Develop. Validate. Apply.
Chemical kinetic mechanisms are needed to represent conventional and next-generation fuels in practical combustion devices like internal combustion engines. Chemical kinetic mechanisms (or models) are developed using ab. initio calculations and fundamental measurements of species-thermodynamic properties and reaction rates (Fig. 1). Mechanism development also requires knowledge of the various reaction paths for fuel pyrolysis and oxidation. This can also be acquired using ab initio calculations and fundamental experiments. Since chemical kinetic models often contain a large number of reactions which need to be assigned rate constants, reactions are usually assigned to reaction classes which have associated reaction-rate rules1. The reaction rules are derived from analysis of relevant ab initio calculations and fundamental measurements of reaction rate constants. Next, the various species and reactions with specified rate constants are assembled into chemical kinetic models. These models are then validated using experimental combustion data from shock tubes, rapid compression machines, laminar flames, jet stirred reactors, flow reactors, and other fundamental and well-characterized combustion devices. Finally, the models are reduced in number of species and reactions to be used in multidimensional simulation codes for application to practical devices like internal combustion engines. With the advent of highly efficient LLNL solvers2, this last step for mechanism reduction for use in simulation codes may no longer be needed.
Practical fuels like gasoline, diesel, and their mixtures with biofuels contain hundreds of fuel components. It is not practicable to simulate the oxidation of all these components. Therefore, it is an attractive approach to introduce a surrogate fuel with a limited number of components to represent the practical fuel. In the case of diesel fuel, a 9-component diesel surrogate has been recently proposed3 (Fig. 2). This surrogate palette is able to reproduce four key properties of FACE (Fuels for Advanced Combustion Engines) diesel fuels4 including Cetane number, distillation curve, density, and compositional characteristics. The LLNL combustion chemistry team is working towards developing chemical kinetic models for all of these 9 components. Once these are developed, these component models can be combined into the 9-component diesel surrogate model. Then, the diesel surrogate model can be reduced or used with LLNL fast solvers in engine simulation codes to predict engine performance.
The LLNL combustion chemistry team is also developing surrogate models for gasoline5-7. A 12-component gasoline surrogate palette has been proposed by the LLNL combustion chemistry team to match the ignition and characteristics of FACE gasoline fuels8 (Fig. 3). The team is in the process of developing chemical kinetic models for all these 12 components. Once these component models have been assembled, they can be combined to make the proposed gasoline surrogate model. This can be used to simulate autoignition and flame propagation under spark ignition and advanced engine combustion conditions such as direct injection spark ignition (DISI) and HCCI engines.
- H. J. Curran; P. Gaffuri; W. J. Pitz; C. K. Westbrook, Combustion and Flame 1998, 114, (1-2) 149-177.
- M. J. McNenly; R. A. Whitesides; D. L. Flowers, Proceedings of the Combustion Institute 2015, (0).
- C. J. Mueller; W. J. Cannella; T. J. Bruno; B. Bunting; H. D. Dettman; J. A. Franz; M. L. Huber; M. Natarajan; W. J. Pitz; M. A. Ratcliff; K. Wright, Energy & Fuels 2012, 26, (6) 3284–3303.
- FACE diesel fuels.
- M. Mehl; J. Y. Chen; W. J. Pitz; S. M. Sarathy; C. K. Westbrook, Energy & Fuels 2011, 25, (11) 5215-5223.
- S. M. Sarathy; G. Kukkadapu; M. Mehl; W. Wang; T. Javed; S. Park; M. A. Oehlschlaeger; A. Farooq; W. J. Pitz; C.-J. Sung, Proceedings of the Combustion Institute 2015, (0).
- G. Kukkadapu; K. Kumar; C.-J. Sung; M. Mehl; W. J. Pitz, Proceedings of the Combustion Institute 2013, 34, (1) 345-352.
- FACE gasoline fuels.
- FACE CRC Fuels for Advanced Combustion Engines Working Group (FACE).