Control of a Directly Driven Four-Stroke Free Piston Engine

The free piston engine is an Internal Combustion Engine (ICE) without crankshaft. This allows driving an independent trajectory of the piston stroke, which adds a new degree of freedom for the control. This trajectory has to be controlled by using a linear electric machine. Most of the reaseach in free piston engines until now was focused on two-stroke cycles. The present paper focuses on a four-stroke cycle and investigates the particular requirements of the linear machine and its control. Especially in a four-stroke free piston engine it is possible to adjust every stroke trajectory to an optimal one depending on the operating point. Herefore a control structure is proposed and validated on a small experimental prototype of the engine. Unlike rotating engines, the directly driven piston provides also the opportuniy to estimate the instantaneous pressure of the cylinder without additional sensors, which allows to optimize the ignition point among others. For this, an observer is proposed and validated on the prototype.