The control of a free-piston engine generator. Part 2: engine dynamics and piston motion control ⋆ R. Mikalsen, A.P. Roskilly * Sir Joseph Swan Institute for Energy Research, Newcastle University, Newcastle upon Tyne, NE1 7RU, England, UK. Abstract Free-piston engines are under investigation by a number of research groups due to potential fuel efficiency and exhaust emissions advantages over conventional technology. The main challenge with such engines is the control of the piston motion, and this has not yet been fully resolved for all types of free-piston engines. This paper builds on the fundamental investigations presented in the accompanying paper and investigates the dynamics of the engine and the feasibility of classical control approaches. The response of the engine to rapid load changes are investigated using decentralised PID, PDF and disturbance feedforward. It is found that the engine is sensitive to rapid load changes but that in constant power applications standard control techniques provide satisfactory performance. The influence of cycle-to-cycle variations in the combustion process are investigated, but not found to be critical for engine operation. Key words: free-piston, linear engine, dynamics, control 1. Introduction A single piston free-piston engine generator cur- rently under development at Newcastle University was presented by Mikalsen and Roskilly [1], and the basic controllability of the engine was investigated in the accompanying paper [2]. It was shown that accurate control of piston dead centre position is critical for the operation of the free-piston engine. Hence, the dynamic response of the free-piston en- gine to changes in the disturbance inputs, predom- inantly engine load, is of high interest. If a control system able to maintain the dead centre positions within specified limits cannot be realised, the engine concept itself will not be feasible. ⋆ This is a preprint version. This paper was published as: Applied Energy, Volume 87, Issue 4, April 2010, Pages 1281– 1287. * Corresponding author. Email addresses: rikard@mikalsen.eu (R. Mikalsen), tony.roskilly@ncl.ac.uk (A.P. Roskilly). This paper builds on the fundamental analy- ses in the accompanying paper and investigates engine dynamic characteristics and controller per- formance. The free-piston engine is a multivariable and non-linear plant, however the successful use of decentralised, single-input single-output (SISO) proportional-integral-derivative (PID) controllers in a similar application has been reported by other authors [3]. This study investigates the use of de- centralised PID control, pseudo-derivative feedback (PDF) control, and disturbance feedforward. 2. Controller design A typical TDC clearance value for an engine such as the one investigated here is 1.5–3 mm. A TDC deviation of ±1 mm is equivalent to a compression ratio range of around 13-18, which is probably ac- ceptable in most cases. Maintaining the TDC posi- tion within ±1 mm of the TDC setpoint is therefore chosen as an initial design guideline. The require- Preprint submitted to Elsevier 18 January 2010