April - June 2000

New Simulation Tool for Designing Cleaner Diesels
[Abstract]

Inside Engine Cylinders: Cleaner Walls
for Lower Emissions, Higher Efficiency
[Abstract]

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New Simulation Tool for Designing Cleaner Diesels

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 numerical analysis, the simulation determines which species and reactions are important in a region and which ones it can leave out. (For example, why examine reactions involving fuel molecules in areas where no fuel remains?) This approach simplifies the computational task without sacrificing accuracy. While much work remains, the new simulation may one day reveal ways to fine-tune a variety of combustion devices and their fuels for cleaner operation, perhaps without major financial investment.

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Inside Engine Cylinders: Cleaner Walls
for Lower Emissions, Higher Efficiency

hen fuel and air burn inside an internal combustion engine, deposits form on the walls inside the cylinders--a real headache for engine designers. The coated walls trap heat inside the cylinder, and the increased temperatures that result affect emissions and impede changes that could increase fuel efficiency. A new Energy Laboratory model may help designers bring about changes in fuels or engines that will discourage deposit formation. The numerical model simulates the chemical reactions that produce deposit "precursors"; the processes that carry those precursors to the wall; and the condensation, evaporation, and chemical interactions that occur at the wall. Using models of precursor-forming reactions designed to bracket real-world engine and fuel conditions, the simulation generates results that are consistent with deposit growth observed experimentally. The results also suggest that the only way to stop precursors from landing on the wall is by preventing their condensation. Raising the wall temperature would prevent condensation but would defeat the overall goal of keeping cylinder temperatures down. The MIT student performing the research is now gathering precise data on precursors and their behavior by using the model plus measurements taken in a specially designed low-pressure flame. With sufficient data, the model may help designers identify practical strategies for keeping the precursors from forming in the first place.

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