David Kennaird

The initial stage of fuel spray penetration

Effects of droplet evaporation, break-up and air entrainment on diesel fuel spray penetration have been studied theoretically at the initial stage of spray penetration when the influence of air entrainment is small (up to 0.1–0.2 ms after the start of injection). Theoretical plots of spray penetration versus time are compared with experimental results obtained using an optical single cylinder rapid compression test rig based on a Ricardo Proteus engine. Three models of spray penetration have been compared. In the first, neither break-up nor air entrainment are taken into account. The break-up processes (bag and stripping) are taken into account in the second model, while in the third model both bag break-up and air entrainment processes are considered. It has been found that the agreement between the predictions of the third model with experimental measurements is better than that for the first two models.

The Influence of Injector Parameters on the Formation and Break-Up of a Diesel Spray

The influences of injector nozzle geometry, injection pressure and ambient air conditions on a diesel fuel spray were examined using back-lighting techniques. Both stills and high-speed imaging techniques were used. Operating conditions representative of a modern turbocharged aftercooled HSDI diesel engine were achieved in an optical rapid compression machine fitted with a common rail fuel injector. Qualitative differences in spray structure were observed between tests performed with short and long injection periods. Changes in the flow structure within the nozzle could be the source of this effect. The temporal liquid penetration lengths were derived from the high-speed images. Comparisons were made between different nozzle geometries and different injection pressures. Differences were observed between VCO (Valve Covers Orifice) and mini-sac nozzles, with the mini-sac nozzles showing a higher rate of penetration under the same conditions.

Laser-induced incandescence study of diesel soot formation in a rapid compression machine at elevated pressures

Simultaneous laser-induced incandescence (LII) and laser-induced scattering (LIS) were applied to investigate soot formation and distribution in a single cylinder rapid compression machine. The fuel used was a low sulfur reference diesel fuel with 0.04% volume 2-ethylhexyl nitrate. LII images were acquired at time intervals of 1 CA throughout the soot formation period, for a range of injection pressures up to 160 MPa, and in-cylinder pressures (ICP) up to 9 MPa. The data collected shows that although cycle-to-cycle variations in soot production were observed, the LII signal intensities converged to a constant value when sufficient cycles were averaged. The amount of soot produced was not significantly affected by changes in in-cylinder pressure. Soot was observed distributed in definite clusters, which were linked to slugs of fuel caused by oscillations in the injector needle. The highest injection pressure exhibited lower soot productions and more homogeneous soot distributions within the flame. Despite diffusion flames lasting longer with lower injection pressure, it appeared that the extended oxidation time was insufficient to oxidize the excess production of soot. In addition, soot particles were detected closer to the nozzle tip with higher injection pressures. The recording of LII sequences at high temporal resolutions has shown that three distinct phases in soot formation can be observed. First, high soot formation rates are observed before the establishment of the diffusion flame. Second, a reduced soot formation rate is apparent from the start of diffusion flame until the end of injection. Finally, high soot oxidation rates occur after the end of injection and for the duration of the flame.

In-Cylinder Penetration and Break-Up of Diesel Sprays Using a Common-Rail Injection System

As part of an ongoing investigation, the influence of in-cylinder charge density, and injector nozzle geometry on the behavior of diesel sprays were examined using high-speed imaging. Both liquid and vapor penetration profiles were investigated in operating conditions representative of a modern turbocharged after-cooled HSDI diesel engine. These conditions were achieved in an optical rapid compression machine fitted with a common-rail fuel injection system. Differences in spray liquid and vapor penetrations were observed for different nozzle geometries and in-cylinder conditions over a range of injection fuelling representative of those in a typical engine map. Investigation into the differences in spray structure formed by multi-hole and single-hole injections were also undertaken. The results of the spray penetration profiles from the experiments were compared to empirical correlations in the literature and differences observed were attributed to flow structures within the nozzle, which are not taken into account by these correlations.

Spray penetration in a turbulent flow

Analytical expressions for mass concentration of liquid fuel in a spray are derived taking into account the effects of gas turbulence, and assuming that the influence of droplets on gas is small (intitial stage of spray development). Beyond a certain distance the spray is expected to be fully dispersed. This distance is identified with the maximum spray penetration. Then the influence of turbulence on the spray stopping distance is discussed and the rms spray penetration is computed from a trajectory (Lagrangian) approach. Finally, the problem of spray penetration is investigated in a homogeneous two-phase flow regime taking into account the dispersion of spray away from its axis. It is predicted that for realistic values of spray parameters the spray penetration at large distances from the nozzle is expected to be proportional to t2/3 (in the case when this dispersion is not taken into account this distance is proportional to t1/2). The t2/3 law is supported by experimental observations for a high pressure injector.

Diesel autogignition at elevated in-cylinder pressueres

Abstract The autoignition of diesel sprays at in-cylinder pressures from 5 to 9 MPa and injection pressures from 100 to 160 MPa was investigated. A pseudo three-dimensional view was obtained by using two high-speed video cameras recording the autoignition process simultaneously from two different viewpoints, so that the positional ambiguity of the ignition sites may be removed. The autoignition recordings were related to spray liquid core and vapour phase video recordings reported previously, in order to obtain a detailed understanding of the structure of the sprays. The autoignition delay was found to decrease with an increase of in-cylinder pressure up to 7 MPa. However, contrary to common belief, a reverse in this trend was observed at higher pressures, with a rapid increase in the delay at in-cylinder pressures above 7 MPa. This effect was related to a decrease in spray penetration, a decrease of diffusion coefficient and an increase in chemical delay.