A Preliminary CFD Investigation of In-Cylinder Stratified EGR for Spark Ignition Engines 2002-01-1734
High exhaust gas recirculation (EGR) tolerance is always pursued not only for its advantages of the pumping loss reduction and fuel economy benefit, but also for stringent emission requirements by using conventional three-way catalytic converter (TWC) instead of costly NOx trap. How to keep fresh charge and EGR separated in the cylinder of a conventional four valve gasoline engine is a critical challenge. This work establishes advanced user subroutines and overall simulation strategies to model engine in-cylinder turbulent flow, temperature, pressure, and EGR concentration fields and to simulate EGR stratification process in a typical pent-roof gasoline engine cylinder during intake and compression strokes.
A series of computational fluid dynamics (CFD) studies - combustion dome, ports, valve and valve mask configurations, EGR tolerances, swirl ratio, boundary and initial conditions, etc., have been conducted to relate the “corndog” flow structure and physical parameters - centrifugal force, heat transfer, swirl ratio, tumble flow, turbulent and burnt gas residual. Based on the CFD results of the four valve-port, pent-roof combustion dome system, the direction to reach the corndog flow structure should be: (1). Reduce the burnt gas residual; (2). Decrease the temperature difference of EGR and air to minimize their heat transfer and centrifugal force; (3). Reduce the turbulent intensity; (4). Design the combustion dome as flat as possible; (5). Avoid using masks; (6). Reduce tumble flow; (7). Keep certain level of swirl ratio.
Another method to stratify EGR in cylinder, named “conjugate vortex EGR stratification” system, has been discovered by the CFD analysis. Two identical inversed helical ports, one intake and another exhaust, are combined together to create two strong inverse gas flow vortices in the cylinder. The double inverse vortices result in a stable separation of in-cylinder fresh charge and EGR flows induced from intake port and exhaust port during the entire intake stroke. A clear stratification interface between the two streams of flow was found and located in the cylinder center. The turbulent kinetic energy at the interface is quite low, indicating that the minimum mixing opportunity between the EGR and fresh charge occurs at this location.