New Modeling Approaches Using Detailed Kinetics for Advanced Engines

Ignition timing and control, as well as predictability of engine emissions, are critical factors in advanced-engine system design. The use of detailed chemical kinetics is key to simulating ignition performance and to predicting emissions. This paper describes a collaborative and systematic effort that is underway to enable computationally efficient use of accurate kinetics in engine simulation. The collaboration is focused on building a database of chemical information and on developing a complementary set of software tools that provide efficient engine-simulation capabilities. The goal is predictive simulations that capture the detailed behavior of complex fuels, such as gasoline and diesel, under homogeneous charge compression ignition (HCCI) and related operating conditions. Since directly accounting for all of the hundreds of constituent molecules in a fuel during simulation of real-fuel combustion is intractable, we employ a “model-fuel” or surrogate-fuel approach instead. Mixtures of model-fuel molecules can be determined to adequately represent important real-fuel properties and engine-combustion characteristics. In this work, model fuel compositions are determined by matching a mixture behavior to that of the real fuel, focusing on distillation-curve characteristics, net-heating value, hydrogen-to-carbon ratio, and octane/cetane numbers. ... mehrThis surrogate-definition process requires detailed chemical-kinetics mechanisms for a variety of model-fuel compounds. To build such a database of model-fuel component mechanisms, we have used a combination of automatic mechanism-generation and manual mechanism-development approaches. These methods adhere to a systematic set of class-based rules in determining elementary reaction rates, as well as thermodynamic and transport properties of species. In addition, a comprehensive validation study of the mechanisms, using a wide variety of both fundamental and engine experiments, has allowed refinement of these rules and improvement of both the mechanisms’ predictions and their consistency across components. Even though model fuels have a small number of components, their detailed mechanisms contain large numbers of species (>1000) and reactions (>10000). Systematic mechanism reduction is therefore required for many engineering applications. To this end, we have also developed a package of automated mechanism-reduction techniques. In addition, we have advanced the solution algorithms used in the kinetics simulations and developed a multi-zone engine model that provides good predictions of ignition behavior and emissions. We report on selected results of this systematic approach to using detailed kinetics in engineering simulation, as well as the challenges encountered.