Development of an In-Cylinder Pressure Observer for Free-Piston Engines

Reconstruction of engine in-cylinder pressure with observers in conventional engines has extensively been studied in the literature. The subject however remains unaddressed for Free-piston Engines (FPEs). An in-cylinder pressure observer suitable for FPEs is developed in this paper. The observer reconstructs in-cylinder pressure from measured piston position and net heat release rate. Observer development starts by examining the commonly adopted high gain observer for nonlinear systems, showing its limitations in a numerical FPE case study. A newly developed sliding mode observer that overcomes these limitations is then proposed. The proposed observer has a finite-time converging error, making it suitable for applications requiring accurate and real-time pressure estimation. Effectiveness of the observer is analytically proven and verified by simulation. The proposed observer is finally generalized into a large family of sliding mode observers.