On-line monitoring of carbon nanostructure and soot reactivity in engine exhaust by dual-pulse laser-induced incandescence

Abstract

In previous work, we introduced the dual-pulse two-color time-resolved laser-induced incandescence (DP-2C-TiRe-LII), which is possibly suitable for on-line monitoring of the nanostructural topology of inhomogeneous soot particle ensembles in transient flows. The method relies on the quantitative relationship between the size of the basic structural units (BSUs) embedded within primary particles and the ratio of the refractive-index function for absorption at two wavelengths E(m,λUV)/E(m,λNIR). As the size of BSUs increases, the band gap energy for an electronic π→π* transition becomes narrower increasing the wavelength for absorption and, hence, forcing E(m,λUV)/E(m,λNIR) to decrease. Once quantitative correlations between the carbon nanostructure, i.e., size of the BSUs, and a structure-associated particle property are available, its on-line monitoring becomes accessible through monitoring of E(m,λUV)/E(m,λNIR). The objective of this paper is to provide the proof-of-concept for on-line monitoring of an exemplary structure-associated particle property, viz. soot reactivity against oxidation, in an application from practice. For this, DP-2C-TiRe-LII is employed downstream of a gasoline direct injection (GDI) engine running under steady-state and transient operating conditions. Intrusive particle sampling in combination with an analytical toolbox comprising high-resolution transmission electron microscopy (HRTEM), pattern recognition methods, elemental analysis (EA), and thermogravimetric analysis (TGA) was applied to validate the results from DP-2C-TiRe-LII. Excellent agreement is found between particle properties derived via ex situ and in situ diagnostics. Moreover, the transformation of the time series of E(m,λUV)/E(m,λNIR) into probability density distributions seems to represent the BSU size distributions of the investigated soot particle ensembles. Likewise, soot reactivity is reflected in the distributions of E(m,λUV)/E(m,λNIR).