Parameters Influencing Soot Oxidation Rates in an Optical Diesel Engine

Soot emissions from diesel engines are the net result of two competing processes: soot formation and soot oxidation. Previous studies have shown poor correlation between soot formation rates and the soot emissions. This article presents a systematic study of a number of parameters affecting soot oxidation rate and how it correlates with the soot emissions. An optical heavy-duty engine is used in conjunction with a laser extinction setup in order to collect time resolved data of the soot concentration in the cylinder during the expansion stroke. Laser extinction is measured using a red (685 nm) laser beam, which is sent vertically through the cylinder and modulated to produce 10 pulses per crank angle degree. Information is obtained about the amount of soot formed and the soot oxidation rate. The parameters studied are the motored density at top dead center (TDC), motored temperature at TDC, injection pressure, engine speed, swirl level and injector orifice diameter. A central composite design is employed to assess the importance of the parameters as well as identifying potential interaction effects. A single exponential decay function is fit to the extinction data to describe the oxidation process and the half-life of the decay is used as a measure of the oxidation rate. The half-lives are compared with engine out emissions and the importance of each parameter is studied using regression analysis. The results suggest that the injection pressure has the strongest effect on the late-cycle soot oxidation rate, while the temperature at TDC has the weakest effect of the parameters studied.