There is a growing body of evidence that in-cylinder surfaces play an important role in determining the nature and quantity of soot emitted by diesel engines. This paper describes recent experimental results which demonstrate the importance of both the deposition of soot on walls during the combustion process and its subsequent reentrainment during exhaust blowdown. Soot deposition was demonstrated both experimentally and theoretically. The principal mechanism of soot deposition during combustion is thermophoresis. Our results suggest that the gross rate of in-cylinder deposition in the indirect injection diesel engine is between 20 and 45 percent of the net soot emission rate. Thus, a significant fraction of the soot emitted may have been stored on combustion chamber surfaces and protected from oxidation. Further evidence of wall deposition and subsequent reentrainment has been obtained by making time-resolved measurements of soot concentrations in the exhaust. These measurements show higher particle concentrations during blowdown than during the remainder of the exhaust stroke.
Our observations are consistent with the hypothesis that a significant quantity of soot is stored on combustion chamber walls and released during the violent expansion which takes place during exhaust blowdown. If this hypothesis is correct, it has important implications in the control of soot emissions from diesel engines. It suggests that a soot control strategy based on enhancement of soot oxidation during the expansion stroke will be ineffective unless care is taken to minimize wall deposition and reentrainment. Furthermore, it suggests that the influence of surface interactions on soot emissions should be carefully investigated.