G. A. Lavoie

A numerical study of laminar flame wall quenching

Abstract Laminar flame quenching at the cold wall of a combustion chamber has been studied, using a numerical model to describe the reactive flow. The model combines an unsteady treatment of the fluid mechanics and a detailed chemical kinetic reaction mechanism. Fuels considered included both methane and methanol. Catalytic reactions at the wall surface are not included in the kinetic model. The one-dimensional case of flame propagation perpendicular to the wall was studied. Two reference cases are described in detail for flame quenching at 10 atm pressure and a wall temperature of 300°K with stoichiometric mixtures of methane-air and methanol-air. In each case a conventional laminar flame propagates toward the wall, approaching to within a distance determined by the thermal flame thickness. Chemical kinetic factors, particularly differences between the temperature dependence of radical recombination reactions and conventional chain branching and chain propagation reactions, are shown to be responsible for quenching the flame near the wall. The flame stagnates, but fuel remaining near the wall diffuses out of the boundary region and is rapidly oxidized away from the wall. Subsequent model calculations demonstrate the effects of variations in pressure, fuel-air equivalence ratio, wall temperature, and type of fuel. Computer results from these methane and methanol flame quenching models indicate that the total unburned hydrocarbon content is considerably smaller than is commonly believed and that thermal wall quenching may not be the major source for hydrocarbon emissions from internal combustion engines at near-stoichiometric conditions.

Phenomenological models for reciprocating internal combustion engines

Abstract In the past 10–15 years there has been a substantial increase in mathematical modeling activity as it relates to improving the design and operation of reciprocating, internal combustion engines. Most of the previous work and a large part of todays efforts center about models which are “phenomenological” in nature. These models attempt to describe complex engine behavior in terms of separate, physically-based submodels of important identifiable phenomena. Typically, they have been built around the First Law of Thermodynamics and involve no explicit spatial dependence. This approach is to be contrasted to the more recent, “detailed” or large scale approach in which the governing conservation equations are solved numerically in either one, two or three dimensions. In the latter approach, the important phenomena should emerge from the rigorous, detailed solution. Given the growing interest in modeling and in the detailed, large scale approach in particular, we have conducted a state-of-the-art review of phenomenological modeling capability to serve as a baseline for future work, be it of a phenomenological or detailed type. For conventional SI engines, stratified charge engines and diesel powerplants we have attempted to indicate those areas in which the phenomenological approach has been or could be successful and those areas in which detailed computations would be of greatest benefit. It is our general conclusion that detailed computations can be most helpful for guiding the development of more sophisticated phenomenological models which can then be used for extensive parametric investigations.

A Fundamental Model for Predicting Fuel Consumption, NOx and HC Emissions of the Conventional Spark-Ignited Engine

Abstract A model of the four-stroke S.I. engine cycle has been developed which predicts fuel consumption, NOx and HC emissions as a function of engine design and operating conditions. The model is primarily thermodynamic in nature containing no formal spatial dependence. The major new features of the model are: first, a treatment of heat transfer which confines heat losses to a boundary layer region surrounding a central adiabatic core; second, an integral boundary-layer analysis of in-cylinder burnup of quenched hydrocarbons; and third, a calculation of exhaust port HC oxidation which considers the temperature history of each element of gas leaving the cylinder. The main adjustable parameters of the model relate to the rate of heat transfer and the ratio of the two-plate quench to the single-wall thickness. An extensive comparison of model predictions with experimental CFR engine data is presented. The results show excellent agreement between predicted and experimental fuel consumption and NOx emissions....

The Effect of Oil Layers on the Hydrocarbon Emissions from Spark-Ignited Engines

Abstract Measured amounts of oil were added to the engine cylinder of a single-cylinder CFR engine to determine the effect of oil layers on exhaust hydrocarbon emissions. The exhaust hydrocarbon concentration increased in proportion to the amount of oil added when the engine was fueled on isooctane. Addition of 0·6 cm3 of oil produced an increase of 1000 ppmC in exhaust hydrocarbon emissions at a coolant temperature of 320°K. Gas chromatographic analysis of the exhaust determined that fuel and fuel oxidation species, not oil oxidation products, were responsible for most of the increase. Similar experiments performed with propane fuel showed no increase in exhaust emissions when oil was added to the cylinder. These measurements have determined that the increase in tailpipe hydrocarbon concentration consists of fuel related species and is proportional to both the amount of oil added and the solubility of the fuel in the oil. Thus, we believe that the principal source of this increase in exhaust hydrocarbon ...

In-Cylinder Measurements of Wall Layer Hydrocarbons in a Spark Ignited Engine

Abstract The hydrocarbon emissions process for the conventional, spark ignited, IC engine has been studied experimentally using a rapid acting gas sampling valve mounted in the combustion chamber wall. The sampling valve was electrohydraulically actuated. Design of the valve specifically allowed sampling in the vicinity of the wall quench layer with minimum leakage and crevice contributions to the measured hydrocarbon concentrations. Experimental results presented give substantial evidence that hydrocarbons remaining in a wall quench layer are not a major source of exhaust hydrocarbon emissions. Measurements of species concentrations as a function of time in the cycle and sample flow rate indicate that after flame arrival and quenching at the cold walls, hydrocarbons in the quench layer are rapidly and extensively oxidized within 2 msec. By use of an analytical model for the gas flow profile into the sampling valve, conservative upper limit calculations have been made of the quench layer contribution to t...

The Effect of Fuel and Operating Variables on Hydrocarbon Species Distributions in the Exhaust from a Multicylinder Engine

Abstract Measurements of the concentrations of individual exhaust hydrocarbon species have been made as a function of engine operating variables (φ, rpm, EGR, spark timing, and coolant temperature) in a 2·3-liter four-cylinder engine. Three fuels were used in these experiments: propane, isooctane (2,2,4-trimethylpentane), and an unleaded gasoline (indolene clear). The results show that a change in operating variable can change not only the total hydrocarbon concentration but also the distribution of species in the exhaust. All three fuels show similar trends when an operating variable is changed. Fuel-air equivalence ratio is a critical parameter in controlling exhaust hydrocarbon emissions. Beginning near stoichiometric, the total hydrocarbon concentration and the percentage contributions of methane and acetylene to the exhaust increase as the mixture becomes richer. These species contribute less than 2 percent to the total hydrocarbon emissions at <0.95. Their contribution rises to 15–25 percent at φ...

A combustion bomb study of the hydrocarbon emissions from engine crevices

Abstract Two combustion bombs manufactured from a Ford 1.6L Escort production engine were used to determine the effects of engine crevice volumes on hydrocarbon emissions. Since these combustion bombs were used as static reactors, the results cannot be directly applied to an operating engine but they focus attention on the major hydrocarbon-producing crevices in an actual engine geometry. During Ihis propane-fueled experiment, the crevices were sequentially filled with epoxy or viton rubber, and after filling each crevice, the exhaust hydrocarbon emission was measured by gas chromatography. This provided a quantitative measurement of the hydrocarbon emission from each crevice. For these reactors, the ring-pack crevices produced approximately 80.5 percent of the total scaled hydrocarbon emission, while the head gasket and spark plug threads produced approximately 12.5 percent and 5 percent, respectively. All other hydrocarbon sources produced less than 2 percent of the total scaled hydrocarbon emissionfrom...

An experimental study of hydrocarbon emissions from closed vessel explosions

Two combustion bombs are used to determine the exhaust hydrocarbon emission after laminar flame propagation through the reactors. Propane and air are used as fuel and oxidizer, and gas chromatography is used to analyze, the emission, gases. Data are taken over an initial pressure range from 50 kPa to 400 kPa and from an equivalence ration of 0.7 to the soot limit at 2.0. During experimentation, extreme care is taken to keep the reactor vessels clean while reducing crevice storage volumes using indium seals and displacement materials. The results under lean conditions indicate that the charging and subsequent outgassing of fuel molecules from storage volumes into a relatively cold bulk gas is the primary cause of exhaust emissions. This is consistent with recent numerical results under near stoichiometric conditions, indicating that flame quench hydrocarbons rapidly diffuse and oxidize, producing less unburnt material than previously thought. Under these conditions, the levels of exhaust hydrocarbons are observed to be two orders of magnitude lower than previously reported in the literature. Furthermore, under rich conditions, >1.2, the results indicate that processes other than storage effects and wall-quenching—possibly occurring in the bulk gas—may be the cause of the exhaust hydrocarbon emission from clean vessels.

LAMINAR HEAD-ON FLAME QUENCHING - A THEORETICAL STUDY.

Results of a numerical investigation of one-dimensional laminar flame quenching with constant and time-dependent pressure variations are reported. A description of flame quenching and postflame oxidation processes for a global reaction is obtained by solving a simplified form of nonlinear conservation of mass, momentum, and energy equations in a planar flow field. These equations are reduced to finite difference form, and time-dependent pressure is input via an integrated form of the energy equation or a third-order polynomial law. Numerical calculations were performed at constant-pressure, combustion bomb-type conditions and under rapid decompression during quenching. One-dimensional head-on quenching distances, hydrocarbon concentration levels, and gas properties were derived for various stoichiometric values in a propane/air-type mixture. Results indicate that quenching distance is primarily controlled by the thermal conduction process; however, residual hydrocarbon levels are intimately related to postquenching diffusion, oxidation kinetics, and the thermodynamic cycle during which they occur. Model calculations exhibit good agreement with experimental flame speed and demonstrate single-wall quench thicknesses which closely follow known experimental trends for the two-plate quench distance. Effects of transient pressure changes on postquench burn-up are examined, and results indicate that pressure variations on a time scale similar to that of the quenching process can account for a factor increase in predicted hydrocarbons, as compared with constant pressure calculations. Additional research is recommended to devise a multistep reduction reaction scheme that adequately describes the quenching process.

Measurements of NO Emissions From a Stratified Charge Engine: Comparison of Theory and Experiment

Abstract Measurements of NO exhaust emissions have been made on a single-cylinder engine with and without stratification. The results were compared with predictions based on an existing computer model of Blumberg (1973). Combustion duration, a critical parameter of the model, was inferred from experimental pressure-time records for each condition studied. In premixed operation at low NO levels the post-flame Zeldovich kinetics, as used by Lavoie et al, (1970), could not account for the observed concentrations. The discrepancy was ascribed to flame-formed NO, and a correction to the chemical kinetics was made based on the measurements of Fenimore (1970). With this correction good qualitative agreement between theory and experiment was obtained for the premixed case. On the basis of these results the modified kinetics were also employed in the stratified charge calculations. For stratified operation, calculations were made with a number of assumed stratification functions and the results compared with exper...