An experimental study of the cycle stability of hydraulic free-piston engines

The piston motion of hydraulic free-piston engine (HFPE) without being restricted as that of conventional crankshaft engine (CE), leads to the cyclic variations, which raises concerns over engine cycle stability. It is therefore necessary to investigate the cycle-to-cycle variations in the piston dynamics and cycle stability of HFPEs. For this purpose, we conducted and evaluated an experimental study of a prototype HFPE. The experimental results indicate that the HFPE was in steady state when it ran at the maximum range of the cyclic variations in the piston dynamic parameters. In addition, variations in the top dead center (TDC) position, bottom dead center (BDC) position, starting piston position and compression ratio were investigated. In this prototype, the maximum variation in the TDC position relative to the total stroke was 1.2%; the starting position of the piston oscillated between 11 and 19 mm, the maximum variation relative to the total stroke was 2.9%; the BDC variation ranged from 4.5 to 13 mm; and the maximum relative variation in the BDC position to the total stroke was 2.7%.