Abstract
In this reported work, a reduced chemical mechanism was developed to provide a comprehensive representation of in-cylinder diesel combustion and soot precursor formation processes. The Combustion Engine Research Center (CERC) mechanism of Chalmers University of Technology was used as the base mechanism in this study since this is the most extensively validated and used mechanism. The reduction scheme was developed by firstly identifying and then eliminating unimportant species/reactions in the ignition and soot precursor formation processes through the associated production rates and temperature sensitivity coefficients. Subsequently, reactions were assimilated based on the quasi-steady state assumption (QSSA). The final reduced mechanism, which consists of 107 elementary reactions with 44 species, was first validated under 48 shock-tube conditions. Ignition delay periods computed by the reduced and base CERC mechanisms were found to be in good agreement, although percentage errors of up to 20% were observed. Further validation was performed by incorporating the reduced mechanism into three-dimensional Computational Fluid Dynamics (3-D CFD) code. Here, simulation results of combustion characteristics and engine-out soot/NO levels were compared against data from parallel experimental programme of a single-cylinder, light-duty diesel engine. Numerical computation was conducted by means of linking a plug-in chemistry solver, CHEMKIN-CFD into ANSYS FLUENT. Simulated peak pressures in all cases were in good agreement with those recorded from experiments, with maximum percentage errors of 6%. Offset in the ignition delay periods of up to two crank angle degrees was also noted. The pattern of variation in the exhaust and soot levels were captured as injection timing was retarded, thus providing significant qualitative relationship between input parameters and engine-out emissions. The implementation of the reduced mechanism has achieved a 32% reduction in terms of computational runtime when compared with that of the base mechanism, whilst maintaining the accuracy of the predictions.
Keywords: Light-duty diesel engine; diesel combustion; soot precursor; reduced mechanism; computational fluid dynamics