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Brake wear emissions can be reduced by altering the surface of brake disks. A parametric study using a gray cast iron and a laser-cladded brake disk was performed in a pin-on-disk experiment with integrated optical pin surface characterization and particle emission measurement. Significant differences in the friction, wear and emission behavior are present. The high wear-resistance of the laser-cladded disk led to a reduction of 70% of the particle number emission relative to the gray cast iron disk, but the coefficient of friction was unstable. The surface of the pin used with the gray cast iron showed an initial large debris extension and protruding patches that were removed at high braking energies, exposing white patches and creating holes. These observations correspond to known processes from the plateau theory. The surface of the pin used with the laser-cladded disk showed a topography dominated by holes with almost no protruding patches. The braking condition did not influence the pin surface, implying that the disk and not solely the pin surface might be governing the friction process, and therefore challenging the applicability of the plateau theory to laser-cladded disks. To further study this aspect, a segmentation method was developed for the pin surface images and topographical data to extract and quantify different features on the pin, such as debris, patches, holes and the tribolayer. The correlation of the surface coverage ratios of the feature classes with the braking conditions (speed and applied pressure), the coefficient of friction and the emissions confirmed the differences between the gray cast iron and laser-cladded brake disk.

Repeated single cavitation bubble experiments were performed primarily on 316L stainless steel, and some on nickel–aluminum–bronze (NAB) and pure aluminum. The bubble dynamics were recorded with two high-speed cameras and correlated with surface images, also acquired in situ. These experiments were performed for a range of stand-off distances γ (the ratio of the distance of the solid surface from the bubble to the bubble’s maximum radius) from 0.3 to 2.15. For all stand-off distances, single pits were the only surface change detected at the beginning of damage formation. Later phases of the collapse are not axisymmetric but show regions of “stronger” collapse, and the pits occur on the material underneath those regions. For γ  < 0.4, the damage is attributed to the second collapse. For γ  > 0.4, the first bubble collapse is most likely responsible for pitting. Shock-wave emission was detected from the collapse regions that were linked to the damage. On 316L, the pitting rate was found to be linearly dependent on the bubble radius, indicating a non-zero lower limit for the bubble radius below which pits do not occur. In terms of stand-off distance, the pitting rate (defined here as average pits per bubble) was non-monotonic, with maxima for bubbles initiated closest to the sample ( γ  = 0.3) and at γ  = 1.4.

To reduce emissions and fuel consumption, the current generation of gasoline engines uses technologies such as direct injection, downsizing and supercharging. All of them require a strong vortical in-cylinder charge motion, usually described as “tumble”, to improve fuel-air mixing and enhance flame propagation. The tumble development strongly depends on the flow field during the intake stroke. This flow field is dominated by the intake jet, which has to be captured well in the simulation. This work investigates the intake jet on a steady-state flow bench, especially in the vicinity of the intake valve. At first, the general flow dynamics of the intake jet for three different valve lifts and three different mass flows were investigated experimentally. For the smallest valve lift (3 mm), flow-field measurements using Particle Image Velocimetry (PIV) show that the orientation of the intake jet significantly depends on the air flow rate, attaching to the pent roof for low flow rates. This phenomenon is less pronounced for higher valve lifts. An intermediate valve lift and flow rate were chosen for further investigations by scale-resolving simulations. Three different meshes (coarse, medium and fine) and two turbulence models (Sigma and Detached Eddy Simulation-Shear Stress Transport (DES-SST)) are applied to consider their effect on the numerical results. An ad-hoc post-processing methodology based on the ensemble-averaged velocity field is presented capturing the jet centerline’s mean velocity and velocity fluctuations as well as its orientation, curvature and penetration depth. The simulation results are compared to each other as well as to measurements by PIV.

Modern spark-ignited internal combustion engines have intake ports designed to introduce high levels of so-called “tumble” charge motion. Correspondingly high shear rates can lead to high fluctuations and turbulence within the combustion chamber. A suitable test case to characterize the intake flow is a steady-state flow bench. Although routinely used in the engine development process to determine the global discharge coefficients, only a few detailed numerical and experimental studies use this test case to analyze the flow in the vicinity of the valve with high spatial and temporal resolution. In this paper, we combined highly resolved two-dimensional, two-component Particle Image Velocimetry (PIV) measurements and numerical simulations using a Detached-Eddy Simulation (DES) model to characterize engine-relevant flow features on a flow bench. The spatial resolution of numerical simulations on two different grids is assessed and compared to that of the PIV measurement. The results of simulations and experiment are then compared in terms of their mean and fluctuation velocity fields and the jet orientation. A detailed study of the area around the valve seats investigates the validity of wall functions in this region. Finally, we examine structures induced by vortex-shedding at the valve stem and if they are transported into the combustion chamber. Les moteurs à combustion interne modernes à allumage commandé disposent de conduits d’admission conçus pour générer des niveaux importants de mouvements de charge dits « tumble ». Les niveaux importants de taux de cisaillement qui en résultent peuvent conduire à de fortes fluctuations et turbulences dans la chambre de combustion. Un cas test adapté pour caractériser l’écoulement d’admission est un banc volute. Bien que son utilisation durant les phases de conception moteur pour déterminer les coefficients de perte de charge globaux soit très répandue, seules quelques études numériques et expérimentales détaillées utilisent ce test pour étudier l’écoulement au voisinage de la soupape avec des résolutions spatiales et temporelles élevées. Dans le présent article, nous avons combiné des mesures PIV bi-composants, bidimensionnelles hautement résolues et des simulations numériques utilisant une approche de type « Detached-Eddy Simulation » pour caractériser des structures d’écoulement d’importance pour le moteur sur un banc volute. La résolution spatiale des simulations numériques est évaluée sur deux maillages et comparée à celle de la mesure « Particle Image Velocimetry », PIV. Les résultats numériques et expérimentaux sont ensuite comparés en termes de champ de vitesse moyen et fluctuant et d’orientation des jets. Une étude détaillée du siège de soupape est réalisée pour vérifier la validité de lois de paroi dans cette zone. Enfin, nous examinons les structures induites par les détachements tourbillonnaires autour de la tige de soupape et si elles sont transportées dans la chambre de combustion.

NRC publication: Yes

Cavitation erosion is typically studied with ultrasonic sonotrodes. Only a few attempts have been made to study cavitation erosion of technical alloys on the level of repeated single bubbles. Such single cavitation bubbles can be induced by a focused laser pulse with high spatio-temporal repeatability. In this work, the surface damage caused by series of laser-induced single bubbles in water is observed with a light microscope in-situ between two successive bubbles. Polished samples from pure aluminum, an austenitic steel (316L, X2CrNiMo18-15-3), and a nickel aluminum bronze (CuAl10Ni5Fe5) were subjected to series of bubbles that typically had a maximum diameter of d  = 2.5 mm and a non-dimensional stand-off distance γ  = 1.4. Via in-situ microscopy, the appearance of individual pits can be assigned to a specific, single bubble collapse event without removing the sample. Consistent with literature, for the chosen parameters the damaged region after many bubbles is circular, with individual pits that are deeper for aluminum than for the bronze and the steel. Additionally, our findings suggest that even high-strength materials can be damaged by the impact of just one single bubble, while not every single bubble causes a pit on the soft aluminum. From series of images after each bubble, the rate of pit accumulation was determined to be 2.6 pits/bubble for aluminum and around 0.3–0.5 pits/bubble for the two technical alloys.

Mixture formation in a hydrogen-fueled heavy-duty engine with direct injection and a nearly-quiescent top-hat combustion chamber was investigated using laser-induced fluorescence imaging, with 1,4-difluorobenzene serving as a fluorescent tracer seeded into hydrogen. The engine was motored at 1200 rpm, 1.0 bar intake pressure, and 335 K intake temperature. An outward opening medium-pressure hollow-cone injector was operated at two different injection pressures and five different injection timings from early injection during the intake stroke to late injection towards the end of compression stroke. Fuel fumigation upstream of the intake provided a well-mixed reference case for image calibration. This paper presents the evolution of in-cylinder equivalence ratio distribution evaluated during the injection event itself for the cylinder-axis plane and during the compression stroke at different positions of the light sheet within the swirl plane. During the injection event, the originally annular jet collapses onto the jet axis within 1°CA after jet emergence and within 10 mm downstream of the nozzle. Multiple shock cells are visible – their size decreases with decreasing pressure ratio. The results of the equivalence ratio distribution show high cyclic variability of mixing for all injection timings during the compression stroke, but only minor variability with early injection during the intake stroke. The ensemble-mean fuel distribution shows that fuel-rich zones shift from the intake side to the exhaust side of the combustion chamber as the injection is advanced. Probability density functions of global equivalence ratio and equivalence ratio at potential spark locations suggest that retarded fuel injection might significantly increase NO emissions and the cyclic variability of early flame kernel development.

Mixture formation in an optically accessible hydrogen-fueled engine was investigated using Planar Laser-Induced Fluorescence (PLIF) of acetone as a fuel tracer. The engine was motored and fueled by direct high-pressure injection. This paper presents the evolution of the spatial distribution of the ensemble-mean equivalence ratio for six different combinations of nozzle design and injector geometry, each for three different injection timings after intake-valve closure. Asymmetric single-hole and 5-hole nozzles as well as symmetric 6-hole and 13-hole nozzles were used. For early injection, the low in-cylinder pressure and density allow the jet to preserve its momentum long enough to undergo extensive jet-wall and (for multi-hole nozzles) jet-jet interaction, but the final mixture is fairly homogeneous. Intermediately timed injection yields inhomogeneous mixtures with surprisingly similar features observed for all multi-hole injectors. Fuel is concentrated near the cylinder wall, an unfavorable scenario were the engine to be fired. Results for late injection depend more on the particular injector configuration. The 13-hole injector shows complete merging of all jets, consistent with results in the literature. The influence of intake-induced bulk-gas tumble is minor for the current injector and combustion-chamber configurations.

UV-chemiluminescence from the excited hydroxyl-radical (OH*) has been used as a marker for the high-temperature reacting zone in spark-ignited engines for quite some time. In research engines with large optical access, sensitive camera systems make it possible to obtain images of the flame that can be used for, e.g., determining the flame-front's propagation speed [Aleiferis et al., Combust. Flame 136 (2004) 283-302]. However, on one hand such optical engines are limited in their speed and load range, on the other, typical UV endoscopes make wide-field imaging at low light levels challenging. Here, a large-aperture UV endoscope is used to capture sequences of OH* chemiluminescence during early flame propagation in a nearly unmodified production engine. We compare three imaging systems: phase-locked single-shot imaging, phase-locked double-frame imaging, and “high-speed” cinematography at kHz repetition rates. The four-cylinder spark-ignition engine can be operated at speeds and loads significantly exceeding the limits of most fully optically accessible engines. During the first 20° crank-angle after ignition, the phase-locked endoscopic images almost match the image quality reported from experiments in a dedicated optically-accessible engine. For later acquisition timings, the flame often exceeds the field of view. From phase-locked imaging the instantaneous size of the apparent burnt area (ABA) can be identified by thresholding after filtering. Its single-shot variant allows only computation of the multi-cycle average of the apparent flame speed (AFS). Acquisition of two successive frames in a single cycle enables determining the instantaneous AFS. High-speed imaging can follow a single cycle and thus the time-resolved ABA can be estimated, but the instantaneous shape of the flame cannot be imaged with much detail, because the detector hardware is less mature.

Quantitative planar laser-induced fluorescence (PLIF) of gaseous acetone as a fuel-tracer has been used in an optically accessible engine, fueled by direct hydrogen injection. The purpose of this article is to assess the accuracy and precision of the measurement and the associated data reduction procedures. A detailed description of the acetone seeding system is given as well. The key features of the experiment are a high-pressure bubbler saturating the hydrogen fuel with acetone vapor, direct injection into an optical engine, excitation of acetone fluorescence with an Nd:YAG laser at 266 nm, and detection of the resulting fluorescence by an unintensified camera. Key steps in the quantification of the single-shot imaging data are an in-situ calibration and a correction for the effect of local temperature on the fluorescence measurement. We assess the accuracy of the measurement in terms of drift in acetone-vapor concentration, linearity of fluorescence with laser energy, absorption of the beam within the probe volume, spatial inhomogeneity in the calibration measurements, and uncertainties in the temperature correction. The precision is impacted by camera read-out noise, shot (quantum) noise, shot-to-shot variations in total laser-pulse energy and the transverse energy profile, and “beam steering” by thermal gradients. Procedures to quantify and if possible minimize all of the above factors are described. Among the factors investigated, the single greatest impact on accuracy and precision has uncertainty in the calibration of global equivalence ratio and refractive beam steering, respectively.