Development of Combustion and Soot Emission Models for Direct-Injection Spark-Ignition Engines

Abstract

Energy issue of fossil fuel depletion and environmental issue of global warming have been the powerful spur to develop a more efficient engine. Direct-injection spark-ignition (DISI) engine combined with turbocharging technology, renowned as one of the most pursued solutions for next-generation powertrain, is capable of increasing thermal efficiency by their abilities to mitigate knock and to reduce pumping loss. Despite these merits, the direct injection deteriorates the homogeneity degree of the air-fuel mixture and induces the fuel film deposition on the wall. Consequently, the unfavorable particulate matter emission increases in significant level compared to the conventional port fuel injection engine. In the light of environmental and public health concerns, the EU imposed a regulatory limit on the particulate number (PN) of 6.E+12/km as of September of 2014, and the target will be tightened to 6.E+11/km in 2017. This goal is challenging to meet without an after-treatment system. Thus, a substantial optimization of the combustion chamber and operating strategies should be conducted systematically. Though the fundamentals of soot formation are same for both Diesel and DISI engines, the air-fuel mixture preparation and the combustion processes are different. In Diesel engine, the turbulent diffusion combustion after the fuel injection is proceeded, and the soot is mainly formed in the core of spray plume. By contrast, in DISI engines, the locally fuel-rich mixture is formed due to the fuel film deposition and the short mixing time, and the soot is produced in the behind of the turbulent propagating flame near wall region. Therefore, it is required to develop the numerical models relevant to DISI engine application. The aim of this study is to develop combustion and soot emission models for DISI engines, and it comprises three major modeling concerns. Firstly, to improve the prediction accuracy of air-fuel mixture field inside the cylinder, a six-component surrogate fuel that covers the wide range of boiling point, as well as the same aromatic content of real gasoline, was developed and validated with the gasoline analysis data. In addition, the Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) model for spray break-up was calibrated against the droplet size distribution and penetration length data obtained from a set of rig-experiments. Secondly, the partially-premixed turbulent combustion in DISI engines was modeled by the G-equation. A new correlation for the laminar burning velocity of gasoline fuel was developed with an emphasis on the prediction improvement of burning velocity in the fuel-rich mixture. In regard to the effect of aromatic hydrocarbons on burning velocity for fuel-rich branch, the laminar burning velocities of three hydrocarbons, iso-octane, n-heptane, and toluene, were calculated by PREMIX code in conjunction with detailed mechanism developed in LLNL, and were blended by the energy fraction based mixing rule to derive the laminar burning velocity of gasoline. Thirdly, a detailed soot modeling framework including the gas-phase polycyclic aromatic hydrocarbons (PAHs) formation as well as the solid-phase soot aerosol dynamics were proposed with the level-set flamelet library approach and two soot models. To determine the chemical composition behind of flame front including the PAHs concentrations, a detailed chemical mechanism which contains the reaction pathway of PAHs up to coronene (C24H12) was employed to calculate the laminar premixed flamelet equation under wide thermos-chemical conditions. From the full solution of flamelet equation, five representative PAHs were adopted for the soot precursors. For the soot evolution, a semi-empirical soot model was developed, in which the soot nucleation was described as PAHs dimerization. Furthermore, the method of moment interpolative closure (MOMIC) was also coupled to the flamelet library to explore the state-of-the-art predictability. The developed models were validated under three sets of engine experiments with various operating conditions. Firstly, the soot emission similarity between the surrogate fuel and real gasoline was verified by conducting the PFI engine experiment with the variation of equivalence ratio at hot operating condition. Secondly, the preliminary evaluation for the sub-models was carried out by comparing the combustion and soot emission results with that measured from the DISI engine experiment under a catalyst heating condition. Finally, the developed models were validated against the DISI engine experiment by varying the injection strategies under cold steady-state operating condition. Based on the modeling and validation work, the combustion and soot emission models developed in this study can be actively used for the engine development and optimization process in the future.