Applying Detailed Kinetics to Realistic Engine Simulation: the Surrogate Blend Optimizer and Mechanism Reduction Strategies

Designing advanced, clean and fuel-efficient engines requires detailed understanding of fuel chemistry. While knowledge of fuel combustion chemistry has grown rapidly in recent years, the representation of conventional fossil fuels in full detail is still intractable. A popular approach is to use a model-fuel or surrogate blend that can mimic various characteristics of a conventional fuel. Despite the use of surrogate blends, there remains a gap between detailed chemistry and its utilization in computational fluid dynamics (CFD), due to the prohibitive computational cost of using thousands of chemical species in large numbers of computational cells. This work presents a set of software tools that help to enable the use of detailed chemistry in representing conventional fuels in CFD simulation. The software tools include the Surrogate Blend Optimizer and a suite of automated mechanism reduction strategies.

We start with a detailed reaction mechanism that contains chemistry for over 26 fuel components (over 3800 species and 15000 reactions) including surrogate components suitable for modeling everything from natural gas to gasoline and diesel, including ethanol. The mechanism is capable of predicting NO_x emissions and soot precursors, and has been validated using fundamental experimental data available in the literature. Using the components in the master mechanism, an optimum blend is generated automatically that can capture the specified physical, combustion, and emission characteristics of conventional fuels. Selected targets can be used to match the specific behavior of real fuels. These targets can include octane number, cetane number, and heating value for combustion characteristics; hydrocarbon class of components and true boiling point curve for physical characteristics; and molar H/C ratio for soot emission characteristics. The master mechanism can then be reduced using the guided mechanism reduction tools based on any reactor model available in the CHEMKIN-PRO software suite, such as the multi-zone IC engine model and flame simulators. Several methods for mechanism reduction, including skeletalization and more severe reduction techniques, have been implemented in a software package that works in conjunction with the CHEMKIN-PRO software. The master mechanism can be reduced for the surrogate blend so that it can reproduce selected targets, such as ignition times, laminar flame speeds, fuel and emission concentration profiles, or any other property from the available reactor model, within a specified level of accuracy requested for the reduced mechanism. Strategies for combining different methods in automated reduction are suggested that result in reduced mechanisms for both gasoline and diesel surrogates, which are being used directly in engine CFD simulations employing fast chemistry solvers.