Free-piston engine generator is an unconventional engine, which abandons the crank and connecting rod mechanism. This paper focused on a numerical simulation for the research on the combustion characteristics of a free-piston diesel engine generator. An iteration numerical simulation method was presented by adopting coupled models of zero-dimensional dynamics, multidimensional scavenging and combustion. According to the coupled parameters of these models, the effects of the piston motion on combustion process were investigated compared with a corresponding traditional crank engine. The results indicate that compared with the conventional engine, the free-piston engine generator has a longer combustion duration due to its faster piston mean velocity in combustion process. While the heat release before top dead center, the isochoric heat release, and the heat release during the premixed combustion period both were lower than the traditional engine, and the post-combustion became more intense for the free-piston engine, thus a slight disadvantage of indicated efficiency was found. In addition, the in-cylinder average gas temperature and pressure were generally lower than the traditional engine, which contribute to the distinct advantages in NO discharge of the free-piston engine generator. (C) 2016 Elsevier Ltd. All rights reserved.