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Technical Paper

Flame Lift-Off on Direct-Injection Diesel Sprays Under Quiescent Conditions

Ambient gas temperature and density, injection pressure, and orifice diameter effects on the flame lift-off length on a direct-injection (DI) diesel spray under quiescent conditions were experimentally investigated. The impacts of the observed lift-off length variations on air entrainment upstream of the lift-off location, soot formation, and the relationship between fuel vaporization and combustion were also examined. The research was conducted in a constant-volume combustion vessel using a common-rail fuel injector and a Phillips research grade #2 diesel fuel. The lift-off length measurements show that lift-off length decreases with increasing ambient gas temperature or density, and increases with increasing injection pressure or orifice diameter. The sensitivity of lift-off length to a change in either temperature or density was non-linear, with the sensitivity to either parameter decreasing as it increased.
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

Determining the Location of End-Gas Autoignition Using Ionization Probes Installed in the Head Gasket

Ionization probes built into the head gasket and uniformly distributed around the cylinder bore of a knocking, spark-ignition engine have been used to locate the autoigniting end-gas region. As normal combustion evolves after spark ignition, the ionization probes individually respond to the arrival of the propagating flame. Then, when autoignition occurs, the probes located in the end-gas region respond in rapid succession. By utilizing pressure transducer measurements to determine when autoignition occurs, the ionization probe response becomes a means to locate the end-gas region. Knowledge of the location of the last ionization probe to detect the normal flame can then be used to infer where, within the end-gas region, autoignition first occurred.
Technical Paper

Cycle-Resolved Measurements of Flame Kernel Growth and Motion Correlated with Combustion Duration

A recently developed spark plug equipped with fiber-optic flame-arrival detectors has been used to measure the motion and rate of growth of the early flame kernel. The cylinder pressure and gas velocity in the spark gap were measured simultaneously with the flame kernel measurements, permitting the data to be analyzed on a cycle-by-cycle basis to identify cause-and-effect correlations between the measured parameters. The data were obtained in a homogeneous-charge research engine that could be modified to produce three very different flow fields: (1) high swirl with high turbulence intensity, (2) tumble vortex with moderate turbulence intensity, and (3) negligible bulk motion with low turbulence intensity. The results presented show a moderate correlation between the combustion duration and the rate of growth of the flame kernel, but virtually no correlation with either the magnitude or direction of movement of the flame kernel away from the spark gap.
Technical Paper

Transmission Electron Microscopy of Soot Particles sampled directly from a Biodiesel Spray Flame

For better understanding of soot formation and oxidation processes in a biodiesel spray flame, the morphology, microstructure and sizes of soot particles directly sampled in a spray flame fuelled with soy-methyl ester were investigated using transmission electron microscopy (TEM). The soot samples were taken at different axial locations in the spray flame, 40, 50 and 70 mm from injector nozzle, which correspond to soot formation, peak, and oxidation zones, respectively. The biodiesel spray flame was generated in a constant-volume combustion chamber under a diesel-like high pressure and temperature condition (6.7 MPa, 1000K). Density, diameter of primary particles and radius of gyration of soot aggregates reached a peak at 50 mm from the injector nozzle and was lower or smaller in the formation or oxidation zones of the spray.
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.
Technical Paper

A Computational Investigation of the Effects of Swirl Ratio and Injection Pressure on Mixture Preparation and Wall Heat Transfer in a Light-Duty Diesel Engine

In a recent study, quantitative measurements were presented of in-cylinder spatial distributions of mixture equivalence ratio in a single-cylinder light-duty optical diesel engine, operated with a non-reactive mixture at conditions similar to an early injection low-temperature combustion mode. In the experiments a planar laser-induced fluorescence (PLIF) methodology was used to obtain local mixture equivalence ratio values based on a diesel fuel surrogate (75% n-heptane, 25% iso-octane), with a small fraction of toluene as fluorescing tracer (0.5% by mass). Significant changes in the mixture's structure and composition at the walls were observed due to increased charge motion at high swirl and injection pressure levels. This suggested a non-negligible impact on wall heat transfer and, ultimately, on efficiency and engine-out emissions.
Technical Paper

The Use of Transient Operation to Evaluate Fuel Effects on Knock Limits Well beyond RON Conditions in Spark-Ignition Engines

Fundamental engine research is primarily conducted under steady-state conditions, in order to better describe boundary conditions which influence the studied phenomena. However, light-duty automobiles are operated, and tested, under heavily transient conditions. This mismatch between studied conditions and in-use conditions is deemed acceptable due to the fundamental knowledge gained from steady-state experiments. Nonetheless, it is useful to characterize the conditions encountered during transient operation and determine if the governing phenomena are unduly influenced by the differences between steady-state and transient operation, and further, whether transient behavior can be reasonably extrapolated from steady-state behavior. The transient operation mode used in this study consists of 20 fired cycles followed by 80 motored cycles, operating on a continuous basis.
Technical Paper

Predicting NOX Emissions from HCCI Engines Using LIF Imaging

Our previous work applied LIF measurements of in-cylinder fuel distribution to predict CO2, CO, and HC emissions from an HCCI engine under low-load stratified-charge conditions. The prediction method is based on the premise that local fuel-air packets at a given equivalence ratio (characterized using LIF imaging) burn as if in a homogeneous charge at the same equivalence ratio. Thus, emissions measured during homogeneous operation provide an emission-versus- equivalence-ratio look-up table for predicting stratified-charge emissions. The present paper extends the technique to predict engine-out NOX emissions. Because of operating-range limitations, NOX look-up data for homogeneous operation cannot adequately be determined by experiment. Instead, a CHEMKIN-based model provides this look-up table data instead.
Technical Paper

Diagnostics for the Study of Cold Start Mixture Preparation in a Port Fuel-Injected Engine

A variety of diagnostic techniques useful for the study of cold start phenomena are presented. Although the tools are demonstrated in a port fuel-injected engine, they are also suitable for direct-injection gasoline engines. A very useful technique, seemingly forgotten in the literature (and applicable to diesel engines as well), is the use of a short focal-length lens inside a Bowditch piston to expand the field-of-view. Rather than being limited by the clear aperture of the window in the piston, this technique permits the entire combustion chamber and the top section of the cylinder liner to be seen. Results using this technique are presented for the imaging of pool fires and laser-induced fluorescence of fuel films.
Technical Paper

Diesel-Spray Ignition and Premixed-Burn Behavior

The temporal and spatial evolution of the ignition and premixed-burn phases of a direct-injection (DI) diesel spray were investigated under quiescent conditions. The diagnostics used included temporally resolved measurements of natural light emission and pressure, and spatially resolved images of natural light emission. Temporally resolved natural light emission measurements were made with a photo-multiplier tube and a photodiode, while the images were acquired with an intensified CCD camera. The experiments were conducted in an optically accessible, constant-volume combustion vessel over a range of ambient gas temperatures and densities: 800-1100 K and 7.3-45.0 kg/m3. The fuel used was a ternary blend of single-component fuels representative of diesel fuel with a cetane number of 45. The fuel was injected with a common-rail injector at high pressure (140 MPa). The results provide new information on the evolution of the two-stage ignition/premixed-burn phases of DI diesel sprays.
Technical Paper

A Qualitative Evaluation of Mixture Formation in a Direct-Injection Hydrogen-Fuelled Engine

In an optically-accessible single-cylinder engine fuelled with hydrogen, OH* chemiluminescence imaging and planar laser induced fluorescence (PLIF) are used to qualitatively evaluate in-cylinder mixture formation. The experiments include measurements for engine operation with hydrogen injection in-cylinder either prior to or after intake valve closure (IVC). Pre-IVC injection is used to produce a near homogeneous mixture in-cylinder to establish a baseline comparison for post-IVC injection. To assess the effects of injection pressure on mixture formation, two injection pressures are used for post-IVC injection. For post-IVC injection with start of injection (SOI) coincident with IVC, mixture distribution is similar to pre-IVC injection and there are little differences between the two injection pressures. With retard of SOI from IVC, mixture inhomogeneities increase monotonically for both injection pressures.
Technical Paper

Acquisition of Corresponding Fuel Distribution and Emissions Measurements in HCCI Engines

Optical engines are often skip-fired to maintain optical components at acceptable temperatures and to reduce window fouling. Although many different skip-fired sequences are possible, if exhaust emissions data are required, the skip-firing sequence ought to consist of a single fired cycle followed by a series of motored cycles (referred to here as singleton skip-firing). This paper compares a singleton skip-firing sequence with continuous firing at the same inlet conditions, and shows that combustion performance trends with equivalence ratio are similar. However, as expected, reactant temperatures are lower with skip-firing, resulting in retarded combustion phasing, and lower pressures and combustion efficiency. LIF practitioners often employ a homogeneous charge of known composition to create calibration images for converting raw signal to equivalence ratio.
Technical Paper

Relationship Between Ignition Processes and the Lift-Off Length of Diesel Fuel Jets

The reaction zone of a diesel fuel jet stabilizes at a location downstream of the fuel injector once the initial autoignition phase is over. This distance is referred to as flame lift-off length. Recent investigations have examined the effects of a wide range of parameters (injection pressure, orifice diameter, and ambient gas temperature, density and oxygen concentration) on lift-off length under quiescent diesel conditions. Many of the experimental trends in lift-off length were in agreement with scaling laws developed for turbulent, premixed flame propagation in gas-jet lifted flames at atmospheric conditions. However, several effects did not correlate with the gas-jet scaling laws, suggesting that other mechanisms could be important to lift-off stabilization at diesel conditions. This paper shows experimental evidence that ignition processes affect diesel lift-off stabilization.
Technical Paper

EGR and Intake Boost for Managing HCCI Low-Temperature Heat Release over Wide Ranges of Engine Speed

Reaching for higher loads and improving combustion-phasing control are important challenges for HCCI research. Although HCCI engines can operate with a variety of fuels, recent research has shown that fuels with two-stage autoignition have some significant advantages for overcoming these challenges. Because the amount of low-temperature heat release (LTHR) is proportional to the local equivalence ratio (ϕ), fuel stratification can be used to adjust the combustion phasing (CA50) and/or burn duration using various fuel-injection strategies. Two-stage ignition fuels also allow stable combustion even for extensive combustion-phasing retard, which reduces the knocking propensity. Finally, the LTHR reduces the required intake temperature, which increases the inducted charge mass for a given intake pressure, allowing higher fueling rates before knocking and NOx emissions become a problem. However, the amount of LTHR is normally highly dependent on the engine speed.
Technical Paper

Investigation of Small-Scale Unintended Releases of Hydrogen

Knowledge of the concentration field and flammability envelope from a small-scale hydrogen leak is an issue of importance for the safe use of hydrogen. A combined experimental and modeling program is being carried out by Sandia National Laboratories to characterize and predict the behavior of small-scale hydrogen releases. In contrast to the previous work performed by Sandia on large, momentum-dominated hydrogen leaks, these studies are focusing on small leaks in the Froude number range where both buoyant and inertial forces are important or, in the limit, where buoyancy dominates leak behavior. In the slow leak regime buoyant forces affect the trajectory and rate of air entrainment of the hydrogen jet leak and significant curvature can occur in the jet trajectory. Slow leaks may occur from leaky fittings or o-ring seals on hydrogen vehicles or other hydrogen-based systems where large amounts of pressure drop occur across the leak path.
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.
Technical Paper

Effect of Intake Pressure on Performance and Emissions in an Automotive Diesel Engine Operating in Low Temperature Combustion Regimes

A single-cylinder, light-duty, diesel engine was used to investigate the effect of changes in intake pressure (boost) on engine performance and emissions in low-temperature combustion (LTC) regimes. Two different LTC strategies were examined: a dilution-controlled regime characterized by high rates of exhaust gas recirculation (EGR) with early-injection (roughly 30° BTDC), and a late-injection (near TDC) regime employing moderate EGR levels. For both strategies, moderate (8 bar IMEP) and low (3 bar IMEP) load conditions were tested at intake pressures of 1.0, 1.5, and 2.0 bar. For both LTC strategies, increased intake pressure reduces emissions of unburned hydrocarbons (UHC) and CO, with corresponding improvements in combustion efficiency and indicated specific fuel consumption (ISFC), particularly at high load. Depending on the operating condition, UHC and CO emissions can stem from either over-lean or over-rich mixtures.
Technical Paper

Characterization of Combustion, Piston Temperatures, Fuel Sprays, and Fuel-Air Mixing in a DISI Optical Engine

A transparent direct-injection spark-ignition engine incorporating a rapid-acting, drop-down cylinder has been built. The design enables access in less than a minute for cleaning windows. Combustion performance of the optical engine is characterized in terms of indicated pressure and coefficient of variation of indicated pressure as a function of injection timing. Piston temperatures are measured and a skip-fire routine is developed so that quartz piston top temperatures agree with a matching non-optical engine. Laser-induced fluorescence imaging of in-cylinder fuel injections highlights the effects of ambient pressure and fuel temperature on spray morphology. Measurements of gasoline vapor distribution provide statistics on heterogeneity of fuel distribution as a function of injection timing. Flame imaging records details of flame development which depend on the degree of fuel mixing.
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.