Bianca Maria Vaglieco

Validation of a Fractal Combustion Model through Flame Imaging

The paper is focused on the development of a fractal combustion model, included within a whole-engine one-dimensional model (1 Dime code). An extensive validation is carried out through the comparison with experimental data. The experimental activity was carried out in the combustion chamber of an optically accessible one-cylinder engine, equipped with a commercial head. Experimental data basically consisted on optical measurements which were also correlated to the instantaneous pressure inside the cylinder. Optical measurements were based on 2D digital imaging and UV chemiluminescence of radical species. The rate of chemical energy release and related parameters were evaluated from the in-cylinder pressure data using interpretation models for heat release analysis. Moreover a post-processing of the optical measurements allowed to define the mean flame radius, and propagation speeds as well, as a function of the crank angle. Theoretical and experimental analyses allowed to fully characterize flame structure and propagation speed as well. In order to estimate prediction levels and limitations of the numerical procedure, different engine speeds and spark timings were experimentally analyzed. A good agreement has been found in the whole tested range.

Study of the multi-injection combustion process in a transparent direct injection common rail diesel engine by means of optical techniques

Abstract A combination of ultraviolet—visible optical diagnostics has been applied in the combustion chamber of a diesel engine in order to study the fuel injection, autoignition, and soot formation processes. Measurements were made in an optically accessible diesel engine equipped with a multivalve cylinder head and a common rail (CR) high-pressure injection system. Several multi-injection strategies, characteristic of new generation CR diesel engines, were tested. They consisted of two (pre + main) and three (pre + main + post) injections per cycle. Fuel injection and visible combustion were studied by imaging the natural flame luminosity, and soot formation and evolution were evaluated by means of the two-colour pyrometry method. The reactions that occur during autoignition and soot formation were investigated by chemiluminescence imaging, while the broadband ultraviolet—visible extinction and scattering spectroscopy (BUVESS) technique was used in order to measure the soot particle size and number concentration. For all the injection strategies investigated, pre injection has contributed to the reduction of the ignition delay of the main injection. Moreover, the present study showed that autoignition can be characterized to some degree by the presence of the OH radical. The effect of post injection on combustion development and soot particle diameter has also been observed and discussed.

The Diesel Exhaust Aftertreatment (DEXA) Cluster: A Systematic Approach to Diesel Particulate Emission Control in Europe

The DEXA Cluster consisted of three closely interlinked projects. In 2003 the DEXA Cluster concluded by demonstrating the successful development of critical technologies for Diesel exhaust particulate after-treatment, without adverse effects on NO x emissions and maintaining the fuel economy advantages of the Diesel engine well beyond the EURO IV (2000) emission standards horizon. In the present paper the most important results of the DEXA Cluster projects in the demonstration of advanced particulate control technologies, the development of a simulation toolkit for the design of diesel exhaust after-treatment systems and the development of novel particulate characterization methodologies, are presented. The motivation for the DEXA Cluster research was to increase the market competitiveness of diesel engine powertrains for passenger cars worldwide, and to accelerate the adoption of particulate control technology.

Spectroscopic analysis and modeling of particulate formation in a diesel engine

Abstract Combined measurements of scattering and extinction coefficients in the UV–visible range have been used to characterize the particulate formed in an optically accessible divided-chamber diesel engine. Multi-wavelength spectroscopic analysis has shown the presence of two classes of particles: soot and organic aerosol. The latter structures absorb light radiation in the UV region and have sizes of 2– 3 nm . They are present just after fuel ignition and their concentration suddenly increases as the combustion proceeds, reaching a maximum value well before the formation of soot particles. Soot particles are characterized by light absorption extending in the visible range and show sizes of about 10 nm in the first stages of the inception process. A detailed kinetic mechanism has been used to model the formation of both classes of particles in high-pressure conditions. The kinetic scheme, coupled to a simplified model of diesel combustion, has been able to correctly predict the total concentration and the size of the particulates.

Optical Diagnostics of Temporal and Spatial Evolution of a Reacting Diesel Fuel Jet

Laser Doppler anemometry, spectral extinction-absorption, and flame chemiluminescence measurements were carried out to characterize the fluid flow and to analyze the temporal and spatial distribution of liquid, vapor and some pollutant species in an optically accessed high swirl combustion chamber Extinction-absorption measurements from UV to visible have shown that the spray, strongly distorted, is mixed downstream by the high swirling flow and the vapor region expands rapidly from the tip of the jet toward the chamber walls. The entrainment of hot air into the jet accelerates the vaporization process and the strong swirling flow transports the vapor around the chamber The OH emission, indicating the spatial location of autoignition, occurs at the same crank angle as that of the minimum of the heat release rate, and in the vapor region far from the tip of the liquid jet. The first appearance of soot occurs later across a wide portion of the leading part of the jet located between the tip of the jet and t...

Multiwavelength ultraviolet absorption spectroscopy of NO and OH radical concentration applied to a high-swirl diesel-like system

Ultraviolet absorption measurements were carried out in an optical research engine by exploiting the emitted plasma kernel of laser-induced breakdown. Temporal evolution and spatial distribution of OH radical and NO absolute concentrations were evaluated by a numerical procedure for retrieving optical data. Spectral measurements were made in an optically accessible external combustion chamber obtained by modifying a single cylinder, four-stroke diesel engine. The experiments were performed using a commercial diesel fuel at two engine speeds varying the ignition delay and fixing the injected fuel amount. In-chamber NO concentrations for each engine operating condition were correlated with those measured at the exhaust by a conventional method.

Knocking diagnostics in the combustion chamber of boosted port fuel injection spark ignition optical engine

High spatial resolution optical techniques have been used to get information about the timing and the location of knocking and about the chemical species involved in this phenomenon. The experiments were realised in the combustion chamber of a boosted single-cylinder spark ignition port-fuel injection optical engine fuelled with commercial gasoline. Engine conditions with different levels of knock were considered from the borderline case onto standard knocking and heavy knocking. Cycle-resolved digital imaging was used to follow the combustion and the flame propagation in normal combustion and knocking conditions. Moreover, the effects of an abnormal combustion due to the firing of fuel deposition near the intake valves and on the piston surface were investigated. The knocking influence on the flame front propagation and combustion speed was investigated following the time evolution of the mean flame radius in the different engine conditions. The appearance of the auto-ignition centres in the end gas during the knock was evaluated in terms of timing, location and frequency of occurrence. Finally, UV-visible natural emission spectroscopy was applied to detect radical species that marked the knock. HCO and OH were identified as markers of the knocking onset and OH of its intensity.

Fuel composition effects on particulate formation in a divided chamber diesel system

Abstract The influence of fuel composition on first stage of combustion and soot formation, in a diesel engine, was analyzed from measurements of spectral extinction and flame intensity using Tetradecane, N -heptane and diesel fuel . Monocomponent fuels with high paraffinic content, such as Tetradecane and N -heptane, free of aromatics, were compared with commercial diesel fuel in order to evaluate the effect of aromatic compounds of diesel fuel on soot formation. The soot formation process, its inception and the amount of the soot volume fraction are strongly dependent on the chemical and physical characteristics of fuels. Monocomponent fuels revealed the presence of species, formed from pyrolysis of fuel and characterized by UV absorption bands, that seem to contribute to soot formation. An evaluation of the soot volumetric fraction was made at different times for all the fuels and at different A/F ratios. N -heptane is the fuel which gives rise to the lowest soot production. The total amount of soot formed during the combustion of Tetradecane was intermediate between N -heptane and diesel fuel.

Spectral extinction measurements of spray combustion in a divided-chamber diesel engine system

Sepctral flame extinction measurements from UV to visible with high temporal and spatial resolution were performed in an optically accessible divided-chamber diesel system. This system was developed ad hoc by modifying a real engine to realize a soot-forming premixed spray combustion at high pressure, high temperature, and high swirl. The large optical accesses of the divided chamber allowed us to follow the progress of fuel injection, vaporization, autoignition, and combustion by direc high-speed photography (8000 frames/s). Light extinetion measurements were carried out in 153 different spatial locations for 250 consecutive combustion cycles from the start of injection to the end of combustion. The photographic sequences have shown that spray is strongly distorted and mixed downstream of the high-swirling flow, resulting in a well premixed region in which the combustion starts. Then, the combustion proceeds rapidly, involving the entire chamber volume. Consequently, the flame luminosity increases, denoting fast soot formation-oxidation processes in the region close to the tangential duet in which the mising is higher. The analysis of the extinction spectra in the UV and visible range has allowed us to follow spatially and temporally the soot formation process and to identify the nature of the particulate matter and the agglomeration degree of soot particles, as well as the time history of the soot volume fraction.

Determination of Size of Fuel Droplets and Soot Particles in a Diesel Engine by Broadband Extinction and Scattering Spectroscopy

The Diesel process was studied and characterized by ultraviolet-visible extinction and scattering spectroscopy. Measurements were performed on an optically accessible Diesel engine, realized by modifying a single-cylinder, air-cooled, 4-stroke Diesel engine, by means of an external combustion chamber with three optical accesses. Two of them were along the longitudinal direction, for the extinction measurements, and the third along the orthogonal direction, for the scattering measurements. The optical measurements were performed with a temporal resolution of 0.05 ms and with a spatial resolution of 0.1×1 mm2. The simultaneous use of broadband extinction and scattering coefficients in the above-mentioned spectral range permitted the real time evaluation of size, concentration of droplets and soot particles and their optical properties. Aromatic hydrocarbons and carbonaceous matter exhibit strong absorption bands in the 200–350 nm spectral range. At fixed time or crank angle, the optical properties change with the wavelength, hence each measurement furnishes independent information on fuel droplets and soot particle size inside the combustion chamber. The experimental results were compared with a model simulation based on Mie theory for the fuel droplets and on Rayleighs theory for soot particles.