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Technical Paper

Effects of Mixture Preparation Characteristics on Four-Stroke Utility Engine Emissions and Performance

A laboratory-based fuel mixture system capable of delivering a range of fuel/air mixtures has been used to observe the effects of differing mixture characteristics on engine combustion through measurement and analysis of incylinder pressure and exhaust emissions. Fuel air mixtures studied can be classified into four different types: 1) Completely homogeneous fuel/air mixtures, where the fuel has been vaporized and mixed with the air prior to entrance into the normal engine induction system, 2) liquid fuel that is atomized and introduced with the air to the normal engine induction system, 3) liquid fuel that is atomized, and partially prevaporized but the air/fuel charge remains stratified up to introduction to the induction system, and 4) the standard fuel metering system. All tests reported here were conducted under wide open throttle conditions. A four-stroke, spark-ignited, single-cylinder, overhead valve-type engine was used for all tests.
Technical Paper

Modeling of NOx Emissions with Comparison to Exhaust Measurements for a Gas Fuel Converted Heavy-Duty Diesel Engine

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.
Technical Paper

Mixture Preparation Effects on Ignition and Combustion in a Direct-Injection Spark-Ignition Engine

Planar instantaneous fuel concentration measurements were made by laser-induced fluorescence of 3-pentanone in the spark gap just prior to ignition in a direct-injection spark-ignition engine operating at a light load, highly stratified condition. The distribution of the average equivalence ratio in a circle of 1.9 mm diameter centered on the spark plug showed that a large fraction of the cycles had an equivalence ratio below the lean limit, yet acceptable combustion was achieved in those cycles. Further, weak correlation was found between the local average equivalence ratio near the spark plug and the time required to achieved a 100 kPa pressure rise above the motoring pressure, as well as other parameters which characterize the early stages of combustion. The cause for this behavior is assessed to be mixture motion during the spark discharge which continually convects fresh mixture through the spark gap during breakdown.
Technical Paper

Effects of Injection Timing on Air-Fuel Mixing in a Direct-Injection Spark-Ignition Engine

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.
Technical Paper

Comparison Between Air-Assisted and Single-Fluid Pressure Atomizers for Direct-Injection SI Engines Via Spatial and Temporal Mass Flux Measurements

Two distinct atomization strategies are contrasted through the measurement of time and spatially dependent mass flux. The two systems investigated include a pressure atomizer (6.9 MPa opening pressure) and an air-assist atomizer. Both systems have potential for use in direct injection spark ignition engines. The mass flux data presented were obtained using a spray patternator that was developed to allow phased sampling of the spray. The temporal mass related history of the spray was reconstructed as volume versus time plots and interpolated mass flux contour plots. Results indicate substantial differences in the distribution of both mass and mass flux in space and time for the two injection systems. For example, the pressure atomizer at high mass delivery rates produced a spray that collapsed into a dispersed cylindrical shape while at low rates, generated a hollow cone structure.
Technical Paper

Transient Spray Characteristics of a Direct-Injection Spark-Ignited Fuel Injector

This paper describes the transient spray characteristics of a high pressure, single fluid injector, intended for use in a direct-injection spark-ignited (DISI) engine. The injector was a single hole, pintle type injector and was electronically controlled. A variety of measurement diagnostics, including full-field imaging and line-of-sight diffraction based particle sizing were employed for spray characterization. Transient patternator measurements were also performed to obtain temporally resolved average mass flux distributions. Particle size and obscuration measurements were performed at three locations in the spray and at three injection pressures: 3.45 MPa (500 psi), 4.83 Mpa (700 psi), and 6.21 MPa (900 psi). Results of the spray imaging experiments indicated that the spray shapes varied with time after the start of injection and contained a leading mass, or slug along the center line of the spray.
Technical Paper

Multidimensional Modeling of Spray Atomization and Air-Fuel Mixing in a Direct-Injection Spark-Ignition Engine

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.
Technical Paper

Modeling the Effects of Valve Lift Profile on Intake Flow and Emissions Behavior in a DI Diesel Engine

Variations in the in cylinder flow field which result from differences in the intake flow are known to have important effects on the performance and emissions behavior of diesel engines. The intake flow and combustion in a heavy duty DI diesel engine with a dual valve port have been simulated using the computational fluid dynamics code KIVA-3. Variation of the in-cylinder flow field has been achieved by varying the intake valve timing. Variations in the in-cylinder flow, including a range of length scales, degrees of inhomogeneity in a number of scalar and vector quantities, and the persistence of various flow structures, are compared, and their significance to combustion and emissions parameters are assessed. The interaction of fuel spray parameters, particularly spray-wall interaction with structures present in the flow field are evaluated.
Technical Paper

Modeling the Effects of Intake Flow Structures on Fuel/Air Mixing in a Direct-injected Spark-Ignition Engine

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.
Technical Paper

Misfire Detection and Cylinder Pressure Reconstruction for SI Engines

Many researchers have studied and developed methods for on-board engine combustion misfire detection in production vehicles. Misfiring can damage the catalytic converter within a short time and can lead to increased emission levels. For that reason, the on-board detection of engine misfire is one requirement of the On Board Diagnosis II (OBDII) Regulation and a recent interest for many researchers. One object in this paper is to propose a misfire detection method for multi-cylinder SI engines. The detection is achieved by examining the estimated cylinder pressures and combustion heat release rates in engine cylinders. The Sliding Observer methodology is applied in these estimations. This detection method provides a reliable and low-cost way to diagnose engine misfires. The other object of the paper is to eliminate large estimation errors due to system unobservability and reconstruct cylinder pressures.
Technical Paper

Predictions of Residual Gas Fraction in IC Engines

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.
Technical Paper

Modeling Combustion in Compression Ignition Homogeneous Charge Engines

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.
Technical Paper

Improvements in 3-D Modeling of Diesel Engine Intake Flow and Combustion

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.
Technical Paper

Comparison of Unburned Fuel and Aldehyde Emissions from a Methanol-Fueled Stratified Charge and Homogeneous Charge Engine

This paper presents the results of an experimental program in which a Texaco L-163S engine was fueled with methanol and operated in its traditional stratified charge mode and then modified to run as a homogeneous charge spark ignited engine. The primary data taken were the aldehyde and unburned fuel emissions (UBF). These data were taken using a continuous time-averaging sampling probe at the exhaust tank and at the exhaust port and with a rotary time-resolving sampling valve located at the exhaust port. The data are for two loads, 138.1 kPa (20 psi) and 207.1 kPa (30 psi) BMEP and three speeds, 1000, 1400 and 1800 rpm. The data indicate that for both the stratified charge and the homogeneous charge modes of operation formaldehyde was the only aldehyde detected in the exhaust and it primarily originated in the cylinder.
Technical Paper

Effect of Equivalence Ratio on the Particulate Emissions from a Spark-Ignited, Direct-Injected Gasoline Engine

The effect of equivalence ratio on the particulate size distribution (PSD) in a spark-ignited, direct-injected (SIDI) engine was investigated. A single-cylinder, four-stroke, spark-ignited direct-injection engine fueled with certification gasoline was used for the measurements. The engine was operated with early injection during the intake stroke. Equivalence ratio was swept over the range where stable combustion was achieved. Throughout this range combustion phasing was held constant. Particle size distributions were measured as a function of equivalence ratio. The data show the sensitivity of both engine-out particle number and particle size to global equivalence ratio. As equivalence ratio was increased a larger fraction of particles were due to agglomerates with diameters ≻ 100 nm. For decreasing equivalence ratio smaller particles dominate the distribution. The total particle number and mass increased non-linearly with increasing equivalence ratio.
Technical Paper

Modeling Knock in Spark-Ignition Engines Using a G-equation Combustion Model Incorporating Detailed Chemical Kinetics

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.
Technical Paper

Fuel Film Temperature and Thickness Measurements on the Piston Crown of a Direct-Injection Spark-Ignition Engine

Fuel film temperature and thickness were measured on the piston crown of a DISI engine under both motored and fired conditions using the fiber-based laser-induced fluorescence method wherein a single fiber delivers the excitation light and collects the fluorescence. The fibers were installed in the piston crown of a Bowditch-type optical engine and exited via the mirror passage. The fuel used for the fuel film temperature measurement was a 2×10-6 M solution of BTBP in isooctane. The ratio of the fluorescence intensity at 515 to that at 532 nm was found to be directly, but not linearly, related to temperature when excited at 488 nm. Effects related to the solvent, solution aging and bleaching were investigated. The measured fuel film temperature was found to closely follow the piston crown metal temperature, which was measured with a thermocouple.
Technical Paper

PIV Measurements of In-Cylinder Flow in a Four-Stroke Utility Engine and Correlation with Steady Flow Results

Large-scale flows in internal combustion engines directly affect combustion duration and emissions production. These benefits are significant given increasingly stringent emissions and fuel economy requirements. Recent efforts by engine manufacturers to improve in-cylinder flows have focused on the design of specially shaped intake ports. Utility engine manufacturers are limited to simple intake port geometries to reduce the complexity of casting and cost of manufacturing. These constraints create unique flow physics in the engine cylinder in comparison to automotive engines. An experimental study of intake-generated flows was conducted in a four-stroke spark-ignition utility engine. Steady flow and in-cylinder flow measurements were made using three simple intake port geometries at three port orientations. Steady flow measurements were performed to characterize the swirl and tumble-generating capability of the intake ports.
Technical Paper

Determination of Flame-Front Equivalence Ratio During Stratified Combustion

Combustion under stratified operating conditions in a direct-injection spark-ignition engine was investigated using simultaneous planar laser-induced fluorescence imaging of the fuel distribution (via 3-pentanone doped into the fuel) and the combustion products (via OH, which occurs naturally). The simultaneous images allow direct determination of the flame front location under highly stratified conditions where the flame, or product, location is not uniquely identified by the absence of fuel. The 3-pentanone images were quantified, and an edge detection algorithm was developed and applied to the OH data to identify the flame front position. The result was the compilation of local flame-front equivalence ratio probability density functions (PDFs) for engine operating conditions at 600 and 1200 rpm and engine loads varying from equivalence ratios of 0.89 to 0.32 with an unthrottled intake. Homogeneous conditions were used to verify the integrity of the method.
Journal Article

Improvements to Combustion Models for Modeling Spark-Ignition Engines Using the G-equation and Detailed Chemical Kinetics

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.