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 pressured fuel 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 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 μs. 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.

**Figure 13.** Comparison of the spray speed results in Type 3 DPIs.

### *3.2. Comparison and Evaluation of Prototype DPIs*

The following prototype DPIs were selected to compare the injection performance, spray, and combustion characteristics. (1) The Type 1 DPI has a 0.8 mm thick steel wire spring with an outer diameter of 4.8 mm, spring rate of five, and a length of 6 mm; (2) For the Type 2 DPI, Case 2-1 (two leaf springs + one thick pressure plate + one thin pressure plate) was selected; (3) For the Type 3 DPI, Case 3-8 (three plate springs + one thick pressure plate + one thin pressure plate) was selected.

### 3.2.1. Comparison of Maximum Injection Pressure Results

The maximum injection pressure with the representative Types 1, 2, and 3, is shown in Figure 14. The Type 3 DPI showed the highest injection pressure at 180 MPa. It was found that the hydraulic application required to build up needle movement is an essential element to obtain an injection performance at high pressures, over 130 MPa. As a result, it can be said that the Type 3 DPI prototype has a similar injection performance to the conventional type DPI, and it has a better flexibility for designing the internal structure of a direct needle-driven piezo-injector.

**Figure 14.** Comparison of the maximum injection pressure for representative Type 1, 2 and 3 DPI.

### 3.2.2. Comparison of Mie-Scattering Spray Image Results

In this study, LED back illumination for Mie-scattering was applied to make the initial liquid spray to full development spray visible. This enabled an accurate SOI determination and simultaneously measured the initial liquid portion of the spray. Additionally, this Mie scattering technique is suited to the spray characteristics in cold conditions, where there is insignificant vaporization [8,9]. The spray image was taken with an exposure time of 38 μs at a resolution of 320 × 320 pixels in 25,000 pps by a high-speed camera.

Figure 15 and Table 8 show the comparative results of the spray speeds with four different DPIs. The Type 1 and 2 DPI had observed-maximum speeds of 91 m/s and 104 m/s, respectively, with an average speed of 59 m/s and 67 m/s. In comparison to the conventional type, Types 1 and 2 have slower spray speeds and a slow SOI response of 200 μs. It is known that the motion condition of a DPI needle is highly affected by the response parameter [9]. Generally, this shows that the use of hydraulic pressure for needle movement enhances the response performance. In this study, Type 1 and 2 DPIs with the needle only being controlled by a mechanical mechanism, without the use of hydraulic pressure, at about 80 μs, would be slower than the conventional type in terms of the SOI response. In particular, by considering the internal actuation structure of Type 2 and the conventional type DPI, it can be seen that the plate spring and the pressure plate are key factors influencing the determination of the spray characteristics.

**Figure 15.** Comparison of spray speed with various DPIs.

characteristics

 with various DPIs.

**Table 8.** Comparison

 of spray


**Figure 16.** Comparison of Mie-scattering spray image between conventional type and Type 3 DPI.

In the case of the Type 3 DPI, the observed-maximum spray speed is 113 m/s, the average speed is 80 m/s, and the SOI response is 80 μs. This value means that the Type 3 DPI has a fast needle movement, since about 40 μs would be faster than the conventional type DPI. In other words, the Type 3 DPI needle is able to start fuel injection at 80 μs, but the conventional type DPI is still closed, maintaining a needle state until the SOI timing is 120 μs.

As shown in Figure 16, there is an obvious difference in the spray characteristics with time after the start of the injection. As stated previously, the Type 3 DPI had a faster needle opening than the other three DPIs, so it displayed a rapid spray behavior until obtaining a full development spray. It is clear that the actuation structure of a direct needle-driven injector significantly affects the initial spray shape. This is due to hydraulic internal flow dynamics within the DPI nozzle. Therefore, when the needle response is relatively high, the spray tip fuel penetration increases in the liquid phase spray characteristics.

### 3.2.3. Comparison of Injection Rate Results

Figures 17 and 18 compare the injection rate results between Types 1, 2, 3, and the conventional type DPI. To confirm the reliability of the injection rate meter, the injection rate obtained from the meter used in this study was first validated with reference data from Delphi [10]. It was observed that the injection quantity of the Type 1 DPI is 3.14 mg, which is relatively smaller than Type 2 or Type 3, as shown in Table 9. Since the spring constant K is relatively higher than the leaf spring, the length of the compressed spring is shortened when the needle is lifted, so the distance of travel of the needle in the Type 1 DPI is also shortened, and the injection quantity is expected to be reduced. In the case of Type 2, the injection amount is high after the start of injection, but then falls sharply. Compared to Type 3, the end of the injection (EOI) of the Type 2 DPI occurs early and pressured fuel moves freely in the needle. So, the Type 2 DPI has a higher injection rate than other DPI types, until it reaches the EOI. This is a key cause of decreasing injection quantity due to the quick closing operation.

**Figure 17.** Comparison of the injection rate with various DPIs.

**Table 9.** Comparison of the injection quantity with various DPIs.


**Figure 18.** Comparison of the injection rate between conventional type and Type 3 DPI.

However, the Type 3 DPI showed a more stable and sufficient injection rate than Type 1 and 2 DPIs. The shape of the injection rate of the Type 3 DPI is relatively horizontal and the fuel injection lasts for the longest time. This means that the Type 3 DPI has a favorable degree of freedom for controlling the injection amount and the injection shape in real injection mapping processes.

Figure 18 shows a comparison of the injection rates between the conventional type and the Type 3 DPI. As can be seen from this graph, the conventional type shows a shorter start of injection than Type 3. However, as compared with the results of Table 10, the injection rate and injection quantity do not show much deviation. This opinion is based on using a different observational method for the injection rate measurement system and the spray visualization measurement system. Moreover, the first "injection signature" marked with one circle in Figure 18 was detected for the Type 3 DPI. They show an almost equal injection performance, even if the Type 3 DPI has more simple structures in the design processes of the direct needle-driven piezo injector.

**Table 10.** Comparison of injection quantity between conventional type and Type 3 DPI.

**Figure 19.** Comparison of HC and NO*x* emissions between conventional type and Type 3 DPI; (**a**) 7 mg/stroke (injection quantity) and BTDC 10 CA (injection timing); (**b**) 3, 7 mg/stroke (pilot/main injection quantity) and BTDC 15, 5 CA (pilot/main injection timing).

The effect of the injection rate on combustion is discussed in Section 3.2.4. It predicts that the Type 2 DPI has a higher NO*x* emission due to its higher injection quantity at the beginning of the combustion. It can be inferred that the Type 3 DPI might have the advantage of multiple injections, because the test results indicated that it had a lower injection delay and faster injection start, with a higher initial injection quantity. Therefore, a pilot injection strategy was applied and described, as shown in Figure 19b.

### 3.2.4. Comparison of Engine HC and NO*x* Emission Results

Figure 19 shows the diesel engine experimental results using the same conditions applied in previous chapters. The speed of the engine was maintained at 1200 rpm, and the injection timing was set to BTDC 10 CA in a single injection case and BTDC 15 CA and 5 CA in a two-injection stage (pilot/main) injection case. The fuel injected was 3 mg and 7 mg with the pilot and main injections, respectively. This was done to maintain the total fuel mass with the single injection experiment. By fixing the pilot injection timing at BTDC 15 CA, it was observed that the combustion characteristics and performance were influenced by the injection timing of the main injection. First, it was observed that HC and NO*x* emissions for the conventional type and the Type 3 DPI were different, regardless of single and two-injection stages. As shown in Figure 19a, in the case of the Type 3 DPI at the injection quantity of 7 mg/stroke, the HC emission increased by about 8 ppm, but the NO*x* value decreased by about 23 ppm, which is more than 50% lower than the conventional type DPI. However when the injection timing of BTDC 10 CA differs for the two-injection stage, as shown in Figure 19b. The NO*x* emission value is about 109 ppm, which is about 4.5 times more than the difference. The reason for this is that when the injection angle of the conventional type DPI is small, the injector response is relatively slow and any special distinction between the pilot injection stage and the main injection stage is not clear. Therefore, it is expected to have a similar effect to that of injecting at the injection stroke. Conversely, Type 3 can exhibit low NO*x* emission values with a relatively fast injection response and spray speed. This shows that HC and NO*x* emissions are significantly reduced in a two-stage injection scheme. The premixed combustion phase and the mixing controlled combustion phase were mostly affected by the injection rate, as stated in Section 3.2.3. It was observed that most of the fuel was injected rapidly following the injection signal at BTDC 15 CA, due to the characteristics of the DPI, a fast SOI, and a high injection rate. Therefore, the premixed combustion phase was promoted. Also, the rest of the remaining fuel was injected after the ignition. Therefore, the combustion was continuous, until the controlled combustion phase. As a result, the air-fuel mixture was saturated, and some fuel in the cylinder was not able to participate in combustion, remaining in an unburned condition. Later, the unburned fuel was used in the late combustion phase. Therefore, a multiple injection strategy could be advantageous in combustion performance assessments. In this sense, a Type 3 DPI specified as Case 3-8 (three plate springs + one thick pressure plate + one thin pressure plate) is a very useful measure for a FIE (fuel injection equipment) system in a newly clean CRDi diesel engine.
