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Numerical Study on Dimethyl Ether as an Ignition Improver for Natural Gas Dual-Fuel Engines
Published August 22, 2005 by Society of Automotive Engineers of Korea in South Korea
The ignition delay of the dual-fuel setup is numerically investigated in a constant-volume combustion vessel at diesel-engine-relevant conditions. A detailed chemical kinetic mechanism, consisting of 28 species and 135 elementary reactions, of DME with methane sub-mechanism has been used in conjunction with the multi-dimensional reactive flow KIVA-3V code to simulate the autoignition process. The start of ignition was defined as the moment when the maximum temperature in the combustion vessel reached to 1900 K with which a best agreement with existing experiment was achieved. Ignition delays of liquid DME injected into air at various high pressures and temperatures compare well with the existing experimental results in a combustion bomb. When a small quantity of liquid DME (10 mg) is injected into premixtures of methane/air, the ignition delay times of dual-fuel setup are longer than that observed with DME only, especially at higher initial temperatures. The variation in the ignition delay between DME only and dual fuel case tend to be constant for lower initial temperatures. It was found also that the predicted values of the ignition delay in dual-fuel operation are dependent on the concentration of the gaseous methane in the chamber charge. Temperature and equivalence ratio contours of the combustion process show that the ignition commonly starts in the boundary at which near stoichiometric mixtures exists. Apart from accurate predictions of ignition delay, the coupling between multi-dimensional flow and multi-step chemistry is essential to reveal detailed features of the ignition process.