This paper presents the measured characteristics of a single piston hydraulic free-piston engine prototype. The detailed results about the piston motion and the fuel combustion are presented. The results show that there is a strong interaction between the piston motion and the fuel combustion in the free-piston engine. The gas force plays a leading role in the innate asymmetric piston motion around top dead centre. The small moving piston inertia makes the asymmetric characteristics more visible. The combustion energy conversion of the single piston hydraulic free-piston engine is fast. Most of the fuel injected burns in 1.0 ms in the pre-mixed combustion phase. Because of the asymmetric characteristics, the mixing-controlled combustion phase in the single piston hydraulic free-piston engine is much shorter than the one in the conventional diesel engine. The short combustion duration decreases the rated power of the free-piston engine. The asymmetric piston motion is unfavourable for the rated power increase of the free-piston engine. The smaller rated power has a negative effect on the free-piston engine application development. (C) 2013 Elsevier Ltd. All rights reserved.