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Piston Bowl Geometry Effects on Combustion Development in a High-Speed Light-Duty Diesel Engine

Ford Motor Company-Eric Kurtz
Sandia National Laboratories-Stephen Busch, Kan Zha
Published 2019-09-09 by SAE International in United States
In this work we studied the effects of piston bowl design on combustion in a small-bore direct-injection diesel engine. Two bowl designs were compared: a conventional, omega-shaped bowl and a stepped-lip piston bowl. Experiments were carried out in the Sandia single-cylinder optical engine facility, with a medium-load, mild-boosted operating condition featuring a pilot+main injection strategy. CFD simulations were carried out with the FRESCO platform featuring full-geometric body-fitted mesh modeling of the engine and were validated against measured in-cylinder performance as well as soot natural luminosity images. Differences in combustion development were studied using the simulation results, and sensitivities to in-cylinder flow field (swirl ratio) and injection rate parameters were also analyzed. In-cylinder mixture formation analysis showed that ignition of the pilot injection mixture develops nearly as it would in a homogeneous adiabatic reactor, being mostly advected, not mixed, by the bowl’s swirling motion, while its timing is influenced by the local flow field. Details of the local in-cylinder flow are also more crucial than injection parameters in igniting the main injection’s premixed fuel, as it…
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Bowl Geometry Effects on Turbulent Flow Structure in a Direct Injection Diesel Engine

Ford Motor Company-Eric Kurtz
General Motors Global R & D-Alok Warey
Published 2018-09-10 by SAE International in United States
Diesel piston bowl geometry can affect turbulent mixing and therefore it impacts heat-release rates, thermal efficiency, and soot emissions. The focus of this work is on the effects of bowl geometry and injection timing on turbulent flow structure. This computational study compares engine behavior with two pistons representing competing approaches to combustion chamber design: a conventional, re-entrant piston bowl and a stepped-lip piston bowl. Three-dimensional computational fluid dynamics (CFD) simulations are performed for a part-load, conventional diesel combustion operating point with a pilot-main injection strategy under non-combusting conditions. Two injection timings are simulated based on experimental findings: an injection timing for which the stepped-lip piston enables significant efficiency and emissions benefits, and an injection timing with diminished benefits compared to the conventional, re-entrant piston.While the flow structure in the conventional, re-entrant combustion chamber is dominated by a single toroidal vortex, the turbulent flow evolution in the stepped-lip combustion chamber depends more strongly on main injection timing. For the injection timing at which faster mixing controlled heat release and reduced soot emissions have been observed experimentally,…
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Experimental and Numerical Studies of Bowl Geometry Impacts on Thermal Efficiency in a Light-Duty Diesel Engine

Ford Motor Company-Eric Kurtz
General Motors LLC-Alok Warey, Richard Peterson
Published 2018-04-03 by SAE International in United States
In light- and medium-duty diesel engines, piston bowl shape influences thermal efficiency, either due to changes in wall heat loss or to changes in the heat release rate. The relative contributions of these two factors are not clearly described in the literature. In this work, two production piston bowls are adapted for use in a single cylinder research engine: a conventional, re-entrant piston, and a stepped-lip piston. An injection timing sweep is performed at constant load with each piston, and heat release analyses provide information about thermal efficiency, wall heat loss, and the degree of constant volume combustion. Zero-dimensional thermodynamic simulations provide further insight and support for the experimental results. The effect of bowl geometry on wall heat loss depends on injection timing, but changes in wall heat loss cannot explain changes in efficiency. Late cycle heat release is faster with the stepped-lip bowl than with the conventional re-entrant bowl, which leads to a higher degree of constant volume combustion and therefore higher thermal efficiency. This effect also depends on injection timing. In general, increasing…
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A Study of Piston Geometry Effects on Late-Stage Combustion in a Light-Duty Optical Diesel Engine Using Combustion Image Velocimetry

SAE International Journal of Engines

Ford Motor Company-Eric Kurtz
General Motors LLC-Alok Warey, Richard C. Peterson
  • Journal Article
  • 2018-01-0230
Published 2018-04-03 by SAE International in United States
In light-duty direct-injection (DI) diesel engines, combustion chamber geometry influences the complex interactions between swirl and squish flows, spray-wall interactions, as well as late-cycle mixing. Because of these interactions, piston bowl geometry significantly affects fuel efficiency and emissions behavior. However, due to lack of reliable in-cylinder measurements, the mechanisms responsible for piston-induced changes in engine behavior are not well understood. Non-intrusive, in situ optical measurement techniques are necessary to provide a deeper understanding of the piston geometry effect on in-cylinder processes and to assist in the development of predictive engine simulation models.This study compares two substantially different piston bowls with geometries representative of existing technology: a conventional re-entrant bowl and a stepped-lip bowl. Both pistons are tested in a single-cylinder optical diesel engine under identical boundary conditions. Utilizing high-speed soot natural luminosity (NL) imaging, 20 kHz time-resolved combustion image velocimetry (CIV) technique is developed to quantify the macro-scale motions of soot clouds during the mixing-controlled portion of combustion.Under a part-load conventional combustion regime, CIV-resolved swirl ratio and the tumble-plane projection of velocity fields confirm that…
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Comparison of Linear, Non-Linear and Generalized RNG-Based k-epsilon Models for Turbulent Diesel Engine Flows

Sandia National Laboratories-Kan Zha, Stephen Busch
University of Wisconsin-Madison-Federico Perini, Rolf Reitz
Published 2017-03-28 by SAE International in United States
In this work, linear, non-linear and a generalized renormalization group (RNG) two-equation RANS turbulence models of the k-epsilon form were compared for the prediction of turbulent compressible flows in diesel engines. The object-oriented, multidimensional parallel code FRESCO, developed at the University of Wisconsin, was used to test the alternative models versus the standard k-epsilon model.Test cases featured the academic backward facing step and the impinging gas jet in a quiescent chamber. Diesel engine flows featured high-pressure spray injection in a constant volume vessel from the Engine Combustion Network (ECN), as well as intake flows in a high-swirl diesel engine. For the engine intake flows, a model of the Sandia National Laboratories 1.9L light-duty single cylinder optical engine was used. An extensive experimental campaign provided validation data in terms of ensemble averages of planar PIV measurements at different vertical locations in the combustion chamber, for different swirl ratio configurations during both the intake and the compression strokes.The generalized RNG k-epsilon model provided the best accuracy trade-off for both swirl and shear flows, thanks to the polynomial…
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Experimental and Numerical Investigations of Close-Coupled Pilot Injections to Reduce Combustion Noise in a Small-Bore Diesel Engine

SAE International Journal of Engines

General Motors Company-Alok Warey, Francesco Pesce, Richard Peterson, Alberto Vassallo
Sandia National Laboratories-Stephen Busch, Kan Zha, Paul C. Miles
  • Journal Article
  • 2015-01-0796
Published 2015-04-14 by SAE International in United States
A pilot-main injection strategy is investigated for a part-load operating point in a single cylinder optical Diesel engine. As the energizing dwell between the pilot and main injections decreases below 200 μs, combustion noise reaches a minimum and a reduction of 3 dB is possible. This decrease in combustion noise is achieved without increased pollutant emissions. Injection schedules employed in the engine are analyzed with an injection analyzer to provide injection rates for each dwell tested. Two distinct injection events are observed even at the shortest dwell tested; rate shaping of the main injection occurs as the dwell is adjusted. High-speed elastic scattering imaging of liquid fuel is performed in the engine to examine initial liquid penetration rates. The penetration rate data provide evidence that rate shaping of the initial phase of the main injection is occurring in the engine and that this rate shaping is largely consistent with the injection rate data, but the results demonstrate that these changes are not responsible for the observed trend in combustion noise.A zero-dimensional model is created to…
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Principal Component Analysis and Study of Port-Induced Swirl Structures in a Light-Duty Optical Diesel Engine

Sandia National Laboratories-Kan Zha, Stephen Busch, Paul Miles
University of Wisconsin-Federico Perini, Rolf D. Reitz
Published 2015-04-14 by SAE International in United States
In this work computational and experimental approaches are combined to characterize in-cylinder flow structures and local flow field properties during operation of the Sandia 1.9L light-duty optical Diesel engine. A full computational model of the single-cylinder research engine was used that considers the complete intake and exhaust runners and plenums, as well as the adjustable throttling devices used in the experiments to obtain different swirl ratios. The in-cylinder flow predictions were validated against an extensive set of planar PIV measurements at different vertical locations in the combustion chamber for different swirl ratio configurations. Principal Component Analysis was used to characterize precession, tilting and eccentricity, and regional averages of the in-cylinder turbulence properties in the squish region and the piston bowl. Complete sweeps of the port throttle configurations were run to study their effects on the flow structure, together with their correlation with the swirl ratio. Significant deviations between the flows in the piston bowl and squish regions were observed. Piston bowl design, more than the swirl ratio, was identified to foster flow homogeneity between these…
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Characterization of Flow Asymmetry During the Compression Stroke Using Swirl-Plane PIV in a Light-Duty Optical Diesel Engine with the Re-entrant Piston Bowl Geometry

SAE International Journal of Engines

Convergent Science, Inc.-Sameera Wijeyakulasuriya, Saurav Mitra, P. K. Senecal
Sandia National Laboratories-Kan Zha, Stephen Busch, Paul C. Miles
  • Journal Article
  • 2015-01-1699
Published 2015-04-14 by SAE International in United States
Flow field asymmetry can lead to an asymmetric mixture preparation in Diesel engines. To understand the evolution of this asymmetry, it is necessary to characterize the in-cylinder flow over the full compression stroke. Moreover, since bowl-in-piston cylinder geometries can substantially impact the in-cylinder flow, characterization of these flows requires the use of geometrically correct pistons. In this work, the flow has been visualized via a transparent piston top with a realistic bowl geometry, which causes severe experimental difficulties due to the spatial and temporal variation of the optical distortion. An advanced optical distortion correction method is described to allow reliable particle image velocimetry (PIV) measurements through the full compression stroke.Based on the ensemble-averaged velocity results, flow asymmetry characterized by the swirl center offset and the associated tilting of the vortex axis is quantified. The observed vertical tilting of swirl center axis is similar for tested swirl ratios (2.2 and 3.5), indicating that the details of the intake flows are not of primary importance to the late-compression mean flow asymmetry. Instead, the geometry of the piston…
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In-Cylinder Wall Temperature Influence on Unburned Hydrocarbon Emissions During Transitional Period in an Optical Engine Using a Laser-Induced Phosphorescence Technique

SAE International Journal of Engines

Georgia Southern Univ.-Valentin Soloiu
Wayne State Univ.-Xi Luo, Xin Yu, Kan Zha, Marcis Jansons
  • Journal Article
  • 2014-01-1373
Published 2014-04-01 by SAE International in United States
Emissions of Unburned Hydrocarbons (UHC) from diesel engines are a particular concern during the starting process, when after-treatment devices are typically below optimal operating temperatures. Drivability in the subsequent warm-up phase is also impaired by large cyclic fluctuations in mean effective pressure (MEP). This paper discusses in-cylinder wall temperature influence on unburned hydrocarbon emissions and combustion stability during the starting and warm-up process in an optical engine. A laser-induced phosphorescence technique is used for quantitative measurements of in-cylinder wall temperatures just prior to start of injection (SOI), which are correlated to engine out UHC emission mole fractions and combustion phasing during starting sequences over a range of charge densities, at a fixed fueling rate.Squish zone cylinder wall temperature shows significant influence on engine out UHC emissions during the warm-up process. Higher surface temperatures correlate with lower levels of engine-out UHC. Moreover, the UHC emissions are more sensitive to wall temperatures at higher charge densities. Engine-out UHC is less sensitive to bowl surface temperatures, which are observed to quickly attain values nearly 100°C greater than those…
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Simulation and Experimental Measurement of CO2*, OH* and CH2O* Chemiluminescence from an Optical Diesel Engine Fueled with n-Heptane

Wayne State University-Xin Yu, Kan Zha, Xi Luo, Dinu Taraza, Marcis Jansons
Published 2013-09-08 by SAE International in United States
A means of validating numerical simulations has been developed which utilizes chemiluminescence measurements from an internal combustion engine. By incorporating OH*, CH2O* and CO2* chemiluminescence sub-mechanisms into a detailed n-heptane reaction mechanism, excited species concentration and chemiluminescence light emission were calculated. The modeled line-of-sight chemiluminescence emission allows a direct comparison of simulation results to experimentally measured chemiluminescence images obtained during combustion in an optically accessible compression ignition engine using neat n-heptane fuel. The spray model was calibrated using in-cylinder liquid penetration length Mie scattering measurements taken from the jets of the high-pressure piezo injector. The experimental, two dimensional images of CH2O* and OH* chemiluminescence during the low and high temperature heat release period were recorded with an intensified CCD camera in a wavelength range covering emission from these species. By interpreting the color content of the images taken from a CMOS high speed camera, crank-angle resolved two dimensional CO2* chemiluminescence distributions were obtained. All the chemiluminescence images taken at the same crank angle degrees were used to generate probability density function (PDF) distributions which can…
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