Effect of split injection on mixture formation and combustion processes of diesel spray injected into two-dimensional piston cavity

Abstract

The objective of this study is to obtain an enhanced understanding of the effect of split injection on mixture formation and combustion processes of diesel spray. A two-dimensional (2D) piston cavity of the same shape as that used in a small-bore diesel engine was employed to form the impinging spray flame. The fuel was injected into a high pressure, high temperature constant volume vessel through a single-hole nozzle with a hole diameter of 0.11 mm. The injection process comprised a pre-injection followed by the main injection. The main injection was carried out either as a single injection of injection pressure 100 MPa (Pre + S100), or by two types of split injection of injection pressure 160 MPa. The latter two types were defined by mass fraction ratios 1:1 and 3:1 (Pre + D160_1-1, Pre + D160_3-1). In order to observe the spray mixture formation process, the tracer laser absorption scattering (LAS) techique was adopted. Tracer LAS fuel with 97.5 vol% of n-tridecane and 2.5 vol% of 1-methylnaphthalene (a-MN) was employed. The spatial distributions of the vapor and liquid phases and the spray mixture formation characteristics in the 2D piston cavity for the three injection strategies were investigated. The diesel spray combustion and soot formation processes were studied using a highspeed video camera. The flame structure and soot formation process were examined using two-color pyrometry. The experimental results revealed that the split-injection vapor distribution was significantly more homogeneous than that of the single injection. The main injection fuel caught up with the pre-injection fuel and provided the spray tip with substantial additional momentum, enabling it to advance further. A high soot concentration and low temperatures appeared near the cavity wall region under the three injection strategies. The soot reduction rate for split injection was higher than that for single injection. The second main injection caught up with the previous injection's flame, which deteriorated the combustion and resulted in higher soot generation. The effect of split injection on the process of soot evolution finished at the same time as that of single injection.

Effects of dwell time of split injection on mixture formation and combustion processes of diesel spray

“…However, for the 0.12 ms dwell time case, the fuel of the second injection passed through only the nozzle tip of the first injection at EOI2. This flame illustration implies that the deterioration did not affect the previous flame and did not result in a higher KL factor distribution in the vicinity of the flame tip location, as shown in Figure 14(b), similar to the result reported in Yang et al 35 By contrast, in the 0.54 ms dwell time case, the KL factor allocation and flame temperature were weaker and lower for every timing, respectively, as illustrated in Figure 14(d). The longer dwell time provided insufficient high temperature burning gases from the combustion of the first injection to that of the second injection.…”

Effects of split ratio of diesel spray injection on mixture formation and combustion process

“…This implies that the marginal injection quantity of the second injection increases the injected fuel mass velocity, which is lower than the evaporation rate. 23…”

“…This flame image suggests that the depletion effect did not influence the preceding flame and did not lead to a higher KL factor allocation near the flame tip region, as mentioned in a previous study. 23…”

Effects of nozzle hole size and rail pressure on diesel spray and mixture characteristics under similar injection rate profile – experimental, computational and analytical studies under non-evaporating spray condition

“…Since the diesel fuel has very little UV light absorbance, therefore a fuel is appropriate for the LAS experiment if it has similar physical properties with the diesel fuel, it absorbs UV light and its UV light absorbance validates the Lambert-Beer Law. 16 Physical properties of Diesel and LAS Tracer fuel are compared in Table 2. Although the current work pays focus on the non-evaporating spray condition only, the LAS tracer fuel is chosen to avoid the impacts of fuel's physical properties on spray characteristics (especially the fuel density which governs the injection quantity 13 ) when the evaporating spray condition is conducted.…”