Impact of the Injection Strategy on Soot Reactivity and Particle Properties of a GDI Engine

The gradual global tightening of emission legislation for particulate matter emissions requires the development of new gasoline engine exhaust aftertreatment systems. For this reason, the development of gasoline direct injection engines aims at the reduction of particulate emissions by application of a Gasoline Particulate Filter (GPF). The regeneration temperature of GPF depend on soot reactivity towards oxidation and therefore on particle properties. In this study, the soot reactivity is correlated with nanostructural characteristics of primary gasoline particles as a function of specific engine injection parameters. The investigations on particle emissions were carried out on a turbocharged 4-cylinder GDI-engine that allows the variation of injection parameters. The emitted engine soot particles have been in-situ characterized towards their number and size distribution using an engine exhaust particle sizer (EEPS). Ex-situ analytics focuses on the analysis of oxidation kinetics and the nanostructural characteristics affecting soot reactivity significantly. The oxidation kinetics were determined by temperature programmed oxidation (TPO) employing thermogravimetric analysis (TGA). The temperature at the maximum of the reaction rate is referred to T_max, where low temperatures are linked to high reactivity. In addition, the nanostructure analysis of primary particles (graphene layer length, tortuosity and separation distance) was also investigated by using high resolution transmission microscopy (HRTEM) and an image analysis algorithm. Findings from this work show that soot reactivity relies significantly on the quality of mixture formation of a GDI engine depending on injection strategy. These findings are in very good agreement with the nanostructural parameters obtained by high-resolution transmission electron microscopy. Soot particles formed in a relatively homogeneous air/fuel mixture due to optimized injection parameters show a high soot reactivity (T_max ≈ 525°C) and an amorphous carbon nanostructure. Such primary particles are characterized by short mean graphene layer length (L_mean ≈ 0.5 nm) and an unordered configuration resulting in an increased separation distance (D_mean ≈ 1 nm). In addition, approximately a comparable number of small (D_p ≈ 5...25 nm) and large particles (D_p ≈ 30...100 nm) are generated. Conversely, inhomogeneous mixtures result in increased formation of primary particles showing long, extended and more ordered graphene (L_mean ≈ 0.55 nm, D_mean = 0.4 nm), which in turn lead to low soot reactivity (T_max ≈ 615°C).