In free piston engines, the stroke of the piston is used directly to produce hydraulic, pneumatic or electric energy. This can be done with a single piston in one combustion chamber, or with more pistons in one or more combustion chambers. The different designs of free piston engines can be divided into three categories: ‘single piston’, ‘dual piston’ and ‘opposed piston’.All three categories of free piston engines have their own specific advantages and disadvantages. This paper evaluates the conceptual differences. The evaluation will be restricted to free piston engines with compression ignition at the combustion chamber side and hydraulic energy production at the user side. The evaluation will be supported with measured data on the diesel hydraulic engine which has been developed by the Dutch company Innas BV.The evaluation shows that the single piston concept is favourable compared to both other concepts. The single piston engine has advantages regarding costs, efficiency, control, and the simplicity of its construction. The evaluation also shows that the single piston concept has no disadvantages regarding vibrations, not even when the piston stroke is not balanced with a counter weight. By means of choosing the correct mounting, almost all vibration transfer to the frame can be avoided.