The RNG κ-e turbulence model derived by Yakhot and Orszag (1986) based on the Renormalization Group theory has been modified and applied to variable-density engine flows in the present study. The original RNG-based turbulence transport approximations were developed formally for an incompressible flow. In order to account for flow compressibility the RNG e-equation is modified and closed through an isotropic rapid distortion analysis. Computations were made of engine compressing/expanding flows and the results were compared with available experimental observations in a production diesel engine geometry. The modified RNG κ-e model was also applied to diesel spray combustion computations. It is shown that the use of the RNG model is warranted for spray combustion modeling since the ratio of the turbulent to mean-strain time scales is appreciable due to spray-generated mean flow gradients, and the model introduces a term to account for these effects. Large scale flow structures are predicted which ar...

Turbulence Modeling of Internal Combustion Engines Using RNG κ-ε Models

Modeling spray atomization with the kelvin-helmholtz/rayleigh-taylor hybrid model

This report describes an extended version of KIVA-3, known as KIVA-3V, that can model any number of vertical or canted valves in the cylinder head of an internal combustion (IC) engine. The valves are treated as solid objects that move through the mesh using the familiar snapper technique used for piston motion in KIVA-3. Because the valve motion is modeled exactly, and the valve shapes are as exact as the grid resolution will allow, the accuracy of the valve model is commensurate with that of the rest of the program. Other new features in KIVA-3V include a particle-based liquid wall film model, a new sorting subroutine that is linear in the number of nodes and preserves the original storage sequence, a mixing-controlled turbulent combustion model, and an optional RNG {kappa}-{epsilon} turbulence model. All features and capabilities of the original KIVA-3 have been retained. The grid generator, K3PREP, has been expanded to support the generation of grids with valves, along with the shaping of valve ports and runners. Graphics output options have also been expanded. The report discusses the new features, and includes four examples of grids with vertical and canted valves that are representative of IC engines in use today.

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KIVA-3V: A Block-Structured KIVA Program for Engines with Vertical or Canted Valves

Mechanism of the Smokeless Rich Diesel Combustion by Reducing Temperature

The development of four-stroke, spark-ignition engines that are designed to inject gasoline directly into the combustion chamber is an important worldwide initiative of the automotive industry. The thermodynamic potential of such engines for significantly enhanced fuel economy, transient response and cold-start hydrocarbon emission levels has led to a large number of research and development projects that have the goal of understanding, developing and optimizing gasoline direct-injection (GDI) combustion systems. The processes of fuel injection, spray atomization and vaporization, charge cooling, mixture preparation and the control of in-cylinder air motion are all being actively researched, and this work is reviewed in detail and analyzed. The new technologies such as high-pressure, common-rail, gasoline injection systems and swirl-atomizing gasoline fuel injectors are discussed in detail, as these technologies, along with computer control capabilities, have enabled the current new examination of an old objective; the direct-injection, stratified-charge (DISC), gasoline engine. The prior work on DISC engines that is relevant to current GDI engine development is also reviewed and discussed.

The fuel economy and emission data for actual engine configurations are of significant importance to engine researchers and developers. These data have been obtained and assembled for all of the available GDI literature, and are reviewed and discussed in detail. The types of GDI engines are arranged in four classifications of decreasing complexity, and the advantages and disadvantages of each class are noted and explained. Emphasis is placed upon consensus trends and conclusions that are evident when taken as a whole. Thus the GDI researcher is informed regarding the degree to which engine volumetric efficiency and compression ratio can be increased under optimized conditions, and as to the extent to which unburned hydrocarbon (UBHC), NOx and particulate emissions can be minimized for specific combustion strategies. The critical area of GDI fuel injector deposits and the associated effect on spray geometry and engine performance degradation are reviewed, and important system guidelines for minimizing deposition rates and deposit effects are presented. The capabilities and limitations of emission control techniques and aftertreatment hardware are reviewed in depth, and areas of consensus on attaining European, Japanese and North American emission standards are compiled and discussed.

All known research, prototype and production GDI engines worldwide are reviewed as to performance, emissions and fuel economy advantages, and for areas requiring further development. The engine schematics, control diagrams and specifications are compiled, and the emission control strategies are illustrated and discussed. The influence of lean-NOx catalysts on the development of late-injection, stratified-charge GDI engines is reviewed, and the relative merits of lean-burn, homogeneous, direct-injection engines as an option requiring less control complexity are analyzed. All current information in the literature is used as the basis for discussing the future development of automotive GDI engines.

Automotive Spark-Ignited Direct-Injection Gasoline Engines

A temperature wall function formulation for variable-density turbulent flows with application to engine convective heat transfer modeling

The Development and Application of a Diesel Ignition and Combustion Model for Multidimensional Engine Simulation

Engine experiments have shown that with highpressure multiple injections (two or more injection pulses per power cycle), the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with the conventional single-pulse injections, reducing both soot and NOx emissions significantly. In order to understand the mechanism of emissions reduction, multidimensional computations were carried out for a heavy-duty diesel engine with multiple injections. Different injection schemes were considered, and the predicted cylinder pressure, heat release rate and soot and NOx emissions were compared with measured data. Excellent agreements between predictions and measurements were achieved after improvements in the models were made. The improvements include using a RNG k-e turbulence model, adopting a new wall heat transfer model and introducing the nozzle discharge coefficient to account for the contraction of fuel jet at the nozzle exit. The present computations confirm that split injection allows significant soot reduction with out a NOx penalty. Based on the computations, it is found that multiple injections have a similar NOx reduction mechanism as single injections with retarded injection timings. Regarding soot reduction, it is shown that reduced soot formation is due to the fact that the soot producing rich regions at the spray tip are not replenished when the injection is terminated and then restarted. With split injections, the subsequently injected fuel burns rapidly and does not contribute significantly to soot production. The present work also demonstrates the usefulness of multidimensional modeling of diesel combustion to reveal combustion mechanisms and to provide design insights for low emission engines. EXTENSIVE RESEARCH is in progress to reduce both nitrogen oxides (NOx) and particulate (soot) emissions from diesel engines due to environmental concerns. One of the emission-control strategies is in-cylinder reduction of pollutant production. It is well known that it is very difficult to reduce both NOx and soot production simultaneously during the combustion process. Many emission-reduction technologies developed so far tend to increase soot emission while reducing NOx emission, and vice versa. For example, retarding fuel injection timing can be effective to reduce NO formation. However, this usually results in an increase of soot production. On the other hand, although increasing fuel injection pressure can decrease soot emissions, it can also cause higher NOx emissions at the same time [1]*. Recently, it has been shown experimentally that with high-pressure multiple injections, the soot-NOx trade-off curves of a diesel engine can be shifted closer to the origin than those with single-pulse injections, reducing both soot and NOx emissions significantly [2-4]. Nehmer and Reitz experimentally investigated the effect of double-pulse split injection on soot and NOx emissions using a single-cylinder Caterpillar heavy-duty diesel engine [2]. They varied the amount of fuel injected in the first injection pulse from 10 percent to 75 percent of the total amount of fuel and found that split injection affected the sootNOx trade-off. In general, their split-injection schemes reduced NOx with only a minimal increase in soot emissions and did not extend the combustion duration. Tow et al. [3] continued the study of Nehmer and Reitz [2] using the same engine, and included different dwells between injection pulses and triple injection schemes in their investigation. They found that at high engine load (75%), particulate could be reduced by a factor of three with no increase in NOx and only a 2.5% increase in BSFC compared to a single injection, using a double injection with a relatively long dwell between injections. They also found that triple injection could reduce NOx and soot emissions at both light and high loads. Another important conclusion of Tow et al. [3] is that the dwell between injection pulses is very important to control soot production and there exits an optimum dwell at a particular engine operating condition. The optimum dwell of a double-injection was found to be about 10 degree crank angles at 75% load and 1600 rev/min for their engine conditions. * Numbers in brackets designate References at the end of the paper. Pierpont et al. [4] confirmed that the amount of fuel injected in the first pulse affects the particulate (smoke) level in experiments where the NOx emission level was held constant. However, the best double injections were found to also depend on the spray nozzle included angle. For a production injector with a 125 included angle, which results in significant wall impingement on the piston bowl, the best double injections were found to be those with 50% to 60% of the fuel injected in the first pulse. They also found that with a combination of EGR and multiple injections, particulate and NOx were simultaneously reduced to as low as 0.07 and 2.2 g/bhp-hr, respectively, at 75% load and 1600 rev/min. Other multiple injection studies can be also found in the open literature [5, 6]. The published experimental works indicate that multiple-injection is an effective mean to control NO and particulate production during the diesel combustion process. In general, multiple injections allow the injection timing to be retarded to reduce NOx emission while holding the particulate at low levels. Both the amount of fuel injected in the first pulse and the dwell between pulses are important for an optimum injection scheme. With the application of multiple injection technology, the goal of improved injection scheme design and better control of engine combustion is made difficult by the fact that design variables are added with flexible injectors. It is thus helpful to simulate the engine processes with the use of computational models, which can provide detailed temporal and spatial information of precisely parameter-controlled injection and combustion processes. Patterson et al. [7] performed multidimensional computations of multiple injections using an improved KIVA code. They tried to reproduce the experimental results of Nehmer and Reitz [2] and achieved a fair success. However, the accuracy of their model prediction deteriorated for double-pulse injections as the amount of fuel injected in the second pulse increased. Kong, Han and Reitz [8] modified the code by including a modified RNG k-e turbulence model and turbulence boundary conditions [9]. Predictions of combustion and emissions of single-injections were shown to be improved significantly [8]. These successes motivated the application of the code to multiple injections in the present study. It is clear that a good model is necessary in order to predict engine combustion and emissions accurately. Accordingly, the submodels used by Kong, Han and Reitz [8] were implemented together with improved heat transfer and injection models. The models were first applied to the experimental results of the double injections of Nehmer and Reitz [2]. For better understanding of the formation of NO and soot during multiple-injection combustion processes, a set of designed singleand double-injection schemes were computed. Based on the computational results, a mechanism of emission reduction using multiple-injection is suggested.

Zhiyu Han

Papers

A temperature wall function formulation for variable-density turbulent flows with application to engine convective heat transfer modeling

The Development and Application of a Diesel Ignition and Combustion Model for Multidimensional Engine Simulation

Mechanism of Soot and NOx Emission Reduction Using Multiple-injection in a Diesel Engine

Modeling atomization processes of pressure-swirl hollow-cone fuel sprays

Modeling fuel preparation and stratified combustion in a gasoline direct injection engine