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Feasibility of Multiple Piston Motion Control Approaches in a Free Piston Engine Generator
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on October 22, 2019 by SAE International in United States
The design optimization and control of Free Piston Linear Engine (FPLE) has been found to be difficult as each independent variable changes the dynamics with respect to time. These dynamics, in turn, alters the alternator and engine response to other governing variables. As a result, the FPLE system necessitates an energy balance control algorithm with high-speed dynamic response for stable operation and perhaps optimized system efficiency. The main objective of this control algorithm is to match the power generated by the engine to the power demanded by the alternator. This energy balance control is similar to the use of a governor to control the crankshaft rotational speed in a conventional crankshaft driven engine. In addition to that, when stiff springs are added to the FPLE system, the dynamics becomes more sinusoidal and more consistent with increasing spring stiffness. To understand the cycle-to-cycle variations, a comprehensive FPLE numerical model with a 1kW target electric power was developed in MATLAB®/Simulink. An FPLE system corresponding to that numerical model has been operated in the laboratory. In this numerical model, two different control methodologies were developed to monitor the compression ratio, mid-stroke velocities and peak in-cylinder pressures of the single cylinder FPLE system. To analyze the FPLE system behavior carefully, the sensitivity analysis and response optimizations were carried out on the proportional, integral and differential values of the controllers. The reactions of the controller coefficients on the cycle-to-cycle variations and energy imbalance were analyzed. The results from these studies provide an assessment of the impacts of modeling assumptions on the reliability of predictions. Moreover, the results are also used as the pathway for improving and optimizing the experimental FPLE design.