Extinction Measurements of In-Cylinder Soot Deposition in a Heavy-Duty DI Diesel Engine 2001-01-1296
The combustion process in diesel engines deposits soot on the in-cylinder surfaces. Previous works have suggested that these soot deposits eventually break off during cylinder blow-down and the exhaust stroke and contribute significantly to exhaust soot emissions. In order to better understand this potential pathway to soot emissions, the authors recently investigated combusting fuel-jet/wall interactions in a diesel engine. This work, published as a companion paper, showed how soot escaped from the combusting fuel jet and was brought in close proximity to the wall so that it could become a deposit. The current study extends this earlier work with laser-extinction measurements of the soot-deposition rate in the same single-cylinder, heavy-duty DI diesel engine.
Measurements were made by passing the beam of a CW-diode laser through a window in the piston bowl rim that was in-line with one of the fuel jets. The change in transmittance of this beam with the number of fired cycles provides a measurement of the net soot deposition rate. Data were obtained for three fuel blends of varying oxygen concentration and for number 2 diesel fuel at eight different operating conditions. Initially, the net deposition rate was nearly constant; however, after the soot-layer thickness increased, the net deposition rate decreased, indicating that the soot was being removed or the deposition rate was slowing. By combining the measured initial deposition rates with the deposition area (as determined from digital images of the combustion chamber), estimates were obtained of the total initial soot deposition rate for the whole combustion chamber. These estimates were compared with steady-state exhaust soot measurements over the eight operating conditions. The results showed that as operating conditions were varied, changes in the initial deposition rate did not correlate with changes in the amount of exhaust soot, and in some cases the initial deposition rate exceeded the exhaust-soot rate. This poor correlation suggests that soot wall-deposition and blow-off is not a major contributor to the changes in engine-out soot emissions over the range of operating conditions examined. It also suggests that the net deposition rate is being reduced from its high initial value to a steady-state value by some mechanism other than by being blown-off into the exhaust gases. Possible alternative mechanisms are suggested and discussed.