Stability analysis of hydraulic free piston engine

Based on the piston force analysis of the hydraulic free piston diesel engine, a nonlinear vibration model is established. Then the solution of the nonlinear vibration model is given by the generalized harmonic KBM method. The reasons that affect the constant amplitude of the nonlinear vibration model are analyzed according to the expression of the constant amplitude. The constant amplitude is limited by many factors and the system will be unstable if the correlated variables are out of range. The influential factors, such as the fuel injection position and quality, are investigated. In addition, the fluctuation of pump chamber caused by the variation of response characteristics of check value also influences the constant amplitude. All these reasons may lead to the unstability of the oscillatory system. Because the combustion condition is influenced by the air intake and fuel injection, the selection of unstable region is decided by the scavenging and combustion. Therefore the scavenging influenced by the port opening is researched and the results are validated by experiment. In order to get the optimal design of HFPE, the stability range of the piston constant amplitude and judging in equation are proposed, and the parameters in the stable region are obtained. All these efforts establish the theoretical foundation for control strategy of stability improvement.