TOWARDS FREE PISTON ENGINE APPLICATIONS

A hydraulic free piston engine (HFPE) combines a diesel engine and a hydraulic pump into one compact component. The piston assembly motion is influenced by pressures and inertia forces only and no mechanical linkage control the motion. The piston assembly moves during operation linearly back and forth between dead centres by means of the fuel energy producing directly hydraulic energy. HFPE do not allow or it is not sensible just to replace the traditional crankshaft engine-pump combination. In the worst case the whole hydraulic system must be redesigned. The features that harm HFPE use are: Reliable compression ratio control requires steady load pressure; The pump displacement is fixed or stepwise variable; High piston accelerations at dead centres cause need of supply pressure i.e. closed hydraulic system. However, the problems can be overcome and some solutions exist. The paper introduces and compares mentioned solutions. The comparison criteria are efficiency, controllability, usability and costs.