Transient motion control for a free-piston engine

The free-piston engine has a unique internal-combustion engine architecture which features unconstrained piston movement because the crankshaft has been removed. The engine design is thus more compact, modular and efficient. One of the technical challenges that hinder the development of free-piston engine technology is the lack of precise control of the piston motion. Previously, a robust repetitive controller was designed and implemented to form a virtual crankshaft to provide stable and precise control of the piston motion. The virtual crankshaft mechanism ensures stable and repeatable free-piston engine operation and opens up an opportunity for controlling the in-cylinder gas pressure–temperature trajectories by altering the piston trajectory. The experimental data from engine motoring tests with the virtual crankshaft demonstrate the effectiveness of the controller. However, the presence of a transient period after combustion prevents the engine from continuous firing. Thus, a transient control scheme is presented in this paper, which consists of a detection and shifting algorithm to remove the transient period, so that continuous engine firing operation can be achieved.