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Fuel film deposits on combustion chamber walls are understood to be the main source of particle emissions in GDI engines under homogenous charge operation. More precisely, the liquid film that remains on the injector tip after the end of injection is a fuel rich zone that undergoes pyrolysis reactions leading to the formation of poly-aromatic hydrocarbons (PAH) known to be the precursors of soot. The physical phenomena accompanying the fuel film deposit, evaporation, and the chemical reactions associated to the injector film are not yet fully understood and require high fidelity CFD simulations and controlled experimental campaigns in optically accessible engines. To this end, a simplified model based on physical principles is developed in this work, which couples an analytical model for liquid film formation and evaporation on the injector tip with a stochastic particle dynamics model for particle formation.

In order to analyze the influence of nozzle geometry on the internal flow characteristics of a Diesel injector, a CFD analysis of the flow through various nozzle geometries has been carried out with a commercial code. This program includes a numerical model simulating the effect of cavitation. For the flow simulation, cylindrical and conical nozzles with different grades of hydro-grinding were used in order to observe the individual effects of these geometrical parameters. The model predicts accurately the onset of cavitation, but is very limited for strongly cavitating flow, so that the analysis of the solution may only be qualitatively assessed. However, the simulations confirm the tendency observed in experiments, that the nozzle geometry significantly influences the inner flow characteristics.

In order to comply with the increasingly restrictive limits of emissions and fuel consumption, researches are focusing on improving the efficiency of combustion engines. In this area, the aging of the injector and its effect on the injection development is not entirely analyzed. In this work, the rate of injection of a diesel injector at different stages of its lifetime is analyzed. To this end, a multi-hole piezoelectric injector was employed, comparing the injection rate measured at the beginning of its lifetime to the rate provided by the injector after aging, maintaining the same boundary conditions in both measurements. Injection pressures up to 200 MPa were used throughout the experiments. The results showed that the steady-state rate of injection was lower after the injector aged. Furthermore, the injector took a longer time to close the needle and end the injection, in comparison with the measurements done at earlier stages of its lifetime.

This publication is the result of a multidisciplinary collaboration between the academia and the industry. An attempt to pre-ionize and influence the trajectory and the fluid mechanics of the injected fuel into an experimental injection system by means of electromagnetic fields was made. This collaboration project started from research proposal, which aims at exploring the effects of a highly ionized environment on the fuel injection event and how the momentum of the injected fuel droplets may be affected by the electromagnetic fields in form of quantified variables, such as spray penetration, spreading angle and the spray axis angle. An influence of the applied electromagnetic field on the fuel spray depending on the electrode configuration was observed and is presented and discussed in this publication.

Most studies about common-rail diesel injection consider the fuel flow along the injector as isothermal. This hypothesis is arguable given the small diameter of the orifices along which the fuel flows, together with the expansions that take place across them. These phenomena may provoke variations in the fuel temperature, which in turn modify the fuel properties (i.e. viscosity, density, speed of sound…), thus influencing injector dynamics as well as the fuel atomization and mixing processes. The present investigation accounts for these effects by means of a 1D model for the fuel flow along a common-rail ballistic injector. Local variations of fuel temperature and pressure are considered by the model thanks to the implementation of the adiabatic flow hypothesis.

Proper initial conditions are essential to successfully perform a simulation, especially for highly transient problems such as Diesel spray injection. Until now, no much attention has been paid to the internal nozzle flow initialization because spray simulations are usually decoupled from the nozzle. However, new homogeneous models like Eulerian Spray Atomization (ESA) model allow to simulate the internal nozzle flow and the spray seamlessly. Therefore, the behavior of the spray for the first microseconds is highly influenced by the initial conditions inside the nozzle. Furthermore, last experiments confirm the presence of gas inside the nozzle between successive injections. This work deals with the initialization procedure in a way that mass flow rate and spray penetration curves are well predicted by the model.

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.

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.

Nowadays the main part of investigations in controlled auto-ignition (CAI) engines are centered on performance or some engine processes simulation, leaving aside particle number (PN) emission. The present work is focused on this last topic: PN emission analysis using two different injectors in a 2-stroke CAI engine, and a global comparison of PN emission of this engine with its homonymous 4-stroke engines at two operating conditions. The study was performed in a single-cylinder gasoline engine with 0.3 l displacement, equipped with an air-assisted direct-injection (DI) fuel injection system. Concerning the injectors evaluated, significant differences in PN emission have been found. When the I160X injector (narrow spray angle) was used, PN emissions were reduced. The spray cone angle during the injection event appears to be a key factor for PN emission reduction.

In order to study differences between Diesel nozzle holes, four methodologies have been tested. The techniques compared in this paper are: the internal geometry determination, hole to hole mass flow measurement, spray momentum flux and macroscopic spray visualization. The first one is capable of obtaining the internal geometry of each of the orifice of the nozzle; the second one is capable of measuring the mass flow of each nozzle hole in both, continuous and real injections. The third one gives the momentum flux of each orifice, and finally, with the macroscopic spray visualization, the spray penetration and spray cone angle of each hole, are obtained. Generally, all these techniques can be used in order to determine the hole to hole dispersion due to different angle inclination of the holes, different internal geometry of orifices, deposits, nozzle needle off-center, needle deflection, etc.

The dense spray region in the near-field of diesel fuel injection remains an enigma. This region is difficult to interrogate with light in the visible range and difficult to model due to the rapid interaction between liquid and gas. In particular, modeling strategies that rely on Lagrangian particle tracking of droplets have struggled in this area. To better represent the strong interaction between phases, Eulerian modeling has proven particularly useful. Models built on the concept of surface area density are advantageous where primary and secondary atomization have not yet produced droplets, but rather form more complicated liquid structures. Surface area density, a more general concept than Lagrangian droplets, naturally represents liquid structures, no matter how complex. These surface area density models, however, have not been directly experimentally validated in the past due to the inability of optical methods to elucidate such a quantity.

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.

The objective of the work is to learn about the internal structure of transient evaporative Diesel sprays. A simple atomization model of a non-stationary evaporative spray, described in [1], is used in this paper to study the effects of the operating variables and atomization parameters, of the primary and secondary models, on the Diesel sprays characteristics. The model is a phenomenological and DDM one, and is realized in a PC with C programming language. There are several sub-models included: A primary atomization model of the jet liquid vein based on the jet interface instabilities caused by the aerodynamic action, including an intact length model. A secondary atomization model of droplets break-up based on the Taylor analogy. A droplet evaporation model including the heat-up and the stationary periods, as well as the natural and forced convection effects.

Several new combustion concepts have been developed during last decade with the aim of reducing pollutant emissions. Specifically, these strategies allow a simultaneous reduction of NOx and soot emissions by reducing the local combustion temperatures, enhancing the fuel/air mixing (PCCI, HCCI…). In spite of their benefits, these concepts present difficulties controlling the appropriate combustion phasing as well as high knocking levels and therefore, their operating range is reduced to low-medium loads. In this work gasoline is considered as a fuel in order to improve combustion strategies based on fully or partially premixed combustion in CI engines. Its use provides more flexibility to achieve lean and low combustion temperature, however the concept has demonstrated difficulty under light load conditions using gasoline with ON up to 95.

The temperature of fuel injectors can affect the flow inside nozzles and the subsequent spray and liquid films on the injector tips. These processes are known to impact fuel mixing, combustion and the formation of deposits that can cause engines to go off calibration. However, there is a lack of experimental data for the transient evolution of nozzle temperature throughout engine cycles and the effect of operating conditions on injector tip temperature. Although some measurements of engine surface temperature exist, they have relatively low temporal resolutions and cannot be applied to production injectors due to the requirement for a specialist coating which can interfere with the orifice geometry. To address this knowledge gap, we have developed a high-speed infrared imaging approach to measure the temperature of the nozzle surface inside an optical diesel engine.

Regulations on emissions from diesel engines are becoming more stringent worldwide. Hence there is a great deal of interest in developing engine combustion systems that offer the fuel efficiency of a diesel engine, but with low smoke and NOx emissions. Thus, premixed compression ignition combustion is an interesting way to achieve a clean and efficient engine. However, using a high reactivity fuel such as diesel fuel leads to a complex and expensive engine design. A proven way to overcome this drawback is to actively control the reactivity of the fuel using low cetane fuels such as gasoline. This strategy has been explored with single and multiple cylinder engines. However no detailed and well conducted studies of the injection process were found related to the effects of gasoline use in a standard commercial compression ignition diesel engine injection system.

A study of the internal flow in the most used nozzle types in Diesel engines (microSAC and valve covered orifice VCO) was carried out in order to compare injection characteristics and understand the differences between them. To determine these differences, several experimental installations will be used, such as the injection rate test rig, steady flow test rig and spray momentum test rig, to obtain a full hydrodynamic characterization. With the help of the silicone methodology and a microscope, it is possible to determine the needle tip geometry (seat). As the geometrical characterization of the components in both nozzles was known, it was possible to carry out a CFD analysis at several needle lifts and thus observe the behavior of the internal flow in the nozzle seats and be able to compare both nozzles

The aim of this paper is to present an experimental study of the influence of using biodiesel blended fuels on a standard injection system taken from a DI commercial Diesel engine. The effects have been evaluated through injection rate measurements, spray momentum and spray visualization at ambient temperature (non-evaporating condition). These tests have been done using five different injection pressures, from 300 to 1600 bar, and three back pressures: 20, 50 and 80 bar. It is well known that fuel properties like density or kinematic viscosity are higher in vegetable oils and strongly affect how injection system operates. The tests showed that the use of biodiesel fuels leads to a higher mass flow when the injector is fully open. The spray pattern is also affected, biodiesel penetrates more and the spray is narrower. Some explanations are provided in this paper in order to understand better the injection process when vegetable oils are used.

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.

A prototype multi-hole diesel injector operating with n-heptane fuel from a high-pressure common rail system is used in a high-pressure and high-temperature test rig capable of reaching 1100 Kelvin and 150 bar under different oxygen concentrations. A novel optical set-up capable of visualizing the soot cloud evolution in the fuel jet from 30 to 85 millimeters from the nozzle exit with the high-speed color diffused back illumination technique is used as a result of the insertion of a high-pressure window in the injector holder opposite to the frontal window of the vessel. The experiments performed in this work used one wavelength provide information about physical of the soot properties, experimental results variating the operational conditions show the reduction of soot formation with an increase in injection pressure, a reduction in ambient temperature, a reduction in oxygen concentration or a reduction in ambient density.