Electron microscopic characterization of soot particulate matter emitted by modern direct injection gasoline engines

Modern direct injection gasoline engines (GDI) generate considerably higher soot particulate matter (PM) emissions than conventional, port fuel gasoline, as well as diesel cars with particle filter. Soot PM generated by a typical state-of-the-art GDI (Euro VI) vehicle during two standard international driving cycles (NEDC, WLTC), as well as during a short experimental sub-cycle was investigated by electron microscopy. The study reveals primary particles between ∼4–55 nm in diameter, the majority being < 20 nm during all driven cycles; sub-20 nm particles are more abundant in the more dynamic WLTC. Monodisperse agglomerates made of small (<20 nm), medium (∼25 nm) and large (∼35 nm) primary particles are distinguished; they usually form parts of polydisperse agglomerates but also occur as distinct entities. The particle groups of different sizes are probably derived under diverse operating conditions, the larger ones being likely associated with the cold start phase. This inference is supported by simultaneously ran FMPS measurements, which reveal in addition that the cold start phase seems to account for ∼25% of the particle agglomerate emissions of the entire cycle. All particles, especially the medium and large fractions, exhibit low degrees of crystallinity indicating relatively high reactivity. The predominance of sub-20 nm primary particles render GDI soot PM potentially hazardous to human health, especially considering the higher surface and surface/volume ratio compared to larger particles of the same total volume. The engine operating parameters seem to be of prime importance for the resulting morphological features of soot.