Validazione di una metodologia basata sulla cinetica chimica per la deduzione della detonazione nei motori GDI. Modellazione del combustibile e confronto fra approccio sintetico e statistico

La narrativa dominante nell'occidente benestante è che il cambiamento climatico rappresenta una "minaccia esistenziale" e per evitarlo sono necessari tagli molto rapidi alle emissioni di gas serra (GHG) e quindi all'uso di combustibili fossili. È improbabile che i livelli di gas serra scendano in modo significativo nei prossimi decenni e anche se lo facessero, gli eventi meteorologici estremi non scompariranno. La ricerca sulla combustione, in particolare sui combustibili fossili e sui motori a combustione interna (ICE) è attualmente considerata non necessaria in molti paesi. Tuttavia, sarà assolutamente necessario, insieme allo sviluppo di alternative, al fine di garantire un migliore utilizzo dell'energia poiché la combustione continuerà a essere fondamentale per la fornitura di energia globale e la guida dei trasporti per i decenni a venire. È comunemente accettato che il rapido passaggio dal veicolo ICE puro al veicolo elettrico a batteria (BEV) possa essere attuato solo tramite lo sviluppo del concetto di veicolo ibrido dotato di motore a combustione interna. Negli ultimi anni le normative sugli inquinanti e le preoccupazioni sul consumo di carburante stanno spingendo i produttori di motori verso la ricerca di una maggiore efficienza termica e potenza specifica per i motori a combustione interna di prossima generazione. Per raggiungere questi obiettivi specifici, i produttori di motori sono impegnati ogni giorno contro molte sfide. Senza ombra di dubbio, la detonazione del motore è il principale aspetto sfidante da considerare nello sviluppo di un'unità motore alimentata a benzina. La detonazione si verifica quando le condizioni davanti al fronte di fiamma in un motore ad accensione comandata danno luogo ad una serie di fenomeni di autoaccensione per via di condizioni di pressione e temperature critiche raggiunte nella zona dei gas non combusti. Da un punto di vista della modellizzazione, la natura stocastica della detonazione del motore, correlata all'instabilità della combustione e alla variabilità da ciclo a ciclo dei flussi turbolenti, suggerirebbe la simulazione Large-Eddy (LES) come l'approccio più appropriato per le simulazioni CFD. Nonostante ciò sia concettualmente vero e diverse pubblicazioni mostrino l'applicabilità di LES sia ai motori da ricerca che a quelli di produzione, la LES rimane ancora un approccio che richiede molto tempo e CPU che difficilmente può essere integrato nel processo di progettazione industriale e tempi per lo sviluppo di nuove unità: nonostante la maturità dei modelli LES per applicazioni motoristiche, l'applicazione diffusa al flusso di lavoro industriale è tutt'altro che possibile. Per limitare i costi e i tempi di calcolo, i modelli Reynolds Averaged Navier-Stokes (RANS) vengono solitamente scelti per rappresentare il comportamento medio del motore. In letteratura sono disponibili numerosi modelli di knock basati sul concetto di assegnare la reattività media cellulare senza considerare una variabilità intrinseca delle proprietà termofisiche come pressione e temperatura ma anche proprietà legate alle condizioni di funzionamento del motore come il rapporto Aria-Combustibile e la percentuale di ricircolo dei gas di scarico. D'altra parte, questa limitazione può essere in parte superata dall'uso di equazioni di varianza per variabili fisiche fondamentali per l’approccio RANS. Le informazioni fornite da questo tipo di modelli sono di natura statistica e si basano sulla varianza generata dalla turbolenza dei campi fisici, che a sua volta influisce sulla velocità di reazione del gas finale verso l'autoaccensione. Tali modelli RANS basati su statistiche sono in grado di ricostruire artificialmente una presunta probabilità di cicli di battito, che può essere un'indicazione molto utile per il progettista del motoreThe dominant narrative in the affluent west is that climate change poses an “existential threat” and very rapid cuts in greenhouse gas (GHG) emissions and hence fossil fuel use are needed to avoid it. Transport is particularly difficult to decarbonize and current policies focusing entirely on battery electric vehicles will not and must not succeed. GHG levels are unlikely to come down significantly in the next several decades and even if they did, extreme weather events will not disappear. Combustion research, particularly of fossil fuels and in internal combustion engines (ICEs) is currently seen as unnecessary in many countries. However, it will be absolutely necessary, along with the development of the alternatives in order to ensure that energy use is improved since combustion will continue to be central to supplying global energy and driving transport for decades to come. It is commonly accepted that the rapid change from pure ICE vehicle to battery electric vehicle (BEV) can be implemented only toward the development of the concept of hybrid vehicle supplied of internal combustion engine. In the latest years pollutant regulations and fuel consumption concerns are pushing engine manufacturers towards the quest for higher thermal efficiency and specific power output for the next generation internal combustion engines. To reach these specific targets, engine manufacturers are busy every day against many challenges. Without shadow of doubt, engine knock is the main challenging aspect to be consider in the development of engine unit fuelled with gasoline. Knock occurs when conditions ahead of the flame front in an spark ignition engine result in one or more of autoignition events in the end-gas region. From a modelling standpoint, the stochastic nature of engine knock, related to combustion instability and cycle-to-cycle variability of turbulent flows, would suggest Large-Eddy Simulation (LES) as the most appropriate approach for CFD simulations. Despite this is conceptually true and several publications show the applicability of LES to both research and production units, LES still remains a very time- and CPU-demanding approach which can hardly be integrated in the industrial design process and timeframe for the development of new SI units: despite the maturity of LES models for engine flows, the widespread application to the industrial workflow is far from being possible. To limit computational costs and times, Reynolds Averaged Navier-Stokes (RANS) models are usually chosen to represent the average engine behaviour. In literature are available numerous knock models based on the concept to assign mean cell-wise reactivity without consider an intrinsic variability of the thermo-physic properties like pressure and temperature but also properties linked to the operating engine conditions like Air-to-Fuel ratio and the percentage of exhaust gas recirculation. On the other side this limitation can be partly overcome by the use of variance equations for fundamental physical variables in RANS. The information given by this kind of models is of statistical nature and it is grounded in turbulence generated variance of physical fields, which in turn affects the end-gas reaction rate towards autoignition. Such statistics-based RANS models are able to artificially reconstruct a presumed probability of knocking cycles, which can be a very useful indication to the engine designer. This manuscript has the aim to highlight the pro and cons of the abovementioned approaches, introducing before the fuel model concept, that is to say, starting from a real gasoline containing a percentage of alcohol, will be defined two different surrogates which mimic real gasoline anti-knock properties. Every surrogate will be tested and used in conjunction with the two approaches to predict knock event for the same tested engine conditions

Preliminary study on the influence of Octane Sensitivity on knock statistics in a GDI engine

In the 3D-CFD practice, actual gasoline fuels are usually replaced by surrogate blends composed of Iso-Octane, n-Heptane and Toluene (Toluene Reference Fuels, TRFs). In this work, the impact of surrogate formulation on the probability of end-gas auto-ignition is investigated in a single cylinder engine. CFD simulations are run on equal charge stratification to discern the effect of fuel reactivity from that of evaporation and mixing. Blends are formulated using an internal methodology, coupled with a proprietary method to predict knock statistical occurrence within a RANS framework. Chemical kinetics calculations of Ignition delay times are performed in a 0D constant pressure reactor using a mechanism for gasoline surrogates, proposed by the Clean Combustion Research Center of King Abdullah University of Science and Technology (KAUST), consisting of 2406 species and 9633 reactions. Surrogates mimic a commercial European gasoline (ULG95). Five different formulations are presented. Three are characterised by equal RON (95) with progressively decreasing Octane Sensitivity S. The fourth and the fifth have a sensitivity of 10 but with lower RON (92.5 and 90). The combinations allow the reader to separate the effects of octane sensitivity from those of RON quality of the tested fuels. Applying the different surrogates, changes in each of autoignition phasing, magnitude and statistical probability are investigated. Results confirm the dependency of knock occurrence on the Octane Sensitivity, as well as the need to include engine-specific and operation-specific characteristics in the analysis of knock. The Octane Index (OI) formulation developed by Kalghatgi is discussed

Impact of Grid Density on the Analysis of the In-Cylinder Flow of an Optical Engine

The evaluation of Internal Combustion Engine (ICE) flows by 3D-CFD strongly depends on a combination of mutually interacting factors, among which grid resolution, closure model, numerics. A careful choice should be made in order to limit the extremely high computational cost and numerical problems arising from the combination of refined grids, high-order numeric schemes and complex geometries typical of ICEs. The paper focuses on the comparison between different grid strategies: in particular, attention is focused firstly on near-wall grid through the comparison between multi-layer and single-layer grids, and secondly on core grid density. The performance of each grid strategy is assessed in terms of accuracy and computational efficiency. A detailed comparison is presented against PIV flow measurements of the Spray Guided Darmstadt Engine available at the Darmstadt University of Technology. As many research groups are simultaneously working on the Darmstadt engine using different CFD codes and meshing approaches, it constitutes a perfect environment for both method validation and scientific cooperation. A motored engine condition is chosen and the flow evolution throughout the engine cycle is evaluated on two different section planes. Pros and cons of each grid strategy are highlighted and motivated

PROPOSAL OF NEW NOISE MAPPING PROCEDURES FOR NOISE ABATEMENT PLANS OF ROAD NETWORKS

In Europe the contents of Noise Action Plans of road networks are established by the Directive 2002/49/EC. In Italy another plan, the Noise Abatement Plan, is required by the national legislation, in particular by the Decree of the Ministry of the Environment of 29/11/2000. The procedures for the definition of both plans are quite similar and are based on the realization of noise maps. This paper analyses the effects of different methodologies used for noise mapping on the outcomes of the noise abatement plan of the road network managed by the Province of Terni in Italy. The road selected as case study is a single carriageway road that is 50 km long and passes nearby several sensitive receivers such as schools and hospitals. Four different methodologies were employed and the outcomes analysed. In particular in the first methodology a regular grid is used for calculations (Grid Noise Maps- GNM), while in the other ones the noise levels are evaluated at 1 meter from each building façade (Façade Noise maps – FNM). Moreover the latter three methodologies differ one from the other by the selection of the number and the position of the calculation points. The first two methodologies are in compliance with the Italian legislation while the other two are improvements developed and tested by the Authors that allow more realistic and accurate estimates of noise impact on the receivers. Usually the main outcome of an Italian noise abatement plan is the ranking of the buildings/areas that most urgently require mitigation and the definition of the anti-noise measures to be realized. The study shows how the different procedures affect these results and influence the activities of the managing authorities in the realization of the mitigation measures

Comparison of library-based and detailed chemistry models for knock prediction in spark-ignition engines

The present engine development pathway for increased specific power and efficiency is moving Spark-Ignition engines towards unprecedented levels of mean thermo-mechanical loading. This in turn promotes undesired abnormal combustion events in the unburnt mixture (also called \u201cengine knock\u201d), leading to solid parts failure and constituting a severe upper constraint to engine efficiency. In this context, CFD simulations are regularly used to investigate the fluid-dynamic reasons for engine knock and to address knock suppression strategies, using dedicated models to simulate the chemical reaction rate of the fuel/air/residual mixture at the same thermodynamics states as those encountered in engines. In this paper three different approaches are coherently compared to simulate knock occurrence on a turbocharged GDI engine, representing some of the most popular choices for modelers in the RANS framework. The first one considers the on-the-fly solution of chemical reactions, which represents the state-of-the-art knock modelling approach albeit its problematic computational cost for industrial turnaround times. The other two methods consider pre-calculated libraries of ignition delay times (calculated at constant pressure and volume, respectively) for the same fuel model, and knock timing is predicted using a classical Livengood-Wu approach coupled to the same main combustion model. All the analyzed models for the end-gas reaction rate are coupled with a dedicated combustion model for propagating flame (G-equation). A comprehensive analysis of computational cost and of knock prediction accuracy is carried out for library-based methods against the detailed chemistry model. Finally, results are critically discussed and explained using combined ignition delay time maps and traces for thermodynamic in-cylinder states, and guidelines for the a priori choice for constant pressure- or volume-generated libraries are provided. In this context, the use of a synthetic knock model combined with libraries of ignition delays calculated at constant volume emerges as an accurate and efficient modelling strategy. The study outlines a method for the well-supported use of simplified CPU-efficient models, with a promoted confidence in simulation results from the comparison with detailed chemistry

Piezoelectric bone surgery compared with conventional rotary instruments in oral surgery and implantology: Summary and consensus statements of the International Piezoelectric Surgery Academy Consensus Conference 2019

Piezoelectric bone surgery was introduced into clinical practice almost 20 years ago as an alternative method for cutting bone in dental surgical procedures, in an attempt to reduce the disadvantages of using conventional rotary instruments. The aim of this Consensus Conference was to evaluate the current evidence concerning the use of piezoelectric surgery in oral surgery and implantology

Materiali e Tecnologie Odontostomatologiche per Igienista Dentale

libro per studenti universitar