Stable compression ratio control for free-piston engine generator

To stably control free-piston engine generator, a model of energy conversation and transfer in continuous cycles was established. The essential energy balance conditions satisfying stable operation were constructed via numerical simulation, including the equation of the compression energy and system energy residual value, then the stability of cyclic energy residual value and compression energy were sequentially achieved. The control system and strategy for dynamic energy balance in continuous cycle were proposed, where the injection quantity and the compression ratio were taken as the control output variables, the proportion-integral-derivative controller was employed to drive the operation, and the continuously stable operation under the controllable compression ratio thus was realized. The results show that the free-piston engine generator operates reliably and stably under the control system and strategy. The compression ratio shortly oscillates once changing load, then gets back to the set value after undergoing several cycles. The peak pressure in-cylinder and operation frequency tend similarly and the parameters vary within the acceptable range.