A limit to TDC turbulence intensity in internal combustion engines

The turbulence level near top dead center (TDC) in an internal combustion engine strongly influences the flame propagation and the combustion processes. The effect of intake system design on TDC turbulence has recently been studied experimentally. This paper presents comparisons of the predictions of a model for the turbulent flowfields with those experimental data taken in a ported engine with and without swirl and in a valved engine without swirl. The model is for an axisymmetric geometry and uses a k-epsilon submodel for turbulent transport. The model constants are the same as those used in several previous studies. The computed results show fairly good agreement with the measurements. The two-dimensional results are also compared with those from a zero-dimensional model and serious limitations of the zero-dimensional model are evidenced. The two-dimensional calculations lead to the important conclusion that there is an upper limit to the magnitude of the TDC turbulence intensity in reciprocating engines with pancake combustion chambers. This limit is independent of the characteristics of the flowfield at the end of the intake processes and is due to the chamber walls. Below the limit, TDC turbulence intensity is influenced primarily by the turbulent diffusivity of the gas at the end of the intake process.