Particle Concentrations in a Diesel Cylinder: Comparison of Theory and Experiment

Soot formation and oxidation within the cylinder of a divided-chamber diesel engine have been studied experimentally and predicted analytically using a diesel combustion model. Experimental measurements of in-cylinder particle concentration were made using a unique sampling system which samples and quenches nearly the entire contents of the cylinder on a time scale of less than 1 ms. The experimental measurements are compared with predictions made using a stochastic combustion model coupled to an Arrhenius-type soot formation model, and 0₂ and OH soot oxidation models. Five engine conditions: low-load standard-timing (base case), high-load standard-timing, low-load advanced-timing, low-load standard-timing + EGR, and low-load standard-timing + 0₂, were examined experimentally, but only the first three were modeled. Particle mass first appears 1-5 CAD after the start of combustion, reaches a peak 10-20 CAD after the start of combustion, and then decreases toward exhaust levels during the next 30-40 CAD. Peak soot mass in the cylinder is 6-15 times exhaust soot mass. Model parameters could be adjusted to give a reasonable match to the in-cylinder and exhaust soot measurements for the base case. When this was done, model predictions for the other two cases did not agree well with experiment. Possible reasons for this are discussed. Both the experimental data and the model predictions indicate that typically more than 90% of the soot initially formed is subsequently oxidized. Thus exhaust soot production results from a relatively small difference between fast formation and oxidation processes. From a modeling standpoint, this implies that sub-models for combustion, soot formation, and soot oxidation must be considerably more precise than heretofore thought necessary to enable quantitative predictions of exhaust soot concentration.