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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.
Journal Article

Particulate Matter Sampling and Volatile Organic Compound Removal for Characterization of Spark Ignited Direct Injection Engine Emissions

2011-08-30
2011-01-2100
More stringent emissions regulations are continually being proposed to mitigate adverse human health and environmental impacts of internal combustion engines. With that in mind, it has been proposed that vehicular particulate matter (PM) emissions should be regulated based on particle number in addition to particle mass. One aspect of this project is to study different sample handling methods for number-based aerosol measurements, specifically, two different methods for removing volatile organic compounds (VOCs). One method is a thermodenuder (TD) and the other is an evaporative chamber/diluter (EvCh). These sample-handling methods have been implemented in an engine test cell with a spark-ignited direct injection (SIDI) engine. The engine was designed for stoichiometric, homogeneous combustion.
Technical Paper

Effect of Flowfield Non-Uniformities on Emissions Predictions in HSDI Engines

2011-04-12
2011-01-0821
The role of the fluid motion in a diesel engine on mixing and combustion was investigated using the CFD code Kiva-3v. The study considered pre-mixed charge compression ignition (PCCI) combustion that is a hybrid combustion system characterized by early injection timings and high amounts of EGR dilution to delay the start and lower the temperature of combustion. The fuel oxidizer mixture is not homogeneous at the start of combustion and therefore requires further mixing for complete combustion. PCCI combustion systems are characterized by relatively high CO and UHC emissions. This work investigates attenuating CO emissions by enhancing mixing processes through non-uniform flowfield motions. The fluid motion was characterized by the amount of average angular rotation about the cylindrical axis (swirl ratio) and the amount of non-uniform motion imparted by the relative amounts of mass inducted through tangential and helical intake ports in a 0.5L HSDI diesel engine.
Journal Article

Optical Diagnostics and Multi-Dimensional Modeling of Spray Targeting Effects in Late-Injection Low-Temperature Diesel Combustion

2009-11-02
2009-01-2699
The effects of spray targeting on mixing, combustion, and pollutant formation under a low-load, late-injection, low-temperature combustion (LTC) diesel operating condition are investigated by optical engine measurements and multi-dimensional modeling. Three common spray-targeting strategies are examined: conventional piston-bowl-wall targeting (152° included angle); narrow-angle floor targeting (124° included angle); and wide-angle piston-bowl-lip targeting (160° included angle). Planar laser-induced fluorescence diagnostics in a heavy-duty direct-injection optical diesel engine provide two-dimensional images of fuel-vapor, low-temperature ignition (H2CO), high-temperature ignition (OH) and soot-formation species (PAH) to characterize the LTC combustion process.
Journal Article

Experiments and Modeling of Dual-Fuel HCCI and PCCI Combustion Using In-Cylinder Fuel Blending

2009-11-02
2009-01-2647
This study investigates the potential of controlling premixed charge compression ignition (PCCI and HCCI) combustion strategies by varying fuel reactivity. In-cylinder fuel blending using port fuel injection of gasoline and early cycle direct injection of diesel fuel was used for combustion phasing control at both high and low engine loads and was also effective to control the rate of pressure rise. The first part of the study used the KIVA-CHEMKIN code and a reduced primary reference fuel (PRF) mechanism to suggest optimized fuel blends and EGR combinations for HCCI operation at two engine loads (6 and 11 bar net IMEP). It was found that the minimum fuel consumption could not be achieved using either neat diesel fuel or neat gasoline alone, and that the optimal fuel reactivity required decreased with increasing load. For example, at 11 bar net IMEP, the optimum fuel blend and EGR rate for HCCI operation was found to be PRF 80 and 50%, respectively.
Technical Paper

Neutron Imaging of Diesel Particulate Filters

2009-11-02
2009-01-2735
This article presents nondestructive neutron computed tomography (nCT) measurements of Diesel Particulate Filters (DPFs) as a method to measure ash and soot loading in the filters. Uncatalyzed and unwashcoated 200cpsi cordierite DPFs exposed to 100% biodiesel (B100) exhaust and conventional ultra low sulfur 2007 certification diesel (ULSD) exhaust at one speed-load point (1500 rpm, 2.6 bar BMEP) are compared to a brand new (never exposed) filter. Precise structural information about the substrate as well as an attempt to quantify soot and ash loading in the channel of the DPF illustrates the potential strength of the neutron imaging technique.
Journal Article

Experimental Investigation of Intake Condition and Group-Hole Nozzle Effects on Fuel Economy and Combustion Noise for Stoichiometric Diesel Combustion in an HSDI Diesel Engine

2009-04-20
2009-01-1123
The goal of this research is to investigate the physical parameters of stoichiometric operation of a diesel engine under a light load operating condition (6∼7 bar IMEP). This paper focuses on improving the fuel efficiency of stoichiometric operation, for which a fuel consumption penalty relative to standard diesel combustion was found to be 7% from a previous study. The objective is to keep NOx and soot emissions at reasonable levels such that a 3-way catalyst and DPF can be used in an aftertreatment combination to meet 2010 emissions regulation. The effects of intake conditions and the use of group-hole injector nozzles (GHN) on fuel consumption of stoichiometric diesel operation were investigated. Throttled intake conditions exhibited about a 30% fuel penalty compared to the best fuel economy case of high boost/EGR intake conditions. The higher CO emissions of throttled intake cases lead to the poor fuel economy.
Journal Article

Effects of Piston Bowl Geometry on Mixture Development and Late-Injection Low-Temperature Combustion in a Heavy-Duty Diesel Engine

2008-04-14
2008-01-1330
Low-temperature combustion (LTC) strategies for diesel engines are of increasing interest because of their potential to significantly reduce particulate matter (PM) and nitrogen oxide (NOx) emissions. LTC with late fuel injection further offers the benefit of combustion phasing control because ignition is closely coupled to the fuel injection event. But with a short ignition-delay, fuel jet mixing processes must be rapid to achieve adequate premixing before ignition. In the current study, mixing and pollutant formation of late-injection LTC are studied in a single-cylinder, direct-injection, optically accessible heavy-duty diesel engine using three laser-based imaging diagnostics. Simultaneous planar laser-induced fluorescence of the hydroxyl radical (OH) and combined formaldehyde (H2CO) and polycyclic aromatic hydrocarbons (PAH) are compared with vapor-fuel concentration measurements from a non-combusting condition.
Technical Paper

Fuel Injection and Mean Swirl Effects on Combustion and Soot Formation in Heavy Duty Diesel Engines

2007-04-16
2007-01-0912
High-speed video imaging in a swirl-supported (Rs = 1.7), direct-injection heavy-duty diesel engine operated with moderate-to-high EGR rates reveals a distinct correlation between the spatial distribution of luminous soot and mean flow vorticity in the horizontal plane. The temporal behavior of the experimental images, as well as the results of multi-dimensional numerical simulations, show that this soot-vorticity correlation is caused by the presence of a greater amount of soot on the windward side of the jet. The simulations indicate that while flow swirl can influence pre-ignition mixing processes as well as post-combustion soot oxidation processes, interactions between the swirl and the heat release can also influence mixing processes. Without swirl, combustion-generated gas flows influence mixing on both sides of the jet equally. In the presence of swirl, the heat release occurs on the leeward side of the fuel sprays.
Technical Paper

Effects of Engine Operating Parameters on near Stoichiometric Diesel Combustion Characteristics

2007-04-16
2007-01-0121
Stoichiometric combustion could enable a three-way catalyst to be used for treating NOx emissions of diesel engines, which is one of the most difficult species for diesel engines to meet future emission regulations. Previous study by Lee et al. [1] showed that diesel engines can operate with stoichiometric combustion successfully with only a minor impact on fuel consumption. Low NOx emission levels were another advantage of stoichiometric operation according to that study. In this study, the characteristics of stoichiometric diesel combustion were evaluated experimentally to improve fuel economy as well as exhaust emissions The effects of fuel injection pressure, boost pressure, swirl, intake air temperature, combustion regime (injection timing), and engine load (fuel mass injected) were assessed under stoichiometric conditions.
Technical Paper

A Computational Investigation into the Effects of Spray Targeting, Bowl Geometry and Swirl Ratio for Low-Temperature Combustion in a Heavy-Duty Diesel Engine

2007-04-16
2007-01-0119
A computational study was performed to evaluate the effects of bowl geometry, fuel spray targeting and swirl ratio under highly diluted, low-temperature combustion conditions in a heavy-duty diesel engine. This study is used to examine aspects of low-temperature combustion that are affected by mixing processes and offers insight into the effect these processes have on emissions formation and oxidation. The foundation for this exploratory study stems from a large data set which was generated using a genetic algorithm optimization methodology. The main results suggest that an optimal combination of spray targeting, swirl ratio and bowl geometry exist to simultaneously minimize emissions formation and improve soot and CO oxidation rates. Spray targeting was found to have a significant impact on the emissions and fuel consumption performance, and was furthermore found to be the most influential design parameter explored in this study.
Technical Paper

Assessment of Diesel Engine Size-Scaling Relationships

2007-04-16
2007-01-0127
Engine development is both time consuming and economically straining. Therefore, efforts are being made to optimize the research and development process for new engine technologies. The ability to apply information gained by studying an engine of one size/application to an engine of a completely different size/application would offer savings in both time and money in engine development. In this work, a computational study of diesel engine size-scaling relationships was performed to explore engine scaling parameters and the fundamental engine operating components that should be included in valid scaling arguments. Two scaling arguments were derived and tested: a simple, equal spray penetration scaling model and an extended, equal lift-off length scaling model. The simple scaling model is based on an equation for the conservation of mass and an equation for spray tip penetration developed by Hiroyasu et al. [1].
Technical Paper

CFD Analysis of Flow Field and Pressure Losses in Carburetor Venturi

2006-11-13
2006-32-0113
A commercial CFD package was used to develop a three-dimensional, fully turbulent model of the compressible flow across a complex-geometry venturi, such as those typically found in small engine carburetors. The results of the CFD simulations were used to understand the effect of the different obstacles in the flow on the overall discharge coefficient and the static pressure at the tip of the fuel tube. It was found that the obstacles located at the converging nozzle of the venturi do not cause significant pressure losses, while those obstacles that create wakes in the flow, such as the fuel tube and throttle plate, are responsible for most of the pressure losses. This result indicated that an overall discharge coefficient can be used to correct the mass flow rate, while a localized correction factor can be determined from three-dimensional CFD simulations in order to calculate the static pressure at locations of interest within the venturi.
Technical Paper

Numerical and Theoretical Fuel Flow Analysis of Small Engine Carburetor Idle Circuits

2006-11-13
2006-32-0111
This paper presents a theoretical analysis of the fuel and air flows within the idle circuit found in simple carburetors. The idle circuit is modeled numerically using a dynamic model that considers the resistances of the flow paths as well as the inertia of the fuel. The modeling methodology is flexible, in that the organization and techniques can be applied to any configuration and geometry. The numerical model calculates the fuel flow response of carburetor idle/transition circuits to pressure variations associated with air flow through the venturi and around the throttle plate. The model is implemented for a typical small engine carburetor and the nominal results are presented for this specific design.
Technical Paper

Implementation of a Theoretical Carburetor Model in One-Dimensional Engine Simulation Software

2006-04-03
2006-01-1543
The main circuits of a small engine carburetor can be represented as a complex, dynamic, two-phase flow fluid network. This paper presents the theoretical characterization of a dynamic one-dimensional model of fuel and air flow in small engine carburetors and its implementation into a one-dimensional engine simulation software package. This implementation allows for studying the effect of changes in individual carburetor parts on engine performance. The characterization of the model indicated that the dynamic behavior of the entire flow network can be captured by the solution of the instantaneous momentum balance equation on the single-phase liquid elements of the network, simplifying the dynamic model considerably. The second part of this work discusses the implementation into the one-dimensional engine simulation package, and shows examples of the studies that the coupled implementation allow for.
Technical Paper

Flamelet Modeling with LES for Diesel Engine Simulations

2006-04-03
2006-01-0058
Large Eddy Simulation (LES) with a flamelet time scale combustion model is used to simulate diesel combustion. The flamelet time scale model uses a steady-state flamelet library for n-heptane indexed by mean mixture fraction, mixture fraction variance, and mean scalar dissipation rate. In the combustion model, reactions proceed towards the flamelet library solution at a time scale associated with the slowest reaction. This combination of a flamelet solution and a chemical time scale helps to account for unsteady mixing effects. The turbulent sub-grid stresses are simulated using a one-equation, non-viscosity LES model called the dynamic structure model. The model uses a tensor coefficient determined by the dynamic procedure and the subgrid kinetic energy. The model has been expanded to include scalar mixing and scalar dissipation. A new model for the conditional scalar dissipation has been developed to better predict local extinction.
Technical Paper

Modeling Early Injection Processes in HSDI Diesel Engines

2006-04-03
2006-01-0056
Numerical simulations were performed to investigate the combustion process in the Premixed Compression Ignition (PCI) regime in a light-duty diesel engine. The CHEMKIN code was implemented into an updated KIVA-3V release 2 code to simulate combustion and emission characteristics using reduced chemistry. The test engine used for validation data was a single cylinder version of a production 1.9L four-cylinder HSDI diesel engine. The engine operating condition considered was 2,000 rev/min and 5 bar BMEP load. Because high EGR levels are required for combustion retardation to make PCI combustion possible, the EGR rate was set at a relatively high level (40%) and injection timing sweeps were considered. Since injection timings were very advanced, impingement of the fuel spray on the piston bowl wall was unavoidable. To model the effects of fuel films on exhaust emissions, a drop and wall interaction model was implemented in the present code.
Technical Paper

Comparison of the Characteristic Time (CTC), Representative Interactive Flamelet (RIF), and Direct Integration with Detailed Chemistry Combustion Models against Optical Diagnostic Data for Multi-Mode Combustion in a Heavy-Duty DI Diesel Engine

2006-04-03
2006-01-0055
Three different approaches for modeling diesel engine combustion are compared against cylinder pressure, NOx emissions, high-speed soot luminosity imaging, and 2-color thermometry data from a heavy-duty DI diesel engine. A characteristic time combustion (KIVA-CTC) model, a representative interactive flamelet (KIVA-RIF) model, and direct integration using detailed chemistry (KIVA-CHEMKIN) were integrated into the same version of the KIVA-3v computer code. In this way, the computer code provides a common platform for comparing various combustion models. Five different engine operating strategies that are representative of several different combustion regimes were explored in the experiments and model simulations. Two of the strategies produce high-temperature combustion with different ignition delays, while the other three use dilution to achieve low-temperature combustion (LTC), with early, late, or multiple injections.
Technical Paper

Development and Application of a Non-Gradient Step-Controlled Search Algorithm for Engine Combustion Optimization

2006-04-03
2006-01-0239
A new search technique, called Non-Gradient Step-Controlled algorithm (NGSC), is presented. The NGSC was applied independently from pre-selected starting points and as a supplement to a Genetic Algorithm (GA) to optimize a HSDI diesel engine using split injection strategies. It is shown that the NGSC handles well the challenges of a complex response surface and factor high-dimensionality, which demonstrates its capability as an efficient and accurate tool to seek “local” convergence on complex surfaces. By directly tracking the change of a merit function, the NGSC places no requirement on response surface continuity / differentiability, and hence is more robust than gradient-dependent search techniques. The directional search mechanism takes factor interactions into consideration, and search step size control is adopted to facilitate search efficiency.
Technical Paper

Stoichiometric Combustion in a HSDI Diesel Engine to Allow Use of a Three-way Exhaust Catalyst

2006-04-03
2006-01-1148
The objectives of this study were 1) to evaluate the characteristics of rich diesel combustion near the stoichiometric operating condition, 2) to explore the possibility of stoichiometric operation of a diesel engine in order to allow use of a three-way exhaust after-treatment catalyst, and 3) to achieve practical operation ranges with acceptable fuel economy impacts. Boost pressure, EGR rate, intake air temperature, fuel mass injected, and injection timing variations were investigated to evaluate diesel stoichiometric combustion characteristics in a single-cylinder high-speed direct injection (HSDI) diesel engine. Stoichiometric operation in the Premixed Charge Compression Ignition (PCCI) combustion regime and standard diesel combustion were examined to investigate the characteristics of rich combustion. The results indicate that diesel stoichiometric operation can be achieved with minor fuel economy and soot impact.
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