## 3 Results and Discussion
### Design Complexity and Specialized Knowledge in Needed for Each DPI Prototype
#### 3.1.3 Type 3 DPI  
As shown in Figure 11, the Type 3 internal structure was analyzed by eight cases with different combinations of pressure plates and leaf springs, including the same nozzle configuration as a conventional type. In Type 3 DPIs, it was found that the available maximum injection pressure was 180 MPa in all cases, except Cases 3-3 and 3-6 in Figure 12. In Case 3-3 with 800 bar and Case 3-6, the pressureful leaked and the fuel spray could not be measured. It was thought that Case 3-3 did not inject more than 800 bar, due to the absence of a plate spring for amplifying the piezo stack force, as seen in the Type 2 DPI. Unlike Case 2-3 of Type 2, the reason for not injecting under 120 MPa is that there is not enough elasticity to use the needle's hydraulic pressure. This indicates that when there is not a large enough elastic force, it is advantageous to spray only when using the needle mechanically, without any hydraulic pressure. In Case 3-6, the difference between Case 3-6 and Case 3-4 is the thickness of the pressure plate. The thickness of the thick pressure plate is 0.65 mm, and the thickness of the thin pressure plate is 0.3 mm. The total length of the variables, excluding the length of the plate, is \(\sim\) 1000 mm.  
Figure 8: Detailed designs of Type 2 DPIs.  
Figure 10: Comparison of the maximum injection pressure of Type 2 DPIs.  
Figure 9: Comparison of needle configuration; (**a**) Type 2, (**b**) Conventional Type DPI.  
the needle and piezo stack, is 1.95 mm for Case 3-4 and 1.6 mm for Case 3-6. Therefore, it was found that the minimum length inside the injector must exceed 1.6 mm.  
For Type 3 DPI Cases that have an injection performance of up to 180 MPa, spray images were acquired by a high-speed camera and the spray speed was compared, as shown in Figure 13. As known, the spray speed was derived by the definition of the spray tip penetration and spray cone angle, with the edge defined as a line of 80% transmittance of back-light in the raw spray images. It was found that Case 3-8 had the best spray performance with an average speed of 107.3 m/s, an observed-maximum speed of 144 m/s, and an SOI (start of injection) value of 80 \(\upmu\)s. The observed-maximum speed in this study means that the spray has not reached the real maximum speed because the bore size of the test engine used in this study is 95 mm, as shown in Table 5, which is about the same as the 100 mm diameter disk shown in Figure 5b. In other words, the high speed camera shoots the fuel stream until it reaches the end of the disk and identifies the highest speed. When the fuel is injected into the cylinder, it is blocked by the inner wall of the cylinder and the maximum speed is not fully achieved in the real spray chamber.  
In some Type 3 DPIs, as the elasticity increases, the needle movement is amplified and its moving speed increases. Therefore, it can be expected to have a good effect on two- or multi-injection schemes that are favorable for enhancing the air-fuel mixing rate of the diffusion combustion in the CI engine.  
Figure 11: Detailed designs of Type 3 DPIs.  
Figure 12: Comparison of the maximum injection pressures of Type 3 DPIs.