Experimental and theoretical study of particulate re-entrainment from the combustion chamber walls of a diesel engine

The purpose of this research was to investigate the influence of the in-cylinder surfaces on the net emission of the particulate matter in the exhaust of a single cylinder, diesel engine. In order to obtain this information, time-resolved sampling was done to characterize the particulate matter emitted in the engine exhaust. A rotating probe sampled the free exhaust plume once each engine cycle. The rotation of the probe was synchronized with the engine cycle in such a way that the samples could be taken at any predetermined crank angle degree window. The sampling probe was designed for isokinetic sampling in order to obtain reliable results. To characterize the exhaust particulate in real time, a filter for mass concentration measurements was used. The results showed about 45 per cent higher mass concentrations as well as particles of larger diameter emitted during blowdown than late in the displacement phase of the exhaust stroke. This suggests that high in-cylinder shear rates and velocities which are associated with the blowdown process, cause the deposited soot to be re-entrained from the surfaces of the combustion chamber, where re-entrainment is favoured by conditions of high surface shear. A mathematical model to predict the amount of soot re-entrained from the cylinder walls is presented. This model is based on information presented in the literature along with the results of the time-resolved measurements of mass concentration. This model supported the hypothesis of soot deposition during the combustion process, with subsequent re-entrainment during the blowdown process of the exhaust stroke.