A free piston engine linear generator (FPELG) is considered a type of contemporary technology for energy conversion. The main features of FPELG are less emission, few parts i.e., less friction, and high efficiency. However, because the FPELG does not have a crankshaft, critical issues e.g., combustion efficiency, and performance were reported. This study examines the influence of various fuel injection durations (FID) on combustion behaviors. According to the results of the experiments, as the time increased of the fuel injection, the in-cylinder pressure and translator velocity both slightly grew. The highest work output and combustion efficiency were at FID17 and FID 14 ms, respectively. Thus, FID plays a significant role to improve FPELG performance and optimal fuel duration is required. © 2023 Novel Carbon Resource Sciences. All rights reserved.