Modeling, Simulation and Implementation of Rectangular Current Commutation for Starting of a Free-Piston Linear Generator

Starting a free-piston linear engine-generator (LG) requires optimal compression pressure of the engine cylinders and favorable amplitude and speed for combustion to occur. The LG can be operated as a linear motor to provide for the required reciprocating motion for starting. The problem arises due to the peak force required to achieve the full stroke of the translator, which is beyond the maximum force that can be effectuated by current injection. Utilizing the air-spring character of the engine cylinders prior to combustion and brushless DC motor operation, a unique starting method is proposed: LG coils are energized with fixed DC voltage provided by standard automotive batteries. Via open-loop, rectangular commutation of injected current into the LG, sufficiently high motoring force is produced to reciprocate the LG 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 and reach the final required values for combustion.

This work discusses the mechanical and electrical aspects of starting the LG and builds the respective models to simulate the starting process. Viability of the mechanical
resonating strategy using constant motoring force is established. Series of simulation and experimentation runs are performed to validate and refine the models. Effectiveness of the resonating strategy using rectangular commutation of current is investigated by comparing two methods - 6-step and square-wave - and by varying the fixed DC bus voltage level. However, due to large inductive current lag, large back emf voltage counteracting the effective current level and pressure leakage at low operating speeds, the resultant current profile is heavily rippled and the air springs are ineffective. Improved
motoring is achieved using higher DC bus voltage. Nevertheless, it is found that motoring the LG with rectangular commutation does not result in cycles of increasing amplitude but steady-state operation after the very first cycle. Thus, for every DC bus voltage level,
certain fixed cyclic amplitude and speed will result, due to the interdependency between the different system parameters. Using 12 batteries (144 Volts), simulation shows that the LG can be reciprocated at the required amplitude for starting but the resultant compression pressure and cyclic speed are much higher than required.