## 4 Conclusions  
In this study, a direct needle-driven piezo injector (DPI) based on the Delphi product was considered to improve injection characteristics, including: a maximum injection pressure and injection response, and manufacturing simplicity. The effects of the injection response and spray characteristics on CI combustion emissions were investigated with three concept designs, in order to ascertain the optimal DPI actuation structure for a clean CRDi diesel engine. First, a maximum injection pressure test and a Mie-scattering spray visualization with a high-speed camera were carried out, to compare the realizable criteria for the prototype DPIs. Second, injection rates based on Bosch tube method were applied for each prototype DPI and compared with a conventional type DPI. Finally, diesel engine experiments were carried out to investigate HC and NO\({}_{x}\) emissions, and to investigate the engine adaptability and combustion performance of the Type 3 DPI prototype. This approach led to the following conclusions:1. In internal design structure for a useful DPI needle response, it is very difficult to apply a high injection pressure over 130 MPa if the hydraulic pressure required to excite the needle's initial state is not applied. From the maximum injection pressure test, it was concluded that a high DPI injection pressure depends on the number of leaf springs. If the leaf spring is not present, the DPI can't spray more than 180 MPa, due to a lack of elasticity. As the number of leaf springs increases, the elasticity increases.
2. Type 1 and 2 DPI prototypes were conceptually designed with non-hydraulic needles. Therefore, they were able to spray up to a maximum injection pressure of 130 MPa. Type 1, with an internal coil spring, had a high spring constant K, which a shortened needle distance of travel and a reduced injection value. Type 2 had the highest injection rate due to free needle movement. However, the Type 3 DPI, with an optimal number of pan-springs, plates, and needle structure for available hydraulics, had a maximum injection pressure of 180 MPa and advantageous degrees of freedom for controlling the injection value and injection rate with a simpler structure in the design process for a direct needle-driven piezo injector.
3. In the spray visualization experiment, the Type 3 DPI showed the fastest spraying speed and responsiveness at an average speed, the observed-maximum speed, and the maximum SOI response. As a result, the key factor influencing the spray characteristics is whether the DPI plate spring and the pressure plate of the needle are controlled by a mechanical mechanism only, or by hydraulic pressure.
4. For the injection rate, the Type 3 DPI had a faster injection rate, which corresponded to the visualization results. Furthermore, by looking at the slope of the injection rate at the beginning and end, the Type 3 DPI had a relatively horizontal slope, and fuel injection lasted the longest.
5. In diesel engine experiments, relatively high fuel injection rates for the Type 3 DPI prototype generated un-burned emission gases such as HC. However, it had a lower injection delay and faster injection start, with a much higher initial injection quantity. Therefore, it is expected to demonstrate a great advantage for multi-stage injections, where a rapid response and the spraying speed are subdivided.  
This research was supported by the CEFV (Center for Environmentally Friendly Vehicle) as Global-Top Project of KMOE (Ministry of Environment, Korea).  
[PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] produced prototype DPI; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data and wrote the paper. [PERSON] supervised and advised all parts.  
The authors declare no conflict of interest.