during starting, limited current rating of stator coils and insufficient motor force constant, it is not possible to push the translator end to end in a single stroke. A strategy is proposed which utilizes the air-spring quality of the engine cylinders prior to combustion. Energizing the coils with fixed DC voltage and open-loop, rectangular commutation of injected current, sufficiently high motoring force is produced to reciprocate the translator 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 speed is expected to grow - due to mechanical resonance - to finally reach the required parameters for combustion. This work discusses the starting problem and its mechanical aspects for a specific LG configuration, builds a mechanical model of LG and presents simulation results on the viability of the starting strategy using different values of constant-magnitude motoring force. © 2008 IEEE.