A free-piston linear generator (FPLG) is an energy conversion device that can generate electrical energy utilizing different types of fuel. Natural gas fields with high carbon dioxide (CO2) content are being simulated for study, knowing that the presence of CO2 in natural gas contributes to a reduction in NOx emission while improving the compression ratio. This paper presents a comparative analysis of the performance and combustion characteristics of a spark ignition (SI) FPLG fuelled with two different fuels, namely compressed natural gas (CNG) and a mixture of CNG and CO2 (CNG-CO2). A test was conducted on the FPLG to study the engine performance and combustion at various lambda ratios. It is clarified that lambda variation significantly affects the piston motion and combustion of CNG and CNG-CO2, resulting in a variation in FPLG engine stability. When a lambda tends toward a rich mixture, the translator attains a peak velocity of 5.0 and 6.25 m/s, for the CNG-CO2 and CNG, respectively, resulting in output power (rms) of 257.2 and 295.7 W at λ = 0.86 and 0.85. Also, the presence of CO2 in CNG increases the compression ratio. The engine could attain ITE of 22.8 and 15.1 for CNG-CO2 and CNG, respectively. © 2022 The Authors.