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Due to the new challenge of meeting number-based regulations for particulate matter (PM), a numerical and experimental study has been conducted to better understand particulate formation in engines fuelled with compressed natural gas. The study has been conducted on a Heavy-Duty, Euro VI, 4-cylinder, spark ignited engine, with multipoint sequential phased injection and stoichiometric combustion. For the experimental measurements two different instruments were used: a condensation particle counter (CPC) and a fast-response particle size spectrometer (DMS) the latter able also to provide a particle size distribution of the measured particles in the range from 5 to 1000 nm. Experimental measurements in both stationary and transient conditions were carried out. The data using the World Harmonized Transient Cycle (WHTC) were useful to detect which operating conditions lead to high numbers of particles. Then a further transient test was used for a more detailed and deeper analysis.

3-D Computational Fluid Dynamics (CFD) simulations have been performed to study particulate formation in a Spark-Ignition (SI) engine under premixed conditions. A semi-detailed soot model and a chemical kinetic model, including poly-aromatic hydrocarbon (PAH) formation, were coupled with a spark ignition model and the G equation flame propagation model for SI engine simulations and for predictions of soot mass and particulate number density. The simulation results for in-cylinder pressure and particle size distribution (PSDs) are compared to available experimental studies of equivalence ratio effects during premixed operation. Good predictions are observed with regard to cylinder pressure, combustion phasing and engine load. Qualitative agreements of in-cylinder particle distributions were also obtained and the results are helpful to understand particulate formation processes.

Improvements to combustion models for modeling spark ignition engines using the G-equation flame propagation model and detailed chemical kinetics have been performed. The improvements include revision of a PRF chemistry mechanism, precise calculation of “primary heat release” based on the sub-grid scale unburned/burnt volumes of flame-containing cells, modeling flame front quenching in highly stratified mixtures, introduction of a Damkohler model for assessing the combustion regime of flame-containing cells, and a better method of modeling the effects of the local residual value on the burning velocity. The validation of the revised PRF mechanism shows that the calculated ignition delay matches shock tube data very well. The improvements to the “primary heat release” model based on the cell unburned/burnt volumes more precisely consider the chemical kinetics heat release in unburned regions, and thus are thought to be physically reasonable.

In this paper, knock in a Ford single cylinder direct-injection spark-ignition (DISI) engine was modeled and investigated using the KIVA-3V code with a G-equation combustion model coupled with detailed chemical kinetics. The deflagrative turbulent flame propagation was described by the G-equation combustion model. A 22-species, 42-reaction iso-octane (iC8H18) mechanism was adopted to model the auto-ignition process of the gasoline/air/residual-gas mixture ahead of the flame front. The iso-octane mechanism was originally validated by ignition delay tests in a rapid compression machine. In this study, the mechanism was tested by comparing the simulated ignition delay time in a constant volume mesh with the values measured in a shock tube under different initial temperature, pressure and equivalence ratio conditions, and acceptable agreements were obtained.

A level set method (G-equation)-based combustion model incorporating detailed chemical kinetics has been developed and implemented in KIVA-3V for Spark-Ignition (SI) engine simulations for better predictions of fuel oxidation and pollutant formation. Detailed fuel oxidation mechanisms coupled with a reduced NOX mechanism are used to describe the chemical processes. The flame front in the spark kernel stage is tracked using the Discrete Particle Ignition Kernel (DPIK) model. In the G-equation model, it is assumed that after the flame front has passed, the mixture within the mean flame brush tends to local equilibrium. The subgrid-scale burnt/unburnt volumes of the flame containing cells are tracked for the primary heat release calculation. A progress variable concept is introduced into the turbulent flame speed correlation to account for the laminar to turbulent evolution of the spark kernel flame.

A level set G-equation model has been developed to model the combustion process in spark ignition engines. The spark ignition process was modeled using an improved version of the Discrete Particle Ignition Kernel (DPIK) model. The two models were implemented into the KIVA-3V code to simulate SI engine combustion under both premixed and direct injection conditions. In the ignition model, the ignition kernel growth is tracked by Lagrangian markers, and spark discharge energy and flow turbulence effects on the kernel growth are considered. Once the ignition kernel grows to a size where the turbulent flame is fully developed, the G-equation model is used to track the mean turbulent flame evolution. When combined with a characteristic time scale combustion model, the models were also used to simulate stratified combustion in DISI engines, where the triple flame structure must be considered.

An improved discrete particle ignition kernel (DPIK) model and the G-equation combustion model have been developed and implemented in KIVA-3V. In the ignition model, the spark ignition kernel growth is tracked by Lagrangian markers and the spark discharge energy and flow turbulence effects on the ignition kernel growth are considered. The predicted ignition kernel size was compared with the available measurements and good agreement was obtained. Once the ignition kernel grows to a size where the turbulent flame is fully developed, the level set method (G-equation) is used to track the mean turbulent flame propagation. It is shown that, by ignoring the detailed turbulent flame brush structure, fine numerical resolution is not needed, thus making the models suitable for use in multidimensional modeling of SI engine combustion.

A computer model that is able to predict the occurrence of knock in spark ignition engines has been developed and implemented into the KIVA-3V code. Three major sub-models were used to simulate the overall process, namely the spark ignition model, combustion model, and end-gas auto-ignition models. The spark ignition and early flame development is modeled by a particle marker technique to locate the flame kernel. The characteristic-time combustion model is applied to simulate the propagation of the regular flame. The autoignition chemistry in the end-gas was modeled by a reduced chemical kinetics mechanism that is based on the Shell model. The present model was validated by simulating the experimental data in three different engines. The spark ignition and the combustion models were first validated by simulating a premixed Caterpillar engine that was converted to run on propane. Computed cylinder pressure agrees well with the experimental data.

A spray model for pressure-swirl atomizers that is based on a linearized instability analysis of liquid sheets has been combined with an ignition and combustion model for stratified charge spark ignition engines. The ignition model has been advanced, such that the presence of dual spark plugs can now be accounted for. Independent validation of the spray model is achieved by investigating a pressure-swirl injector inside a pressure bomb containing air at ambient temperature. In a second step, the complete model is used to estimate the performance of a small marine DISI Two-Stroke engine operating in stratified charge mode. Simulation results and experimental data are compared for several different injection timings and the agreement is generally good such that there is confidence in the predictive quality of the combustion model. Finally the model is applied in a conceptual study to investigate possible benefits of split injections.

A new ignition and combustion model has been developed and tested for use in premixed spark-ignition engines. The ignition model is referred to as the Discrete Particle Ignition Kernel (DPIK) model, and it uses Lagrangian markers to track the flame-front growth. The model includes the effects of electrode heat transfer on the early flame kernel growth process, and it is used in conjunction with a characteristic-time-scale combustion model once the ignition kernel has grown to a size where the effects of turbulence on the flame must be considered. A new term which accounts for the effect of air-fuel ratio, was added to the combustion model for modeling combustion in very lean and very rich mixtures. The flame kernel size predicted by the DPIK model was compared with measurements of Maly and Vogel. Furthermore, predictions of the electrode heat transfer were compared with data of Kravchik and Heywood. In both comparisons the model predictions were in good agreement with the experiments.

Fuel preparation and stratified combustion were studied for a conceptual gasoline Direct-Injection Spark-Ignition (GDI or DISI) engine by computer simulations. The primary interest was on the effects of different injector orientations and the effects of tumble ratio for late injection cases at a partial load operating condition. A modified KIVA-3V code that includes improved spray breakup and wall impingement and combustion models was used. A new ignition kernel model, called DPIK, was developed to describe the early flame growth process. The model uses Lagrangian marker particles to describe the flame positions. The computational results reveal that spray wall impingement is important and the fuel distribution is controlled by the spray momentum and the combustion chamber shape. The injector orientation significantly influences the fuel stratification pattern, which results in different combustion characteristics.

Fuel spray atomization and breakup processes within a direct-injection spark-ignition (DISI) engine and outside the engine were modeled using a modified KIVA-3V code with improved spray models. The structures of the predicted sprays were qualitatively compared with planar images. The considered sprays were created by a prototype pressure-swirl injector and the planar images were obtained by laser sheet imaging in an optical DISI engine. In the out-of-engine case, the spray was injected into atmospheric air, and was modeled in a two dimensional bomb. In the engine case, the injection started from 270° ATDC, and full 3-D computations in the same engine were performed. In both cases, two liquid injection pressure conditions were applied, that is, 3.40 MPa and 6.12 MPa. The model gives good prediction of the tip penetration, and external spray shape, but the internal structure prediction has relatively lower accuracy, especially near the spray axis.

A numerical study of air-fuel mixing in a direct-injection spark-ignition engine was carried out. In this paper, the numerical models are described and grid generation methods to represent a realistic port-valve-chamber geometry is discussed. To model a vaporizing hollow-cone spray resulting from an automotive pressure-swirl injector, a newly developed sheet spray atomization model was used to compute the processes of disintegration of the liquid sheet and breakup of the subsequent drops. Computations were performed of a particular 4-valve pent-roof engine configuration in which the intake process and an early fuel injection scheme were considered. After an analysis of the intake-generated flow structures in this engine configuration, the spray behavior and the spatial and temporal evolution of fuel liquid and vapor phases are characterized.

Multidimensional modeling is used to study air-fuel mixing in a direct-injection spark-ignition engine. Emphasis is placed on the effects of the start of fuel injection on gas/spray interactions, wall wetting, fuel vaporization rate and air-fuel ratio distributions in this paper. It was found that the in-cylinder gas/spray interactions vary with fuel injection timing which directly impacts spray characteristics such as tip penetration and spray/wall impingement and air-fuel mixing. It was also found that, compared with a non-spray case, the mixture temperature at the end of the compression stroke decreases substantially in spray cases due to in-cylinder fuel vaporization. The computed trapped-mass and total heat-gain from the cylinder walls during the induction and compression processes were also shown to be increased in spray cases.

It is well known that the accuracy of simulations of combustion processes in diesel and spark ignited (SI) engines depends on the initial conditions within the cylinder at intake valve closure (IVC). Residual gas affects the engine combustion processes through its influence on charge mass, temperature and dilution. In SI engines, there is little oxygen in the residual gas, and thus the dilution effect on flame propagation is more significant than in compression ignited (CI) engines. However, in CI engines, the ignition delay depends strongly on the in-cylinder gas temperature, which is proportional to the gas temperature at IVC. Furthermore, ignition delay is significantly affected by how much oxygen is present, which is also partly determined by the residual gas fraction. Therefore, it is of extreme importance to determine residual gas concentrations accurately.

In previous work the KIVA-II code has been modified to model modem DI diesel engines and their emissions of particulate soot and oxides of nitrogen (NOx). This work presents results from a program to further validate the NOx emissions models against engine experiments with a well characterized modern engine. To facilitate a simplified comparison with experiments, a single cylinder research version of the Caterpillar 3406 heavy duty DI diesel engine was retrofitted to run as a naturally-aspirated, propane-fueled, spark-ignited engine. The retrofit includes installing a low compression ratio piston with bowl, adding a gas mixer, replacing the fuel injector assembly with a spark plug assembly and adding spark and fuel stoichiometry control hardware. Cylinder pressure and engine-out NOx emissions were measured for a range of speeds, exhaust gas residual (EGR) fractions, and spark timing settings.

Multidimensional computations were carried out to simulate the in-cylinder fuel/air mixing process of a direct-injection spark-ignition engine using a modified version of the KIVA-3 code. A hollow cone spray was modeled using a Lagrangian stochastic approach with an empirical initial atomization treatment which is based on experimental data. Improved Spalding-type evaporation and drag models were used to calculate drop vaporization and drop dynamic drag. Spray/wall impingement hydrodynamics was accounted for by using a phenomenological model. Intake flows were computed using a simple approach in which a prescribed velocity profile is specified at the two intake valve openings. This allowed three intake flow patterns, namely, swirl, tumble and non-tumble, to be considered. It was shown that fuel vaporization was completed at the end of compression stroke with early injection timing under the chosen engine operating conditions.

A three-dimensional computer code (KIVA) is being modified to include state-of-the-art submodels for diesel engine flow and combustion: spray atomization, drop breakup/coalescence, multi-component fuel vaporization, spray/wall interaction, ignition and combustion, wall heat transfer, unburned HC and NOx formation, soot and radiation and the intake flow process. Improved and/or new submodels which have been completed are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops.

The combustion mechanism in a Compression Ignition Homogeneous Charge (CIHC) engine was studied. Previous experiments done on a four-stroke CIHC engine were modeled using the KIVA-II code with modifications to the combustion, heat transfer, and crevice flow submodels. A laminar and turbulence characteristic time combustion model that has been used for spark-ignited engine studies was extended to allow predictions of ignition. The rate of conversion from one chemical species to another is modeled using a characteristic time which is the sum of a laminar (high temperature) chemistry time, an ignition (low temperature) chemistry time, and a turbulence mixing time. The ignition characteristic time was modeled using data from elementary initiation reactions and has the Arrhenius form. It was found to be possible to match all engine test cases reasonably well with one set of combustion model constants.

Combustion and flow were calculated in a spark-ignited two-stroke crankcase-scavenged engine using a laminar and turbulent characteristic-time combustion submodel in the three-dimensional KIVA code. Both premixed-charge and fuel-injected cases were examined. A multi-cylinder engine simulation program was used to specify initial and boundary conditions for the computation of the scavenging process. A sensitivity study was conducted using the premixed-charge engine data. The influence of different port boundary conditions on the scavenging process was examined. At high delivery ratios, the results were insensitive to variations in the scavenging flow or residual fraction details. In this case, good agreement was obtained with the experimental data using an existing combustion submodel, previously validated in a four-stroke engine study.