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Modeling plume interaction and collapse for direct-injection gasoline sprays is important because of its impact on fuel-air mixing and engine performance. Nevertheless, the aerodynamic interaction between plumes and the complicated two-phase coupling of the evaporating spray has shown to be notoriously difficult to predict. With the availability of high-speed (100 kHz) Particle Image Velocimetry (PIV) experimental data, we compare velocity field predictions between plumes to observe the full temporal evolution leading up to plume merging and complete spray collapse. The target “Spray G” operating conditions of the Engine Combustion Network (ECN) is the focus of the work, including parametric variations in ambient gas temperature. We apply both LES and RANS spray models in different CFD platforms, outlining features of the spray that are most critical to model in order to predict the correct aerodynamics and fuel-air mixing.

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. Experimental validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California (USC). Companion computer simulations are being performed by Sandia National Laboratories (SNL), Lawrence Livermore National Laboratory (LLNL), and California Institute of Technology (Caltech) using state-of-the-art techniques.

Simultaneous measurements of the radial and the tangential components of velocity are obtained in a high-speed, direct-injection diesel engine typical of automotive applications. Results are presented for engine operation with fuel injection, but without combustion, for three different swirl ratios and four injection pressures. With the mean and fluctuating velocities, the r-θ plane shear stress and the mean flow gradients are obtained. Longitudinal and transverse length scales are also estimated via Taylor's hypothesis. The flow is shown to be sufficiently homogeneous and stationary to obtain meaningful length scale estimates. Concurrently, the flow and injection processes are simulated with KIVA-3V employing a RNG k-ε turbulence model. The measured turbulent kinetic energy k, r-θ plane mean strain rates ( 〈Srθ〉, 〈Srr〉, and 〈Sθθ〉 ), deviatoric turbulent stresses , and the r-θ plane turbulence production terms are compared directly to the simulated results.

This work presents turbulent premixed combustion modeling in spark ignition engines using G-equation based turbulent combustion model. In present study, a turbulent flame speed expression proposed and validated in recent years by two co-authors of this paper is applied to the combustion simulation of spark ignition engines. This turbulent flame speed expression has no adjustable parameters and its constants are closely tied to the physics of scalar mixing at small scales. Based on this flame speed expression, a minor modification is introduced in this paper considering the fact that the turbulent flame speed changes to laminar flame speed if there is no turbulence. This modified turbulent flame speed expression is implemented into Ford in-house CFD code MESIM (multi-dimensional engine simulation), and is validated extensively.

RNG k-ε closure turbulence dissipation equations are evaluated employing the CFD code KIVA-3V Release 2. The numerical evaluations start by considering simple jet flows, including incompressible air jets and compressible helium jets. The results show that the RNG closure turbulence model predicts lower jet tip penetration than the "standard" k-ε model, as well as being lower than experimental data. The reason is found to be that the turbulence kinetic energy is dissipated too slowly in the downstream region near the jet nozzle exit. In this case, the over-predicted R term in RNG model becomes a sink of dissipation in the ε-equation. As a second step, the RNG turbulence closure dissipation models are further tested in complex engine flows to compare against the measured evolution of turbulence kinetic energy, and an estimate of its dissipation rate, during both the compression and expansion processes.

Well-mixed lean SI engine operation can provide improvements of the fuel economy relative to that of traditional well-mixed stoichiometric SI operation. This work examines the use of two methods for improving the stability of lean operation, namely multi-pulse transient plasma ignition and intake air preheating. These two methods are compared to standard SI operation using a conventional high-energy inductive ignition system without intake air preheating. E85 is the fuel chosen for this study. The multi-pulse transient plasma ignition system utilizes custom electronics to generate 10 kHz bursts of 10 ultra-short (12ns), high-amplitude pulses (200 A). These pulses were applied to a custom spark plug with a semi-open ignition cavity. High-speed imaging reveals that ignition in this cavity generates a turbulent jet-like early flame spread that speeds up the transition from ignition to the main combustion event.

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.

Recently, high-speed optical imaging data for a single operating point of a spray-guided gasoline engine has, along with the flamelet model and the G-equation theory, enabled the development of the new spark-ignition model SparkCIMM. Within its framework, detailed chemistry flamelet models capture the experimental feature of multiple localized ignition events along the excessively stretched and restriking spark channel, as well as the observations of non-spherical highly corrugated early turbulent flame fronts. The developed flamelet models account for the substantial turbulent fluctuations in equivalence ratio and enthalpy present under spray-guided conditions. A non-unity Lewis number formulation captures the deficient species diffusion into the highly curved flame reaction zone.

A one-dimensional model for crevice HC post-flame oxidation is used to calculate and understand the effect of operating parameters and fuel type (propane and isooctane) on the extent of crevice hydrocarbon and the product distribution in the post flame environment. The calculations show that the main parameters controlling oxidation are: bulk burned gas temperatures, wall temperatures, turbulent diffusivity, and fuel oxidation rates. Calculated extents of oxidation agree well with experimental values, and the sensitivities to operating conditions (wall temperatures, equivalence ratio, fuel type) are reasonably well captured. Whereas the bulk gas temperatures largely determine the extent of oxidation, the hydrocarbon product distribution is not very much affected by the burned gas temperatures, but mostly by diffusion rates. Uncertainties in both turbulent diffusion rates as well as in mechanisms are an important factor limiting the predictive capabilities of the model.

Significant reductions of particulate matter (PM) and nitrogen oxides (NOx) emissions from diesel engines have been realized through fueling with water-fuel emulsions. However, the physical and chemical in-cylinder mechanisms that affect these pollutant reductions are not well understood. To address this issue, laser-based and chemiluminescence imaging experiments were performed in an optically-accessible, heavy-duty diesel engine using both a standard diesel fuel (D2) and an emulsion of 20% water, by mass (W20). A laser-based Mie-scatter diagnostic was used to measure the liquid-phase fuel penetration and showed 40-70% greater maximum liquid lengths with W20 at the operating conditions tested. At some conditions with low charge temperature or density, the liquid phase fuel may impinge directly on in-cylinder surfaces, leading to increased PM, HC, and CO emissions because of poor mixing.

Formaldehyde (HCHO) near the lift-off location in a reacting diesel jet was visualized using planar laser-induced fluorescence (PLIF). Simultaneous imaging of OH chemiluminescence identified the high-temperature combustion region (lift-off). Experiments were performed in a constant-volume combustion vessel at ambient gas conditions (temperature and oxygen concentration) that generate no-soot, low-soot and moderate-soot diesel jets during mixing-controlled combustion. For no-soot conditions, results show that HCHO is formed upstream of the lift-off location and is consumed downstream of the lift-off length in fuel-rich premixed reaction zones at the jet center. Despite the fuel-rich combustion, and downstream regions that are surrounded by a high-temperature diffusion flame, there is no detectable PAH formation in the no-soot condition.

High resolution planar laser-induced fluorescence (PLIF) measurements were performed in an optically accessible internal combustion (IC) engine to investigate the behavior of scalar dissipation and the fine-scale structures of the turbulent scalar field. The fluorescent tracer fluorobenzene was doped into one of the two intake streams and nitrogen was used as the carrier gas to permit high signal-to-noise ratio fluorescence measurements without oxygen quenching effects. The resulting two-dimensional images allowed for an analysis of the structural detail of the scalar and scalar dissipation fields defined by the mixing of the two adjacent intake streams. High levels of scalar dissipation were found to be located within convoluted, sheet-like structures in accordance with previous studies. The fluorescence data, which were acquired during the intake stroke, were also used to examine the scalar energy and dissipation spectra.

Engine experiments have revealed the importance of fuel condensation on the emission characteristics of low temperature combustion. However, direct in-cylinder experimental evidence has not been reported in the literature. In this paper, the in-cylinder condensation processes observed in optically accessible engine experiments are first illustrated. The observed condensation processes are then simulated using state-of-the-art multidimensional engine CFD simulations with a phase transition model that incorporates a well-validated phase equilibrium numerical solver, in which a thermodynamically consistent phase equilibrium analysis is applied to determine when mixtures become unstable and a new phase is formed. The model utilizes fundamental thermodynamics principles to judge the occurrence of phase separation or combination by minimizing the system Gibbs free energy.

A novel algebraic similarity model for subgrid scalar dissipation rate has been developed as part of the Large Eddy Simulation (LES) package KIVA3V-LES for diesel engine study. The model is proposed from an a priori study using Direct Numerical Simulation (DNS) of forced isotropic turbulence. In the a posteriori test, fully resolved turbulent passive scalar field measurements are used to validate the model in actual engine flows. For reason of the length limit by SAE and the specific interest in engine applications, only a prior test and a posteriori test in engine flows are included in this paper. A posteriori tests in isotropic cube flow, turbulent round jet and flame cases will be presented in separate papers. An engine LES simulation of multi consecutive cycles was performed in this study.

Recent experiments of diesel injection processes have demonstrated that mixing accelerates after the end of injection (EOI). This finding has significant implications for low-temperature combustion (LTC) diesel engines. Previous simulations using a one-dimensional model of a single-pulsed air jet, which is analogous in many aspects to diesel jets, suggest that the rapid mixing after EOI in diesel jets is due to a temporary increase in the entrainment rate as the jet decelerates. In the present study, we performed a high-fidelity large eddy simulation (LES) of an unsteady air jet identical to that used for the one-dimensional model. The LES calculation agrees well with available experimental data and provides both spatially and temporally resolved details of the three-dimensional transient mixing field. Results show that entrainment increases during deceleration.

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.

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.

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.

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. In addition, greater use of newly developed computational tools holds promise for reducing the number of prototype tests, for cutting manufacturing costs, and for reducing overall time to market. Experimental verification and validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California. Companion computer simulations are being performed by Sandia National Laboratories, Lawrence Livermore National Laboratory, and California Institute of Technology using state-of- the-art techniques, with the intention of implementing more complex methods in the future.

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.