# Effect of Fuel Mass Flow at the End of Injection on Cavitation and Gas Ingestion in the Nozzle
## 4 Results and Discussion
### Influence of Mass Flux  
In each complete working cycle, the amount of fuel injected into the internal combustion engine depended on the width of the fuel injection pulse. Nevertheless, during the experiment, the electronic control unit (ECU) could not control the width of the fuel injection pulse. It was controlled by fuel injection feedback, thus not only influencing fuel atomization but also affecting the engine power, economy, and emission performance.  
Figures 9 and 10 show the changes in gas volume fraction with time for various fuel mass fluxes (12 mg/str, 14 mg/str, 18 mg/str). As can be seen, the effect of mass flux on cavitation was not obvious at the beginning of the EOI, but it became obvious as the EOI progressed. The sac with a larger fuel mass flow had larger cavitation and lagged at the corresponding time. The reason for this was that because of the larger mass flux fuel with a greater inertia force, the kinetic energy of fuel increased sharply near the sealing surface of the needle valve and inlet of the orifice, resulting in the increased kinetic energy at the nozzle inlet, the rapidly decreased mass flow rate, and the fast velocity of the needle valve, thus making the cross-sectional area of fuel flow near the sealing surface reduce quickly. Therefore, considering the throttle effect and the fuel inertia outflow, a larger velocity and a lower pressure appeared, i.e., the higher fuel mass flux was, the greater cavitation was.  
In Figure 9, cavitation in the nozzle reaches its maximum volume at 0.3 ms. The total cavitation had two peaks--the first time at 0.3 ms and the second time at 0.4 ms--which was caused by pressure waves in the nozzle. Analyzing Figures 9 and 10, total cavitation in the nozzle was closed to that in the sac at three different mass fluxes, and the cavitation in the nozzle corresponding to three different mass fluxes was up to the sac after 0.4 ms. After 0.4 ms and until 0.6 ms, the larger the mass flux in the nozzle, the less cavitation reduction there was, which also meant more cavitation. This is because the higher the  
Figure 10: The change of cavitation under different mass fluxes.  
Figure 9: The change of total cavitation in the nozzle under different mass fluxes.  
mass flux near the nozzle inlet was, the more obvious the stretching effect was, and the less viscous dissipation in the wall-attached layer flow was reduced, the more likely it was that the kinetic energy in the nozzle would produce a lower pressure zone. The higher the mass flux, the longer the low-pressure duration was.  
Figure 11 shows the effect of different mass fluxes on gas ingestion. As can be seen, the bigger the mass flux, the longer the gas ingestion duration, and the greater the gas ingestion volume. This indicates that the larger mass flux could compensate for the viscous dissipation and stretching effect of fuel in the sac, resulting in lagged gas ingestion. Compared with Figures 10 and 11, the time of cavitation collapse was synchronized with the time of gas ingestion increase, which proves that gas ingestion was mainly caused by cavitation collapse in the nozzle.