A prototype of hydraulic free piston engine has been developed to achieve efficient energy conversion directly from chemical energy of fuel to hydraulic energy. In stable running condition, in order to verify the features of this kind of engine, the piston dynamics, combustion process and hydraulic characteristics are investigated through series of tests of the prototype. The experimental results show that the majority of fuel burns in the rapid combustion phase which is due to the high piston velocity in the final part of the compression stroke. The combustion is characterized by a constant volume process. Furthermore, the indicated thermal efficiency and the indicated mean effective pressure of the prototype presented in this study are 41% and 5.2 bar respectively. (C) 2012 Elsevier Ltd. All rights reserved.