Youngchul Ra

The application of a multicomponent droplet vaporization model to gasoline direct injection engines

Abstract A model for unsteady droplet vaporization is presented that considers the droplet temperature range from flash-boiling conditions to normal evaporation. The theory of continuous thermodynamics was used to model the properties and compositions of multicomponent fuels such as gasoline. In order to model the change of evaporation rate from normal to boiling conditions more realistically, an unsteady internal heat flux model and a new model for the determination of the droplet surface temperature are proposed. An explicit form of the equation to determine the heat flux from the surrounding gas mixture to the droplet/gas interface was obtained from an approximate solution of the quasi-steady energy equation for the surrounding gas mixture, with the interdiffusion of fuel vapour and the surrounding gas taken into account. The model was applied to calculate normal and boiling evaporation processes of droplets for various ambient temperatures and droplet temperatures. Single-droplet evaporation calculated using the present model was compared with the results calculated by using the standard evaporation routine of the KIVA-3V code. Also, simulations of the vaporization of a single-component fuel (iso-octane) were compared with multi-component fuel cases. The vaporization of a hollow cone spray of fuel injected into a cylindrical chamber was simulated for both normal and flash-boiling conditions using the KIVA-3V code implemented with the present model. In addition, the model was applied to a realistic gasoline direct injection engine.

Effects of Fuel Physical Properties on Diesel Engine Combustion using Diesel and Bio-diesel Fuels

A computational study is performed to investigate the effects of physical property on diesel engine combustion characteristics using bio-diesel fuels. Properties of typical bio-diesel fuels that were either calculated or measured are used in the study and the simulation results are compared with those of conventional diesel fuels. Sensitivity of the computational results to individual physical properties is also investigated, and the results can provide information for desirable characteristics of the blended fuels. The properties considered in this study include liquid density, vapor pressure, surface tension, liquid viscosity, liquid thermal conductivity, liquid specific heat, latent heat, vapor specific heat, vapor diffusion coefficient, vapor viscosity and vapor thermal conductivity. The results show significant effects of the fuel physical properties on ignition delay and burning rates at various engine operating conditions. It is seen that there is no single physical property that dominates differences of ignition delay between diesel and bio-diesel fuels. However, among the 11 properties considered in the study, the simulation results were found to be most sensitive to the liquid fuel density, vapor pressure and surface tension through their effects on the mixture preparation processes.

Premixed Diesel Combustion Analysis in a Heavy-Duty Diesel Engine

Optimizations were performed on a Heavy-Duty diesel engine equipped with a conventional electronic unit injector in order to minimize fuel consumption, and emissions of NOx and particulate matter. A low speed light load case and a high speed light load case were optimized with these considerations in mind. Exhaustive parametric studies were performed in order to find sets of operating conditions that resulted in low emissions and high fuel economy. It was found for the low speed light load case (Mode 2, 25% load and 821 rev/min) that low emissions operating conditions existed at either very early or very late start-of-injection timings and high EGR (PM = 0.018 g/kW-hr, NOx + HC = 1.493 g/kW-hr with SOI = -21 degrees ATDC, 48% EGR; or 0.085 g/kW-hr PM, 1.02 g/kW-hr NOx with SOI = 4 degrees ATDC, 39% EGR). For the high speed light load case, it was found that low emissions were available when operating with an early start-of-injection timing and high EGR (0.059 g/kW-hr PM, 2.52 g/kW-hr NOx with SOI = -28 degrees ATDC, 49% EGR). Particulate is known to be formed in fuel rich regions, but if the local equivalence ratio is kept below approximately 2, very little particulate is formed. Analysis of the present data shows that if there is an optimal delay between the end of injection and the start of combustion (2 to 4 crank angle degrees), then enough mixing takes place so that the maximum local equivalence ratios are reduced. NOx is shown to decrease steadily as the EGR rate is increased, or as the start-of-injection timing is retarded.

Optimization of Injection Rate Shape Using Active Control of Fuel Injection

The effect of injection rate shape on spray evolution and emission characteristics is investigated and a methodology for active control of fuel injection is proposed. Extensive validation of advanced vaporization and primary jet breakup models was performed with experimental data before studying the effects of systematic changes of injection rate shape. Excellent agreement with the experiments was obtained for liquid and vapor penetration lengths, over a broad range of gas densities and temperatures. Also the predicted flame lift-off lengths of reacting diesel fuel sprays were in good agreement with the experiments. After the validation of the models, well-defined rate shapes were used to study the effect of injection rate shape on liquid and vapor penetration, flame lift-off lengths and emission characteristics. A consistent trend was observed over the entire range of densities and temperatures, which lead to the conclusion that the fuel distribution can be controlled by modifying the injection rate shape. Since it is believed that the NOx and soot emissions are substantially affected by the equivalence ratio distribution of the gas mixture prior to the combustion, the variation of fuel distribution via the modification of injection rate shape offers a useful way of emission control. From the knowledge gained by this study, a control approach was devised to maintain the local equivalence ratios below a certain level using active control of fuel injection, thus preventing soot formation.

Numerical Parametric Study of Diesel Engine Operation with Gasoline

Parametric studies of direct injection (DI) compression ignition (CI) engine combustion fueled with gasoline are presented. A multi-dimensional CFD code, KIVA-ERC-Chemkin, coupled with improved sub-models and the Chemkin library, was employed. The oxidation chemistry of the fuel was calculated using a reduced mechanism for primary reference fuel combustion. The results show that high pressure DI gasoline engine combustion and emissions are successfully predicted and are in good agreement with available experimental measurements under various operating conditions. It is seen that gasoline has a much longer ignition delay than diesel fuel for the same combustion phasing; thus, oxides of nitrogen (NOx) and particulate emissions are significantly reduced compared to corresponding diesel-fueled cases. The results also suggest possible methods for expanding the operating conditions of DI gasoline compression ignition (CI) combustion. This indicates that the application of gasoline fueling to compression ignition engines is likely to lead to low-emission engine concepts.

The Use of Variable Geometry Sprays With Low Pressure Injection for Optimization of Diesel HCCI Engine Combustion

A numerical study of the effects of injection parameters and operating conditions for diesel-fuel HCCI operation is presented with consideration of Variable Geometry Sprays (VGS). Methods of mixture preparation are explored that overcome one of the major problems in HCCI engine operation with diesel fuel and conventional direct injection systems, i.e., fuel loss due to wall impingement and the resulting unburned fuel. Low pressure injection of hollow cone sprays into the cylinder of a production engine with the spray cone angle changing during the injection period were simulated using the multi-dimensional KIVA-3V CFD code with detailed chemistry. Variation of the starting and ending spray angles, injection timing, initial cylinder pressure and temperature, swirl intensity, and compression ratio were explored. As a simplified case of VGS, two-pulse, hollow-cone sprays were also simulated. The results show that VGS is effective in minimizing wall wetting and allows wall wetting to be decoupled from ignition timing control. Using VGS, the initial gas temperature, boost/throttling pressure, and compression ratio are effective parameters in ignition timing control. Variation of swirl intensity is effective to ensure fuel vapor-air mixing, and to prevent the formation of rich regions with high NOx emissions. The results indicate that VGS is a promising methodology to be used to control diesel-fueled HCCI engine operation, and it deserves further experimental and numerical study.

Physical Properties of Bio-Diesel and Implications for Use of Bio-Diesel in Diesel Engines

In this study we identify components of a typical biodiesel fuel and estimate both their individual and mixed thermo-physical and transport properties. We then use the estimated mixture properties in computational simulations to gauge the extent to which combustion is modified when biodiesel is substituted for conventional diesel fuel. Our simulation studies included both regular diesel combustion (DI) and premixed charge compression ignition (PCCI). Preliminary results indicate that biodiesel ignition is significantly delayed due to slower liquid evaporation, with the effects being more pronounced for DI than PCCI. The lower vapor pressure and higher liquid heat capacity of biodiesel are two key contributors to this slower rate of evaporation. Other physical properties are more similar between the two fuels, and their impacts are not clearly evident in the present study. Future studies of diesel combustion sensitivity to both physical and chemical properties of biodiesel are suggested.

Parametric study of combustion characteristics in a direct-injection diesel homogeneous charge compression ignition engine with a low-pressurefuel injector

Abstract Homogeneous charge compression ignition (HCCI) combustion is an alternative to current engine combustion systems and is used as a method to reduce emissions. It has the potential nearly to eliminate engine-out NOx emissions while producing diesel-like engine efficiencies, when a premixture of gas-phase fuel and air is burned spontaneously and entirely by an autoignition process. However, when direct injection is used for diesel fuel mixture preparation in engines, the complex in-cylinder flow field and limited mixing times may result in inhomogeneity of the charge. Thus, in order to minimize non-uniformity of the charge, early injection of the fuel is desirable. However, when fuel is injected during the intake or early compression stroke, the use of high-pressure injection is limited by the relatively low in-cylinder gas pressure because of spray impingement on the cylinder walls. Thus, it is also of interest to consider low-pressure injectors as an alternative. In the present paper, the parametric behaviour of the combustion characteristics in an HCCI engine operated with a low-pressure fuel injector were investigated through numerical simulations and engine experiments. Parameters including the start-of-injection (SOI) timing and exhaust gas recirculation were considered, and diesel and n-heptane fuels were used. The results show good agreement of behaviour trends between the experiments and the numerical simulations. With its lower vaporization rates, significant effects of the SOI timing and intake gas temperature were seen for diesel fuel due to the formation of wall films. The modelling results also explained the origin of high-temperature NO x-producing regions due to the effect of the gas density on the spray.

Combustion simulations of the fuels for advanced combustion engines in a homogeneous charge compression ignition engine

Numerical simulations of combustion have been performed for a homogeneous charge compression ignition engine operating on multi-component reference diesel fuels. Two surrogate representation methods were used to describe the nine fuels for advanced combustion engines. The first method of surrogates, denoted as physical-surrogate components, was formulated to describe the fuel’s physical properties by matching its distillation profile, specific gravity, lower heating value, hydrogen-to-carbon ratio, and cetane index with measured data. The second method of surrogates, denoted as chemical-surrogate components, was introduced to represent the chemistry of the fuel components using a new group chemistry representation model. In the model, the fuel components in the physical-surrogate components and chemical-surrogate components are related through the classification of chemical structures of the components, i.e. the component in the physical-surrogate components are grouped based on their chemical classes and the chemistry of each group is calculated using a chemical kinetics mechanism (MultiChem) that represents the combustion characteristics of its chemical class. This MultiChem mechanism includes reduced reaction mechanisms for the four main surrogate hydrocarbon chemistry components of diesel fuel, n-paraffins, iso-paraffins, naphthenes, and aromatics, with two n-paraffins to provide the ability to mimic molecular weight effects. The computations were performed using a multi-dimensional computational fluid dynamic code, KIVA-ERC-CHEMKIN, and the results show that the predictions of the present multi-component combustion models are in good agreement with experimental measurements. The combustion characteristics of the fuels for advanced combustion engines are well represented and the present models are well suited for practical engine computations.

Computationally Efficient Simulation of Multicomponent Fuel Combustion Using a Sparse Analytical Jacobian Chemistry Solver and High-Dimensional Clustering

The need for more efficient and environmentally sustainable internal combustion engines is driving research towards the need to consider more realistic models for both fuel physics and chemistry. As far as compression ignition engines are concerned, phenomenological or lumped fuel models are unreliable to capture spray and combustion strategies outside of their validation domains – typically, high-pressure injection and high-temperature combustion. Furthermore, the development of variable-reactivity combustion strategies also creates the need to model comprehensively different hydrocarbon families even in single fuel surrogates. From the computational point of view, challenges to achieving practical simulation times arise from the dimensions of the reaction mechanism, that can be of hundreds species even if hydrocarbon families are lumped into representative compounds, and thus modeled with nonelementary, skeletal reaction pathways. In this case, it is also impossible to pursue further mechanism reductions to lower dimensions. CPU times for integrating chemical kinetics in internal combustion engine simulations ultimately scale with the number of cells in the grid, and with the cube number of species in the reaction mechanism. In the present work, two approaches to reduce the demands of engine simulations with detailed chemistry are presented. The first one addresses the demands due to the solution of the chemistry ODE system, and features the adoption of SpeedCHEM, a newly developed chemistry package that solves chemical kinetics using sparse analytical Jacobians. The second one aims to reduce the number of chemistry calculations by binning the CFD cells of the engine grid into a subset of clusters, where chemistry is solved and then mapped back to the original domain. In particular, a high-dimensional representation of the chemical state space is adopted for keeping track of the different fuel components, and a newly developed bounding-box-constrained k-means algorithm is used to subdivide the cells into reactively homogeneous clusters. The approaches have been tested on a number of simulations featuring multi-component diesel fuel surrogates, and different engine grids. The results show that significant CPU time reductions, of about one order of magnitude, can be achieved without loss of accuracy in both engine performance and emissions predictions, prompting for their applicability to more refined or full-sized engine grids.