Stability, Control, and Constraint Enforcement of Piston Motion in a Hydraulic Free-Piston Engine

A free-piston engine (FPE) requires stabilizing feedback control to manage piston motion and ensure safe operation. A high-fidelity model, which simulates the FPE behavior by combining dynamic, thermodynamic, and hydraulic submodels, is exploited as an engine surrogate for control development. A control-oriented model implicitly describes the FPE piston turnaround position using energy balance principles for an idealized Otto cycle. Local eigenvalues of the control-oriented model suggest that the FPE system is open-loop unstable. The control system relies on dynamic inversion and state feedback to stabilize the plant and track a target piston turnaround position. A robust reference governor is introduced to manage load changes, such that the piston position does not violate constraints, even in the presence of parameter uncertainty. In high-fidelity simulations, the reference governor constrains piston turnaround position to ±0.5 mm of the target during a load change.