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Technical Paper

Turbulence and Residual Gas Effects on Mixing, Combustion, and Emissions in Split Injection of Gaseous Fuel

Combustion and pollutant formation in split injection may be influenced by interaction between fuel pulses. Specifically, the interest here is in two aspects of that interaction: turbulence effects and residual gas effects. The objective of this work is to understand these two aspects of the interaction between multiple fuel pulses, in isolation from other effects, while employing widely accepted computational methods. Residual gas effects on combustion in the jets are studied using two combustion models: a characteristic time combustion model within Reynolds-averaged Navier-Stokes simulations and an interactive flamelet model. Findings indicate that dilution and thermal effects of residual gases are dominant. Regarding the turbulence effects, this work does not predict mixing enhancement due to turbulence from prior injection pulses. Rather, the jet is accelerated by the bulk flow field established by prior injections.
Technical Paper

A Model for Multicomponent Droplet Vaporization in Sprays

A simplified model for multicomponent droplet vaporization is developed and implemented in a multidimensional model for flows, sprays and combustion in engines. The model is applied to study the vaporization characteristics of a multicomponent droplet under Diesel conditions, the distribution of the vapor components in a Diesel spray and the distribution of the components in a Diesel engine. It is shown that for typical warm Diesel engine operating conditions, the droplets vaporize sufficiently rapidly that the stratification of the different components in the spray is not significant. However, under engine starting conditions and, in particular, cold starting conditions, there is a significant stratification of the different components of the fuel. When the species are stratified, the heavier and slower vaporizing components are predicted to be on the periphery of the spray envelope. However, these components also take longer to reach there.
Technical Paper

Comparisons of Computed and Measured Pressure in a Premixed-Charge Natural-Gas-Fueled Rotary Engine

The combustion chamber pressure computed with a three-dimensional model is compared with the measured one in a rotary engine fueled with mixtures of natural gas and air. The rotary engine has a rotor displacement of 654 cm3, a compression ratio of 9.4 and uses 2 ignition sparks. The model incorporates a k-ϵ submodel for turbulence, wall function submodels for turbulent wall boundary layer transport, and a hybrid laminar/mixing controlled submodel for species conversion and energy release. Nine cases are considered that cover a wide range of engine operating conditions: rpm of 2503-5798, volumetric efficiency of 35.7-100.5% and equivalence ratio of 0.59-1.15. In all cases the computed and measured pressures agree within 12%.
Technical Paper

Simple Modeling of Autoignition in Diesel Engines for 3-D Computations

For practical, extensive 3-D computations for engine improvements, each physical submodel needs to be the simplest that is compatible with the accuracy of all other physical submodels and of the numerics. The addition of one progress variable controlled by one Arrhenius term is shown to be adequate to reproduce Diesel ignition delay in 2-D and 3-D computations. The rest of the model is that used for years by the authors to optimize combustion in reciprocating and rotary engines with premixed and non-premixed charges, including all of its model constants. This minimal Diesel autoignition submodel reproduces well trends and magnitudes of ignition delay versus chamber temperature and pressure. As in experiments, it is found that multiple ignition sources develop in rapid succession at various locations around the fuel spray after the first ignition event.
Technical Paper

3-D Computations to Improve Combustion in a stratified-Charge Rotary Engine Part II: A Better Spray Pattern for the Pilot Injector

A three-dimensional combustion model of a direct-injection stratified-charge rotary engine is used to identify modifications that might lead to better indicated efficiency. The engine, which has a five-hole main injector and a pilot injector, is predicted to achieve better indicated efficiency if a two-hole ‘rabbit-ear’ pilot injector is used instead of its present single-hole pilot injector. This rabbit-ear arrangement is predicted to increase the surface area of the early flame (on account of better distribution of the fuel), and thereby result in an increased overall burning rate. Computations were made at high and low engine speeds and loads, encompassing the practical operating range. It is concluded that the modified pilot injector will increase indicated efficiency by about 5% within the computed operating range.
Technical Paper

Fuel-Air Mixing and Distribution in a Direct-Injection Stratified-Charge Rotary Engine

A three-dimensional model for flows and combustion in reciprocating and rotary engines is applied to a direct-injection stratified-charge rotary engine to identify the main parameters that control its burning rate. It is concluded that the orientation of the six sprays of the main injector with respect to the air stream is important to enhance vaporization and the production of flammable mixture. In particular, no spray should be in the wake of any other spray. It was predicted that if such a condition is respected, the indicated efficiency would increase by some 6% at higher loads and 2% at lower loads. The computations led to the design of a new injector tip that has since yielded slightly better efficiency gains than predicted.
Technical Paper

Combustion Optimization Computations-Part I: Swirl and Squish Effects in Air-Assist Injection Engines

Results are presented of two-dimensional computations of air-assist fuel injection into engines with bowl-in-piston and bowl-in-head, with and without swirl and for early and late injection but without combustion. The general finding is that swirl tends to destroy the head vortex of the air/fuel jet and results in a faster collapse of the spray cone toward its axis. Faster collapse is also promoted by high density of the chamber gas (e.g. late injection) and bowl-in-head design (limited availability of chamber gas around the spray, presence of walls and delayed influence of squish by the injector). With enhanced collapse, fuel-rich regions are formed around the axis and away from the injector. With reduced collapse, the radial distribution of the fuel is more uniform. Thus swirl tends to lead to both slower vaporization and richer vapor mixtures. Also, with strong swirl the rich mixtures tend to end up by the injector; without swirl, by the piston.
Technical Paper

3-D Computations to Improve Combustion in a Stratified-Charge Rotary Engine Part IV: Modified Geometries

A three-dimensional model for a direct injection stratified-charge rotary engine has been employed to study two modifications to the pocket geometry of the engine. In one modification, a pocket is located towards the leading edge of the rotor and is shown to produce recirculation within the pocket and faster burning. In the second modification, a two pocket rotor with two injectors and two spark plugs is studied. It appears that this should result in better utilization of the chamber air. It also appears that both modifications rhould result in higher efficiency of the direct-injected stratifiedcharge rotary engine. However extensive computations are required before a final conclusion is reached and before specific recommendations can be made.
Technical Paper

3-D Steady-State Wall Heat Fluxes and Thermal Analysis of a Stratified-Charge Rotary Engine

A three-dimensional model is used to compute the flow,sprays and combustion in a stratified-charge rotary engine. Wall temperatures estimated from available measurements are used as boundary conditions for the energy equation. The computations provide local and instantaneous heat fluxes on the rotor and the rotor housing. The instantaneous heat fluxes are integrated in time over one cycle of the rotor to obtain estimates of local cycle averaged heat flux through the rotor and the rotor housing. These are then used as boundary conditions in a thermal analysis of the rotor and rotor housing with known coolant-side flow rates and heat transfer coefficients. The thermal analysis is done using a finite-element three-dimensional code which provides updated estimates of the rotor and rotor housing wall temperatures. These wall temperatures agree within ±20°C of the measured wall temperatures.
Technical Paper

3-D Computations of Premixed-Charge Natural Gas Combustion in Rotary Engines

A three-dimensional model for premixed- charge naturally-aspirated rotary engine combustion is used to identify combustion chamber geometries that could lead to increased indicated efficiency for a lean (equivalence ratio =0.75) natural gas/air mixture. Computations were made at two rpms (1800 and 3600) and two loads (approximately 345 Kpa and 620 Kpa indicated mean effective pressure). Six configurations were studied. The configuration that gave the highest indicated efficiency has a leading pocket with a leading deep recess, two spark plugs located circumferentially on the symmetry plane (one after the minor axis and the other before), a compression ratio of 9.5, and an anti-quench feature on the trailing flank.
Technical Paper

Swirl-Spray Interactions in a Diesel Engine

Swirl in Diesel engines is known to be an important parameter that affects the mixing of the fuel jets, heat release, emissions, and overall engine performance. The changes may be brought about through interactions of the swirling flow field with the spray and through modifications of the flow field. The purpose of this paper is to investigate the interaction of the swirl with sprays in a Diesel engine through a computational study. A multi-dimensional model for flows, sprays, and combustion in engines is employed. Results from computations are reported with varying levels of swirl and initial turbulence in two typical Diesel engine geometries. It is shown that there is an optimal level of swirl for each geometry that results from a balance between increased jet surface area and, hence, mixing rates and utilization of air in the chamber.
Technical Paper

3-D Computations to Improve Combustion in a Stratified-Charge Rotary Engine - Part III: Improved Ignition Strategies

A three-dimensional combustion model for a direct-injection stratified-charge rotary engine is used to identify modifications to the engine that should lead to better indicated efficiency. The engine has a single spark plug positioned alongside a single-hole pilot injector in a cavity located after the minor axis and a five-hole main injector that is located before the minor axis. It is predicted that a second ignition source located upstream of the main injector will lead to an increase in indicated efficiency of 6-8% if it ignites the mixture consistently. The computations were made at high and low engine speeds and loads, covering a significant part of the practical operating range of the engine. It is also predicted that the gain in efficiency of the engine with two ignition sources would be 7-10%, instead of 6-8%, if a two-hole pilot injector is also used instead of the one-hole pilot.
Technical Paper

Effects of Combustion on In-Cylinder Mixing of Gaseous and Liquid Jets

In a previous study, the authors compared the fuel-air mixing characteristics of gas jets and sprays in Diesel engine environments in the absence of combustion. A three-dimensional model for flows and sprays was used. It was shown that mixing was slower in gas jets relative to fast-evaporating sprays. In this study, which is an extension of the previous one, the direct-injection of gasesous methane, gaseous tetradecane and liquid tetradecane are studied using the same three-dimensional model. This study concentrates on combustion. It is shown that the fuel-air mixing rate and hence the burning rate are initially slower with gas injection.
Technical Paper

Three-Dimensional Computations of Diesel Sprays in a Very High Pressure Chamber

Results of three-dimensional computations of non-vaporizing and vaporizing Diesel sprays in a very high pressure (up to 18.4 MPa without combustion) environment are presented. These pressures and corresponding density ratios of ambient gas to injected liquid are about a factor of two greater than those in current Diesel engines. The spray model incorporates a line source for drops, heat, mass and momentum exchange between the gas and liquid phases, turbulent dispersion of drops, collisions and coalescences, and drop breakup. The accuracy of the model is assessed by making comparisons of computed and measured spray penetrations. Reasonable agreement is obtained for a broad range of conditions. A scaling for time and axial distance clarifies these results.
Technical Paper

Effects of Ignition Cavity Flows on the Performance of a Stratified-Charge Rotary Engine: Initial 3-D Predictions

Computations of combustion in a stratified-charge rotary engine are presented. A three-dimensional model for flows, sprays and combustion which includes the ignition cavity of the engine is used to make these computations. The geometric complexity of the cavity and its coupling with the main chamber is handled by using an unsteady generalized curvilinear coordinate system. The grid is generated using an algebraic grid generator in the main chamber and by solving an elliptic equation in the cavity. Computations of the flows in the cavity are presented for different arrangements of the pilot injector and spark plug and for different timings and fuel injection rates from pilot and main injectors. The dominant feature of the flowfield in the cavity is shown to be the presence of a vortex, induced by the flow in the main chamber, which controls the distribution of the fuel and also the burning rate in the cavity.
Technical Paper

Three-Dimensional Modeling of Soot and NO in a Direct-injection Diesel Engine

Results of comparisons of computed and measured soot and NO in a direct-injection Diesel engine are presented. The computations are carried out using a three-dimensional model for flows, sprays and combustion in Diesel engines. Autoignition of the Diesel spray is modeled using an equation for a progress variable which measures the local and instantaneous tendency of the fuel to autoignite. High temperature chemistry is modeled using a local chemical equilibrium model coupled to a combination of laminar kinetic and turbulent characteristic times. Soot formation is kinetically controlled and soot oxidation is represented by a model which has a combination of laminar kinetic and turbulent mixing times. Soot oxidation appears to be controlled near top-dead-center by mixing and by kinetics as the exhaust is approached. NO is modeled using the Zeldovich mechanism.
Technical Paper

Modeling of Diesel Sprays in a Very High Pressure Chamber, Part II: Effects of Combustion

Results of two- and three-dimensional computations of combustion of Diesel sprays in a very high-pressure chamber are presented. A wide range of experimental conditions are considered. Peak chamber pressure with combustion range from about 6.0 MPa to about 20 MPa. Computed and measured spray penetrations and chamber pressures are compared and shown to be in adequate agreement. Autoignition is modeled using an equation for a progress variable which measures the local and instantaneous tendency of the fuel to autoignite. High temperature chemistry is modeled using a local equilibrium model coupled to a combination of laminar and turbulent characteristic times. It is shown that scaling rules which were found to apply in vaporizing and non-vaporizing sprays also apply in the combusting sprays. The fuel-air mixing rates and burning rates increase as the ratio of the ambient density to injected density increases.
Technical Paper


The first three-dimensional rotary-engine computations are reported of exhaust, intake (with side and peripheral ports, and with different intake turbulence intensities and length scales), compression, homogeneous-charge combustion, dual liquid fuel injection, and dual liquid fuel injection and combustion. The model includes a k-ε submodel for turbulence, a stochastic treatment of the fuel drops and a hybrid laminar and mixing-controlled submodel for the conversion of reactant to products. The code is an extensively modified version of KIVA. The latter was developed at the Los Alamos National Laboratory for reciprocating engines.
Technical Paper

Pressure Non-Uniformity and Mixing Characteristics in Stratified-Charge Rotary Engine Combustion

Studies are reported of stratified-charge combustion in rotary engines. They were performed with a three-dimensional model that computes intake, compression, liquid fuel injection, combustion, expansion, and exhaust. Comparisons are shown of computed and measured chamber pressures for two engines and seven conditions. They are the first comparisons of three-dimensional computations for rotary engines. The agreement is adequate for the purpose of interpreting the main features of the combustion flowfield. Then two subjects are considered: the mixing of injected-fuel and air, and the pressure non-uniformity within the combustion chamber. It is found that the TDC turbulence diffusivity of rotary engines in general is smaller than in corresponding reciprocating engines because of the longer time between intake and TDC. The pressure non-uniformity is shown to be caused by large fluid acceleration around TDC.