Investigation of linear generator starting modes by mechanical resonance and rectangular current commutation

Starting a free-piston linear engine-generator (LG) requires favorable compression pressure and piston speed for combustion to occur. To produce the required reciprocating motion, the LG is operated as a brushless linear motor. However, the peak force required to achieve the full stroke is beyond the maximum motor force that can be produced, limited by the coils' current capacity and LG's motor constant. A starting method is proposed, utilizing mechanical resonance and the air-spring character of engine cylinders prior to combustion. Energizing the coils with fixed DC bus voltage and open-loop, rectangular current commutation, the LG is reciprocated in small amplitudes initially. Due to repeated compression-expansion of the engine cylinders and constant application of motoring force in the direction of natural resonating motion, the translator's amplitude and speed is expected to grow and reach the final required values for combustion. To investigate viability of the proposed strategy, an integrated model of LG - consisting of mechanical and electrical subsystems - is developed and real-time simulation is performed on Matlab Simulink. The individual models are validated against field experimentation before final simulation and experimentation are implemented. This work discusses simulation and experimental results of the proposed starting strategy. © 2009 IEEE.