A three-dimensional simulation model of an ignited hydraulic free piston engine(HFPE) was established by using a CONVERGE simulation platform. The pressure acting on the top surface of piston was solved in real time to promote the coupling of the combustion gas force in the cylinder with the hydraulic pressure. On this basis

the indicated thermal efficiency and knocking conditions of the pressurized multi-point ignition HFPE were also studied. Results show that the use of multi-point ignition improves the indicated thermal efficiency of the pressurized HFPE

which results in a smaller compression ratio for the best indicated thermal efficiency and a higher indicated thermal efficiency at a higher boost ratio. Use of multi-point ignition also expands the boost ratio in the high efficiency zone to over 4. Moreover

an appropriate increase in the mass of piston assembly can extend the residence time of the piston near the top dead center

thus further improving the indicated thermal efficiency.