Ronald D. Matthews

The necessity of using detailed kinetics in models for premixed combustion within porous media

Abstract Models for premixed combustion within porous inert media (PIM) are complicated by the highly nonlinear radiative exchange terms in the energy equation for the solid matrix in addition to the stiffness of the set of gas phase equations. Therefore, prior researchers have simulated the gas-phase reactions using single-step chemistry. In the present work, predictions are made using both single-step and multistep kinetics mechanisms. It is concluded that it is essential to use multistep kinetics if accurate predictions of the temperature distributions, energy release rates, and total energy release are sought. Obviously, this is also true if predictions of the composition profiles and emissions are sought. Single-step kinetics is shown to be adequate for predicting all the flame characteristics except the emissions for the very lean conditions under which equilibrium favors the more complete combustion process dictated by global chemistry. The first predictions of NO and CO emissions from PIM burners are presented and compared with experimental data. The model predicts the CO emissions very accurately and predicts the NO trend correctly but overpredicts the NO emissions for φ > 0.8. The present multistep PIM burner model does not accurately reproduce the data for the burning speed and NO emissions for nondilute mixtures. These discrepancies can be only partially attributed to experimental uncertainties and/or imprecise knowledge of the properties of the solid matrix. Thus, it is concluded that important aspects of the physical processes within PIM combustors are not well simulated at present.

A Numerical Investigation of Premixed Combustion Within Porous Inert Media

A numerical investigation of premixed combustion within a highly porous inert medium is reported. Specifically, results of a numerical model using detailed chemical kinetics and energy exchange between the flowing gas and the porous solid are presented. The current formulation differs from prior models of this type of combustion in that multistep kinetics is used and a better description of the thermophysical properties of the solid is applied in the present model. It was found that the preheating effect increases strongly with increasing convectiue heat transfer and with increasing effective thermal conductivity of the solid

Use of Fractal Geometry to Model Turbulent Combustion in SI Engines

Abstract Use of fractal geometry to model the effects of turbulence on flame propagation in an engine is explored using a quasidimensional, 4-stroke, homogeneous charge, SI engine code. This application of fractal geometry requires a new interpretation of the effect of turbulence on the combustion process in an engine. Specifically, flame wrinkling, rather than entrainment, is assumed to be the dominant effect of turbulence on the combustion process. Various simplifications are made in the formulation of the engine model to allow this fractal technique to be investigated as expeditiously as possible. Model predictions are compared to experimental data from an engine with an axisymmetric pancake-shaped combustion chamber. The sensitivity of the model predictions to the fractal dimension, to the effects of flame stretch, and to the ratio of maximum-to-minimum flame wrinkling scales is investigated. It is shown that the predicted initial rate of pressure rise is a strong function of the fractal dimension but...

Diluents and Lean Mixture Combustion Modeling for SI Engines with a Quasi-Dimensional Model

Lean mixture combustion might be an important feature in the next generation of SI engines, while diluents have already played a key role in the reductions of emissions and fuel consumption. Lean burning modeling is even more important for engine modeling tools which are sometimes used for new engine development. The effect of flame strain on flame speed is believed to be significant, especially under lean mixture conditions. Current quasi-dimensional engine models usually do not include flame strain effects and tend to predict burn rate which is too high under lean burn conditions. An attempt was made to model flame strain effects in quasi-dimensional SI engine models. The Ford model GESIM was used as the platform. A new strain rate model was developed with the Lewis number effect included. A 2.5L V6 4-valve engine and 4.6L V8 2-valve modular engine were used to validate the modified turbulent entrainment combustion model in GESIM. Results showed that the current GESIM can differ by as much as 10 crank angle degrees compared with test data. The modified GESIM can predict burn duration to within 1--2 CA of experimental data, which is considered very good for engine models.

The significance of intermediate hydrocarbons during wall quench of propane flames

A one-dimensional model is used to study end-on wall quench using a detailed chemical kinetics mechanism for propane. Previous models using detailed chemical kinetics mechanisms for methane, methanol, and acetylene revealed that intermediate hydrocarbons exist at much lower levels than unreacted fuel molecules during quench, and thus led to the general conclusion that one-step global chemistry (which accounts only for the rate of disappearance of the fuel) is adequate for studying hydrocarbon evolution during wall quench. However, these fuels are extremely simple, low molecular weight molecules with very limited paths available for forming intermediate hydrocarbons. In the present study, wall quench is studied for propane-air mixtures at equivalence ratios of 0.9, 1.0, and 1.1; pressures of 1, 10, and 40 atmospheres; and wall temperatures of 400 and 500 K. It is shown that intermediate hydrocarbons exist at higher levels and at greater distances from the wall during quench than unreacted fuel. Furthermore, the intermediate hydrocarbons are oxidized less rapidly and persist at significant levels much longer after quench. The persistence of the intermediate hydrocarbons is aggravated by lower wall temperatures, lower pressures, and equivalence ratios both greater than and less than stoichiometric. At lower pressures, the rate of oxidation of the intermediate hydrocarbons is slowed to an even greater extent than is the fuel oxidation rate. The conclusion that intermediate hydrocarbons contribute significantly to wall quench hydrocarbon evolution indicates that one-step global chemistry is inadequate for studying turbulent wall quench, bulk quench, and crevice volume quench of higher hydrocarbon fuels and thus casts doubt on the use of the results of previous theoretical wall quench studies to obtain general conclusions regarding wall quench hydrocarbon evolution in practical engines using practical fuels.

Liquid Fuel Impingement on In-Cylinder Surfaces as a Source of Hydrocarbon Emissions From Direct Injection Gasoline Engines

Hydrocarbon (HC) emissions from direct injection gasoline (DIG) engines are significantly higher than those from comparable port fuel injected engines, especially when late direct injection (injection during the compression stroke) is used to produce a fuel economy benefit via unthrottled lean operation. The sources of engine-out hydrocarbon emissions for late direct injection are bulk flame quench, low temperatures for postcombustion oxidation, and fuel impingement on in-cylinder walls. An experimental technique has been developed that isolates the wall impingement source from the other sources of HC emissions from DIG engines. A series of steady-state and transient experiments is reported for which the HC emissions due to operation with a premixed charge using a gaseous fuel are compared to those when a small amount of liquid fuel is injected onto an in-cylinder surface and the gaseous fuel flow rate is decreased correspondingly. The steady-state experiments show that wetting any in-cylinder surface dramatically increases HC emissions compared to homogeneous charge operation with a gaseous fuel. The results of the transient fuel injection interrupt tests indicate that liquid-phase gasoline can survive within the cylinder of a fully warmed-up firing engine and that liquid fuel vaporization is slower than current computational models predict. This work supports the argument that HC emissions from DIG engines can be decreased by reducing the amount of liquid fuel that impinges on the cylinder liner and piston, and by improving the vaporization rate of the fuel that is deposited on these surfaces.

The Texas diesel fuels project, part 1: Development of TxDOT-specific test cycles with emphasis on a "route" technique for comparing fuel/water emulsions and conventional diesel fuels

The Texas Department of Transportation (TxDOT) began using an emulsified diesel fuel in July 2002. They initiated a simultaneous study of the effectiveness of this fuel in comparison to 2D on-road diesel fuel, which they use in both their on-road and off-road equipment. The study also incorporated analyses for the fleet operated by the Associated General Contractors (AGC) in the Houston area. Some members of AGC use 2D off-road diesel fuel in their equipment. The study included comparisons of fuel economy and emissions for the emulsified fuel relative to the conventional diesel fuels. Cycles that are known to be representative of the typical operations for TxDOT and AGC equipment were required for use in this study. Four test cycles were developed from data logged on equipment during normal service: 1) the TxDOT Telescoping Boom Excavator Cycle, 2) the AGC Wheeled Loader Cycle, 3) the TxDOT Single-Axle Dump Truck Cycle, and 4) the TxDOT Tandem-Axle Dump Truck Cycle. As is conventional for heavy-duty engines, the first two of these cycles are specified in terms of percent torque and percent engine speed versus time for engine dynamometer testing. The latter two cycles are specified in terms of vehicle speed versus time for chassis dynamometer testing. Due to the torque loss associated with the water in the emulsified fuel, there was concern that conventional means for comparing the two fuels would result in less work performed by the engine over the cycle when operating on the emulsified fuel. The inadequacies of traditional speed versus time test cycles, when applied to heavy-duty vehicles where power-to-weight ratio can change greatly, have been recognized for some time (1, 2). Speed versus distance test routes have been developed using icons as simple driver instructions (3), using free accelerations in a traditional speed versus time environment (4), and using sequences of distanced-based phases (5). For this study, a route technique was developed for testing the dump trucks. The route technique assures equal distances traveled for each micro-trip and for the overall cycle independent of the fuel. For engine dynamometer testing, the same command cycle was used to assure the same work was requested over the cycle independent of the fuel.

Ignition of polyoxymethylene

Abstract An experimental opposed flow diffusion flame system has been developed with which polymer ignition may be carefully investigated. The effects of ignition source intensity, oxidizer flow rate, and oxidizer composition on the ignition of polyoxymethylene have been studied. Comparisons with available theoretical models have been made where possible. It is concluded that the polymer surface temperature at ignition is independent of ignition source intensity, oxidizer composition, and oxidizer flow rate; that the ignition delay time is independent of oxidizer velocity over the range of velocities for which a flat flame can be established in the opposed flow diffusion flame system; that gas phase absorption of radiation may affect the ignition process even for relatively small optical depths and low source intensities; that the ignition delay time decreases linearly with increasing oxygen concentration in oxygen nitrogen mixtures for low flame stretch rates; that ignition occurs during the transition stage when the free stream oxygen mass fraction is less than the oxygen mass fraction in the solid, and for higher free stream oxygen mass fractions it appears that oxygen diffusion to the surface is enhancing the gasification rate during the transition stage and/or decreasing the period of the transition stage; and that the various processes occurring in the gas phase during the ignition event appear to result in a net liberation of energy.

Friction predictions for piston ring-cylinder liner lubrication

This paper presents two piston ring and cylinder liner lubrication models and compares the friction predictions against the experimental results from a corresponding bench test. The first model aims to solve the average Reynolds equation with corrective flow factors, which describe the influence of surface irregularities on the lubricant flow under mixed lubrication condition. The second model takes account of the lubricant film rupture and cavitation. Meanwhile, a stochastic rough contact sub-model quantifies the relation between contact pressure and mean surface separation in both cases. Numerical results on the top compression ring simulation show that both models capture hydrodynamic, mixed, and boundary lubrication regimes, which depend on the real surface topographies of the piston ring and the cylinder liner. Whenever hydrodynamic action is insufficient to maintain the equilibrium position of the ring, the restoring force will be augmented by multi-asperity contacts lubricated by a thin boundary film. Total friction will originate mainly from shearing of viscous lubricant and shearing of asperity conjunctions. The purpose of this modeling effort is to compare both lubrication models to data from an experimental test-rig. This test rig eliminates many of the factors that can make analysis of predictions for real engine operating conditions difficult.Copyright

Use of Railplugs to Extend the Lean Limit of Natural Gas Engines

Ignition of extremely lean mixtures is a very challenging problem, especially for the low speed, high load conditions of large-bore natural gas engines. This paper presents initial results from testing a high energy ignition system, the railplug, which can assure ignition of very lean mixtures by means of its high energy deposition and high velocity jet of the plasma. Comparisons of natural gas engine tests using both a spark plug and a railplug are presented and discussed in this paper. The preliminary engine test show that the lean stability limit (LSL) can be extended from an equivalence ratio, φ, of ∼0.63 using a spark plug down to 0.56 using a railplug. The tests show that the railplug is very promising ignition system for lean burn natural gas engines and potentially for other engines that operate with very dilute mixtures. The ignition characteristics of different railplug geometric and circuit designs are also discussed.Copyright