Effect of fuel injection rate shapes on mixture formation and combustion characteristic in a free-piston diesel engine generator

The free-piston engine generator is a new power machinery, which removes the crank-shaft mechanism. This article aims to study the impact of different injection rate shapes (left triangle, rectangle, and right triangle) on the free-piston engine generator spray and combustion characteristics. An iteration numerical model was established and validated by experiment. The results reveal that there is a positive correlation between injection rate and evaporation rate, and the faster evaporation rate is more conducive to total evaporated fuel mass. The left triangle and rectangle injection modes have the better level of atomization and mixing in the early injection due to stronger turbulent kinetic energy, longer penetration, and smaller Sauter mean diameter, while the right triangle has opposite results, which impairs the mixture formation, and its higher injection pressure in the late injection duration causes more impinged fuel mass. Moreover, the temperature, pressure, peak heat release rate, and indicated thermal efficiency for the left triangle are greater than the other two injection modes. Therefore, for the free-piston engine generator, faster injection rate in the early injection stage enables to improve the fuel evaporation, atomization, and mixing processes, meanwhile enhances the level of isochoric heat release around top dead center, resulting in higher indicated thermal efficiency.