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Technical Paper

Experimental and Numerical Analyses for the Characterization of the Cyclic Dispersion and Knock Occurrence in a Small-Size SI Engine

2010-09-28
2010-32-0069
In this paper, an experimental and numerical analysis of combustion process and knock occurrence in a small displacement spark-ignition engine is presented. A wide experimental campaign is preliminarily carried out in order to fully characterize the engine behavior in different operating conditions. In particular, the acquisition of a large number of consecutive pressure cycle is realized to analyze the Cyclic Variability (CV) effects in terms of Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV). The spark advance is also changed up to incipient knocking conditions, basing on a proper definition of a knock index. The latter is estimated through the decomposition and the FFT analysis of the instantaneous pressure cycles. Contemporary, a quasi-dimensional combustion and knock model, included within a whole engine one-dimensional (1D) modeling framework, are developed. Combustion and knock models are extended to include the CV effects, too.
Journal Article

Generation of Turbulence in a RCEM towards Engine Relevant Conditions for Premixed Combustion Based on CFD and PIV Investigations

2017-09-04
2017-24-0043
The interaction of turbulent premixed methane combustion with the surrounding flow field can be studied using optically accessible test rigs such as a rapid compression expansion machine (RCEM). The high flexibility offered by such a test rig allows its operation at various thermochemical conditions at ignition. However, limitations inherent to such test rigs due to the absence of an intake stroke do not allow turbulence production as found in IC-engines. Hence, means to introduce turbulence need to be implemented and the relevant turbulence quantities have to be identified in order to enable comparability with engine relevant conditions. A dedicated high-pressure direct injection of air at the beginning of the compression phase is considered as a measure to generate adjustable turbulence intensities at spark timing and during the early flame propagation.
Technical Paper

Development and Application of S.I. Combustion Models for Emissions Prediction

2006-04-03
2006-01-1108
The s.i. combustion process and its corresponding pollutant formation are investigated by means of a quasiD approach and a CFD model. This work has been motivated by the need to better understand the reliability of such models and to assess their accuracies with respect to the prediction of engine performances and emissions. An extended dissertation about the fundamental mechanisms governing the pollutant formation in the turbulent premixed combustion which characterizes the s.i. engines is given. The conclusion of such analysis is the definition of a new reduced chemical scheme, based on the application of partial-equilibrium and steady-state assumptions for the radicals and the solution of a transport equation for each specie which is kinetically controlled. For this purpose the CFD code OpenFOAM [1, 2, 3] and the thermo-fluid dynamic code GASDYN [4, 5] have been applied and enhanced.
Technical Paper

In-Cylinder Flow Measurements by LDA and Numerical Simulation by KIVA-II Code

1992-02-01
920155
The fluid-mechanic behaviour of straight-sided and re-entrant chamber geometries has been studied using laser doppler anemometry (LDA) technique. Measurements have been carried out during the compression stroke in a direct injection diesel engine, representative of medium size family, operating at 1000 rpm under motored conditions. The mean motion and turbulence intensity have been computed using a filtering procedure on the LDA data. Using the second version of KIVA code, the air flow field evolution during the same crank angle period has been also computed. To perform proper comparisons between measured and computed values of mean velocity and turbulence intensity, a careful choice of the initial conditions for computations has been performed. Reasonable agreement has been found between computed and measured mean swirl velocities for both combustion chamber geometries tested. On the contrary, the computed turbulence intensities underestimate those measured.
Technical Paper

Effects of Fuel Temperature and Ambient Pressure on a GDI Swirled Injector Spray

2000-06-19
2000-01-1901
The effects of fuel temperature on both the geometry and the droplet size and velocity of a GDI swirled injector spray were investigated by means of visualizations and PDA measurements. Isooctane was used as model fuel and was injected in a quiescent bomb at injection pressure of 7 MPa. Bomb pressure ranged from 40 kPa to 800 kPa with injector nozzle temperature ranging from 293 K to 393 K. A drastic change in spray geometry was observed when conditions above the vaporization curve were reached. The temperature increase has two macroscopic effects on the spray geometry: at the nozzle exit the liquid flash boiling strongly enlarges the spray angle, at a certain distance from the nozzle the air entrainment collapses the spray. Raising the fuel temperature up to flash boiling conditions causes a significant decrease of the average droplet size.
Technical Paper

Effect of Gas Density and Temperature on Air Entrainment in a Transient Diesel Spray

1996-02-01
960862
The air entrainment in a transient diesel spray was studied using laser Doppler anemometry to provide information on the effect of gas density and temperature. The spray was injected vertically into a confined quiescent atmosphere and the entrained mass flow rate was evaluated by measuring the air velocity component normal to a cylindrical geometric surface surrounding the spray, and extending to about 200 nozzle diameters (50 mm). The experimental results, relative to a density range from 0.84 to 7.02 kg/m3 and a temperature range from 293 to 473 K, indicate that the non dimensional entrainment rate, averaged in time over the main injection period, depends on the distance from the nozzle and both gas density and temperature. A first analysis, based on the available data, allowed to quantify the dependence and provided a correlation with such variables.
Technical Paper

Assessment of k-ε Turbulence Model in KIVA-II by In-Cylinder LDV Measurements

1995-10-01
952385
In-cylinder measurements of turbulent integral length scales, carried out during the last 60 degrees of the compression stroke at 600 and 1,000 rpm by a two-probe volume LDV system, were used to assess the capability of the k-ε model used in KIVA-II code. The objective of the paper is to address the following question: what is the most reasonable definition of turbulent length scale in the k-ε model for engine applications? The answer derived from the comparison between KIVA predictions and experiments that showed a fair agreement between the computed turbulent length scale and the measured lateral integral length scale. The agreement is a result of proper choice of the initial swirl ratio and turbulent kinetic energy at inlet valve closure (IVC) by taking into account the LDV measurements and the value of the constant Cμε in the k-ε model equations that relates the turbulent length scale to k and ε.
Journal Article

Detailed Kinetic Analysis of HCCI Combustion Using a New Multi-Zone Model and CFD Simulations

2013-09-08
2013-24-0021
A new multi-zone model for the simulation of HCCI engine is here presented. The model includes laminar and turbulent diffusion and conduction exchange between the zones and the last improvements on the numerical aspects. Furthermore, a new strategy for the zone discretization is presented, which allows a better description of the near-wall zones. The aim of the work is to provide a fast and reliable model for carrying out chemical analysis with detailed kinetic schemes. A preliminary sensitivity analysis allows to verify that 10 zones are a convenient number for a good compromise between the computational effort and the description accuracy. The multi-zone predictions are then compared with the CFD ones to find the effective turbulence parameters, with the aim to describe the near-wall phenomena, both in a reactive and non-reactive cases.
Journal Article

Full-Cycle CFD Modeling of Air/Fuel Mixing Process in an Optically Accessible GDI Engine

2013-09-08
2013-24-0024
This paper is focused on the development and application of a CFD methodology that can be applied to predict the fuel-air mixing process in stratified charge, sparkignition engines. The Eulerian-Lagrangian approach was used to model the spray evolution together with a liquid film model that properly takes into account its effects on the fuel-air mixing process into account. However, numerical simulation of stratified combustion in SI engines is a very challenging task for CFD modeling, due to the complex interaction of different physical phenomena involving turbulent, reacting and multiphase flows evolving inside a moving geometry. Hence, for a proper assessment of the different sub-models involved a detailed set of experimental optical data is required. To this end, a large experimental database was built by the authors.
Journal Article

Numerical Investigation of Non-Reacting and Reacting Diesel Sprays in Constant-Volume Vessels

2009-06-15
2009-01-1971
A numerical investigation on a series of Diesel spray experiments in constant-volume vessels is proposed. Non reacting conditions were used to assess the spray models and to determine the grid size required to correctly predict the fuel-air mixture formation process. To this end, not only computed liquid and vapor penetrations were compared with experimental data, but also a detailed comparison between computed and experimental mixture fraction distributions was performed at different distances from the injector. Grid dependency was reduced by introducing an Adaptive Local Mesh Refinement technique (ALMR) with an arbitrary level of refinement. Once the capabilities of the current implemented spray models have been assessed, reacting conditions at different ambient densities and temperatures were considered. A Perfectly Stirred Reactor (PSR) combustion model, based on a direct integration of complex chemistry mechanisms over a homogenous cell, was adopted.
Technical Paper

A Method for the Characterization of Off-Road Terrain Severity

2006-10-31
2006-01-3498
Highway and roadway surface measurement is a practice that has been ongoing for decades now. This sort of measurement is intended to ensure a safe level of road perturbances. The measurement may be conducted by a slow moving apparatus directly measuring the elevation of the road, at varying distance intervals, to obtain a road profile, with varying degrees of resolution. An alternate means is to measure the surface roughness at highway speeds using accelerometers coupled with high speed distance measurements, such as laser sensors. Vehicles out rigged with such a system are termed inertial profilers. This type of inertial measurement provides a sort of filtered roadway profile. Much research has been conducted on the analysis of highway roughness, and the associated metrics involved. In many instances, it is desirable to maintain an off-road course such that the course will provide sufficient challenges to a vehicle during durability testing.
Technical Paper

Analysis of In-Cylinder Turbulent Air Motion Dependence on Engine Speed

1994-03-01
940284
In-cylinder cycle-resolved LDV measurements have been made in a diesel engine having a high-squish re-entrant combustion chamber with compression ratio of 21:1. The engine has been motored in the range of 1000 to 3000 rpm thanks to the use of self-lubricating seeding particles. Conventional ensemble-averaging and filtering techniques have been used for analyzing instantaneous velocity data obtained at two points along a diameter located in a horizontal plane at 5 mm below the engine head. The dependence of the mean motion and turbulence on engine speed has been evaluated. The effect of cut-off frequency selection on turbulence values has been also analyzed. Moreover, the Kolmogorov's -5/3 power domain has been investigated in detail by spectral analysis on the instantaneous velocity data.
Technical Paper

A Study of Physical and Chemical Delay in a High Swirl Diesel System via Multiwavelength Extinction Measurements

1998-02-23
980502
The characterization of a turbulent diesel spray combustion process has been carried out in a divided chamber diesel system with optical accesses. Laser Doppler Anemometry, spectral extinction and flame intensity measurements have been performed from U.V., to visible from the start of injection to the end of combustion, at fixed air/fuel ratio and different engine speeds. Spatial distribution of fuel and vapor as well as the ignition location and soot distribution have been derived in order to study the mechanism of the air-fuel interaction and the combustion process. The analysis of results has shown that the high swirling motion transports the fuel towards the left part of the chamber and breaks up the jet into small droplets of different sizes and accelerates the fuel vaporization. Then, chemical and physical overlapped phases were observed during the ignition delay, contributing both to autoignition.
Technical Paper

LDV Measurements of Integral Length Scales in an IC Engine

1996-05-01
961161
Tangential component of velocity and turbulence were measured in three locations in the re-entrant combustion chamber of a motored single-cylinder d.i. Diesel engine (0.435 liter, 21:1 compression ratio) using a Laser Doppler Velocimetry system. Moreover, a modified LDV system with two-probe volume was used to measure directly lateral integral length scales of the velocity tangential component at two engine speeds. The measurements were made on a horizontal plane at 5 mm below the engine head from 100 degrees before TDC to 60 degrees after TDC of both the compression and expansion strokes. The engine was motored at 1,000 and 1,500 rpm respectively. An ensemble-averaging technique was performed to analyze the instantaneous velocity information supplied by two Burst Spectrum Analyzers. The lateral integral length scale was obtained from the integral of the spatial correlation coefficient of the velocity fluctuation for different separation.
Technical Paper

Integral and Micro Time Scales Estimate in a D.I. Diesel Engine

1997-05-01
971678
The present paper aims at developing a general method to estimate integral and microtime scales of turbulent in-cylinder flow field in reciprocating engines. The ensemble average technique was used to compute the integral time scale from the single point time autocorrelation function, whereas the microtime scale, representative of the most rapid changes that occur in the fluctuation, was computed as the intercept of the parabola that matches the autocorrelation function at the origin. Further, the microtime scale was also estimated by spectral analysis through the energy spectral density function of the ensemble turbulent fluctuation and the results obtained by the two methods were compared. The procedures were applied to the tangential component of the instantaneous velocity data collected, at different engine speeds (1,000, 1,500, 2,000 rpm), within a motored d.i. diesel engine equipped with a re-entrant combustion chamber, using the Laser Doppler Anemometry (LDA) technique.
Journal Article

Experimental and Numerical Analyses of Liquid and Spray Penetration under Heavy-Duty Diesel Engine Conditions

2016-04-05
2016-01-0861
The modeling of fuel sprays under well-characterized conditions relevant for heavy-duty Diesel engine applications, allows for detailed analyses of individual phenomena aimed at improving emission formation and fuel consumption. However, the complexity of a reacting fuel spray under heavy-duty conditions currently prohibits direct simulation. Using a systematic approach, we extrapolate available spray models to the desired conditions without inclusion of chemical reactions. For validation, experimental techniques are utilized to characterize inert sprays of n-dodecane in a high-pressure, high-temperature (900 K) constant volume vessel with full optical access. The liquid fuel spray is studied using high-speed diffused back-illumination for conditions with different densities (22.8 and 40 kg/m3) and injection pressures (150, 80 and 160 MPa), using a 0.205-mm orifice diameter nozzle.
Journal Article

Geometric and Fluid-Dynamic Characterization of Actual Open Cell Foam Samples by a Novel Imaging Analysis Based Algorithm

2017-10-05
2017-01-9288
Metallic open-cell foams have proven to be valuable for many engineering applications. Their success is mainly related to mechanical strength, low density, high specific surface, good thermal exchange, low flow resistance and sound absorption properties. The present work aims to investigate three principal aspects of real foams: the geometrical characterization, the flow regime characterization, the effects of the pore size and the porosity on the pressure drop. The first aspect is very important, since the geometrical properties depend on other parameters, such as porosity, cell/pore size and specific surface. A statistical evaluation of the cell size of a foam sample is necessary to define both its geometrical characteristics and the flow pattern at a given input velocity. To this purpose, a procedure which statistically computes the number of cells and pores with a given size has been implemented in order to obtain the diameter distribution.
Technical Paper

Effects of Turbulence Modulation Addition in OpenFOAM® Toolkit on High Pressure Fuel Sprays

2011-04-12
2011-01-0820
The OpenFOAM® CFD methodology is nowadays employed for simulation in internal combustion engines and a lot of work has been done for an appropriate description of all complex phenomena. At the moment in the RANS turbulence models available in the OpenFOAM® toolbox the turbulence modulation is not yet included, and the present work analyzes the predictive capabilities of the code in simulating high injection pressure fuel sprays after modeling the influence of the dispersed phase on the turbulence structure. Different experiments were employed for the validation. At first, non-evaporating diesel spray was considered in a constant volume and quiescent vessel. The validation was performed via the available experimental spray evolution in terms of penetrations and spatial/temporal fuel distributions. Then the Sandia combustion chamber was chosen for diesel spray simulation in non-reacting conditions.
Journal Article

Improved Analytical Model of an Outer Rotor Surface Permanent Magnet Machine for Efficiency Calculation with Thermal Effect

2017-03-28
2017-01-0185
In this paper, an improved analytical model accounting for thermal effects in the electromagnetic field solution as well as efficiency map calculation of an outer rotor surface permanent magnet (SPM) machine is described. The study refers in particular to an in-wheel motor designed for automotive electric powertrain. This high torque and low speed application pushes the electric machine close to its thermal boundary, which necessitates estimates of winding and magnet temperatures to update the winding resistance and magnet remanence in the efficiency calculation. An electromagnetic model based on conformal mapping is used to compute the field solution in the air gap. The slotted air-gap geometry is mapped to a simpler slotless shape, where the field solution can be obtained by solving Laplace's equation for scalar potential. The canonical slottless domain solution is mapped back to the original domain and verified with finite element model (FEM) results.
Technical Paper

A LES Study on the Evolution of Turbulent Structures in Moving Engine Geometries by an Open-Source CFD Code

2014-04-01
2014-01-1147
The dynamics and evolution of turbulent structures inside an engine-like geometry are investigated by means of Large Eddy Simulation. A simplified configuration consisting of a flat-top cylinder head with a fixed, axis-centered valve and low-speed piston has been simulated by the finite volume CFD code OpenFOAM®; the standard version of the software has been extended to include the compressible WALE subgrid-scale model, models for the generation of synthetic turbulence, some improvements to the mesh motion strategy and algorithms for LES data post-processing. In order to study both the initial transient and the quasi- steady operating conditions, ten complete engine cycles have been simulated. Phase and spatial averages have been performed over cycles three to ten in order to extract first and second moment of velocity; these quantities have then been used to validate the numerical procedure by comparison against experimental data.
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