Modeling and Validation of Engine Cylinder Pressure During Starting of Free-Piston Linear Generator by Brushless Motor Commutation

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.