# Study on the Common Rail Type Injector Nozzle Design Based on the Nozzle Flow Model
## 3 Results and Discussion  
### Nozzle Hole Diameter and Needle Lift  
Figure 6 shows the peak injection rate (\(\dot{m}_{max}\)) for injection pressure of the diesel and DME systems. It is based on solenoid injector with 0.14 (mm) nozzle diameter and eight holes in a common rail system. The design requirement for the injection pressure of DME was set to be \(P_{max}=40\) (MPa), while that of diesel was \(P_{max}=160\) (MPa). DME gasifies immediately during injection, due to its low boiling point, even though it is injected as a liquid. Therefore, the high fuel injection pressures, such as \(P_{max}=50\)-\(160\) (MPa) used in modern diesel injection systems are not required for DME [5]. The peak injection rates were experimentally measured by the Bosch tube method and numerically predicted by the zero-dimensional nozzle flow model. In the experiment, it was difficult to measure nozzle injection pressure in the nozzle inlet, as shown in Figure 1c; the pressure in the common rail system is regarded as the inlet injection pressure. In the experiment, peak injection rate was calculated using the average  
Figure 4: Experimental setup for injection rate measurement. Figure 5: Control volume chosen for analysis. value of 100 cycles from an injection rate at time of 0.5 (ms). The trend predictions for peak injection rate are in good agreement with the experimental results. However, the peak injection rate using the nozzle flow model was found to be overpredicted, compared to those of the experiment results. There are certain assumptions in the nozzle flow model. Because the fuel in the common rail system enters the nozzle inlet through the orifices in the injector, a pressure drop occurs through the orifices. Therefore, the nozzle inlet pressure, which determines the peak injection rate, is low in the experiment. In the nozzle flow model, the inlet injection pressure is assumed as a sine function, as shown in Figure 2. In general, the peak injection rate occurs after T/2. In addition, the nozzle flow model presented in this work did not consider the effect of the flow reduction due to the surface roughness inside the nozzle. Thus, the peak injection rates of the nozzle flow model are higher than those of the experiment. In Figure 6, the peak injection rate is 0.0559 (g/s) for the required maximum pressure in the diesel common rail system, \(P_{max}=160\) (MPa). The peak injection rate is 0.0278 (g/s) with the designed maximum pressure of 50 (MPa) in the DME common rail system, which is 49.7% of the diesel system. In addition, since the lower heating value of DME (\(Q_{LHV}\)) is 27.6 (MJ/kg) and 65% of diesel, more DME is required in the combustion chamber to obtain the equivalent engine power. To increase the injection rate, injection duration should be expanded. However, it is necessary to secure a peak injection rate since there are limits to increasing injection duration. Therefore, the injector nozzle design should be changed to compensate for insufficient DME heating value when applying DME common rail system of \(P_{max}=50\) (MPa) to a conventional diesel common rail system of \(P_{max}=160\) (MPa). Figure 7 shows the peak injection rate (\(\dot{m}\)) according to nozzle hole diameter for DME injection pressure of 50 (MPa). To ensure sufficient injection rate in the nozzle, an increase in nozzle diameter is required, as shown in Figure 6. As the diameter of the nozzle increases by 0.2 (mm), the peak injection rate becomes 0.0559 (g/s), which is the same amount of that of 160 (MPa). When the diameter of the nozzle is 0.245 (mm), the peak injection rate is 0.086 (g/s). It is capable of obtaining equivalent engine power considering the lower heating value, as shown in Figure 7. However, as the diameter of the hole increases from 0.14 (mm) to 0.245 (mm), the distance between the holes decreases from 1.64 (mm) to 0.8 (mm). The minimum distance between holes (\(D_{H-H}\)) has a significant effect on the reliability of the nozzle operating at high pressure; thus, it is recommended to maintain a certain distance.