Prediction of homogeneous combustion by modification of the fuel combustion mechanism in a DME fueled CI engine

Abstract

This study was performed to predict numerically the homogeneous combustion characteristics where the fuel combustion mechanism was modified in a DME fueled CI engine. To achieve this, detailed chemical kinetic reaction mechanisms were used for the prediction of homogeneous combustion of hydrocarbon fuels (diesel and DME). The results were compared in terms of cylinder pressure, heat release rate, and CO and NOX emissions. Calculations were performed using the detailed chemical kinetic mechanism, including 671 species and 2933 reactions for n-heptane (n-C7H16) and 96 species and 453 reactions for dimethyl ether (CH3OCH3). In addition, a modified NOX mechanism was added to investigate the accumulated results of NOX production. The engine experiments were conducted using a single cylinder CI engine with a common-rail injection system. The fuel injection quantity was set to 8 mg, and the injection pressure was maintained at 50 MPa at 1500 RPM. In this study, DME and diesel fuels were injected for inducing the homogeneous combustion at BTDC 60 and BTDC 40 degree. The modified mechanism hardly affected the overall chemical reactions, which was almost identical in comparison to the results of the conventional mechanism. In addition, the ignition delay was slightly faster in the calculation of the n-heptane mechanism because it showed that a high reaction rate was involved in the fuel oxidation and reaction of the NTC region. The combustion and exhaust emission characteristics of the homogeneous combustion simulation were in good agreement with the engine experimental results at BTDC 60 degree of injection timing, but the CO emission was over predicted when the n-heptane mechanism was employed in the DME combustion simulation. The chemical reactions of the DME combustion process were dominated by the forward reaction, which were CH3OCH3+O→CH3OCH2+OH and CH3OCH3+OH→CH3OCH2+H2O reactions. Furthermore, this study confirmed that an increase of OH radicals promotes the fuel oxidation reaction, and it can influence the combustion temperature, which also affected the processes of NOX production and CO oxidation reactions.