**2. Experimental Apparatus and Procedure**

### *2.1. Design Concept of Prototype DPIs*

By using a solenoid coil or a piezo stack as an actuator, diesel injectors have recently been designed in response to stringent emission regulations. A solenoid-driven injector, operated by peak and hold current control, is still commonly used for its cost effectiveness, reliability, and smaller unit size. However, this type of injector is likely to exhibit an injection delay. Therefore, a number of studies [6,7] have focused on the characteristics and injection flexibility of a piezo-driven injector.

A DPI directly utilizes a longer piezo stack to lift the needle for injections. The longer piezo stack generates enough displacement for the lifting using an adapting amplifier located between the needle and the piezo stack. The elongation generated by the piezo stack is used to directly control the needle. During DPI operation, the piezo stack shrinks when its electric energy is discharged. This causes the needle to move upward and reveal previously blocked injector holes. Then, high-pressure fuel is injected through the injector holes. This allows the DPI to have a low fuel consumption by controlling various injection rates. Therefore, as shown in Figure 1, the prototype DPI design concepts in this study focused on the internal DPI construction with three types. Type 1 uses a needle body, a cylindrical shell, and a spring. This type is simple to make, produces a good response, and reduces the unit cost. Type 2 consists of pan-springs, plates, a needle body, a needle cylindrical shell, and a small spring. There is no cap to decrease the hydraulic influence and increase the needle response. Type 3 is constructed without limiting the number of pan-springs and plates. It is designed using a needle that has a needle body, a cylindrical shell, a spring, and cap. This design allows hydraulics to be used. This Type 3 design is used to analyze and study the injector influence by varying the number of plate springs and pressure plates.

**Figure 1.** DPI proposed by this study; (**a**) Three prototype DPIs, (**b**) Principle of the inverse piezo-electric effect.
