1,721,021 research outputs found
Further Investigation of RNG k-Eps Model Capabilities in the Simulation of In-Cylinder Turbulent Flows
No full textFluid-dynamic and numerical aspects in the simulation of direct CNG injection in spark-ignition engines
No full textThis 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
No full textThis 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 possibl
Development and assessment of a new methodology for end of combustion detection and its application to cycle resolved heat release analysis in IC engines
No full textThe heat release analysis has proved to be a powerful diagnostic tool for the analysis of the combustion process in spark ignition engines. Still, a fine tuning of the heat transfer correlations embedded in the heat release models is necessary for a correct diagnostic analysis of the pressure signal. To that end, a new methodology has been developed and assessed to properly locate the end of combustion on the basis of the heat release intensity. The results produced by the proposed method have been compared to those obtained by applying different methodologies available in the literature. The newly developed method has proved to be accurate and consistent and has allowed a reliable estimation of the end of combustion on a cycle-by-cycle basis. An extensive burn rate analysis has also been accomplished by means of a heat release model previously developed and purposely modified to embed the new end of combustion detection procedure. The main combustion related quantities have been considered for the experimental investigation to appropriately quantify the engine cyclic variability as a function of the relative air-to-fuel ratio. The experimental tests have been performed on a naturally aspirated 2L engine featuring a fast-burn combustion chamber and running on gasoline and natural gas as well as on a 1.2L turbocharged natural gas engine displaying a disk shaped combustion chamber. The diagnostic tool has proved to properly match the nonlinear behavior of the quantities related to the combustion duration in the cycle-resolved analysis and a general good agreement with previous works has emerged as far as the coefficient of variations of the main combustion parameters are concerned. Moreover, thanks to the automatic facet the proposed methodology retains, it is strongly recommended when an extensive cycle-by-cycle and cylinder-to-cylinder analysis needs to be performed. Finally, regardless of the considered fuel, the heat release model embedding the EOC detection procedure proved to be capable of properly detecting the combustion features induced by a fast-burn combustion chamber with respect to a traditional one. As a matter of fact, smaller Δθ10-90% values and an overall reduced cyclic dispersion were highlighted for the 2L engin
Development of a method for the estimation of the behavior of a CNG engine over the NEDC cycle and its application to quantify for the effect of hydrogen addition to methane operations
No full textThe current pollutant regulations and policies have set the need to consider the use of alternative fuels capable of complying with the emission limits still retaining appreciable engine performance. Natural gas has been considered as an effective alternative to gasoline but the drawbacks connected to its use forced the researchers to investigate into fuel additives, dual fuel solutions and innovative engine control strategies. The present work analyzes the use of hydrogen as an additive to CNG for a natural gas production engine of a C-segment vehicle and carries out a thorough investigation into the engine response over a selection of operating key points. The actual focus is set on the investigation into the vehicle as well as into the engine response and performance over driving cycles. Still, the simulation of real driving conditions would set the need to properly quantify for the effect of the hydrogen enriched blends on the full engine map over varying powers and speeds within the vehicle driving cycle. Such an approach often turns out to be too demanding in terms of time and costs and an alternative solution has been hereafter proposed by properly selecting a reduced number of operating points on the basis of the correspondent residence time and frequency over the NEDC. The selection has been performed by matching the actual engine map to the readings from the NEDC vehicle testing. Different selections have been considered and compared so as to assess for the one embedding the minimum number of working points. The key points are not meant to substitute the accomplishment of the NEDC cycle but are to be used as driving factors in the engine design so as to allow for detecting the optimal hardware and ECU configurations. The so considered engine key points have hence been extensively studied by reproducing the engine performance at the test bench and by performing a detailed heat release analysis. Different composition of the hydrogen-methane blends have been considered up to a 25% by volume of hydrogen in the mixture and specific attention has been paid to the main combustion parameters and to their optimization. As a matter of fact, such a study would allow for detecting major trends in the engine design and control strategies to compensate for the poor behavior of some of the considered points. As an example, low load and speed operations assessed for the need of a better control of the engine parameters to diminish the cylinder-to-cylinder as well as the cycle-to-cycle variabilit
Development and application of a method for characterizing mixture formation in a port-injection natural gas engine
Full text linkNatural gas has been identified as one of the most promising alternative fuels. Port injection of natural gas, due to
its advantages of costs, manufacturing complexity and mixture homogeneity, is relevant to the current and future
engine development. The present work aims to provide a comprehensive characterization of gas fuel injection
and mixing process, by developing a modeling method for the injector and the engine as a whole serving as a
diagnostic tool for further expounding on the basis of experimental results. The injector is modeled by the source
cell approach that allows for cost-efficient and physics-consistent description of the underexpanded gas jet, by
which a set of fuel injection timings is investigated and then compared with a conventional premixed case. Two
mechanisms peculiar to gas port injection are characterized, being firstly the two-stage mixing process that involves immediate induction of the residual fuel from the previous engine cycle and delayed induction of the fuel
injected in the current cycle, and secondly the limited fuel penetration speed along the intake ports with associated delay of charge induction. Additional information on volumetric efficiency, mixture quality, coherent flow
motion and turbulence level is highlighted. It is concluded that the otherwise intuitive correlation between injection timing and mixture homogeneity for port injection is complicated by those two mechanisms, and,
depending on specific engine design and operating point, differences resulted from modeling the engine operation with fuel injection and with premixed charge may prove combustion-significant. The method and the
underlying mechanisms found herein are equally applicable to other combustion systems involving port injection
of gaseous fuels
Nonlinear Versus Linear Stress-Strain Relations in Engine Turbulence Modeling Under Swirl and Squish Flow Conditions
No full textA general form of the stress-strain constitutive relation was introduced for the application of two nonlinear k- turbulence models, namely, the algebraic Reynolds stress model of Gatski and Speziale (1993, "On Explicit Algebraic Stress Models for Complex Turbulent Flows," J. Fluid Mech., 254, pp. 59-78) and the cubic model of Lien et al. (1996, "Low Reynolds Number Eddy-Viscosity Modeling Based on Non-Linear Stress-Strain/Vorticity Relations," Proceedings of Third Symposium on Engineering Turbulence Modeling and Measurements, Crete, Greece), to the numerical analysis of flow fields in a test engine with flat-piston and bowl-in-piston arrangements, under swirling and no-swirling flow motored conditions. The model capabilities in capturing turbulent flow features were compared to those of the upgraded linear RNG k- model, which was previously indicated as a good compromise between accuracy and computational cost. Evaluations were made on the basis of the predicted flow evolution throughout the whole engine cycle, as well as of the comparison between the numerical and experimental results. Furthermore, the effect of the stress-strain relationship on the predicted averaged turbulence quantities and anisotropy-invariant values were examined, in addition to the sensitivity of the nonlinear models to the mesh quality. Finally, prospects concerning possible improvements of turbulence eddy-viscosity models were presented. The predictions were made by a newly developed CFD code embedding various accuracy-order finite-volume discretization schemes. Modified wall boundary conditions with respect to the conventional logarithmic-function approach were used, so as to make the local equilibrium hypothesis virtually ineffectiv
Hydraulic Circuit Design Rules to Remove the Dependence of the Injected Fuel Amount on Dwell Time in Multijet CR Systems
No full textIn multijet common rail (CR) systems, the capability to manage multiple injections with full flexibility in the choice of the dwell time (DT) between consecutive solenoid current pulses is one of the most relevant design targets. Pressure oscillations triggered by the nozzle closure after each injection event induce disturbances in the amount of fuel injected during subsequent injections. This causes a remarkable dispersion m the mass of fuel injected when DT is varied. The effects of the hydraulic circuit layout of CR systems were investigated with the objective to provide design rules for reducing the dependence of the injected fuel amount on DT. A multijet CR of the latest solenoid-type generation was experimentally analyzed at different operating conditions on a high performance test bench The considerable influence that the injector-supplying pipe dimensions can exert on the frequency and amplitude of the injection-induced pressure oscillations was widely investigated and a physical explanation of cause-effect relationships was found by energetics considerations, starting from experimental tests. A parametric study was performed to identify the best geometrical configurations of the injector-supplying pipe so as to minimize pressure oscillations. The analysis was carried out with the aid of a previously developed simple zero-dimensional model, allowing the evaluation of pressure-wave frequencies as functions of main system geometric data. Pipes of innovative aspect ratio and capable of halving the amplitude of injected-volume fluctuations versus DI were proposed. Purposely designed orifices were introduced into the rail-pipe connectors of a commercial automotive injection system, so as to damp pressure oscillations. Their effects on multiple-injection performance were experimentally determined as being sensible The resulting reduction in the injector fueling capacity was quantified. It increased by lowering the orifice diameter. The application of the orifice to the injector inlet-pipe with innovative aspect ratio led to a hydraulic circuit solution, which coupled active and passive damping of the pressure waves and minimized the disturbances in injected fue volumes. Finally, the influence of the rail capacity on pressure-wave dynamics was studied and the possibility of severely reducing the rail volume (up to one-fourth was assessed. This can lead to a system not only with reduced overall size but also with a prompter dynamic response during engine transients
Computational and Experimental Analysis of Direct CNG Injection and Mixture Formation in a SI Research Engine
No full text- …
