Abstract
Due to worldwide increasingly sharpened emission regulations, the development of Gasoline Direct Injection and Diesel Direct Injection engines not only aims at the reduction of the emission of nitrogen oxides but also at the reduction of particulate emissions. Regarding present regulations, both tasks can be achieved solely with the help of exhaust after treatment systems. For the reduction of the emission of particulates, Gasoline (GPF) and diesel Particulate Filters (DPF) offer a solution and their implementation is intensely promoted. Under optimal conditions particulates retained on particulate filters are continuously oxidized with the exhaust residual oxygen so that the particulate filter (PF) is regenerated possibly without any additional intervention into the engine operating parameters. The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at exhaust gas temperatures. The reaction rates of soot particles from internal combustion engines (ICE) often are discussed in terms of order/disorder on the particle nanoscale, the concentration and kind of functional groups on the particle surfaces, and the content of (mostly polycyclic aromatic) hydrocarbons in the soot. In this work the reactivity of different kinds of soot (soot from flames, soot from ICE, carbon black) under oxidation conditions representative for PF regeneration is investigated. Soot reactivity is determined in dynamic Temperature Programmed Oxidation (TPO) experiments and the soot primary particle morphology and nanostructure is investigated by High-Resolution Transmission Electron Microscopy (HRTEM). An image analysis method based on known methods from the literature and improving some infirmities is used to evaluate morphology and nanostructural characteristics. From this, primary particle size distributions, length and separation distance distributions as well as tortuosities of fringes within the primary particle structures are obtained. Further, UV–visible spectroscopy and Raman scattering and other diagnostic techniques are used to study the properties connected to the reactivity of soot and to corroborate the experimental findings. It is found that nanostructural characteristics predominantly affect reactivity. Oxidation rates are derived from TPO and interpreted on a molecular basis from quantum chemistry calculations revealing a replication/activation oxidation mechanism.
Highlights
Due to the increasingly stringent emission regulations (The European Parliament and the Council of the European Union 2007), the development of Gasoline (GDI) and Diesel Direct Injection (DDI) engines aims at the reduction of particulate matter (PM) emissions by application of Gasoline (GPF) or Diesel Particulate Filters (DPF)
Particulate filters (PF) trap soot particles present in the exhaust gases resulting in minimized PM emission
The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at engine exhaust gas temperatures (573 K to approximately 1073 K)
Summary
Due to the increasingly stringent emission regulations (The European Parliament and the Council of the European Union 2007), the development of Gasoline (GDI) and Diesel Direct Injection (DDI) engines aims at the reduction of particulate matter (PM) emissions by application of Gasoline (GPF) or Diesel Particulate Filters (DPF). Particulate filters (PF) trap soot particles present in the exhaust gases resulting in minimized PM emission. To allow for a continuous operation of PF, the captured soot is removed periodically by a regeneration procedure or continuously by oxidation with residual O2 in the exhaust gas (Fang and Lance 2004). The reaction rates of soot against oxidation by O2 determine the frequency and efficiency of this kind of PF-regeneration (Fang and Lance 2004; Bhardwaj et al 2014). The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at engine exhaust gas temperatures (573 K to approximately 1073 K). Time scales of reactions of soot in oxidation determine the reactivity of soot and the reactivity of soot may be expressed through the reciprocal over-all rate coefficient of the oxidation reaction
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