Rodney John Tabaczynski

A Turbulent Entrainment Model for Spark-Ignition Engine Combustion

A turbulent entrainment model for the turbulent combustion process in spark-ignition engines is described. The model uses the basic quantities of turbulent flow, i.e., the integral length scale, micro length scale, and turbulent intensity. The characteristic reaction time for a large eddy tau was calculated using the characteristic reaction time tau/sub c/ for the microscale, lambda/S/sub l/, where S/sub l/ is the laminar flame speed and propagation of ignition sites within a coherent turbulent structure. Tau is related to the flame kernel development time and shows similar trends to the ignition delay time. The combustion model is demonstrated by calculations showing the typical trend behavior of combustion duration with equivalence ratio, exhaust gas recirculation, spark timing and engine speed.

FURTHER REFINEMENT AND VALIDATION OF A TURBULENT FLAME PROPAGATION MODEL FOR SPARK-IGNITION ENGINES.

A turbulent flame propagation model that is dependent on the structure of the turbulent flow field is formulated and applied to combustion in a spark-ignition engine. The turbulence structure is modeled after the work of Tennekes and assumes that the flow is composed of vortex tubes the diameter of the Kolmogorov scale and the spacing of the Taylor mircoscale. Combustion is assumed instantaneous over the Kolmogorov scale and the burned gases in the vortex tubes are assumed to propagate at a rate equal to U′ + SL. Combustion is assumed to proceed in a laminar fashion across the microscale. In applying the model to a spark-ignition engine, we conserve the turbulent kinetic energy and angular momemtum in the unburned gases. Validation of the model is presented in the form of mass fraction burned versus crank angle curves. Comparisons of predicted versus experimental data show good agreement for variations in equivalence ratio, dilution, speed, load, and spark advance.

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.

Premixed turbulent flame blowoff velocity correlation based on coherent structures in turbulent flows

Abstract A correlation for the blowoff velocity of premixed turbulent flames stabilized by bluff bodies is developed using a simple model for coherent structures in turbulent flows. The correlation shows the correct trends for variations in equivalence ratio, free stream pressure and temperature, flameholder characteristic size, and turbulent Reynolds number, and good agreement with published experimental data is obtained.

Radiation-affected laminar flame propagation

Abstract Increased laminar flame thickness and flame speed under the influence of radiation is shown in terms of an original heat transfer number H= ntp 1+3t 2 /(1−ω) where η = ( κ p κ R ) 1 2 is the weighted nongreyness, κp and κR are the Planck mean and the Rosseland mean of the absorption coefficient, τ = κMδK is optical thickness, κ M = (κ p κ R ) 1 2 the mean absorption, δK the conduction flame thickness, P = 4σT M 3 ( λ δ K ) the Planck number, TM the adiabatic flame temperature, λ the thermal conductivity, and ω the albedo, the ratio of scattering to extinction.

COMBUSTION CHAMBER EFFECTS ON BURN RATES IN A HIGH SWIRL SPARK IGNITION ENGINE

Experimental measurements of burn rates have been carried out in a single cylinger homogeneous charge engine. Three different combustion chambers were investigated (75 % and 60 % squish bowl-in-piston chambers and a disk chamber) using a cylinder head with a swirl producing intake port and near central spark location. Data were obtained with each combustion chamber as a function of spark timing, EGR, and load at 1500 RPM. The combustion rate is strongly influenced by chamber shape. The 10-90 % burn durations of the 75 % and 60 % squish chambers are respectively about 40 % and 60 % that of the disk chamber. Chamber configuration had less effect on 0-10 % burn duration. The disk had about 25 % longer 0-10 % burn time than the bowl-in-piston chambers. Modifications to the GESIM model enabled good overall agreement between predictions and experimental data, a rather severe test of the model because the coupling of fluid mechanics, combustion and chamber geometry must be properly modeled. An improved basic understanding of the influence of combustion chamber shape on burn rate has been achieved through the interactive use of experimental data and modeling. The results suggest that differences in turbulence intensity and flame area development due to changes in chamber shape are responsible for the observed burn rate differences.

Radiation-Affected Laminar Flame Quenching

Abstract Under the influence of radiation, the increase in Peclet number characterizing the flame quench distance Δ, Pe = ρ u c p S u T b 0 λT b 0 Δ − adiabatic flame enthalpy flow conduction , and the decrease in flame temperature are shown in terms of an original radiation number R w = ητ(1− e w 2 )B b 0 1+3τ 2 ( 2 e w −1 ) (1−ω) − total radiation adiabatic flame ethalpy flow , where ϱ is the density, cp the specific heat at constant pressure, Su the laminar flame speed, Tb the flame temperature, subscript u the unburned gas and superscript 0 the adiabatic gas, λ the thermal conductivity, η = ( κ P κ R ) 1 2 the weighted nongrayness, κP and κR being the Planck mean and the Rosseland mean of the absorption coefficient, ϵw the wall emissitivity, τ=κMl the optical thickness, κ M =(κ P κ R ) 1 2 being the mean absorption coefficient and l a characteric length (related to geometry or quench distance), ω the albedo of single scattering, and Bb0 the adiabatic flame Boltzmann number. B b 0 = 4E b 0 ρ u c p S u 0 T b 0 ∼ emission adiabatic flame enthalpy flow , where Eb is the blackbody emissive power. It is qualitatively shown that the contribution of radiation to the heat transfer and the laminar flame quenching in small diesel engines can be as much as 35%.

Verification of LDA and seed generator performance

An essential step in the application of laser-Doppler anemometry (LDA) to turbulent flow research is to verify the capability of the measurement system to respond adequately to the anticipated velocity fluctuations. Problems of seed particle velocity fidelity can arise due to inadequate detectability of micron-size seed particles by the LDA system or improper seeding techniques. As a result of the present investigation, we caution that reported LDA measurements in piston engines could be in error. A convenient method to check for the occurrence of these problems is described.

THE EFFECT OF INTAKE VALVE LIFT ON TURBULENCE INTENSITY AND BURNRATE IN S.I. ENGINES-MODEL VERSUS EXPERIMENT

An Engine Simulation Model was used to study the effect of changing the maximum intake valve lift to control in-cylinder turbulence intensity and burn rate. Experimental measurements of burn rate for two different valve lift profiles were obtained and compared with predictions. The standard K-epsilon turbulence model was found to be inadequate for predicting the proper behavior of turbulence level during compression and expansion. Further investigation showed that the dissipation of turbulence calculated by the standard k-epsilon model was inadequate, thus causing the turbulence levels and burn rates to be approximately independent of the intake valve lift. A new turbulent dissipation model is proposed which uses the eddy angular momentum to scale the dissipation constant. Turbulent intensity predictions from this model resulted in acceptable agreement between the measured and predicted burn rates as the intake valve lift was changed. The effect of throttling the engine using intake valve lift was investigated and predictions made of turbulence intensity, burn rate, combustion efficiency and brake specific fuel consumption (BSFC) as a function of air-fuel ratio and load. Results showed a significant reduction in BSFC at 13 BMEP, 1500 RPM when conventional throttling was compared with intake valve throttling at equal burn rates. In addition, the effect of B/S ratio on turbulence intensity, burn rate and ISFC was investigated and results showed the independent effects of engine geometry and turbulence on burn rate when the engine was stroked holding the cylinder bore constant.

A Model for the Lean Misfire Limit in Spark-Ignition Engines

Abstract The combined effect of compression ratio, engine speed, intake air temperature, nitrogen diluent an spark-timing on the lean misfire limit is investigated experimentally for a CFR engine with propane as a fuel. The misfire limit is defined as the point where 5 to 8 cycles out of 1,000 consecutive cycles have firing pressure traces equal to the motoring presssure trace. A balance between energy produced in the entrained volume of the developing flame and the energy diffused to unburnt gas is proposed as a model for the flame initiation. The initial conditions for the turbulent flame extinction process are obtained from a k∊ model which includes the effect of compressibility