Optimization and Analysis of Mixing Strategies in Diesel Engines

Bergin, M. J. Optimization and Analysis of Mixing Strategies in Diesel Engines. University of Wisconsin-Madison, 2010.

The role of the fluid motion in a diesel engine on mixing and combustion was investigated using the CFD code Kiva-3v. The fluid motion was characterized by the amount of average angular rotation about the cylindrical axis (swirl ratio) and the amount of non-uniform motion imparted by the relative amounts of mass inducted through the tangential and helical intake ports. Non-uniform fluid motion, characterized by counter rotating vortices, was found to contribute to a decrease in the amount of pollutant formation at varying levels of swirl ratio. The attenuation of the pollutant CO could be significant (?15%) but insufficient to decrease the pollutant levels to desired levels without further after-treatment. Accordingly, the role of swirl in attenuation of pollutant formation and promotion of combustible mixture formation was examined further. Swirl was found to alter the details of the impact of the fuel spray on the combustion chamber (piston bowl) walls. The asymmetry of the fuel spray created by swirl was found to improve the mixing processes compared to simulations without swirl. The impact of the jet with the engine walls in the presence of swirl was found to lead to a change in the circulation, ?. Tracking the terms in the vorticity transport equation indicated the change in ? was due to the baroclinic torque, which is due to the interaction between pressure and density gradients.

The convection of oxygen due to the flow motions was found to track the mixing enhancement caused optimal fuel spray – wall interactions.