Numerical Simulation Of Engine Combustion Under Oxygen-enriched Air

As the development of automobile industry, the automobile amount increases sharply, which brings big pressure to environment and energy sources. More and more strict emission regulations have been enacted all over the world. In order to release the pressure of energy source and meet the requirements of the regulations, it needs to improve engine performance, decrease fuel consumption and emissions.Oxygen-enriched combustion with the potential of high efficiency and low emission has been one of the hottest topics in the world. Along the experimental works, oxygen-enriched simulation on the combustion model, combustion mechanism and variable parameters had been paid much attention.The reaction mechanism of oxygen-enriched combustion was investigated with gas reaction chemical kinetics software-CHEMKIN combined with n-heptanes detail reaction mechanism. The effects of intake charge temperature and the oxygen concentration in the air were studied and the results show:1. The n-heptanes oxidation reaction can be divided into low temperature reaction, NTC reaction and high temperature reaction. The reaction, R + O2→products dominates in the low temperature reaction stage. The products Isomerization and secondary oxygenase from reaction 2 to reaction process 5 determine the oxidation in low temperature reaction stage. The reaction, C7 H 14 + O2→C7 H 14 +HO2, products Stable olefin and as the temperature rises, the oxidation slow down. The reaction of C7H15 is mainly pyrolysis to generate smaller molecular structure of the alkyl at the high temperature stage and the cell reaction accelerates and the combustion proceeds quickly.2. When the intake charge temperature is kept constant, as the oxygen enriches, the peak in-cylinder pressure raises, the pressure rise ratio raises and maximum pressure rise ratio is up to 0.75MPa/°CA,the combustion advances and the combustion timing can be adjusted through variation of oxygen concentration. Further, the OH occurrence timing in first stage advances from -35.06/°TDC to -36.2/°TDC and its peak value increases from 3.53E-5 to 3.6E-5. The conclusion can be made that the combustion can be improved by enriching the oxygen volume fraction in the air and the total amount of the two species varies a little when the oxygen concentration rises.3. When the oxygen concentration is kept constant, as the intake charge temperature rises, the peak in-cylinder pressure raises, the pressure rise ratio raises and the pressure rise ratio reaches 0.748 MPa/°CA when the temperature is 303K and the oxygen volume fraction is 25%. The OH occurrence timing in first stage advances from -25.98/°TDC to -37.2/°TDC and its peak value increases from 2.94E-5 to 3.65E-5, the CH2O occurrence timing in first stage advances from-3.54/°TDC to -17.82/°TDC, the peak CO value occurrence timing in first stage advances from 15.84/°TDC to -0.48/°TDC. The density of OH and CH2O increases and the CO emission decreases as the intake charge temperature rises and the conclusion that the combustion can be improved with higher intake charge temperature can be made. However, too high intake temperature under the oxygen-enriched air would result in too high the pressure rise ratio value, leading to damage on the engine.