A Computationally Efficient Approach using Detailed Chemical Kinetics in Multidimensional Simulation of HCCI and PCCI Engine Combustion

April 27, 2007 - 12:00pm

Homogeneous Charge Compression Ignition (HCCI) engines are currently of great interest as a future alternative to diesel and spark ignition engines, due to HCCI's potential to achieve high efficiency with very low NOx emissions. HCCI engines can be said to combine diesel and spark-ignition engines, using compression ignition (as in diesel engines) on a premixture of fuel and air (as in spark-ignition engines). However, significant technical barriers remain to practical implementation of HCCI engines: difficult-to-control combustion, low power density, rapid pressure rise, and high hydrocarbon and carbon monoxide emissions. To overcome some of these barriers, operational strategies that involve relaxing the constraint of truly "homogeneous" HCCI combustion are being studied. The phrase "Premixed Charge Compression Ignition" or "PCCI" combustion can be used to describe this class of combustion processes, in which combustion occurs similarly to HCCI engines as a non-mixing controlled, chemical kinetics dominated, autoignition process, but the fuel, air, and residual gas mixture need not be homogeneous. Combustion in HCCI and PCCI engines occurs as a bulk autoignition process. Simulating hydrocarbon fuel autoignition typically requires using detailed chemical kinetic mechanisms, which can contain hundreds of species and thousands of reactions and present very stiff systems of differential equations. Because of the computational intensity of autoignition simulation, solving detailed autoignition chemistry for complex hydrocarbon fuels in every cell of a computational fluid dynamics simulation is beyond the capability of today's computers. To overcome this problem, we have developed a novel "multi-scale" approach that allows for solving the detailed chemistry in a computationally efficient and accurate way within the CFD simulation. The approach involves solving CFD with a high-resolution discretization, while simultaneously solving chemistry in a transformed coordinate system that has a lower resolution discretization. We apply our simulation approach to the analysis of HCCI engines, as well as investigating the effect of charge non-uniformity upon combustion in PCCI engines. Using HCCI experiments with isooctane fuel as a baseline, we first conduct simulations to investigate truly homogeneous engine operation. Further simulations have been conducted to assess the effect of charge non-uniformity on the heat release and emissions for PCCI operation. The results show that charge non-uniformity used in the PCCI strategy may sometimes have a beneficial effect of reducing carbon monoxide and hydrocarbon emissions. However, charge non-uniformity within the combustion chamber has the potential to increase NOx emissions. For more information about this seminar, please contact: Robert Cheng(510) 486-5438

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