Effects of Fuel Composition, Turbulence, and Turbulence/Chemistry Interactions on Emissions from Compression-Ignition Engines

摘要

There is increasing interest in alternatives to conventional petroleum-derived fuels for piston engines and other applications. A significant issue with alternative fuels is their influence on pollutant emissions. Recent studies have shown that turbulence and turbulence/chemistry interactions affect emissions from compression-ignition engines. To capture the sometimes subtle influences of fuel composition on emissions using CFD-based models, it is expected that these complex interactions will need to be accounted for explicitly. In this research, skeletal chemical mechanisms and a transported composition probability density function (PDF) method are used to capture chemistry and turbulence/chemistry interactions in direct-injection compression-ignition engines. Two examples of applications that explore fuel composition effects are discussed: NO_x emissions for hydrogen-assisted diesel combustion, and the increase in NO_x emissions that has been observed when biodiesel fuel is substituted for petroleum-derived diesel fuel in common-rail diesel engines. For hydrogen-assisted diesel combustion, the model is able to reproduce the experimentally observed trends for some operating conditions, in spite of the significant simplifications that were made; a model that explicitly accounts for turbulence/chemistry interactions (using a PDF method) does somewhat better than a model that neglects turbulence/chemistry interactions. In the second example, the sensitivity of NO_x emissions to variations in the physical properties of the fuel (here density and viscosity) has been explored to assess the origins of the biodiesel-NO_x effect; NO_x is found to increase with increasing fuel density, with all other parameters held fixed. Complex turbulence/chemistry/soot/radiation interactions are being explored in ongoing work aimed at understanding the biodiesel-NO_x effect.