Modelling of combustion and knock onset risk in a high-performance turbulent jet ignition engine

The reduction of CO2 emissions, and hence of fuel consumption, is currently a key driver for the development of innovative SI engines for passenger car applications. In recent years, motorsport technical regulations in the highest categories have seen the introduction of limits concerning the fuel flow rate and the total amount of fuel per race, thus driving engine development toward further reduction of specific fuel consumption. Among the different techniques that can be shared between conventional and high-performance SI engines, turbocharging, compression ratio increase and Turbulent Jet Ignition (TJI) have shown a significant potential for fuel consumption reduction. The combination of turbocharging and compression ratio increase, however, can promote the onset of knocking combustion, with detrimental effects on engine’s efficiency and durability. Additionally, engines equipped with TJI systems show unusual combustion development and knock onset. In this study a methodology for the 3D-CFD modelling of combustion and knock onset risk was developed for a high-performance turbocharged engine featuring a passive TJI system. First, a comprehensive numerical study was carried out in a commercially available software, CONVERGE 2.4, in order to develop a 3D-CFD model able to reproduce the available experimental data. The resulting 3D-CFD model was then validated on different working conditions featuring different spark advances. Lastly, a methodology for the assessment of knock onset risk was developed, which led to the definition of two novel knock-risk indexes based on the progress of chemical reactions within the combustion chamber. The proposed knock-risk indexes showed good agreement with the experimental data

Comparison Between Different Hydrogen Fuelled Powertrains for Urban Busses

In the compelling need for the decarbonization of the transport sector, hydrogen could play a crucial role, especially in heavy duty applications where the limited specific energy of chemical batteries can significantly reduce either the payload or the operative range. Moreover, the possibility to use Hydrogen not only within Fuel Cells (FCs) systems but also as a fuel in Internal Combustion Engines (ICEs) makes it even more attractive for future sustainable transport systems. In such a framework, this work aims to compare, through numerical simulation, different hydrogen powertrain configurations designed for an urban bus application. In particular, a series hybrid architecture was chosen as a reference considering three different technologies for its Auxiliary Power Unit: two internal combustion engines fuelled with Diesel and Hydrogen respectively, and a Fuel Cell featuring almost the same power level of the internal combustion engines. The study was carried out in real world driving condition and it showed the benefits of both hydrogen powertrains on the vehicle fuel economy. Finally, in order to provide a more comprehensive overview, an analysis of the Total Cost of Ownership (TCO) was performed demonstrating that the H2-engine could achieve a significant improvement of the powertrain efficiency with investments and operating costs closer to the Diesel configuration

Large Eddy Simulations (LES) towards a comprehensive understanding of Ducted Fuel Injection concept in non-reacting conditions

The diesel combustion research is increasingly focused on Ducted Fuel Injection (DFI), a promising concept to abate engine-out soot emissions in Compression-Ignition engines. A large set of experiments and numerical simulations, at medium-low computational cost, showed that the duct adop- tion in front of the injector nozzle activates several soot mitigation mechanisms, leading to quasi-zero soot formation in several engine-like operating conditions. However, although the simplified CFD mod- elling so far played a crucial role for the preliminary understanding of DFI technology, a more accurate turbulence description approach, combined with a large set of numerical experiments for statistical pur- poses, is of paramount importance for a robust knowledge on the DFI physical behavior. In this context, the present work exploits the potential of Large Eddy Simulations (LES) to analyze the non-reacting spray of DFI configuration compared with the unconstrained spray. For this purpose, a previously developed spray model, calibrated and validated in the RANS framework against an exten- sive amount of experimental data related to both free spray and DFI, has been employed. This high- fidelity simulation model has been adapted for LES, firstly selecting the best grid settings, and then carrying out several numerical experiments for both spray configurations until achieving a satisfying statistical convergence. With this aim, the number of independent samples for the averaging procedure has been increased exploiting the axial symmetry characteristics of the present case study. The relia- bility of this methodology has been herein proven, highlighting an impressive runtime saving without any remarkable worsening of the accuracy level. Thanks to this approach, a detailed description of the main DFI-enabled soot mitigation mechanisms has been achieved, bridging the still open knowledge gap in the physical understanding of the impact of spray-duct interaction

Investigation of Ducted Fuel Injection Implementation in a Retrofitted Light-Duty Diesel Engine through Numerical Simulation

Ducted Fuel Injection (DFI) is a concept of growing interest to abate soot emissions in diesel combustion, based on a small duct within the combustion chamber in front of the injector nozzle. Despite the impressive potential of the DFI has been proven in literature, its application for series production and the complexity for the adaptation of existing compression-ignition (CI) engines need to be extensively investigated. In this context, the aim of this study is to numerically assess the potential of DFI implementation in a CI engine for light-duty applications, highlighting the factors which can limit or facilitate its integration in existing combustion chambers. The numerical model for combustion simulation was based on a 1D/3D-CFD coupled approach relying on a calibrated spray model, extensively validated against experimental data. Once assessed the coupling procedure by comparing the numerical results with experimental in-cylinder pressure and heat release rate data for both low and high load operating conditions, the duct impact was investigated introducing it in the computational domain. It was observed that DFI did not yield any significant advantage to engine-out soot emissions and fuel consumption with the existing combustion system. Although the soot formation was generally reduced, the soot oxidation process was partially inhibited by the duct adoption maintaining fixed the engine calibration, suggesting the need for complete optimization of the combustion system design. On the other hand, a preliminary variation of engine calibration highlighted several beneficial trends for DFI, whose operation improved with a simplified injection strategy. Present numerical results indicate that DFI retrofit solutions without specific optimization of the combustion system design do not guarantee soot reduction. Nevertheless, wide room for improvement remains in terms of DFI-targeted combustion chamber design and engine calibration towards the complete success of this technology for soot-free CI engines

Is it possible to improve existing sample-based algorithm to compute the total sensitivity index?

Variance-based sensitivity indices have established themselves as a reference among practitioners of sensitivity analysis of model output. It is not unusual to consider a variance-based sensitivity analysis as informative if it produces at least the first order sensitivity indices ¿¿j and the so-called total-effect sensitivity indices ¿¿j for all the uncertain factors of the mathematical model under analysis. Computational economy is critical in sensitivity analysis. It depends mostly upon the number of model evaluations needed to obtain stable values of the estimates. While efficient estimation procedures independent from the number of factors under analysis are available for the first order indices, this is less the case for the total sensitivity indices. When estimating Tj , one can either use a sample-based approach, whose computational cost depends on the number of factors, or approaches based on meta-modelling/emulators, e.g. based on Gaussian processes. The present work focuses on sample-based estimation procedures for Tj and tries different avenues to achieve an algorithmic improvement over the designs proposed in the existing best practices. We conclude that some proposed sample-based improvements found in the literature do not work as claimed, and that improving on the existing best practice is indeed fraught with difficulties. We motivate our conclusions introducing the concepts of explorativity and efficiency of the design

Numerical and experimental investigation of a piston thermal barrier coating for an automotive diesel engine application

This paper investigates the potential of coated pistons in reducing fuel consumption and pollutant emissions of a 1.6l automotive diesel engine. After a literary review on the state-of-the-art of the materials used as Thermal Barrier Coatings for automotive engine applications, anodized aluminum has been selected as the most promising one. In particular, it presents very low thermal conductivity and heat capacity which ensure a high “wall temperature swing” property. Afterwards, a numerical analysis by utilizing a one-dimensional Computational Fluid Dynamics engine simulation code has been carried out to investigate the potential of the anodized aluminum as piston Thermal Barrier Coating. The simulations have highlighted the potential of achieving up to about 1% in Indicated Specific Fuel Consumption and 6% in heat transfer reduction. To confirm the simulation results, the coated piston technology has been experimentally evaluated on a prototype engine and compared to the baseline aluminum pistons. Despite the promising potential for Indicated Specific Fuel Consumption reduction highlighted by the numerical simulation, the experimental campaign has indicated a slight worsening of the engine efficiency (up to 2% at lower load and speed) due to the slowdown of the combustion process. The primary cause of these inefficiencies is attributed to the roughness of the coating

Printability study by selective laser sintering of bio-based samples obtained by using PBS as polymeric matrix

The emerging request to reduce the environmental impact of plastics encourages scientists to use novel sustainable polymeric materials for many applications fields. The present paper aims to use for the first-time poly (butylene succinate) (PBS), a biodegradable and compostable polymer, for Selective Laser Sintering (SLS) applications. PBS is a flexible semicrystalline aliphatic polyester, which can represent a very good alternative to the traditional thermoplastic polymers obtained by fossil sources. The present work started from a lab-scale production of PBS powders by means of an emulsion solvent evaporation/precipitation method, with the purpose to increase the number of polymeric powders available for SLS. The obtained PBS powders were first characterized by morphological and thermal point of view, and then employed as innovative polymeric material in SLS to realized 3D printed parts with increasing geometrical complexity. To confirm PBS cytocompatibility, cell proliferation and cell viability assays (MTT and Live&Dead) were measured using a lung adenocarcinoma epithelial cell line (H1299). The in vitro cytotoxicity of the 3D printed material was also investigated, showing no harm on cells