## 3 Results and Discussion
### Spray Process Analysis  
The injector of the engine with six nozzles was installed on the side of intake port and a certain amount of fuel was injected into the cylinder at a certain injection timing. Due to the high temperature and the strong air flow in the cylinder, the fuel was rapidly atomized. The fuel droplets gradually absorbed heat and evaporated during the interaction with the gas in the cylinder. In the early stage of fuel injection, the trajectory of the spray changed significantly. The installation angle of injector was at an angle of 45\({}^{\circ}\) to the cylinder head plane. Figure 6 shows an image of IC8H18 fuel composition cloud diagram and the instantaneous velocity fiedom the tangent plane parallel to the centerline of the cylinder and passing through the center of the spark plug. It can be seen that the spray trajectory has an obvious shift due to the turbulence caused by spray at \(-210\)\({}^{\circ}\)CA. As illustrated in Figure 6, the installation angle of injector did not adapt to fuel injection advance angle, so most of fuel was injected towards the cylinder liner, which induced the fuel can't mix with the air well. Later the fuel moved to the intake side because of the tumble flow and the swirl flow. It formed the fuel lean region around the exhaust side, therefore the distribution of the fuel was non-uniformly.  
The counterflow induced by the tumble flow at \(-245\)\({}^{\circ}\)CA as shown in Figure 7 was retarded at \(-240\)\({}^{\circ}\)CA since the injection of the fuel. Beside after \(-240\)\({}^{\circ}\)CA the injection of the fuel caused airflow perturbance and two distinct swirl flow regions were formed at \(-235\)\({}^{\circ}\)CA. After that the above mentioned two swirl flow regions broke up and recombined again. At \(-220\)\({}^{\circ}\)CA the swirl flow near the cylinder liner could be observed clearly in Figure 7. The tumble flow and the swirl flow were enhanced because of the direct injection, which was able to increase laminar flame speed and the turbulence flame speed at the same time as illustrated in literature [20]. The flow ratio of tumble flow and the swirl flow were induced by fuel direct injection, in addition, tumble flow produced turbulence and increased the turbulent flame speed.  
Figure 5: In-cylinder tumble ratio.  
Figure 6: IC\({}_{8}\)H\({}_{18}\) spray cloud and instantaneous velocity field (clip through the spark plug). (a) \(-210\)\({}^{\circ}\)CA; (b) \(-155\)\({}^{\circ}\)CA; (c) \(-110\)\({}^{\circ}\)CA; (d) \(-30\)\({}^{\circ}\)CA.  
At the end of the spray model calculation, the fuel particles did not evaporate completely and the evaporation ratio was 99.13%. Figure 8 shows the equivalence ratio distribution of fuel/air at \(-20\)\({}^{\circ}\)CA. As shown in Figure 8, a fuel lean region was formed because large amounts of fuel were injected towards the spark electrode, and the fuel lean region on the left side was the end-gas region. A fuel lean region was produced, and the knock propensity in this region was totally suppressed by longer ignition delay time of the fuel lean condition. Nevertheless, by analyzing the distribution and the percentage of equivalence ratio, it could also be seen that the fuel and air distribution was relatively dispersed at the \(-20\)\({}^{\circ}\)CA, and a certain of fuel was attached to the cylinder wall.