Because of the cetane number of n-heptane and diesel are the same, the simulation study of diesel engine usually choose n-heptane as a substitute for diesel combustion calculation. Most of the reaction kinetics models are supplied from the Lawrence Livermore national laboratory in n-heptane detailed oxidation combustion mechanism [11] [12] . However, as a result of the mechanism is too complicated for coupling calculation, in this study, the n-heptane-butanol-PAH reduced mechanism which proposed by Hu Wang [13] was selected. The mechanism is verified by the experimental data of the shock tube, the constant volume combustion bomb and the engine , and it can well predict the process of the fuel combustion and the generation of the emissions. As fuel for pure diesel, the new mechanism should get rid of the reduced mechanism of n-butanol in the mechanism part, finally obtained a mechanism including 60 species and 272 elementary reactions to simulate the diesel combustion process and emissions generated.

According to the geometric parameters and operating conditions of the diesel engine which was selected by Haozhong Huang [14] for experimental research, the model is established. Diesel engine basic parameters see Table 1.

Because of the full size model calculation needs to consume too long and the combustion chamber is in a central symmetry, the 1/7 combustion chamber model is selected according to the number of injector holes. The computational grid is shown in Figure 1.

Due to just the combustion characteristics of diesel engine are researched, the calculation will start from the valve closing time of 583 deg to the exhaust valve opening time 845 deg. In the boundary conditions of the model, there are three surface temperatures needing to be set, including the wall of the combustion chamber, the piston head and the inner wall of the cylinder. The temperature of the three surfaces is assumed to be constant, and the temperature value is calculated by reference to empirical formula. In the initial calculation, the temperature and pressure of the cylinder are uniform, the pressure value is set according to the test values, and the temperature value is estimated by the corresponding empirical formula [15] . In addition, according to the engine speed, intake flow and other parameters, the initial kinetic energy intensity is calculated and its direction is defined. In the selection of calculation model, according to the structure characteristics of diesel engine, gas turbulence model selected k-zeta-f, spray model selected Spray Turbulent Diffusion, oil mist evaporation model selected Dukowicz and fuel crushing model selected Wave.

The accuracy of the model is verified by using the cylinder pressure curve measured by Haozhong Huang [14] . The fuel used in the experiment was pure diesel, and the test conditions were in agreement with the simulation conditions (see Table 2).

Figure 2 shows the comparison between the test data and the calculated values of the diesel engine at the intake air (21% O₂), It shows that the pressure in the cylinder is simulated value agrees well with the test results, especially the ignition timing prediction is accurate. It indicates that the boundary conditions are reasonable, calculation model and calculation result is believable. Therefore the model can be used to simulate the combustion process of diesel engine with different intake oxygen concentration.

Figure 3 is the calculation of cylinder pressure curve. It shows that with the increase of inlet oxygen concentration, there is a slight increment in cylinder peak pressure, a slight advancement in peak pressure point and ignition starting point accordingly. Because of the increase of the oxygen concentration, the fuel and oxygen mix better, the ignition period is shortened and fuel combustion more fully, resulting in the increase of peak cylinder pressure. In addition, in the process of inlet oxygen concentration increasing from 21% to 30%, with the oxygen concentration improving 3% each time, the peak cylinder pressure rise by 1.48%, 1.04% and 0.77% in turn. It can be seen that compared to other inlet oxygen concentration interval, the influence of inlet oxygen concentration on burning process is most significant when the oxygen concentration improve from 21% to 24%. This is because the effect of oxygen concentration decreases with the increase of inlet oxygen concentration, cased a declined in the influence of the oxygen concentration on the combustion process.

Figure 4 shows the rate of instantaneous heat release in the combustion chamber model of 1/7, which indicates that enhanced the oxygen concentration in the intake air makes the cylinder heat release slightly forward. With the promoting of the inlet oxygen concentration, the ignition time is advanced, and then, the heat release of the premixed combustion stage is reduced. As the combustion continued, it shows that the fuel injection continues while mixed diffusion combustion appears, so the heat release of the diffusion combustion stage is increased.

Figure 5 and Figure 6 respectively show the curve of the average temperature of the combustion chamber and the wall heat flux changes with the Crank Angle under the different inlet oxygen concentration. With the promoting of oxygen concentration, the number of high-energy collisions between molecules combustible mixture molecules and oxygen molecules increases, cased an exacerbation in the combustion and fuel combustion more fully. The average temperature of the cylinder in the ignition stage and the rapid burning stage show an increasing trend, which is favorable for the improvement of the combustion efficiency. At the same time, the temperature of the cylinder increases, cased an increment in the heat flux of the combustor wall. It indicate that the heat loss of the combustor wall is increased, which hinders the improvement of the effective thermal efficiency of the diesel engine.

Figure 7 and Figure 8 respectively show the combustion chamber temperature field and mole fraction distribution of the NO at the average temperature in cylinder peak point (13 deg a TDC). With the promoting of inlet oxygen concentration, the maximum temperature inside the cylinder is improved and the high temperature region is expanded. Thereinto, the change of oxygen concentration of 24% was the most significant. By comparing the temperature field and NO concentration distribution in cylinder, the changed trend of both is consistent

with the increasing of inlet oxygen concentration. Under oxygen enrichment, the formation rate of NO increases rapidly in the high temperature region.

Figure 9 shows the curve of the average mass fraction of the NO with the crank angle at different inlet oxygen concentrations. From the picture, it can be seen that with the increase of the oxygen concentration, the NO production volume increases sharply, and the curve is getting steeper. This is because the formation conditions of NO are high temperature, rich oxygen and the residence time in high temperature. See Figure 7, with the increase of the oxygen concentration, the range of N₂ in high temperature and oxygen enriched conditions becomes larger and the retention time is lengthened, so, the NO emissions increase greatly.

The molecular of the polycyclic aromatic hydrocarbon (PAH) generated by combustion is considered as the precursor of soot, so PAH oxidation reaction mechanism is used to predict soot emissions. However, the changing trend of A1 and soot generation is basically consistent [16] , so the changes of soot formation in different oxygen concentrations can be qualitatively studied by observing the changing trend of A1.

As can be seen from Figure 10, with the increase of oxygen concentration, the formation of A1 is greatly reduced and the generation time in advance. Among them, when the oxygen concentration increased from 21% to 24%, oxygen enrichment had the most significant effect on the inhibition of A1 formation. This is because the increase of the intake oxygen enriched oxygen makes the fuel burning more fully and the lack of oxygen burning area significantly smaller, caused a decrement in the formation of soot.