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Technical Paper

Effect of Partial Needle Lift on the Nozzle Flow in Diesel Fuel Injectors

An experimental study has been performed to evaluate the impact of different needle partial lifts on the nozzle orifices flow. A prototype injector with a multi-orifice nozzle was used. This injector allows controlling the needle lift at different percentages of maximum lift. Different measures of mass flow rate and spray momentum flow at several rail pressures (up to 200 MPa) were made in order to observe the effect of different needle lift percentages on the amount of fuel injected and momentum of the jet. The influence of partial needle lifts on nozzle orifices flow was estimated by using non-dimensional parameters of discharge coefficient (Cd), velocity coefficient (Cv) and area coefficient (Ca). The results show an interesting reduction in nozzle discharge coefficient at partial lift. This reduction of Cd value at partial needle lift is not only due to the drop in the velocity coefficient but also to a reduction of the effective orifice area.
Technical Paper

Understanding Diesel Injection Characteristics in Winter Conditions

Starting and operating of diesel engines in cold conditions is a common and important problem. Many factors such as ambient conditions, fuel properties, fuel injection, cranking speed, etc, affect cold engine functionality. In order to improve diesel engine cold start, it is essential to understand better these problems. In this paper the injection development at cold temperatures is studied, since it is an important parameter that affects the fuel interaction with the air, so the future combustion process would also be influenced. In particular, a hydraulic characterization of diesel injection is made, using specialized test rigs that simulate real engine in-cylinder air pressure and density; the fuel is injected from three axi-symmetric convergent nozzles at several injection pressures (30, 50, 80, 120 and 180 MPa), two chamber densities and two temperatures of 255 K (winter) and 298 K (reference).
Technical Paper

Cavitation effects on spray characteristics in the near-nozzle field

In this paper, a special technique for visualizing the first 1.5 millimetres of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behaviour in the near nozzle field. For this purpose, a real Diesel axi-symmetric nozzle has been analyzed. Firstly, the nozzle has been geometrically and hydraulically characterized. Mass flow measurements at stationary conditions have allowed the detection of the pressure conditions for mass flow choking, usually related with cavitation inception in the literature. Nevertheless, with the objective to get a deeper knowledge of cavitation phenomenon, near nozzle field visualization technique has been used to detect cavitation bubbles injected in a pressurized chamber filled with Diesel fuel. Using backlight illumination, the differences in terms of density and refractive index allowed the distinction between vapour and liquid fuel phases.
Technical Paper

Combined CFD-Phenomenological Approach to the Analysis of Diesel Sprays under Non-Evaporative Conditions

In the present work a CFD analysis of formation and spray evolution emerging from a Multi-holes-Common Rail Diesel injector under non-evaporative conditions has been carried out. The aim of the work was to set up a tool for fuel spray simulation that could offer reasonable accuracy for the prediction of the spray tip penetration, droplet size and a reduction of CPU time. For this purpose the influence of different primary and secondary break-up models as well as drop interaction has been investigated. Phenomenological relationships have also been implemented in the code in order to enhance the prediction of the stable diameter inside the break-up models, allowing the mean drop size to be better predicted and a reduction of the time necessary to set-up the model.
Technical Paper

Modeling the Exhaust System in Two-Stroke Small Engines

The purpose of this paper is to describe a methodology based on experimental and theoretical studies for the modeling of typical exhaust systems used in two-stroke small engines. The steady and dynamic behaviors of these systems have been measured in a flow test rig and in an impulse test rig, respectively. Information obtained from these experiments is used in two ways: to find a suitable geometric model to be used in a finite-difference scheme code, and to provide a mean pressure and a frequency domain reflecting boundary, in the frame of a hybrid method. A complete 50cc engine was modeled and comparisons between predicted and measured instantaneous pressure at the exhaust port show a fair agreement, the results of the hybrid approach being more accurate.
Technical Paper

Study of the Influence of Geometrical and Injection Parameters on Diesel Sprays Characteristics in Isothermal Conditions

This paper deals with the problem of quantifying and predicting the macroscopic spray behaviour as a function of the parameters governing the injection process. The parameters studied were ambient gas density as a representative parameter external to the system, and nozzle hole diameter and injection pressure as influential system parameters. The main purpose of this research is to validate and extend the different correlations available in the literature to the actual Diesel engine conditions, i.e. high injection pressure, small nozzle holes, severe cavitating conditions, etc. The sprays from five axi-symmetrical nozzles with different diameters are characterized in two different test rigs that can reproduce the real engine in-cylinder air density and pressure. The wide parametric study that was performed has permitted to quantify the effects of the injection pressure, nozzle hole diameter and environment gas density on the spray tip penetration.
Technical Paper

Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles

In Diesel injection Systems, cavitation often appears in the injection nozzle holes. This paper analyses how cavitation affects the Diesel spray behavior. For this purpose two spray parameters, mass flux and momentum flux, have been measured at different pressure. We know that cavitation brings about the mass flux choke, but there are few studies about how the cavitation affects the momentum and the outlet velocity. The key of this study is just the measurement of the spray momentum under cavitation conditions.
Journal Article

Macroscopic Behavior of Diesel Sprays in the Near-Nozzle Field

The objective of the paper is the characterization of the macroscopic behavior of Diesel sprays by focusing in at the first instants of the injection process at which the spray is clearly affected by the injector needle dynamic. There are several works dealing with the characterization of Diesel sprays in stationary conditions. Most of them conclude with empirical correlations which predict spray tip penetration as a function of the most important parameters involved in the injection process, such as: injection pressure, gas ambient density, hole diameter and time elapsed from the start of injection. In all these experiments, authors find similar power law dependencies with more or less high level of confidence. Nevertheless, few works have tried to validate or to obtain new correlations for the first instants of the injection process where the spray develops in not stationary conditions because of the influence of injector needle lift.