This paper describes mechanical modeling of a specific configuration of free-piston linear engine-generator (FPLG) during starting, by brushless commutation of linear motor.To account for piston blow-by, or air leakage through piston rings at low starting speeds, the model incorporates an air mass transfer algorithm for the air compression-expansion process in the engine cylinders.The model is verified with experimental data.Pressure profiles obtained during experimental validation of the FPLG are discussed, for different motoring conditions.The improved model shows a reduction of 28 in compression pressure compared to the ideal case.In single-stroke tests, the airflow corrective factor value of C = 3.7 � 10�7 is found to give the closest profile to experimental pressure for the right cylinder and C = 5.5 � 10�7 for the left cylinder, while C = 4.1 � 10�7 best matches the data of cyclic motoring tests.Results show that by increasing motoring voltage, the piston�s amplitude remains unchanged, since increasing motoring energy leads to better compression-expansion process, more effective air spring and a pressure profile closer to the lossless system. © 2023, Institute of Technology PETRONAS Sdn Bhd.