The free-piston engine generator (FPEG) is regarded as a substitute of conventional combustion engine (CCE) because of special operation mechanism and potential advantages. This article presents an investigation to analyze the fuel spray and mixture formation characteristics of a direct injection gasoline FPEG by comparing a corresponding CCE. A full-cycle multi-dimensional fuel spray model is established by coupling with dynamic, and then it is validated experimentally to predict fuel diffusion and mixture formation in the FPEG. Results indicate that compared with the CCE, the FPEG operates with lower in cylinder gas turbulence, pressure and temperature during injection stage, so the fuel spray in FPEG receives a smaller gas resistance and behaves more impingement, longer penetration, slower evaporation, larger sauter mean diameter (SMD), and lower mixture uniformity in this stage than the CCE. However, the slower compression process of FPEG not only makes that the fuel droplets evaporates earlier, but also provides a longer duration for fuel-air mixing and diffusion. Therefore, at the moment of spark ignition, a more homogeneous mixture is formed in the FPEG, and it shows smaller SMD and higher concentration around spark plug than the CCE. This phenomenon may facilitate the spark ignition and high efficiency combustion. (C) 2019 Elsevier Ltd. All rights reserved.