Stochastic Reactor Models for Engine Simulations

University dissertation from Division of Combustion Physics, Department of Physics, Lund University

Abstract: The aim of the thesis work is the further development of practical engine simulation tools based on Stochastic Reactor Models, SRMs. Novel and efficient implementations were made of a variety of SRMs adapted to different engine types. The models in question are the HCCI-SRM, the TwoZone SI-SRM and the DI-SRM. The specific models developed were incorporated into two different interfaces: DARS-ESSA, which is a stand-alone tool, and DARS-ESM through which all the models can be operated in a simple and effective manner with use of several commercial 1-D engine simulation tools. The tools and couplings to commercial 1-D codes were successfully developed and employed to simulate such complex combustion processes as of HCCI engines with NVO combustion. SRMs are able to model cyclic variations, but these may be overpredicted if discretization is too coarse. It was found that for studies of cyclic variations in HCCI engines, by using the HCCI-SRM, discretization needs to have a level of resolution of 500 particles and of 0.5 CAD time steps, to provide the correct range of the cyclic variations. To get correct predictions of average values, of for example the pressure, temperature and species mass fractions, as few as 10 cycles are usually required, even when employing as coarse discretization of 100 particles and time steps of 0.5 CAD. Investigations to study the effects of turbulence and heat transfer in HCCI combustion were performed. In the case of high levels of turbulence and evenly distributed heat transfer, the in-cylinder conditions become homogeneous more quickly. The results indicate that in HCCI engines, inhomogeneties tend to promote earlier ignition and lower pressure rates as well as more stable operating conditions with lesser cyclic variations. Turbulence and the heat transfer distribution had little impact on the duration of combustion or on the amount of HC and NO at EVO. The calculated concentrations of hydroxyl radicals and formaldehyde were compared with LIF-measurements made in an optically accessed iso-octane / n-heptane fuelled HCCI engine. The averaged and distributed concentrations of CH2O and OH could be predicted with quite high accuracy by the SRM. This clearly proves the validity of the stochastic reactor model. The formation of exothermic centers was modeled with the SRM to investigate their impact on HCCI combustion. By varying the exhaust valve temperature, and thus assigning more realistic wall temperatures, the formation of exothermic centers and the ignition timing was shifted in time. It was shown that promoting exothermic centers provide more inhomogeneous conditions before ignition, and lead to earlier ignition. This in turn leads to more homogeneous conditions after combustion, counteracting emissions of hydrocarbons and CO which are a problem in HCCI engines. Studies involving the use of a novel approach with adaptive chemistry, POSM, were performed. Incorporated into the Two-Zone SI-SRM code, calculations showed almost no accuracy to be lost, while there was a decrease in calculation time by a factor of 3. For a further gain in calculation speed of a factor of 12, clear losses in accuracy were experienced, although the global conditions were well captured. Simulations of diesel engine combustion, DICI, using the newly developed DI-SRM coupled with a 1-D full engine simulation tool were found to agree well with the results of experiments that were conducted. Parametric studies were performed to indicate the sensitivity of the modeling parameters. The DI-SRM behaved as predicted, and even with use of coarse discretization the results were comparable to those of the experiments.