The CHIRON is a hydraulic free piston engine developed by the Dutch companies NOAX and Innas. In the CHIRON the energy of the combustion process is almost directly converted into hydraulic energy. The CHIRON features a direct electronic control of the injection parameters, the flow and the compression ratio. The flow output is controlled by means of Pulse Pause Modulation of the piston frequency. The CHIRON is designed for application in common pressure rail systems. In these systems the hydraulic energy is supplied through a common rail. This paper describes the design of the CHIRON. Special attention will be paid on the specific characteristics of the CHIRON compared to conventional engines and pumps.