3.1.1. Type 1 DPI

In Type 1, the spring (70C hardened SW-C steel wire) was fabricated according to the wire thickness, outer diameter, number of spring revolutions, and length. It was inserted into the injector inner-body, and then the needle body and needle cylinder were also inserted. Experimental results showed that there is no problem in spraying up to the available injection pressure of 120 MPa. However, at 130 MPa, normal injection becomes intermittent. For this reason, the injector normally injected only 20 of the total injection signals.

As shown in Figure 7, each graph is divided into steel wire thicknesses of 0.7 mm and 0.8 mm. It is shown that the production spring constant (K) increases toward the right of the X-axis, and the actual number of injections continually increases. The reason for this is that the number of actual injections is expected to increase as the spring stiffness and K increases. The spring with high K becomes harder than the lower K springs, such that the pressure inside the injector provides the force to lift the injector needle. It was found that the best Type 1 performance was achieved by the spring with a steel wire thickness of 0.8 mm, outer diameter of 4.8 mm, five spring revolutions, and a length of 6 mm.

**Figure 7.** Actual number of injections according to the thickness of the steel wire; (**a**) 0.7 mm, (**b**) 0.8 mm.
