A model-based control design approach for linear free-piston engines

A general design approach is presented for model-based control of piston position in a free-piston engine (FPE). The proposed approach controls either ‘bottom-dead-centre’ (BDC) or ‘top-dead-centre’ (TDC) position. The key advantage of the approach is that it facilitates controller parameter selection, by way of deriving parameter combinations that yield both stable BDC and stable TDC. Driving the piston motion towards a target compression ratio is therefore achieved with sound engineering insight, consequently allowing repeatable engine cycles for steady power output. The adopted control design approach is based on linear control-oriented models derived from exploitation of energy conservation principles in a two-stroke engine cycle. Two controllers are developed: A Proportional Integral (PI) controller with an associated stability condition expressed in terms of controller parameters, and a Linear Quadratic Regulator (LQR) to demonstrate a framework for advanced control design where needed. A detailed analysis is undertaken on two FPE case studies differing only by rebound device type, reporting simulation results for both PI and LQR control. The applicability of the proposed methodology to other common FPE configurations is examined to demonstrate its generality