A Multi Zone Spray and Combustion Model for Formation of Polycyclic Aromatic Hydrocarbons and Soot in Diesel Engines

Abstract

Considering the certain impact of exposing to soot particles in increasing the risk of lung cancer and mortality in human being and the inevitable role of Polycyclic Aromatic Hydrocarbons (PAHs) on these carcinogenic effects, formation of selected PAHs during diesel cycle combustion is investigated. To accomplish this, a detailed chemical mechanism is embedded with a multi zone diesel cycle model. In development of the multi zone diesel cycle model, emphasis is given to including most of the processes taking place in the cylinder during the closed cycle and to describing as many as possible of these processes by correct modelling of physical laws. Diesel fuel combustion chemistry is modelled under the light of a detailed mechanism consisting of 1696 reactions, forward and backward counted separately, and 155 species. The mechanism is capable of modelling combustion of hydrocarbons up to C₁₀ and considers most recent developments in gas-phase reactions and aromatic chemistry. It includes sub-mechanisms for normal (n-decane and n-heptane), ramified (iso-octane) and aromatic (benzene and toluene) species to correctly model the combustion feature of engine relevant fuels. Formation of selected PAH species (Phenanthrene and Pyrene) in a diesel engine and how they are affected with change in engine speed and load are studied. Both these PAHs are cancerogenic themselves. Meanwhile many studies have shown that soot particle nuclei, smallest visible solid particles, are dimers which form through collision of Pyrene molecules. Hence, correct prediction of Pyrene concentration will be a large step toward correctly solving the first mode of bimodal soot Number Density Function (NDF), which can be regarded as one of the engines sooting characteristics. Despite lower computational costs of the new model comparing to multidimensional simulations, the predicted data are in good agreement with the results obtained experimentally. This proves that soot nucleation is a chemically controlled process, therefore bearing in the mind that coupling detailed reaction schemes with multidimensional simulations is not feasible due to very high computational cost, multi zone methodology could serve as a powerful tool.

F2012-A06-037