Effects of Stepped-Lip Combustion System Design and Operating Parameters on Turbulent Flow Evolution in a Diesel Engine

摘要

Interactions between fuel sprays and stepped-lip diesel piston bowls can produce turbulent flow structures that improve efficiency and emissions, but the underlying mechanisms are not well understood. Recent experimental and simulation efforts provide evidence that increased efficiency and reduced smoke emissions coincide with the formation of long-lived, energetic vortices during the mixing-controlled portion of the combustion event. These vortices are believed to promote fuel-air mixing, increase heat-release rates, and improve air utilization, but they become weaker as main injection timing is advanced nearer to the top dead center (TDC). Further efficiency and emissions benefits may be realized if vortex formation can be strengthened for near-TDC injections. This work presents a simulation-based analysis of turbulent flow evolution within a stepped-lip combustion chamber. A conceptual model summarizes key processes in the evolution of turbulent flow for main injections starting after TDC. Differences in turbulent flow evolution are described for a near-TDC main injection, and potential variations in combustion system design and operating parameters to enhance vortex formation under these conditions are hypothesized. The parametric studies executed to test these hypotheses reveal that while intake pressure and spray targeting play important roles in turbulent flow evolution, they are not capable of fundamentally changing the late-cycle flow topology for near-TDC injection timings. A dimpled stepped-lip (DSL) piston design is developed that supports the hypothesis that increasing space in the squish region promotes vortex formation for near-TDC injection timings. Further analyses reveal the mechanisms by which the DSL piston strengthens vortex formation.