Physical and Chemical Characterization of Gasoline and Diesel Engine Carbonaceous Particulate

Randy  Vander Wal, Penn State University

We report observations of nanoscale microstructural changes in soot from an experimental light-duty diesel engine, produced with varying levels of biodiesel fuel blending. Based on these observations and current information in the literature, we propose a mechanistic hypothesis to explain the effects of biodiesel blending. Our underlying assumption is that particulate nanostructure is closely coupled to the local chemistry at the time the soot is formed. In the context of in-cylinder soot formation, this implies that changes in nanostructure may aid in diagnosing important changes in fuel-air mixing.

It is hypothesized here that fuel pyrolysis chemistry is reflected in the soot nanostructure and that the oxygenated fuel brings about secondary mixing effects that change the path of fuel decomposition [1]. Models [2] and experiments [3] have shown that 5-membered rings are the essential component to impart high curvature to lamella, as for example in the fullerene family of carbon allotropes [4]. Five-membered rings in soot nanostructure are readily traceable to the gas phase chemistry [5, 6, 7]. Chemistry and mixing are thus intimately linked. With increasing biodiesel content, fuel decomposition changes from thermal pyrolysis to oxidation assisted pyrolysis (partial oxidation). Odd numbered carbon species are produced and, in particular, paths are opened towards C5 production [1, 8]. The incorporation of cyclopentadiene and related C5 containing species into carbon lamella account for curvature in lamella as measured by tortuosity in the soot nanostructure.

Therein the levels of soot nanostructural tortuosity and fullerenic structure observed in this study are roughly correlated to biodiesel blend level. This implies that changes in nanostructure may aid in diagnosing important changes in fuel-air mixing associated with biodiesel fueling. Our observations also appear consistent with the established effects of biodiesel on key diesel combustion characteristics such as ignition time delay, lift-off, and flame length. We hypothesize that a primary chemical factor involved in the formation of the observed nanostructural features is the relative amount of C5 precursors present during soot formation [9].

Many diesel combustion strategies have focused upon methods that enhance partial premixing to realize increases in power while minimizing emissions by requiring less fuel and/or lowering temperature. Complimentary to these efforts are studies using oxygenated fuels or additives, which seek to modify fuel chemistry to reduce emissions. Observations here indicate that fuel chemistry might indeed be combined synergistically with in-cylinder fluid mixing to achieve better pre-mixing and reduced emissions. More specifically, fuel oxygenation might be used to promote soot with a more reactive nanostructure. We expect that more reactive soot will tend to be more completely consumed downstream, as long as the soot surface is sufficiently accessible to oxidizer penetration.

Keywords: Soot, Biodiesel, Nanostructure, HRTEM, In-cylinder mixing
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This work was funded by DOE OVT under contract with Oak Ridge National Laboratory. The authors acknowledge the support of DOE Sponsors Gurpreet Singh, Ken Howden and Kevin Stork.

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