Development of Reduced Fuel Oxidation Mechanisms and Application in Low Temperature Combustion (LTC) Diesel Engine by CMC-ISR Model

Abstract

It is difficult to apply large detailed mechanisms of fuel oxidation in simulation of complex combustion phenomena in practical combustion devices. There are systematic methodologies that allow reduction of detailed chemical mechanisms to smaller sizes involving less computational load. Skeletal mechanisms are produced by directed relation graph with specified accuracy requirement and sensitivity analysis based on species data from the directed relation graph. The skeletal mechanism is optimized through elimination of unimportant reaction steps by computational singular perturbation importance index. Reduction is performed for the mechanism of methyl butanoate consisting of 264 species and 1219 elementary reactions. Validation shows acceptable agreement for auto-ignition delays in wide parametric ranges of pressure, temperature and equivalence ratio. Methyl butanoate has been proposed as a simple biodiesel surrogate although the alkyl chain consists of four carbon atoms, which is much shorter than that of major biodiesel esters. It was suggested to resolve the problem by employing a mixture of MB and n-heptane in previous work. The resulting surrogate mechanism for n-heptane and MB consists of 76 species and 226 reaction steps including those for NOx. It has become essential to consider realistic chemistry in computational simulation, since there is a trend of more significant roles of chemical kinetics in new types of internal combustion engines. A typical example is low temperature combustion (LTC) diesel engine which aims at simultaneous reduction of NOx and soot with acceptable compromise in the efficiency. Simulation is performed in this work as a preliminary step for development of an LTC diesel engine for off-highway construction vehicles. The oxidation mechanisms of fossil diesel and soybean biodiesel fuels involve both low and high temperature reaction including NOx chemistry. Validation is performed for major physical models of diesel combustion against measurements in LTC conditions. Conditional moment closure (CMC) for incompletely stirred reactor (ISR) is employed to address coupling between chemistry and turbulence by a uniform conditional flame structure in KIVA-CMC. Submodels in KIVA-CMC include Kelvin-Helmholtz/Rayleigh-Taylor model for spray breakup and two-equation soot model for the prediction of particulate matter. Results are in good agreement for pressure traces, NOx and trends of variation of CO and PM in the given operating conditions. A possible range of LTC operation is identified through parametric study with proper understanding of relevant physical phenomena in the given engine conditions.