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This paper shows a numerical comparison between DBI and focused shadowgraph imaging techniques applied in the characterization of macroscopic parameters and optical thickness of a multi-hole GDI spray. Simulated experiments were designed around a synthetic spray, whose jets have been modelled through the Musculus and Kattke model. The DBI configuration was in line with the experimental constraints for beam steering suppression reported in the literature. A simple ray-tracing programme based on geometrical optics and radiometric laws was developed, and the attenuation of light through the spray was evaluated according to Lambert-Beer law. The results show that diffuse backlighting can appreciably bias the measurement of the optical thickness and the macroscopic spray parameters, net of any beam steering effects. Significant discrepancies between techniques occur when the spray develops out of the focal plane and away from the optical axis, especially when the spray develops asymmetrically to the focal plane.

A major challenge for internal combustion engines (ICEs), and diesel engines, in particular, is the reduction of exhaust emissions, essentially nitrogen oxides (NOx) and particulate matter (PM). In this regard, the potential of LPG-diesel blends was evaluated in this work. The LPG and diesel blends were externally prepared by exploiting their perfect miscibility at high pressures. Two diesel-LPG mixtures with 20% and 35% by mass LPG concentrations were tested. In terms of spatial and temporal evolution, the spray characterization was performed for the two blends and pure diesel fuel through high-speed imaging technique. The combustion behavior, engine performance and exhaust emissions of LPG-diesel blends were evaluated through a test campaign carried out on a single-cylinder diesel engine. Diesel/LPG sprays penetrate less than pure diesel. This behavior results from a lower momentum, surface tension and viscosity, of the blend jets in comparison to diesel which guarantee greater atomization. The addition of LPG to diesel tends to proportionally increase the spray cone angle, due to the stronger turbulent flow interaction caused by, the lower density and low flash-boiling point. Because of improved atomization and mixing during the injection phase, the blends have shown great potential in reducing PM emissions, without affecting engine performance (CO2 emissions). The addition of LPG resulted in a significant smoke reduction (about 95%) with similar NOx emissions and acceptable THC and CO emissions. Furthermore, the low cetane number (CN) and high low-heating value (LHV) ensuring leaner air-fuel mixture, and improvements in terms of efficiency, particularly for a blend with a higher concentration of LPG.

The DI (Direct Injection) in SI (Spark Ignition) engines is rapidly developing and seems very attractive offering the possibility of multi-mode operation, homogeneous and stratified charge, with valuable benefits respect to conventional PFI (Port Fuel Injection). One of the major drawbacks of the GDI-fueled engine is the impingement of liquid fuel on the combustion chamber walls that generally produces an increasing of HC emissions and detrimental effects on the combustion process, like soot formation in the wall guided engines due to the diffusive combustion of fuel film deposits on the piston head. This paper investigates the structure of gasoline spray-wall interaction generated by a single-hole injector for GDI engines in an optically-accessible quiescent vessel. The axially-disposed hole diameter is 0.200 mm with a L/d ratio of 1.0 while the injection pressure and the wall temperature varied in the range of 10.0–20.0 MPa and ambient to 300 °C, respectively. Injection duration of 1.0 ms was investigated and the nozzle tip-wall distance was 22.5 mm. Optical investigations were carried out at atmospheric backpressure by an in-line schlieren optical setup using a LED source while the cycle-resolved images of the evolving impact were captured by a C-Mos high-speed camera with a time resolution of 80 μs and a window size of 640 × 464 pixels. The optic-geometrical setup realized a spatial resolution of 0.039 mm/pixel. The images were treated by a digital processing software for background subtraction and both liquid and vapor contour extractions. The spatial and temporal evolution of the liquid and vapor phases as a function of the wall temperature is described for the different operative conditions.

The paper describes the results of a co-operative research project aimed at the energetic exploitation of the biodiesel derived glycerol. The research objective is the definition and validation of a synthesis process for conversion of glycerol in an oxygenated fuel additive (glycerol alkyl–ether) suitable for blending with diesel and biodiesel. The employed facilities, methodologies and first results from engine tests are described, including a preliminary Life Cycle Assessment (LCA). Etherification of glycerol with tert-butyl alcohol and isobutylene allowed to identify the suitable experimental conditions required to obtain a mixture predominantly composed of higher glycerol ethers (GEM). Novel catalysts, based on perfluorosulphonic ionomers as active species and spherical silica as support, were found to be stable and easy reusable allowing to obtain mixtures containing very low amount of monoethers and oligomers considered as undesired products. Engine tests with blends of GEM in diesel fuels revealed good combustion and emission performances when compared both to neat diesel fuel and a blend diesel/biodiesel. LCA analysis indicates that the GEM/diesel blend generates an environmental impact which is lower than the one of neat diesel.

The present work is focused on multi-dimensional simulations of combustion in diesel engines. The primary objective was to test, in a diesel engine framework, a soot particle size model to represent the carbon particle formation and calculate the corresponding size distribution function. Simulations are performed by means of a parallel version of the KIVA3V numerical code, modified to adopt detailed kinetics reaction mechanisms. A skeletal reaction scheme for n-heptane autoignition has been extended, to include PAH kinetics and carbonaceous particle formation and consumption rates: the full reaction set is made up of 82 gas species and 50 species accounting for the particles, thus the complete reaction scheme comprises 132 species and 2206 reaction steps. Four different engine operative conditions, varying engine speed and load, are taken into account and experimentally tested on a single cylinder diesel engine fuelling pure n-heptane. Computed particle size distribution functions are compared with corres...

This article introduces an innovative method for the segmentation of Mie-scattering and schlieren images of GDI sprays. The contours of the liquid phase are obtained by segmenting the scattering images of the spray by means of optimal filtering of the image, relying on variational methods, and an original thresholding procedure based on an iterative application of Otsu's method. The segmentation of schlieren images, to get the contours of the spray vapour phase, is obtained by exploiting the surface curvature of the image to strongly enhance the intensity texture due to the vapour density gradients. This approach allows one to unambiguously discern the whole vapour phase of the spray from the background. Additional information about the spray liquid phase can be obtained by thresholding filtered schlieren images. The potential of this method has been substantiated in the segmentation of schlieren and scattering images of a GDI spray of isooctane. The fuel, heated to 363 K, was injected into nitrogen at a density of 1.12 and 3.5 kg m-3 with temperatures of 333 K and 573 K.

In internal combustion engines, the direct injection at high pressures produces a strong impact of the fuel on the combustion chamber wall, especially in small-bore sizes used for passenger cars. This effect is relevant for the combustion process resulting in an increase of the pollutant emissions and in a reduction of the engine performances. This paper aims to report the effects of the injection pressure and wall temperature on the macroscopic behavior and atomization of the impinging sprays on the wall. The gasoline spray-wall interaction was characterized inside an optically accessible quiescent chamber using a novel make ready Z-shaped schlieren-Mie scattering set-up using a high-speed C-Mos camera as imaging system. The arrangement was capable to acquire alternatively the schlieren and Mie-scattering images in a quasisimultaneous fashion using the same line-of-sight. This methodology allowed complementing the Mie scattering images, adapting to the liquid phase, with the schlieren ones for the determination both of the liquid and vaporing phase during the single cycle. A single-hole axially disposed injector was used, 0.200 mm in diameter L/d=1.0, while the injection pressure and the wall temperature ranged between 5.0 - 20.0 MPa and ambient to 573 K, respectively. Optical investigations were carried out at atmospheric backpressure and ambient gas temperature. The images of the impact, resolved in the cycle, were processed by a customized algorithm able to catch the contours of the liquid phase and the vapor/atomized zone. Spatial and temporal evolutions were measured for both the phases in terms of width penetration and thickness growth.

The soot formation process in the high temperature - high pressure pyrolysis of ethylene has been studied in a conventional diaphragm-type shock lube behind reflected shock waves. The investigated ranges of temperature and pressure were 1600-2200 K and 9-13 bar, respectively. Light scattering/extinction measurements in the visible, and extinction measurements in the near infrared have been analysed to characterise the soot formation process. Pyrolytic products, formed prior lo sool particles, have been evidenced by continuous absorption measurements, with high time/spectral resolution, in the UV-visible range. The absorption spectra have been analysed according to the Tauc optical band-gap model. In the investigated experimental conditions, the characteristic optical band-gaps of aromatic structures with 2-4 rings at most have been observed before soot-inception. After the soot-inception, a sharp decrease of the band-gap has been revealed, indicating a high degree of aromaticity of sool particles

An extensive investigation has been carried out on the thermal behavior of biomass pyrolysis oil derived from different feedstocks (mixed hardwood, pine, and poplar). Experiments were performed on streams of monodispersed droplets (50–100 μm) and on large suspended droplets (300–600 μm). The small droplets were generated by a single-droplet generator and injected in a drop-tube furnace, the temperature of which ranged from 300 to 850°C. Light-scattering methods have been used to follow the process undergone by the pyrolysis oil droplets. The solid material, residual of the droplet heating and vaporization processes, has been examined by means of optical and scanning electron microscopy. Two different morphologies of residual particles have been observed at the exit of the furnace: (a) more compact, mechanically resistant spheres, with typical diameter of 10–40 μm, and (b) fragile, glasslike cenospheres with thin walls and menisci, with diameter of 100–200 μm. The large droplets were suspended at the exit of the furnace, using both a quartz fiber and an exposed junction thermocouple. The furnace temperature was varied between 400 and 1200°C. The phenomena, occurring during the vaporization and burning of each droplet, were followed by coupling the thermocouple measurement to high-speed visualization. The imaging was performed by means of fast digital video recording (50–250 frames/s) and high-speed cinematography (400–1000 frames/s). Temperature-time curves of the burning droplets show a stepped behavior, with two zones at constant temperature, at ∼100 and ∼450°C. The first step is due to the vaporization of water. The second plateau corresponds to the heating of heavy compounds due to selective vaporization and liquid-phase pyrolysis. Swelling, shrinking, and microexplosions with ejection of matter characterize this phase. The combustion of the droplets starts with an enveloping blue flame. The flame then develops a yellow tail of increasing size. After the flame extinction, the remaining char particle burns without residual.

This paper deals with the evaluation of the effect of fuel properties on soot formation in a GDI (gasoline direct injection) engine. Experimental investigation was carried out in an optical 4-stroke small single cylinder engine for twowheel vehicles. The engine displacement was 250 cc. It was equipped with an elongated piston with a wide sapphire window in the head and a quartz cylinder liner. The engine was fuelled with pure gasoline and ethanol, and ethanol/gasoline blends at 20% v/v, 50% v/v and 85% v/v. Optical techniques based on 2D-digital imaging were used to follow the combustion process and soot formation. Spectroscopic measurements were carried out in order to assess the soot evolution. Radical species such as OH and CH, related to fuel quality and to soot formation/oxidation process, were detected. Measurements were carried out at various engine speeds and loads in order to allow optical measurements and to test the engine in real conditions. Regulated gas emissions and opacity were measured at the exhaust by conventional equipment. Moreover, particle emissions were characterized also in terms of number concentration and size distribution using a differential mobility sizer.

Some oxygenated hydrocarbons were tested as pure fuels in two different DI diesel engines, and their emission potential was compared to n-tetradecane combustion. Two colours pyrometry method was used to infer in-cylinder sooting tendency of tested fuels. Pure pyrolysis of the same fuels was also investigated in a conventional shock lube at high temperature (1700–2500 K) and pressure (9–13 bar), using light scattering/extinction methods. All oxygenated compounds tested in the engines exhibited a strong decrease of soot loading compared with tetradecane combustion. The long soot induction times, as measured in shock tube experiments, and the oxygen content of the fuel molecules seem to provide a plausible explanation of soot lacking in oxygenated fuels combustion. Emission measurements at the exhaust of a four cylinder engine fully confirm the trends obtained by two colours pyrometry and shock tube experiments. As a matter of fact, the oxygenated synthetic fuels strongly reduce both gaseous and total partic...

Soot aggregates have been produced in a series of experiments based on the pyrolysis of ethylene in a shock tube facility, for pressure and temperature varying, respectively, in the range ∼ 10–15 bar and ∼ 1800–2000 K. In situ light scattering/extinction coupled measurements have been performed and the process has been followed with high spatial and temporal resolution (μm/μs). On the basis of the Kumar and Tien approach for randomly oriented aggregates [Kumar and Tien (1989) Combust. Sci. Technol. 66, 199–216], we investigated the sensitivity of the angular scattering pattern and of the dissymmetry ratio for straight chain and random cluster morphologies to the refractive indices, the number and diameters of primary spheres (spherules) in the agglomerate and the solid volume fraction of the cluster. The time variation of the scattered light intensity measured at different scattering angles θ (30°, 90°, 150°) is shown to enable deduction of the prevailing morphology of the (self-organizing) aggregate. At the end of the examined agglomeration process, the resulting soot aggregates appear to be structured on average as chains of 5–10 spherules with spherule diameter of ≈ 25–30 nm.

The use of oxygenated and renewable fuels is nowadays a widespread means to reduce regulated pollutant emissions produced by internal combustion engines, as well as to reduce the greenhouse impact of transportation. Besides PM, NOx and HC emissions, also the size distribution of particles emitted at the engine exhaust represent a meaningful information, considering its adverse effects on the environment and human health. In this work, the results of a comprehensive investigation on the combustion characteristics and the exhaust emissions of a GDI high performance engine, fuelled with pure Bio-ethanol and European gasoline, are shown. The engine is a 4-cylinder, 4 stroke, 1750 cm3 displacement, and turbocharged. The engine was operated at different speed/load conditions and two fuel injection strategies were investigated: homogeneous charge mode and stratified charge mode. The particle size distribution at the engine exhaust was measured in the range 5-1000 nm using a differential mobility spectrometer. Simultaneously, the air-fuel mixing and the combustion process inside the cylinder were followed by means of high-speed imaging in the UV-visible. To this aim, the last cylinder was optically accessible through a sapphire window (5mm wide) in the engine head. The particles size distributions measured at the engine exhaust for both gasoline and bio-ethanol show similar trends, suggesting a common route for the particles formation. For bio-ethanol the accumulation mode is shifted towards smaller sizes, which could be ascribed to the chemical and physical properties of the fuel, as observed by optical investigation of the fuel injection and combustion process. On the other hand, an increase of particles number concentration for stratified charge operating mode was observed for both fuels, due to the poorer air fuel mixing and the diffusive combustion of the fuel films deposited on the piston and cylinder wall.

In the present paper, the effect of the clean and cold EGR flow on the performance of a diesel engine running under Low Temperature Combustion conditions is investigated by means of experimental tests on a single-cylinder research engine. The engine layout was “ad hoc” designed to isolate the effect of the clean and cold recirculated gas flow on the combustion quality. The results have shown the possibility to increase the EGR rate by EGR flow temperature reduction, with a decrement of both NOx and PM emissions, at the same fuel consumption, highlighting that the intake manifold temperature is the main factor affecting the engine performances.

Combusiton of pine pyrolysis oil droplets was studied at different pressures up to a maximum of 60 bar in a single-droplet combustion chamber. Oil droplets, with diameters between 400 \gmm and 120 \gmm were suspended to a thermocouple of to a quartz fiber. Their behavior was followed by means of high-speed digital imaging based on a shadowgraph scheme. About two thousands of frames were collected during every test with acquisition speed between 400 and 4000 frames/s. Droplets were easy to ignite at normal as well as at high pressure. Increasing the pressure the intensity of swelling phenomena, undergone by droplets, decreased and completely disappeared at pressures higher than 20 bar. However, bubbling and foaming were always observed. Liquid-phase pyrolysis and the formation of cenospheres as combustion residual were observed under all the pressure conditions.

A light-scattering anaysis based on Lorenz-Mie theory shows that the size and refractive index of transparent droplets can be determined by measuring the polarized components of the scattered light at two angles in the forward direction. The horizontally polarized cross section C(HH)(33°) depends exclusively on the droplet diameter, whereas the ratio C(HH)(33°)/C(HH)(60°) is a sensitive function of the refractive index and hence of the temperature. On this basis, a new optical system for measuring the temperature, size, and velocity of transparent droplets has been developed. This system can make possible the determination of droplet temperature within a few degrees centigrade. In addition, a critical review of the rainbow method to determine droplet temperature is also presented. These techniques have been applied to vaporizing tetradecane droplets (D(0) = 72 µm), which are heated up in a tube furnace with a temperature range of 20°-200 °C.

The present work relates to the investigation of the basic oxidation characteristics of iron and aluminium nanoparticles as well as the feasibility of their combustion under both Internal Combustion Engine (ICE)-like and real engine conditions. Based on a series of proof-of-concept experiments, combustion was found to be feasible taking place in a controllable way and bearing similarities to the respective case of conventional fuels. These studies were complimented by relevant in-situ and ex-situ/post-analysis, in order to elaborate the fundamental phenomena occurring during combustion as well as the extent and ‘quality’ of the process. The oxidation mechanisms of the two metallic fuels appear different and -as expected- the energy release during combustion of aluminium is significantly higher than that released in the case of iron. The peak gas temperatures recorded during combustion of iron under engine-like and real engine conditions indicate that the NOx formation threshold is not reached. Combustion of aluminium yields substantially higher peak combustion temperatures and thus it is likely that is accompanied by NOx emissions. In addition, the experimental results provided indication that the approach of Exhaust Gas Recirculation (EGR) could be an efficient means of controlling the combustion process.