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he fuel-efficient diesel engine may face a bleak future unless engineers can dramatically reduce its particulate emissions to meet likely future regulations. Low-cost solutions may emerge if engineers can rethink engine and fuels technologies in tandem, tailoring the fuel properties to the engine design and vice versa. To help them achieve that match, Energy Laboratory researchers are formulating a simulation tool that will predict the effects of changes in both engine design and fuel composition on emissions and efficiency. The simulation generates equations that describe chemical reactions occurring throughout the combustion chamber and links them to reflect the interdependency of chemical composition, flows, and temperatures in adjacent regions. Representing all the molecular species in a combustor would require thousands of chemical models, so the simulation instead deals with “functional groups”—groups of atoms that always act as a unit and are building blocks for many types of molecules. Most important, the simulation uses “adaptive chemistry,” a novel concept that involves using the simplest possible chemical model to analyze a given region. Based on T