Simulation on the effect of the combustion parameters on the piston dynamics and engine performance using the Wiebe function in a free piston engine

Abstract

Numerical simulations were conducted to observe the relationship between the combustion phase and the piston dynamics in a free piston engine. The simulations were conducted with commercial software, MATLAB/SIMULINK®. The Wiebe function was used to simulate the combustion process. The combustion parameters such as combustion duration and the spark timing were varied at various piston initial velocities at compression stroke. The indicated mean effective pressure (IMEP) and the mass fraction burned (MFB) were analyzed as indicators of the engine performance.Under given combustion duration conditions, the minimum ignition advance for best torque (MBT) timing was first retarded towards top dead center (TDC) and consequently advanced away from TDC as the piston initial velocity increased. An MBT timing curve was plotted against various spark timings and piston initial velocities in a map. There existed a peak IMEP value along this MBT curve. Longer combustion duration brought negative effects on the IMEP due to larger deviation from the quasi-constant volume combustion, thus lowering the efficiency.The velocity profile of the piston was plotted against the displacement with a contour of every 10% increment of mass fraction burned in order to provide a clear visualization of the combustion process. The spark timing had to be advanced with high piston initial velocity or a long combustion duration condition in order to complete the combustion process near TDC.Mass fraction burned values at TDC showed a linear relationship with piston initial velocity values. This provided a general idea of how to vary the spark timing under a given piston initial velocity condition to achieve the best conversion efficiency from combustion to work regardless of the combustion duration.