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Technical Paper

The Influence of Swirl on HSDI Diesel Combustion at Moderate Speed and Load

Heat release analysis of the in-cylinder pressure records and images of the naturally occurring combustion luminosity obtained in an optical engine are used to explore the effect of variable swirl ratio on the diesel combustion process. Swirl ratios Rs at IVC of 1.5, 2.5, and 3.5 were investigated. The engine is equipped with common-rail fuel injection equipment, and the combustion chamber geometry is maintained as close as possible to typical engines intended for automotive applications. The operating condition employed was 2000 rpm, with a gross IMEP of 5.0 bar and 800 bar injection pressure. Swirl ratio is found to exert a measurable influence on most of the combustion process, from ignition to late-cycle oxidation. Ignition delay decreases with increasing Rs, as do the magnitudes of the initial premixed burn, the peak rates of heat release, and the maximum rates of pressure rise.
Technical Paper

The Influence of Swirl Ratio on Turbulent Flow Structure in a Motored HSDI Diesel Engine - A Combined Experimental and Numerical Study

Simultaneous two-component measurements of gas velocity and multi-dimensional numerical simulation are employed to characterize the evolution of the in-cylinder turbulent flow structure in a re-entrant bowl-in-piston engine under motored operation. The evolution of the mean flow field, turbulence energy, turbulent length scales, and the various terms contributing to the production of the turbulence energy are correlated and compared, with the objectives of clarifying the physical mechanisms and flow structures that dominate the turbulence production and of identifying the source of discrepancies between the measured and simulated turbulence fields. Additionally, the applicability of the linear turbulent stress modeling hypothesis employed in the k-ε model is assessed using the experimental mean flow gradients, turbulence energy, and length scales.
Technical Paper

The Influence of Charge Dilution and Injection Timing on Low-Temperature Diesel Combustion and Emissions

The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a small-bore single-cylinder diesel engine over a wide range of injection timing. The fresh air was diluted with additional N2 and CO2, simulating 0 to 65% exhaust gas recirculation in an engine. Diluting the intake charge lowers the flame temperature T due to the reactant being replaced by inert gases with increased heat capacity. In addition, charge dilution is anticipated to influence the local charge equivalence ratio ϕ prior to ignition due to the lower O2 concentration and longer ignition delay periods. By influencing both ϕ and T, charge dilution impacts the path representing the progress of the combustion process in the ϕ-T plane, and offers the potential of avoiding both soot and NOx formation.
Journal Article

The Impact of Fuel Mass, Injection Pressure, Ambient Temperature, and Swirl Ratio on the Mixture Preparation of a Pilot Injection

Fuel tracer-based planar laser-induced fluorescence is used to investigate the vaporization and mixing behavior of pilot injections for variations in pilot mass of 1-4 mg, and for two injection pressures, two near-TDC ambient temperatures, and two swirl ratios. The fluorescent tracer employed, 1-methylnaphthalene, permits a mixture of the diesel primary reference fuels, n-hexadecane and heptamethylnonane, to be used as the base fuel. With a near-TDC injection timing of −15°CA, pilot injection fuel is found to penetrate to the bowl rim wall for even the smallest injection quantity, where it rapidly forms fuel-lean mixture. With increased pilot mass, there is greater penetration and fuel-rich mixtures persist well beyond the expected pilot ignition delay period. Significant jet-to-jet variations in fuel distribution due to differences in the individual jet trajectories (included angle) are also observed.
Technical Paper

The Effect of Swirl Ratio and Fuel Injection Parameters on CO Emission and Fuel Conversion Efficiency for High-Dilution, Low-Temperature Combustion in an Automotive Diesel Engine

Engine-out CO emission and fuel conversion efficiency were measured in a highly-dilute, low-temperature diesel combustion regime over a swirl ratio range of 1.44-7.12 and a wide range of injection timing. At fixed injection timing, an optimal swirl ratio for minimum CO emission and fuel consumption was found. At fixed swirl ratio, CO emission and fuel consumption generally decreased as injection timing was advanced. Moreover, a sudden decrease in CO emission was observed at early injection timings. Multi-dimensional numerical simulations, pressure-based measurements of ignition delay and apparent heat release, estimates of peak flame temperature, imaging of natural combustion luminosity and spray/wall interactions, and Laser Doppler Velocimeter (LDV) measurements of in-cylinder turbulence levels are employed to clarify the sources of the observed behavior.
Journal Article

Sources of UHC Emissions from a Light-Duty Diesel Engine Operating in a Partially Premixed Combustion Regime

Sources of unburned hydrocarbon (UHC) emissions are examined for a highly dilute (10% oxygen concentration), moderately boosted (1.5 bar), low load (3.0 bar IMEP) operating condition in a single-cylinder, light-duty, optically accessible diesel engine undergoing partially-premixed low-temperature combustion (LTC). The evolution of the in-cylinder spatial distribution of UHC is observed throughout the combustion event through measurement of liquid fuel distributions via elastic light scattering, vapor and liquid fuel distributions via laser-induced fluorescence, and velocity fields via particle image velocimetry (PIV). The measurements are complemented by and contrasted with the predictions of multi-dimensional simulations employing a realistic, though reduced, chemical mechanism to describe the combustion process.
Technical Paper

Quantitative Measurements of Residual and Fresh Charge Mixing in a Modern SI Engine Using Spontaneous Raman Scattering

Line-imaging of Raman scattered light is used to simultaneously measure the mole fractions of CO2, H2O, N2, O2, and fuel (premixed C3H8) in a modern 4-valve spark-ignition engine operating at idle. The measurement volume consists of 16 adjacent sub-volumes, each 0.27 mm in diameter × 0.91 mm long, giving a total measurement length of 14.56 mm. Measurements are made 3 mm under the centrally-located spark plug, offset 3 mm from the spark plug center towards the exhaust valves. Data are taken in 15 crank angle degree increments starting from top center before the intake stroke (-360 CAD) through top center of the compression stroke (0 CAD).
Technical Paper

Quantitative In-Cylinder Fluid Composition Measurements Using Broadband Spontaneous Raman Scattering

A measurement technique is described which permits quantitative, multi-point, in-cylinder fluid composition measurements in an internal combustion engine. The technique is based on spontaneous Raman scattering with broadband signal collection, and provides simultaneous determination of the mole fractions of H2O, CO2, N2, O2, and fuel (C3H8). Calibration and data reduction procedures which largely eliminate interference between the signals from the various species are discussed. Mean mole fractions obtained in an optical access engine from the beginning of the intake stroke to the end of the compression stroke are reported. The mean mole fractions of the individual species are used to obtain independent estimates of the mean mole fraction of burnt residual gases, which, in turn, are used to evaluate the accuracy and consistency of the data. An over-prediction of the CO2 mole fraction in hot exhaust gases is found to be the most significant shortcoming of the technique.
Journal Article

Pilot Injection Ignition Properties Under Low-Temperature, Dilute In-Cylinder Conditions

Measurements of ignition behavior, homogeneous reactor simulations employing detailed kinetics, and quantitative in-cylinder imaging of fuel-air distributions are used to delineate the impact of temperature, dilution, pilot injection mass, and injection pressure on the pilot ignition process. For dilute, low-temperature conditions characterized by a lengthy ignition delay, pilot ignition is impeded by the formation of excessively lean mixture. Under these conditions, smaller pilot mass or higher injection pressures further lengthen the pilot ignition delay. Similarly, excessively rich mixtures formed under relatively short ignition delay conditions typical of conventional diesel combustion will also prolong the ignition delay. In this latter case, smaller pilot mass or higher injection pressures will shorten the ignition delay. The minimum charge temperature required to effect a robust pilot ignition event is strongly dependent on charge O2 concentration.
Journal Article

PIV Measurements in the Swirl-Plane of a Motored Light-Duty Diesel Engine

Particle image velocimetry (PIV) is used to investigate the structure and evolution of the mean velocity field in the swirl (r-θ) plane of a motored, optically accessible diesel engine with a typical production combustion chamber geometry under motoring conditions (no fuel injection). Instantaneous velocities were measured were made at three swirl-plane heights (3 mm, 10 mm and 18 mm below the firedeck) and three swirl ratios (2.2, 3.5 and 4.5) over a range of crank angles in the compression and expansion strokes. The data allow for a direct analysis of the structures within the ensemble mean flow field, the in-cylinder swirl ratio, and the radial profile of the tangential velocity. At all three swirl ratios, the ensemble mean velocity field contains a single dominant swirl flow structure that is tilted with respect to the cylinder axis. The axis of this structure precesses about the cylinder axis in a manner that is largely insensitive to swirl ratio.
Journal Article

Optical Investigation of UHC and CO Sources from Biodiesel Blends in a Light-Duty Diesel Engine Operating in a Partially Premixed Combustion Regime

The influence of soy- and palm-based biofuels on the in-cylinder sources of unburned hydrocarbons (UHC) and carbon monoxide (CO) was investigated in an optically accessible research engine operating in a partially premixed, low-temperature combustion regime. The biofuels were blended with an emissions certification grade diesel fuel and the soy-based biofuel was also tested neat. Cylinder pressure and emissions of UHC, CO, soot, and NOx were obtained to characterize global fuel effects on combustion and emissions. Planar laser-induced fluorescence was used to capture the spatial distribution of fuel and partial oxidation products within the clearance and bowl volumes of the combustion chamber. In addition, late-cycle (30° and 50° aTDC) semi-quantitative CO distributions were measured above the piston within the clearance volume using a deep-UV LIF technique.
Technical Paper

On the Cyclic Variability and Sources of Unburned Hydrocarbon Emissions in Low Temperature Diesel Combustion Systems

The cycle-to-cycle variability and potential sources of unburned hydrocarbon (UHC) emissions are examined in a single-cylinder, light-duty diesel test engine operating in low-temperature combustion regimes. A fast flame ionization detector (FID) was employed to examine both cycle-to-cycle variations in UHC emissions and intra-cycle emissions behavior. A standard suite of emissions measurements, including CO, CO2, NOx, and soot, was also obtained. Measurements were made spanning a broad range of intake O2 concentrations-to examine the UHC behavior of dilution-controlled combustion regimes-and spanning a broad range of injection timings-to clarify the behavior of increased UHC emissions in late-injection combustion regimes. Both low- and moderate-loads were investigated. The cycle-resolved UHC data showed that the coefficient of variation of single-cycle UHC did not increase with increases in UHC emissions as either O2 concentration or injection timing was varied.
Technical Paper

Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine

This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 × 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 1283 cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid.
Technical Paper

Modeling the Effects of EGR and Injection Pressure on Soot Formation in a High-Speed Direct-Injection (HSDI) Diesel Engine Using a Multi-Step Phenomenological Soot Model

Low-temperature combustion concepts that utilize cooled EGR, early/retarded injection, high swirl ratios, and modest compression ratios have recently received considerable attention. To understand the combustion and, in particular, the soot formation process under these operating conditions, a modeling study was carried out using the KIVA-3V code with an improved phenomenological soot model. This multi-step soot model includes particle inception, surface growth, surface oxidation, and particle coagulation. Additional models include a piston-ring crevice model, the KH/RT spray breakup model, a droplet wall impingement model, a wall heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process.
Technical Paper

Modeling the Effects of EGR and Injection Pressure on Emissions in a High-Speed Direct-Injection Diesel Engine

Experimental data is used in conjunction with multi-dimensional modeling in a modified version of the KIVA-3V code to characterize the emissions behavior of a high-speed, direct-injection diesel engine. Injection pressure and EGR are varied across a range of typical small-bore diesel operating conditions and the resulting soot-NOx tradeoff is analyzed. Good agreement is obtained between experimental and modeling trends; the HSDI engine shows increasing soot and decreasing NOx with higher EGR and lower injection pressure. The model also indicates that most of the NOx is formed in the region where the bulk of the initial heat release first takes place, both for zero and high EGR cases. The mechanism of NOx reduction with high EGR is shown to be primarily through a decrease in thermal NOx formation rate.
Technical Paper

Measurement of Instantaneous Flamelet Surface Normals and the Burning Rate in a SI Engine

A recently developed technique, crossed-plane imaging, is extended to measure instantaneous flamelet surface normals in a single-cylinder, optical SI engine. Two simultaneous, orthogonal acetone PLIF images are used to measure the instantaneous flamelet orientation in three dimensions. The images are also used to measure contours of constant mean reaction progress variable < c> and the mean flamelet crossing density. Statistics of the flamelet surface normal are presented in spherical coordinates in terms of a polar angle, f, and an azimuthal angle,q; the pole is aligned with the normal to a constant surface. The data are used to estimate marginal probability density functions (PDF's) in f and q. The estimated marginal PDF's are found to be well represented by the same functional forms applied previously to turbulent V-flames. The flamelet surface density and the mean fractional increase in flamelet surface area due to turbulence are also estimated.
Journal Article

Measurement of Equivalence Ratio in a Light-Duty Low Temperature Combustion Diesel Engine by Planar Laser Induced Fluorescence of a Fuel Tracer

The spatial distribution of the mixture equivalence ratio within the squish volume is quantified under non-combusting conditions by planar laser-induced fluorescence (PLIF) of a fuel tracer (toluene). The measurements were made in a single-cylinder, direct-injection, light-duty diesel engine at conditions matched to an early-injection low temperature combustion mode. A fuel amount corresponding to a low load (3.0 bar indicated mean effective pressure) operating condition was introduced with a single injection. Data were acquired during the mixture preparation period from near the start of injection (-22.5° aTDC) until the crank angle where the start of high-temperature heat release normally occurs (-5° aTDC). Despite the opposing squish flow, the fuel jets penetrate through the squish region to the cylinder bore. Although rapid mixing is observed in the head of each jet, rich regions remain at the head at the start of high-temperature heat release.
Technical Paper

Late-Cycle Turbulence Generation in Swirl-Supported, Direct-Injection Diesel Engines

Cycle-resolved analysis of velocity data obtained in the re-entrant bowl of a fired high-;speed, direct-injection diesel engine, demonstrates an unambiguous, approximately 100% increase in late-cycle turbulence levels over the levels measured during motored operation. Model predictions of the flow field, obtained employing RNG k-ε turbulence modeling in KIVA-3V, do not capture this increased turbulence. A combined experimental and computational approach is taken to identify the source of this turbulence. The results indicate that the dominant source of the increased turbulence is associated with the formation of an unstable distribution of mean angular momentum, characterized by a negative radial gradient. The importance of this source of flow turbulence has not previously been recognized for engine flows. The enhanced late-cycle turbulence is found to be very sensitive to the flow swirl level.
Technical Paper

Influence of Spray-Target and Squish Height on Sources of CO and UHC in a HSDI Diesel Engine During PPCI Low-Temperature Combustion

Laser induced fluorescence (LIF) imaging during the expansion stroke, exhaust gas emissions, and cylinder pressure measurements were used to investigate the influence on combustion and CO/UHC emissions of variations in squish height and fuel spray targeting on the piston. The engine was operated in a highly dilute, partially premixed, low-temperature combustion mode. A small squish height and spray targeting low on the piston gave the lowest exhaust emissions and most rapid heat release. The LIF data show that both the near-nozzle region and the squish volume are important sources of UHC emissions, while CO is dominated by the squish region and is more abundant near the piston top. Emissions from the squish volume originate primarily from overly lean mixture. At the 3 bar load investigated, CO and UHC levels in mixture leaving the bowl and ring-land crevice are low.
Journal Article

In-cylinder CO and UHC Imaging in a Light-Duty Diesel Engine during PPCI Low-Temperature Combustion

Two-dimensional planar imaging and one-dimensional, spectrally-resolved line-imaging of laser-induced fluorescence from CO and UHC are performed to help identify the sources of these emissions in a light-duty diesel engine operating in a partially-premixed compression ignition combustion regime. Cycle-averaged measurements are made in the clearance volume above the piston crown at a 3 bar IMEP, 1500 rpm baseline operating condition. Sweeps of injection timing, load, and O2 concentration are performed to examine the impact of these parameters on the in-cylinder spatial distributions of CO and UHC. At the baseline operating condition, the main contributions to UHC from the clearance volume stem from regions near the cylinder centerline and near the cylinder wall, where UHC likely emanates from the top ring-land crevice. Broadly distributed CO within the squish volume dominates over CO observed near the cylinder centerline.