A Study on Diesel Spray Characteristics for Small- Quantity Injection

Abstract

Multi-stage injection with pilot injection and post injection has been widely used for the noise and emissions reduction of diesel engines. Considering many parameters to be decided for optimal combustion, computer simulations such as three dimensional computational fluid dynamics (3D-CFD) and lower dimensional codes should play a role for optimal selection of intervals and quantity ratios. However, the data for the sprays are insufficient for reproducing the actual fuel-air mixture formation process related to pilot and post injection. Hence, there is a need for experimental data with a small-quantity injection. The small-quantity injection is characterized with an injection rate shape similar to a triangle rather than a rectangle. This study is mainly focused on the spray characteristics of diesel sprays in which the entire process is dominated by unsteady injection processes. The effects of injection parameters and nozzle hole diameter on spray penetration, spray angle, and fuel concentration are studied with the help of a rapid compression and expansion machine. A hybrid of shadowgraph and Mie scattering imaging setup is used to visualize both spray liquid phase and vapor phase at the same time. A highspeed camera with a frame rate of 90,000 fps is used to acquire spray images. Two injectors with a nozzle hole diameter of 0.12 mm and 0.14 mm are used. The studies are performed at the injection pressure of 40, 80, and 120 MPa while environmental temperature of 850 K. The experimental results show that the development of the spray tip is proportional to t and then to t 1/2 at the later stage, and after the end of the injection, the spray tip penetration is found to follow t 1/4. Also, the spray liquid penetration, spray dispersion, and air-fuel mixing processes are evaluated and compared with various injection parameters. In addition, the instantaneous behavior of the near-nozzle spray angle is studied carefully in order to provide reliable input data for spray models.

Improvement of Thermal Efficiency in a Diesel Engine with High-Pressure Split Main Injection

“…In such an injection process, the influence of the transient needle motion on the nozzle-exit flow is significant. Several studies show that the nozzle-exit flow spreads at the start and end of injection when the needle lift is quite low [15][16][17]. This is because the flow inside the nozzle hole has a radial component that comes from the vortex in the sac [16,17].…”

Investigation on an Injection Strategy Optimization for Diesel Engines Using a One-Dimensional Spray Model

“…m f,0 (t) is the injection mass rate, µ is the flow coefficient of the nozzle orifice, d n is the geometric orifice diameter. m f,0 (t) is assumed to have a trapezoidal pattern, as shown in Figure 2, and it can be calculated according to Equation (12).…”

“…is assumed to have a trapezoidal pattern, as shown in Figure 2, and it can be calculated according to Equation (12).…”

“…However, there is little relative research about this. Inversely, as the common used and typical injection rate shapes in diesel engines, scholars have devoted a great deal of effort to the triangle and rectangle as well as trapezoid shapes [9][10][11][12][13]. In order to compare the potential of varying injection rate shapes to lower the pollutant emissions of diesel engines more objectively and comprehensively and find the optimized injection strategy, five injection rate shapes (including triangle, ramping-up, ramping-down, rectangle and trapezoid) were selected in this paper, as shown in Figure 4.…”

“…The shorter the injection duration, the higher the proportion of the ramping part. In the case of a small fuel injection quantity with a short injection duration, the injection rate shape always tends to be triangular [11,12], whose spray mixing characteristics are quite different from that of the commonly used injection rate shapes, like the square, boot or rectangle [9,10,13]. Besides, the advanced innovation of piezoelectric injectors makes a significant contribution to the extremely fast needle response time [14,15], meaning it is easer for diesel engines to adopt highly defined injection rate shapes to improve their performance.…”