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Technical Paper

An Investigation of Real-Gas and Multiphase Effects on Multicomponent Diesel Sprays

2020-04-14
2020-01-0240
Lagrangian spray modeling represents a critical boundary condition for multidimensional simulations of in-cylinder flow structure, mixture formation and combustion in internal combustion engines. Segregated models for injection, breakup, collision and vaporization are usually employed to pass appropriate momentum, mass, and energy source terms to the gas-phase solver. Careful calibration of each sub-model generally produces appropriate results. Yet, the predictiveness of this modeling approach has been questioned by recent experimental observations, which showed that at trans- and super-critical conditions relevant to diesel injection, classical atomization and vaporization behavior is replaced by a mixing-controlled phase transition process of a dense fluid. In this work, we assessed the shortcomings of classical spray modeling with respect to real-gas and phase-change behavior, employing a multicomponent phase equilibrium solver and liquid-jet theory.
Technical Paper

Experimental Evaluation of a Custom Gasoline-Like Blend Designed to Simultaneously Improve ϕ-Sensitivity, RON and Octane Sensitivity

2020-04-14
2020-01-1136
ϕ-sensitivity is a fuel characteristic that has important benefits for the operation and control of low-temperature gasoline combustion (LTGC) engines. However, regular gasoline is not very ϕ-sensitive at low-pressure conditions, meaning that intake boosting (typically Pin ≥ 1.3 bar) is required to take advantage of this property. Thus, there is strong motivation to design a gasoline-like fuel that simultaneously improves ϕ-sensitivity, RON and octane sensitivity, to make an improved fuel suitable for both LTGC and modern SI engines. In a previous study [SAE 2019-01-0961], a 5-component regulation-compliant fuel blend (CB#1) was computationally designed; and simulations showed promising results when it was compared to a regular E10 gasoline (RD5-87). The current study experimentally evaluates CB#1 in the Sandia LTGC engine and compares the results with those of RD5-87. The RON and octane sensitivity were improved 1.3 and 3.6 units by CB#1, respectively.
Technical Paper

Measurements and Correlations of Local Cylinder-Wall Heat-Flux Relative to Near-Wall Chemiluminescence across Multiple Combustion Modes

2020-04-14
2020-01-0802
Minimizing heat-transfer (HT) losses is important for both improving engine efficiency and increasing exhaust energy for turbocharging and exhaust aftertreatment management, but engine combustion system design to minimize these losses is hindered by significant uncertainties in prediction. Empirical HT correlations such as the popular Woschni model have been developed and various attempts at improving predictions have been proposed since the 1960s, but due to variations in facilities and techniques among various studies, comparison and assessment of modelling approaches among multiple combustion modes is not straightforward. In this work, simultaneous cylinder-wall temperature and OH* chemiluminescence high-speed video are all recorded in a single heavy-duty optical engine operated under multiple combustion modes. OH* chemiluminescence images provide additional insights for identifying the causes of near-wall heat flux changes.
Technical Paper

Numerical Investigation of Near Nozzle Flash-Boiling Spray in an Axial-Hole Transparent Nozzle

2020-04-14
2020-01-0828
Understanding and prediction of flash-boiling spray behavior in gasoline direct-injection (GDI) engines remains a challenge. In this study, computational fluid dynamics (CFD) simulations using the homogeneous relaxation model (HRM) for not only internal nozzle flow but also external spray were evaluated using CONVERGE software and compared to experimental data. High-speed extinction imaging experiments were carried out in a real-size axial-hole transparent nozzle installed at the tip of machined GDI injector fueled with n-pentane under various ambient pressure conditions (Pa/Ps = 0.07 - 1.39). The width of the spray during injection was assessed by means of projected liquid volume, but the structure and timing for boil-off of liquid within the sac of the injector were also assessed after the end of injection, including cases with different designed sac volumes.
Technical Paper

Transient Internal Nozzle Flow in Transparent Multi-Hole Diesel Injector

2020-04-14
2020-01-0830
An accurate prediction of internal nozzle flow in fuel injector offers the potential to improve predictions of spray computational fluid dynamics (CFD) in an engine, providing a coupled internal-external calculation or by defining better rate of injection (ROI) profile and spray angle information for Lagrangian parcel computations. Previous research has addressed experiments and computations in transparent nozzles, but less is known about realistic multi-hole diesel injectors compared to single axial-hole fuel injectors. In this study, the transient injector opening and closing is characterized using a transparent multi-hole diesel injector, and compared to that of a single axial hole nozzle (ECN Spray D shape). A real-size five-hole acrylic transparent nozzle was mounted in a high-pressure, constant-flow chamber. Internal nozzle phenomena such as cavitation and gas exchange were visualized by high-speed long-distance microscopy.
Technical Paper

Identifying the Driving Processes of Diesel Spray Injection through Mixture Fraction and Velocity Field Measurements at ECN Spray A

2020-04-14
2020-01-0831
Diesel spray mixture formation is investigated at target conditions using multiple diagnostics and laboratories. High-speed Particle Image Velocimetry (PIV) is used to measure the velocity field inside and outside the jet simultaneously with a new frame straddling synchronization scheme. The PIV measurements are carried out in the Engine Combustion Network Spray A target conditions, enabling direct comparisons with mixture fraction measurements previously performed in the same conditions, and forming a unique database at diesel conditions. A 1D spray model, based upon mass and momentum exchange between axial control volumes and near-Gaussian velocity and mixture fraction profiles is evaluated against the data.
Technical Paper

Combined Experimental/Numerical Study of the Soot Formation Process in a Gasoline Direct-Injection Spray in the Presence of Laser-Induced Plasma Ignition

2020-04-14
2020-01-0291
Combustion issued from an eight-hole, direct-injection spray was experimentally studied in a constant-volume pre-burn combustion vessel using simultaneous high-speed diffused back-illumination extinction imaging (DBIEI) and OH* chemiluminescence. DBIEI has been employed to observe the liquid-phase of the spray and to quantitatively investigate the soot formation and oxidation taking place during combustion. The fuel-air mixture was ignited with a plasma induced by a single-shot Nd:YAG laser, permitting precise control of the ignition location in space and time. OH* chemiluminescence was used to track the high-temperature ignition and flame. The study showed that increasing the delay between the end of injection and ignition drastically reduces soot formation without necessarily compromising combustion efficiency. For long delays between the end of injection and ignition (1.9 ms) soot formation was eliminated in the main downstream charge of the fuel spray.
Technical Paper

Two-Scale Command Shaping for Reducing NVH during Engine Shutdown

2020-04-14
2020-01-0411
Two-scale command shaping is a recently proposed feedforward control method aimed at mitigating undesirable vibrations in nonlinear systems. The TSCS strategy uses a scale separation to cancel oscillations arising from nonlinear behavior of the system, and command shaping of the remaining linear problem. One promising application of TSCS is in reducing engine restart and shutdown vibrations found in conventional and in hybrid electric vehicle powertrains equipped with start-stop features. The efficacy of the TSCS during internal combustion engine restart has been demonstrated theoretically and experimentally in the authors’ prior works. The present article presents simulation results and describes the verified experimental apparatus used to study TSCS as applied to the ICE shutdown case. The apparatus represents a typical HEV powertrain and consists of a 1.03 L three-cylinder diesel ICE coupled to a permanent magnet alternating current electric machine through a spur gear coupling.
Journal Article

On the Role of Nitric Oxide for the Knock-Mitigation Effectiveness of EGR in a DISI Engine Operated with Various Gasoline Fuels

2019-12-19
2019-01-2150
The knock-suppression effectiveness of exhaust-gas recirculation (EGR) can vary between implementations that take EGR gases after the three-way catalyst and those that use pre-catalyst EGR gases. A main difference between pre-and post-catalyst EGR gases is the level of trace species like NO, UHC, CO and H2. To quantify the role of NO, this experiment-based study employs NO-seeding in the intake tract for select combinations of fuel types and compression ratios, using simulated post-catalyst EGR gases as the diluent. The four investigated gasoline fuels share a common RON of 98, but vary in octane sensitivity and composition. To enable probing effects of near-zero NO levels, a skip-firing operating strategy is developed whereby the residual gases, which contain trace species like NO, are purged from the combustion chamber. Overall, the effects of NO-seeding on knock are consistent with the differences in knock limits for preand post-catalyst EGR gases.
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

Investigation of Fuel Condensation Processes under Non-reacting Conditions in an Optically-Accessible Engine

2019-04-02
2019-01-0197
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.
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.
Journal Article

Dilution and Injection Pressure Effects on Ignition and Onset of Soot at Threshold-Sooting Conditions by Simultaneous PAH-PLIF and Soot-PLII Imaging in a Heavy Duty Optical Diesel Engine

2019-04-02
2019-01-0553
Although accumulated in-cylinder soot can be measured by various optical techniques, discerning soot formation rates from oxidation rates is more difficult. Various optical measurements have pointed toward ways to affect in-cylinder soot oxidation, but evidence of effects of operational variables on soot formation is less plentiful. The formation of soot and its precursors, including polycyclic aromatic hydrocarbons (PAHs), are strongly dependent on temperature, so factors affecting soot formation may be more evident at low-temperature combustion conditions. Here, in-cylinder PAHs are imaged by planar laser-induced fluorescence (PAH-PLIF) using three different excitation wavelengths of 355, 532, and 633 nm, to probe three different size-classes of PAH from 2-3 to 10+ rings. Simultaneous planar laser-induced incandescence of soot (soot-PLII) using 1064-nm excitation provides complementary imaging of soot formation near inception.
Technical Paper

Ignition and Soot Formation/Oxidation Characteristics of Compositionally Unique International Diesel Blends

2019-04-02
2019-01-0548
With the global adoption of diesel common rail systems and the wide variation in composition of local commercial fuels, modern fuel injection systems must be robust against diverse fuel properties. To bridge the knowledge gap on the effects of compositional variation for real commercial fuels on spray combustion characteristics, the present work quantifies ignition and soot formation/oxidation in three unique, international diesel blends. Schlieren imaging, excited-state hydroxyl radical (OH*) chemiluminescence imaging and diffused back-illumination extinction imaging were employed to quantify vapor penetration, ignition, and soot formation and oxidation for high-pressure sprays in a constant-volume, pre-burn chamber. The three fuels were procured from Finland, Japan and Brazil and have cetane numbers of 64.1, 56.1 and 45.4, respectively.
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

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

Combustion-Timing Control of Low-Temperature Gasoline Combustion (LTGC) Engines by Using Double Direct-Injections to Control Kinetic Rates

2019-04-02
2019-01-1156
Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies and extremely low NOx and particulate emissions, but controlling the combustion timing remains a challenge. This paper explores the potential of Partial Fuel Stratification (PFS) to provide fast control of CA50 in an LTGC engine. Two different compression ratios are used (CR=16:1 and 14:1) that provide high efficiencies and are compatible with mixed-mode SI-LTGC engines. The fuel used is a research grade E10 gasoline (RON 92, MON 85) representative of a regular-grade market gasoline found in the United States. The fuel was supplied with a gasoline-type direct injector (GDI) mounted centrally in the cylinder. To create the PFS, the GDI injector was pulsed twice each engine cycle. First, an injection early in the intake stroke delivered the majority of the fuel (70 - 80%), establishing the minimum equivalence ratio in the charge.
Journal Article

Using Chemical Kinetics to Understand Effects of Fuel Type and Compression Ratio on Knock-Mitigation Effectiveness of Various EGR Constituents

2019-04-02
2019-01-1140
Exhaust gas recirculation (EGR) can be used to mitigate knock in SI engines. However, experiments have shown that the effectiveness of various EGR constituents to suppress knock varies with fuel type and compression ratio (CR). To understand some of the underlying mechanisms by which fuel composition, octane sensitivity (S), and CR affect the knock-mitigation effectiveness of EGR constituents, the current paper presents results from a chemical-kinetics modeling study. The numerical study was conducted with CHEMKIN, imposing experimentally acquired pressure traces on a closed reactor model. Simulated conditions include combinations of three RON-98 (Research Octane Number) fuels with two octane sensitivities and distinctive compositions, three EGR diluents, and two CRs (12:1 and 10:1). The experimental results point to the important role of thermal stratification in the end-gas to smooth peak heat-release rate (HRR) and prevent acoustic noise.
Technical Paper

Φ-Sensitivity for LTGC Engines: Understanding the Fundamentals and Tailoring Fuel Blends to Maximize This Property

2019-04-02
2019-01-0961
Φ-sensitivity is a fuel characteristic that has important benefits for the operation and control of low-temperature gasoline combustion (LTGC) engines. A fuel is φ-sensitive if its autoignition reactivity varies with the fuel/air equivalence ratio (φ). Thus, multiple-injection strategies can be used to create a φ-distribution that leads to several benefits. First, the φ-distribution causes a sequential autoignition that reduces the maximum heat release rate. This allows higher loads without knock and/or advanced combustion timing for higher efficiencies. Second, combustion phasing can be controlled by adjusting the fuel-injection strategy. Finally, experiments show that intermediate-temperature heat release (ITHR) increases with φ-sensitivity, increasing the allowable combustion retard and improving stability. A detailed mechanism was applied using CHEMKIN to understand the chemistry responsible for φ-sensitivity.
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.
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