<div class="htmlview paragraph">The C<span class="small-caps">HIRON</span> is a hydraulic free piston engine developed by the Dutch companies NOAX and Innas. In the C<span class="small-caps">HIRON</span> the energy of the combustion process is almost directly converted into hydraulic energy. The C<span class="small-caps">HIRON</span> 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 C<span class="small-caps">HIRON</span> 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 C<span class="small-caps">HIRON</span>. Special attention will be paid on the specific characteristics of the C<span class="small-caps">HIRON</span> compared to conventional engines and pumps.</div>