Combustion engine PDF

In the limit of homogeneous reactants and adiabatic combustion, ignition timing and pollutant emissions in homogeneous-charge compression-ignition (HCCI) engines would be solely governed by chemical kinetics. As one moves away from this idealization, turbulence and turbulence/chemistry interactions (TCI) play increasingly important roles. Here, the influence of TCI on autoignition and emissions of CO and unburned hydrocarbon (UHC) is examined using a three-dimensional time-dependent computational fluid dynamics (CFD) model that includes detailed chemical kinetics. TCI is accounted for using a transported probability density function (PDF) method: the joint PDF of 40 chemical species and mixture enthalpy is considered. The modeled joint PDF equation is solved using a consistent hybrid particle/finite-volume method. Variations in global equivalence ratio, wall temperature, swirl level, degree of mixture inhomogeneity (premixed versus direct injection, and start-of-injection timing for direct-injection cases), and a top-ring-land crevice (TRLC) are investigated. For low-swirl nearly homogeneous conditions and no TRLC, TCI has little effect on ignition timing; however, even in that case the influence of TCI on emissions is not negligible. With increasing swirl, higher degrees of mixture inhomogeneity, and for cases that include a TRLC, TCI effects become increasingly important and result in significant changes in ignition timing and emissions. Unburned fuel is a non-negligible contribution to UHC for high swirl and in cases where a TRLC is considered. In addition to providing new insight into HCCI combustion processes, this work also demonstrates the feasibility of bringing transported PDF methods to bear in modeling a geometrically complex three-dimensional time-dependent turbulent combustion system.

Keywords

• HCCI engine;
• Autoignition;
• Emissions;
• Multidimensional modeling;
• PDF method