Hydraulic free piston engine — Challenge for control

A hydraulic free piston engine, HFPE, combines a diesel engine and a hydraulic pump into one compact component. In practical view there is only one linearly moving part in the HFPE, the piston assembly, which contains combustion and hydraulic pistons and piston rod. The piston assembly moves during operation linearly back and forth between left and right extremes by means of the fuel energy. Simultaneously hydraulic pistons produce directly hydraulic energy. This study deals with control task of the hydraulic free piston engine, and especially dual piston case. The piston motion of the HFPE differs significantly from the piston motion of the conventional crankshaft engine because there is no mechanical linkages to control the motion of the piston assembly. The piston of the HFPE has very high accelerations at the dead centres. This creates high requirements for the sensors, control devices and hydraulic valves as well. The piston motion is possible to control by fuel amount and hydraulic load. The proper operation of the HFPE requires constant pressure.