Study on soot formation characteristics in the diesel combustion process based on an improved detailed soot model

Abstract

Although much research has been done on soot formation in engines, the mechanisms involved in the process are poorly understood and models of the process are overly simplistic. An improved detailed soot model is developed that couples a reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene, implemented into KIVA-3V2 code, for the numerical investigation of soot formation, mass concentration, and size distribution in diesel engines. This detailed soot model incorporates the effects of soot precursors, including isomers of acetylene and polycyclic aromatic hydrocarbons (PAHs), and the physical processes of PAH deposition on the particle surface, soot formation, and particle surface growth. Compared with experiment results in an optical engine and a single-cylinder diesel engine, the improved detailed soot model was effective: the simulated in-cylinder combustion pressure, heat release rate, and ignition timing were in excellent agreement with the experimental results. The simulated two-dimensional, transient distribution of soot concentration was in good agreement with that obtained by using the two-color method, and the simulated changing trend of soot emission was consistent with the experimental results. Therefore, the detailed soot model can be used to accurately simulate and predict soot emission at different conditions in diesel engines. Furthermore, at the initial stage of combustion, large amounts of small-size soot particles were produced by the pyrolysis reactions and polymerization of the hydrocarbon fuel, and the particle size was in the range of 5–40nm. At the middle stage of combustion, soot particles continued to grow by particle coagulation, surface growth, and the deposition of PAHs, which greatly expanded the particle size range and caused many large-size particles to be produced in the engine cylinder. At the late stage of combustion, the range of particle size distribution stabilized under the influence of further oxidation reactions, and the particle size ranged from 5 to 20nm. The details of soot-relevant quantities (e.g., particle size, number concentration, and mass) provide valuable insights into soot formation and oxidation processes in diesel engines.