This article highlights animated three-dimensional computer models that show how to control the biggest pollutants coming from diesel engines. Increasingly strict regulations have led engineers to search for new ways of controlling the polluting emissions from diesel engines without compromising fuel economy. The past experiments tended to yield the same disappointing results-nitrogen oxide emissions increased if soot emissions were reduced and vice versa. Using animated three-dimensional computer models, researchers at the University of Wisconsin's Engine Research Center (ERC) in Madison found that multiple high-pressure injections enabled soot and nitrogen oxide emissions to be reduced at the same time. The discovery of the new multiple-injection method is significant for diesel-engine design. The combustion models developed at the ERC are now being used at major engine and automotive companies. Because engineers now know how to reduce nitrogen oxide and soot simultaneously, they can apply this mechanism to improve injection-scheme designs.
Increasingly strict regulations have led engineers to search for new ways of controlling the polluting emissions from diesel engines without com-promising fuel economy. Past experiments tended to yield the same disappointing results- nitrogen oxide emissions increased if soot emissions were reduced and vice versa. Using animated three-dimensional computer models, researchers at the University of Wisconsin's Engine Research Center (ERC) in Madison found that multiple high-pressure injections enabled soot and nitrogen oxide emissions to be reduced at the same time.
The diesel combustion process is complex and difficult to visualize because a variety of factors affect it, including turbulence, spray characteristics, and air/fuel mixing. For engineers at the ERC, a good model was needed to make accurate predictions of engine combustion and emissions.
Researchers have been trying to create an integrated spray combustion model, based on the public-domain hydrodynamic KIVA codes developed at Los Alamos National Laboratory in Los Alamos, N.M., for more than five years. The ERC model comprised many submodels to simulate the physical and chemical phenomena present in diesel-spray combustion, among them gas turbulence, wall h eat transfer, spray atomization and evaporation, fuel autoignition, and mixing- controlled high-temperature combustion. Models for nitrogen oxide, soot formation, and oxidization were also used.
Seeing the Results
While ERC researchers had to have a good model to work with, they also needed a way to visualize their test results quickly and accurately. Traditional engineering postprocessing software h as critical limitations: Results are displayed in two dimensions, and it is extremely time- consuming to generate and work with visualizations. To view the transient process of combust ion, engineers needed a 3-D postprocessor with strong animation capabilities. The program chosen was the EnSight postprocessor from Computational Engineering International in Research Triangle Park, N. C. This software represented the composite results in three dimensions from many different views at one time and cut the postprocessing time by a factor of four.
The ERC researchers simulated a single-cylinder version of the Caterpillar 3406 heavy-duty truck engine for the experiment. The engine has a bore (diameter) of 137.6 n1.illimeters, a stroke of 161.6 millimeters, and a compression ratio of 15.0. It operates at 1,600 rpm, and the intake manifold is pressurize d to 1.8 atmospheres to simulate a turbocharged engine. The combustion chamber has a Mexican-hat-type bowl in the piston and a flat cylinder head. Six spray plumes are distributed evenly in th e chamber. To save computing time and storage, one- sixth of the chamber, containing one spray plume, was used as the computational domain. A wide range of operating conditions were simulated, including high speed, low speed, and idle.
EnSight's 3-D animations of the mechanism provided a window into the solution to the problem. When single- injection combustion is used, the injected fuel penetrates into the fuel- rich, relatively low-temperature region at the tip of the spray plume and continuously replenishes it. The combination of low temperature and rich fuel is the perfect environment for producing soot. The EnSight animations showed that multiple injections with delayed injection timings reduce both soot and nitrogen oxide. The reason is relatively simple: The soot-producing rich regions at the spray tip are not replenished when the injection is stopped and restarted. The subsequently injected fuel is rapidly consumed by combustion before a new rich soot- producing region can accumulate. In addition, the soot cloud of the first spray plume is not replenished with fresh fuel but instead continues to oxidize.
Split injections enable the injected fuel to burn rapidly and do not leave time to generate a significant amount of soot. The soot production with multiple injections is most greatly reduced if the pause between injection pulses is optimized. The pause should be long enough that the soot-formation region of the first injection is not replenished with fresh fuel, but short enough that the gas temperature in the cylinder during the second injection remains high enough to prompt fast combustion.
A considerable reduction in particulate is found when a longer pause is used between the injections. Even larger particulate reductions have been achieved by injecting the fuel in three or even four pulses with appropriate pauses between each pulse.
The EnSight animations enabled ER e researchers to experiment with a wide range of possible factors and clearly see the results of different scenarios. They were therefore able to determine the optimal solution for each injection scheme.
Regarding nitrogen oxide emissions, exhaust-gas recirculation (EGR) is known to be effective at reducing nitrogen oxide, but at high loads there is usually a corresponding increase in particulate. Since multiple injections are effective at reducing particulate, a promising concept would be to combine EGR and multiple injections to control both nitrogen oxide and particulate emissions.
Recent experimental-engine test results obtained at the ERC have demonstrated the utility of this concept. For example, nitrogen oxide emissions have been reduced from 5.0 grams per brake horsepower-hour, for the baseline single-injection, zero-EGR engine, to as low as 1.5 grams per brake horsepower- hour, while particulate levels remain below 0.1 grams per brake horsepower-hour. This was achieved using quadruple injections together with 10- percent cooled EGR. Such extremely low emissions levels are in the range of future federally mandated ultralow-emissions-vehicle standards.
The discovery of the new multiple-injection method is significant for diesel-engine design. The combustion models developed at the ERC are now being used at major engine and automotive companies. Because engineers now know how to reduce nitrogen oxide and soot simultaneously, they can apply this mechanism to improve injection-scheme designs. This lays the groundwork for continuing emissions reductions with acceptable levels of fuel economy.
To save computing time and storage, researchers at the University of Wisconsin's Engine Research Center used one-sixth of the combustion chamber, containing one spray plume, as the computational domain.