Effect of advanced intake valve closing on the thermo-chemical characteristics of the homogeneous combustion in a DME fueled HCCI engine

Abstract

This numerical study was performed to analyze the thermo-chemical characteristics of homogeneous combustion on the effect of advanced intake valve closing timing in a DME fueled HCCI engine. To achieve this, the engine simulation was performed using the detailed chemical kinetic mechanism, including 453 reactions for dimethyl ether (CH3OCH3), and the modified NOX mechanism was applied to analyze the NOX reaction pathway of the DME combustion. The experimental study was performed to validate the engine analysis using a single cylinder CI engine with the injection timing of BTDC 40 degree and IVC timing of BTDC 128 degree. The injection quantity and pressure were set to 8 mg and 50 MPa at 1500 RPM, respectively. The results of engine performance such as cylinder pressure, cylinder temperature, heat release rate, and IMEP were decreased and the ISFC and ignition delay were increased when the IVC timing was advanced since the overall rates of a chemical reaction and the reaction rate of OH radicals are reduced. Especially, the reduction of CH2OCH2O2H radicals was a very important factor it affects the ignition delay in the low temperature reaction. Consequently, it influences the LTC and HTC reactions and makes a difference in the fuel oxidation rate and combustion reaction. Furthermore, the OH radicals also had the greatest effect to the CO oxidation and NOX production. The CO and NOX emissions were primarily occurred by the forward reactions of HCO + O2 → CO + HO2 and HO2 + NO → NO2 + OH during the combustion period, respectively. The chemical reactivity and CO oxidation reaction were reduced as the fuel mass fraction increased in the low combustion temperature. In addition, the reaction path of NOX emission related to the thermal NOX reaction did not occur due to the low combustion temperature. Thus, the thermo-chemical characteristics of the DME homogeneous combustion of this work suggest that the chemical reaction of fuel oxidation and exhaust emissions are limited as the effect of chemical oxidation rate, reaction rate, and the OH radical depending on the IVC timing changes.