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Technical Paper

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

2019-09-09
2019-24-0167
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.
Technical Paper

Limitations of Sector Mesh Geometry and Initial Conditions to Model Flow and Mixture Formation in Direct-Injection Diesel Engines

2019-04-02
2019-01-0204
Sector mesh modeling is the dominant computational approach for combustion system design optimization. The aim of this work is to quantify the errors descending from the sector mesh approach through three geometric modeling approaches to an optical diesel engine. A full engine geometry mesh is created, including valves and intake and exhaust ports and runners, and a full-cycle flow simulation is performed until fired TDC. Next, an axisymmetric sector cylinder mesh is initialized with homogeneous bulk in-cylinder initial conditions initialized from the full-cycle simulation. Finally, a 360-degree azimuthal mesh of the cylinder is initialized with flow and thermodynamics fields at IVC mapped from the full engine geometry using a conservative interpolation approach. A study of the in-cylinder flow features until TDC showed that the geometric features on the cylinder head (valve tilt and protrusion into the combustion chamber, valve recesses) have a large impact on flow complexity.
Technical Paper

Identification and Characterization of Steady Spray Conditions in Convergent, Single-Hole Diesel Injectors

2019-04-02
2019-01-0281
Reduced-order models typically assume that the flow through the injector orifice is quasi-steady. The current study investigates to what extent this assumption is true and what factors may induce large-scale variations. Experimental data were collected from a single-hole metal injector with a smoothly converging hole and from a transparent facsimile. Gas, likely indicating cavitation, was observed in the nozzles. Surface roughness was a potential cause for the cavitation. Computations were employed using two engineering-level Computational Fluid Dynamics (CFD) codes that considered the possibility of cavitation. Neither computational model included these small surface features, and so did not predict internal cavitation. At steady state, it was found that initial conditions were of little consequence, even if they included bubbles within the sac. They however did modify the initial rate of injection by a few microseconds.
Technical Paper

A Visual Investigation of CFD-Predicted In-Cylinder Mechanisms That Control First- and Second-Stage Ignition in Diesel Jets

2019-04-02
2019-01-0543
The long-term goal of this work is to develop a conceptual model for multiple injections of diesel jets. The current work contributes to that effort by performing a detailed modeling investigation into mechanisms that are predicted to control 1st and 2nd stage ignition in single-pulse diesel (n-dodecane) jets under different conditions. One condition produces a jet with negative ignition dwell that is dominated by mixing-controlled heat release, and the other, a jet with positive ignition dwell and dominated by premixed heat release. During 1st stage ignition, fuel is predicted to burn similarly under both conditions; far upstream, gases at the radial-edge of the jet, where gas temperatures are hotter, partially react and reactions continue as gases flow downstream. Once beyond the point of complete fuel evaporation, near-axis gases are no longer cooled by the evaporation process and 1st stage ignition transitions to 2nd stage ignition.
Technical Paper

Detailed Investigation into the Effect of Ozone Addition on Spark Assisted Compression Ignition Engine Performance and Emissions Characteristics

2019-04-02
2019-01-0966
The impact of 50 ppm intake seeding of ozone (O3) on performance and emissions characteristics was explored in a single-cylinder research engine operated under lean spark assisted compression ignition (SACI) conditions. Optical access into the engine enabled complementary crank angle resolved measurements of in-cylinder O3 concentration via ultraviolet (UV) light absorption. Experiments were performed at moderate loads (4 - 5 bar indicated mean effective pressure) and low-to-moderate engine speeds (800 - 1400 revolutions per minute). Each operating condition featured a single early main injection and maximum brake torque spark timing. Intake pressure was fixed at 1.0 bar, while intake temperatures were varied between 42 - 80 °C. Moderate amounts of internal residuals (12 - 20%) were retained through the use of positive valve overlap. Ozone addition was to found stabilize combustion relative to similar conditions without O3 addition by promoting end gas auto-ignition.
Technical Paper

Discrete-Direct Model Calibration and Propagation Approach Addressing Sparse Replicate Tests and Material, Geometric, and Measurement Uncertainties

2018-04-03
2018-01-1101
This paper introduces the “Discrete Direct” (DD) model calibration and uncertainty propagation approach for computational models calibrated to data from sparse replicate tests of stochastically varying systems. The DD approach generates and propagates various discrete realizations of possible calibration parameter values corresponding to possible realizations of the uncertain inputs and outputs of the experiments. This is in contrast to model calibration methods that attempt to assign or infer continuous probability density functions for the calibration parameters-which incorporates unjustified information in the calibration and propagation problem. The DD approach straightforwardly accommodates aleatory variabilities and epistemic uncertainties in system properties and behaviors, in input initial and boundary conditions, and in measurement uncertainties in the experiments.
Journal Article

The Effects of Injector Temperature on Spray Characteristics in Heavy-Duty Diesel Sprays

2018-04-03
2018-01-0284
This work investigates the impact of injector temperature on the characteristics of high-pressure n-dodecane sprays under conditions relevant to heavy-duty diesel engines. Sprays are injected from a pair of single-hole diesel injectors belonging to the family of “Spray C” and “Spray D” Engine Combustion Network (ECN) injectors. Low and high injector temperature conditions are achieved by activating or deactivating a cooling jacket. We quantify spray spreading angle and penetration using high-speed shadowgraphy and long-distance microscopy imaging. We evaluate differences in fuel/air mixture formation at key timings through one-dimensional modeling. Injections from a cooled injector penetrate faster than those from a higher temperature injector, especially for an injector already prone to cavitation (Spray C).
Journal Article

Standardized Optical Constants for Soot Quantification in High-Pressure Sprays

2018-04-03
2018-01-0233
Soot formation in high-pressure n-dodecane sprays is investigated under conditions relevant to heavy-duty diesel engines. Sprays are injected from a single-hole diesel injector belonging to the family of engine combustion network (ECN) Spray D injectors. Soot is quantified using a high-speed extinction imaging diagnostic with incident light wavelengths of 623 nm and 850 nm. Previously, soot measurements in a high-pressure spray using 406-nm and 520-nm incident light demonstrated a minimal wavelength dependence in the complex refractive index of soot (m), as demonstrated by a near unity ratio of the non-dimensional extinction coefficients (ke,406 nm/ke,520 nm). The present work, however, demonstrates a significant difference in m for measurements with infrared incident light. During the quasi-steady period of the spray combustion event, the experimentally determined ke ratio (ke,623 nm/ke,850 nm) is 1.42 ± 0.27.
Technical Paper

Mechanisms of Post-Injection Soot-Reduction Revealed by Visible and Diffused Back-Illumination Soot Extinction Imaging

2018-04-03
2018-01-0232
Small closely-coupled post injections of fuel in diesel engines are known to reduce engine-out soot emissions, but the relative roles of various underlying in-cylinder mechanisms have not been established. Furthermore, the efficacy of soot reduction is not universal, and depends in unclear ways on operating conditions and injection schedule, among other factors. Consequently, designing engine hardware and operating strategies to fully realize the potential of post-injections is limited by this lack of understanding. Following previous work, several different post-injection schedules are investigated using a single-cylinder 2.34 L heavy-duty optical engine equipped with a Delphi DFI 1.5 light-duty injector. In this configuration, adding a closely-coupled post injection with sufficiently short injection duration can increase the load without increasing soot emissions.
Journal Article

A Study of Piston Geometry Effects on Late-Stage Combustion in a Light-Duty Optical Diesel Engine Using Combustion Image Velocimetry

2018-04-03
2018-01-0230
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.
Technical Paper

Experimental and Numerical Studies of Bowl Geometry Impacts on Thermal Efficiency in a Light-Duty Diesel Engine

2018-04-03
2018-01-0228
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.
Technical Paper

Large-Eddy Simulations of Spray Variability Effects on Flow Variability in a Direct-Injection Spark-Ignition Engine Under Non-Combusting Operating Conditions

2018-04-03
2018-01-0196
Large-eddy Simulations (LES) have been carried out to investigate spray variability and its effect on cycle-to-cycle flow variability in a direct-injection spark-ignition (DISI) engine under non-reacting conditions. Initial simulations were performed of an injector in a constant volume spray chamber to validate the simulation spray set-up. Comparisons showed good agreement in global spray measures such as the penetration. Local mixing data and shot-to-shot variability were also compared using Rayleigh-scattering images and probability contours. The simulations were found to reasonably match the local mixing data and shot-to-shot variability using a random-seed perturbation methodology. After validation, the same spray set-up with only minor changes was used to simulate the same injector in an optically accessible DISI engine. Particle Image Velocimetry (PIV) measurements were used to quantify the flow velocity in a horizontal plane intersecting the spark plug gap.
Journal Article

Mechanisms of Enhanced Reactivity with Ozone Addition for Advanced Compression Ignition

2018-04-03
2018-01-1249
Mechanisms responsible for enhanced charge reactivity with intake added ozone (O3) were explored in a single-cylinder, optically accessible, research engine configured for low-load advanced compression ignition (ACI) experiments. The influence of O3 concentration (0-40 ppm) on engine performance metrics was evaluated as a function of intake temperature and start of injection for the engine fueled by iso-octane, 1-hexene, or a 5-component gasoline surrogate. For the engine fueled by either the gasoline surrogate or 1-hexene, 25 ppm of added O3 reduced the intake temperature required for stable combustion by 65 and 80°C, respectively. An ultraviolet (UV) light absorption diagnostic was also used to measure crank angle (CA) resolved in-cylinder O3 concentrations for select motored and fired operating conditions. The O3 measurements were compared to results from complementary 0D chemical kinetic simulations that utilized detailed chemistry mechanisms augmented with O3 oxidation chemistry.
Journal Article

Energy Analysis of Low-Load Low-Temperature Gasoline Combustion with Auxiliary-Fueled Negative Valve Overlap

2017-03-28
2017-01-0729
In-cylinder reforming of injected fuel during an auxiliary negative valve overlap (NVO) period can be used to optimize main-cycle auto-ignition phasing for low-load Low-Temperature Gasoline Combustion (LTGC), where highly dilute mixtures can lead to poor combustion stability. When mixed with fresh intake charge and fuel, these reformate streams can alter overall charge reactivity characteristics. The central issue remains large parasitic heat losses from the retention and compression of hot exhaust gases along with modest pumping losses that result from mixing hot NVO-period gases with the cooler intake charge. Accurate determination of total cycle energy utilization is complicated by the fact that NVO-period retained fuel energy is consumed during the subsequent main combustion period. For the present study, a full-cycle energy analysis was performed for a single-cylinder research engine undergoing LTGC with varying NVO auxiliary fueling rates and injection timing.
Journal Article

Increasing the Load Range, Load-to-Boost Ratio, and Efficiency of Low-Temperature Gasoline Combustion (LTGC) Engines

2017-03-28
2017-01-0731
Low-temperature gasoline combustion (LTGC) has the potential to provide gasoline-fueled engines with efficiencies at or above those of diesel engines and extremely low NOx and particulate emissions. Three key performance goals for LTGC are to obtain high loads, reduce the boost levels required for these loads, and achieve high thermal efficiencies (TEs). This paper reports the results of an experimental investigation into the use of partial fuel stratification, produced using early direct fuel injection (Early-DI PFS), and an increased compression ratio (CR) to achieve significant improvements in these performance characteristics. The experiments were conducted in a 0.98-liter single-cylinder research engine. Increasing the CR from 14:1 to 16:1 produced a nominal increase in the TE of about one TE percentage unit for both premixed and Early-DI PFS operation.
Journal Article

Conceptual Investigation of the Origins of Hydrocarbon Emissions from Mixing-Controlled, Compression-Ignition Combustion

2017-03-28
2017-01-0724
Experiments conducted with a set of reference diesel fuels in an optically accessible, compression-ignition engine have revealed a strong correlation between hydrocarbon (HC) emissions and the flame lift-off length at the end of the premixed burn (EOPMB), with increasing HC emissions associated with longer lift-off lengths. The correlation is largely independent of fuel properties and charge-gas O2 mole fraction, but varies with fuel-injection pressure. A transient, one-dimensional jet model was used to investigate three separate mechanisms that could explain the observed impact of lift-off length on HC emissions. Each mechanism relies on the formation of mixtures that are too lean to support combustion, or “overlean.” First, overlean regions can be formed after the start of fuel injection but before the end of the premixed burn.
Technical Paper

Comparison of Linear, Non-Linear and Generalized RNG-Based k-epsilon Models for Turbulent Diesel Engine Flows

2017-03-28
2017-01-0561
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.
Technical Paper

Efficiency Improvement of Boosted Low-Temperature Gasoline Combustion Engines (LTGC) Using a Double Direct-Injection Strategy

2017-03-28
2017-01-0728
For lean or dilute, boosted gasoline compression-ignition engines operating in a low-temperature combustion mode, creating a partially stratified fuel charge mixture prior to auto-ignition can be beneficial for reducing the heat-release rate (HRR) and the corresponding maximum rate of pressure rise. As a result, partial fuel stratification (PFS) can be used to increase load and/or efficiency without knock (i.e. without excessive ringing). In this work, a double direct-injection (D-DI) strategy is investigated for which the majority of the fuel is injected early in the intake stroke to create a relatively well-mixed background mixture, and the remaining fuel is injected in the latter part of the compression stroke to produce greater fuel stratification prior auto-ignition. Experiments were performed in a 1-liter single-cylinder engine modified for low-temperature gasoline combustion (LTGC) research.
Journal Article

Investigation of Fuel Effects on In-Cylinder Reforming Chemistry Using Gas Chromatography

2016-04-05
2016-01-0753
Negative Valve Overlap (NVO) is a potential control strategy for enabling Low-Temperature Gasoline Combustion (LTGC) at low loads. While the thermal effects of NVO fueling on main combustion are well-understood, the chemical effects of NVO in-cylinder fuel reforming have not been extensively studied. The objective of this work is to examine the effects of fuel molecular structure on NVO fuel reforming using gas sampling and detailed speciation by gas chromatography. Engine gas samples were collected from a single-cylinder research engine at the end of the NVO period using a custom dump-valve apparatus. Six fuel components were studied at two injection timings: (1) iso-octane, (2) n-heptane, (3) ethanol, (4) 1-hexene, (5) cyclohexane, and (6) toluene. All fuel components were studied neat except for toluene - toluene was blended with 18.9% nheptane by liquid volume to increase the fuel reactivity.
Technical Paper

Experimental Evaluation of a Prototype Free Piston Engine - Linear Alternator (FPLA) System

2016-04-05
2016-01-0677
This paper describes the experimental evaluation of a prototype free piston engine - linear alternator (FPLA) system developed at Sandia National Laboratories. The opposed piston design was developed to investigate its potential for use in hybrid electric vehicles (HEVs). The system is mechanically simple with two-stroke uniflow scavenging for gas exchange and timed port fuel injection for fuel delivery, i.e. no complex valving. Electrical power is extracted from piston motion through linear alternators which also provide a means for passive piston synchronization through electromagnetic coupling. In an HEV application, this electrical power would be used to charge the batteries. The engine-alternator system was designed, assembled and operated over a 2-year period at Sandia National Laboratories in Livermore, CA.
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