Gas Exchange and Injection Modeling of an Advanced Natural Gas Engine for Heavy Duty Applications

The scope of the work presented in this paper was to apply the latest open source CFD achievements to design a state of the art, direct-injection (DI), heavy-duty, natural gas-fueled engine. Within this context, an initial steady-state analysis of the in-cylinder flow was performed by simulating three different intake ducts geometries, each one with seven different valve lift values, chosen according to an estabilished methodology proposed by AVL. The discharge coefficient (Cd) and the Tumble Ratio (TR) were calculated in each case, and an optimal intake ports geometry configuration was assessed in terms of a compromise between the desired intensity of tumble in the chamber and the satisfaction of an adequate value of Cd. Subsequently, full-cycle, cold-flow simulations were performed for three different engine operating points, in order to evaluate the in-cylinder development of TR and turbulent kinetic energy (TKE) under transient conditions. The latest achievements in open source mesh generation and motions were applied, along with time-varying and case-fitted inizialization values for the fields of intake pressure and temperature. Finally, direct-injection of natural gas in the cylinder was incorporated in full-cycle simulations, to evaluate the effects of injection on charge motions and charge homogeneity at the estimated spark timing. Three specific engine operating points were simulated and different combinations of turbochargers and valve lift laws were tested. Results consistency was verified by means of validations with data from 1D simulations and literature

Modeling of Aqueous Urea Solution injection with characterization of spray-wall cooling effect and risk of onset of wall wetting

AbstractThe definition of a sufficiently resolved heat transfer model with spray cooling effect as a function of each droplet kinetic and thermal parameters is a key factor in the numerical simulation of aqueous urea (AUS) based Selective Catalytic Reduction (SCR) exhaust after-treatment systems.A consolidated spray-wall interaction model [1] has been implemented on the open source 3D finite volume software OpenFOAM and a critical investigation of its behaviour in engine representative conditions is reported.A simplified test case is used to highlight the influence of the chosen model on the numerical simulation of the system, reducing the importance of the other spray sub-models in the Lagrangian-Eulerian computational framework. The coupling between the droplet evaporation heat flux and the gas-solid interface thermal boundary condition has been studied, pointing out the significance of each contribution.The main focus of this work is to present reference conditions to simulate the spray-dry wall spray impingement behavior to determine the ‘onset of wall wetting’ thermal conditions

A comprehensive analysis of natural gas direct injection and of mixture formation in spark ignition engines

The transportation sector is becoming increasingly linked to environmental problems, with a technology relying heavily on the exploitation of not renewable energies: the combustion of hydrocarbons in internal combustion engine (ICE). Since this is still expected to be the main technical solution to fulfil the mobility demand, more secure and sustainable fuel sources are needed. Natural gas (NG), which is composed primarily of methane, is regarded as one of the most promising alternative fuels because of its interesting chemical properties with high H/C ratio and high research octane number. The use of NG allows the reduction of pollutant emissions, while the gap in performance with respect to gasoline engines can be recovered by means of turbocharging devices. NG engine performance can be further improved by adopting a direct injection (DI) concept that can also extend the fuel-lean operating limit of normal engine operation, compared to port fuel injection. Within this framework a research activity was carried out at the Politecnico di Torino within the InGAS Collaborative Project (VII FP), Sub-project A2, dealing with the development of a turbo DI CNG engine, optimized for mono-fuel CNG operation. The present thesis focuses on the development of a methodology for the numerical simulation (by means of 3D CFD) of the CNG direct injection process, in view of its application to internal combustion engines, and on the analysis of the physical processes related to the jet formation and development and the formation of the air-fuel mixture. The results of the numerical analysis are then correlated with the outcomes of an experimental activity performed on the developed multi-cylinder engine and on an optical access single-cylinder engine, in order to obtain more detailed knowledge of the DI CNG engine behaviour. The thesis is divided into six chapters. After a short introduction related to the transportation sector environmental impact (Chapter 1, section 1.1) an overview of the CNG Vehicle technology (section 1.2) is presented. The results achieved during the InGAS CP (SP A2) are summarized in section 1.3. Chapter 2 focuses on the development of the numerical model for CNG direct injection simulation in an engine combustion chamber. Firstly the theoretical background of supersonic underexpanded jets is summarized (section 2.1), followed by a literary review of previous investigations regarding the numerical simulation of gaseous direct injection in internal combustion engines (section 2.2). In section 2.3, the test case of a two-dimensional compressible flow issuing from a nozzle is examined to characterize the underexpanded-jet phenomenon independently from any geometrical constraint, imposed by the specific application. The main guidelines drawn in this preliminary study are then applied to the development of the finite volume numerical model of the engine, that is thoroughly described in section 2.4. Chapter 3 is devoted to the validation of the numerical model. Section 3.1 describes the planar laser induced fluorescence (PLIF) technique, employed for the experimental investigation of the mixture formation process in the optical access SCE, whose setup is presented in section 3.2. Finally the post-processed results of the PLIF experiments are used to validate the developed numerical model in section 3.3. A comprehensive analysis of the mixture formation process in the InGAS DI-CNG engine is discussed in Chapter 4, with reference to relevant operation conditions and injection strategies. The homogeneous operation at partial and full load is analyzed in section 4.1 and 4.2 respectively. The lean stratified operation at partial load is studied in section 4.3. Chapter 5 is related to the injection system development and focuses on the optimization of the control phase of the InGAS injector, paying specific attention to its behavior at small injected-fuel amounts. The development of a CFD model of the actual injector geometry is described and the dynamic behaviour of the injector is analyzed. Finally, Chapter 6 draws the main conclusions of this study

Fluid-dynamic and numerical aspects in the simulation of direct CNG injection in spark-ignition engines

This paper presents a detailed discussion on the numerical simulation of the underexpanded gas efflux from an outward-opening poppet-valve injector into an engine combustion chamber. The aim of the paper is to optimize the numerical simulation strategy for direct gas injection, in view of its application to internal combustion (IC) engines. In the first part of the paper, the widely studied case of a two-dimensional compressible flow is examined, and the main guidelines for the development of an effective numerical model for compressed natural gas (CNG) direct injection simulation are given, with specific reference to IC engines. The second part of the paper is devoted to the description of the numerical model developed and validated by the authors within the Star-CD environment, which is characterized by the presence of two distinct meshes. The first is built manually and covers the region surrounding the injector exit, whereas the second one covers most of the engine chamber and is built using the Es-ICE tool. A careful grid-independence study has been carried out in both the first and second part of the paper, and the influence of the spatial discretization of the convective fluxes has been discussed as well. The analyses have shown that a resolution of 40 cells in the nozzle height should be adopted to describe the typical phenomena that characterize an underexpanded free jet, unless a second order scheme can be implemented. However, as far as the simulation of the jet penetration time-history and its mixing with the surrounding air is concerned, sufficiently accurate results can also be obtained by using 20 cells per nozzle diameter and the first-order upwind scheme. As for the direct injection engine model, 16 cells across the nozzle lift represent a good compromise between accuracy and reliability of the results and the required computational time. The model has been validated with the support of experimental PLIF images in an optical-access engine, and has shown overall good accuracy and reliability, thus suggesting it is suitable for mixture formation analysi

Mixture formation analysis in a direct-injection NG SI engine under different injection timings

This paper investigates into the mixture formation in a direct injection, turbocharged, spark-ignition, CNG engine. The engine features a pent-roof combustion chamber, a bowl in piston and an outward-opening poppet valve injector, which is located centrally in the chamber dome. In the last few years, many studies have been conducted focusing on direct injection natural gas engines, and the end-of-injection timing has been identified as the main parameter affecting the quality and completeness of the mixture formation process. This paper aims at contributing to the progress of this research field, by means of the presentation and discussion of a large number of experimental and numerical data. The results obtained from the authors’ CFD model, which has been developed and validated within the InGAS Collaborative Project of the EC, are in fact introduced and correlated to the outcomes of the experimental activity done by AVL GmbH, Graz, as part of the same research project. This synergy allowed a deep understanding of the mixture formation process, over a wide range of operating conditions. As a matter of fact, the mixture formation process in a direct injection gaseous-fuel engine differs significantly from direct-injection engines fuelled by gasoline. In fact, the gas jet momentum is lower, reducing the penetration, and the mixture formation strongly relies on the charge motion generated during the intake stroke. More precisely, the work presented in this paper showed that several factors exert an influence on the fuel-air mixing process: jet shape, interaction with piston and/or with the charge motion, and time available for mixing between the end-of-injection and the spark timing, and these may combine differently depending on the specific working point. On an average, at low load and low-medium speeds, the injection should better take place during the second part of the induction stroke. On the other hand, at high speed or high load the injection timing needs to be advanced till around 250°-300° CA degrees before firing TDC, in order to increase the time available for mixing as much as possible

Internet Use and Access, Behavior, Cyberbullying and Grooming. Revised questionnaire

The way children use the Internet and mobile technologies has changed in the last few years according to the Digital Agenda for Europe. The growing phenomenon of cyberbullying is among the risks associated with the increasing Internet use. Methods of access and use by teenagers are highly important factors in assessing those risks.In order to fight cyberbullying we expanded our previous study adding to our questionnaire a new cluster of 5 more items, specifically developed to investigate cyberbullying and cyber-grooming among the teenage population. This new cluster, proposed by the “Cyber Expert” program created by the Spanish Policia Nacional, is structured to gauge the students’ awareness and knowledge of what cyberbullying is through multiple choice questions. We submitted this updated survey to students of the Liceo Scientifico “G. Keplero” in Rome, while also inviting them to provide their feedback on the study itself. On the methodological side, since the data we gathered strongly hinted at the need to spread surveys and informational material using language and concepts adolescents can relate to more easily, we verified the need to review and update the questionnaire as often as required by the constant evolution of browsing habits in daily life. Here we present our revised questionnair