Numerical simulation of the effect of spring dynamics on the combustion of free piston linear engine

Abstract

Free piston linear engine (FPLE) is a new potential alternative to conventional engine, and the mechanical spring of the engine is a key energy storage device to maintain reciprocating operation and then affect combustion and thermodynamic. This article presents a study to reveal the effect of spring dynamics on the combustion performances of a linear diesel engine. A system model which couples spring dynamic model and multi-dimensional combustion model is proposed to discuss the variable spring stiffness effect on the combustion and heat release characteristics of the FPLE. The results indicate that the spring stiffness gives a positive excitation on the reciprocating frequency and engine compression ratio, although it brings about low fuel-air mixture uniformity for combustion reaction. Moreover, the great spring stiffness leads to high combustion pressure and fast combustion process due to its large compression ratio. The results suggest that properly increasing the spring stiffness is beneficial to increase the thermal efficiency of the engine, but too high spring stiffness will also result in higher NO emission and soot generation.