Soot formation and oxidation mechanisms during diesel combustion: Analysis and modeling impacts

Abstract

Soot dynamics during diesel combustion is a complex process and consists mainly of the competing mechanisms soot formation and soot oxidation. For future crank angle–based closed loop combustion control design it is not sufficient to focus only on the soot measured with standard sensors in the tailpipe, which is similar to the soot amount in the combustion chamber at exhaust valve opening, but to understand how soot is formed and oxidized during combustion. Hence, this article focuses on analyzing soot dynamics inside the combustion chamber with an optical sensor and how the analyzed effects can be captured with crank angle–based simulation models. An optical sensor was installed on the engine to measure the actual soot concentration and to analyze it together with the results of a thermodynamic engine process calculation based on the measured cylinder pressure. Based on the measured results, two observed effects are analyzed in detail and hypotheses to explain the behavior are sketched. First, it is shown that the soot formation after start of combustion must be subdivided into three phases. The second hypothesis shows that the impact of increased swirl level is mainly present during the soot oxidation phase and that it almost has no impact during the formation phase. As a main innovation, data-based methods were used to underline the formulated hypotheses. Additionally, the crank angle–based soot model equation structure according to Hiroyasu was extended with suitable inputs to capture these effects. Locally, the extended structure is able to catch the effects well with only one set of parameters.