A Novel Approach to Free-Piston Engine Control Using an FPGA Based Control System

The variable stroke length of the free-piston engine poses an interesting problem for the controls engineer. At a low level, the control system must be able to track piston position and address changes in top and bottom dead center positions. To accomplish this, a new FPGA (field programmable gate array) based engine position tracking software was developed, along with a simple method of mapping from a conventional engine control system to a free-piston control system. The tracking software was integrated into a complete rapid prototyping control system that was responsible for all control actions of the engine. The control system was laboratory tested on the HiPerTEC, an opposed, free-piston engine with a circular piston arrangement (as opposed to linear free-piston engines) developed by Applied Thermal Sciences, Inc (ATS). The control system has been demonstrated to run 8 cylinders up to an effective speed of 2,200rpm in spark ignition mode.