Grantee Research Project Results

1997 Progress Report: Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines

EPA Grant Number: R824970C001
Subproject: this is subproject number 001 , established and managed by the Center Director under grant R824970
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (1989) - Northeast HSRC
Center Director: Sidhu, Sukh S.
Title: Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines
Investigators: Hochgreb, Simone
Institution: Massachusetts Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1992 through June 30, 1998
Project Period Covered by this Report: January 1, 1996 through June 30, 1997
Project Amount: Refer to main center abstract for funding details.
RFA: Center on Airborne Organics (1993) Recipients Lists
Research Category: Targeted Research

Objective:

The project objectives are to understand what controls the process of post-flame oxidation of unburned hydrocarbons in spark ignition engines. The questions addressed by the project are: (a) under what conditions (engine operation and relevant temperatures) the production of products of incomplete combustion (particularly important ozone precursors and toxic products) are most likely, (b) the relative roles of mixing and reaction on oxidation during the different phases of post flame oxidation and (c) how well predictions of unburned hydrocarbon product hold against experimental data.

Rationale: Hydrocarbon emissions from spark-ignition engines are responsible for a large fraction of ozone precursors in the atmosphere as well as some important regulated toxic hydrocarbons. The process of oxidation of unburned hydrocarbons in spark ignition engines involves the emergence of hydrocarbons from cold engine walls (crevices, oil layers and deposits) and subsequent diffusion (turbulent and molecular) through the thermal boundary layer into the hot burned gases.

Experimental evidence (Green, 1995) and calculations (Wu et al., 1991; Lee and Morley, 1994) indicate that oxidation the oxidation process can be represented by a one-dimensional process during the gas expansion phase through bottom dead center, and a more complex large scale vortex mixing processes after bottom dead center. In order to characterize the hydrocarbon oxidation process, and identify what phases control the formation of ozone precursors and toxic hydrocarbons, one must combine chemical kinetic models to fluid mechanical mixing models. Given the complexity of the task, it is necessary to simplify the fluid mechanics during each phase of the process (expansion and displacement), and to identify simplifications to the chemical kinetic models that might be useful in more complex fluid mechanical simulations of the gas reaction.

Approach: The processes of hydrocarbon emergence and oxidation taking place inside the engine cylinder and exhaust system involve molecular and turbulent diffusion away from the walls into the hot core gases, total or partial reaction and subsequent convective transport out of the cylinder. The simulations are primarily concerned with the oxidation during the expansion process, during which hydrocarbons emerge from the various cold wall and oil sources, and are confined to a fairly thin (sub-millimeter) layer adjacent to the wall. This allows the simulations to be kept one-dimensional, so that detailed chemistry computations are feasible.

The model is a one-dimensional mixing layer model that includes convective and diffusive transport of species and energy, as well as chemical production and destruction of species, and chemical energy release (thoroughly described in Wu and Hochgreb, 1997a, 1997b). The initial and boundary conditions are set to match conditions expected around the engine walls. The evolution of the species released at different times during the expansion process can then be investigated, and the contributions of the different factors (reaction, diffusion and convection) to the overall process can be understood.

Progress Summary:

We have made significant progress in implementing the simulations, and using sensitivity and flux analysis to understand the roles of reaction, diffusion and chemistry under different conditions and with various fuels. The results are reported in two recent publications (Wu and Hochgreb, 1997a , 1997b). The important results are summarized as follows:

1. General features of the reactive/ diffusive process: Simulations show that unburned fuel is transported towards the hot burned gases, where intermediate species are quickly generated. The cold region near the walls acts as a buffer, which preserves the intermediate species from quick oxidation. Radicals are generated close to the burned gas during the oxidation process, at much higher concentrations than the original burned gas radical concentrations, indicating that radicals in the burned gas do not have a significant effect on the oxidation level of unburned hydrocarbons, except to initiate the oxidation reactions.
2. Roles of diffusion, convection, and reaction: Flux analysis indicates that the convective term can be neglected in comparison with diffusive and reactive terms. Simulation results show that the in- cylinder process is largely controlled by diffusion rates for temperatures above a transition burned gas temperature, and by reaction for temperatures below that level. These temperatures are of the order of 1700 K for isooctane, 1400 K for propane, 1350 K for ethane and 1300 K for ethene. Diffusion acts by transporting radicals generated at higher temperatures towards lower temperature regions, and moving intermediate species towards both higher temperature regions for destruction, or towards the colder wall regions, where they accumulate unreacted. The mechanism of generation (at lower temperatures) and destruction (at higher temperatures) of intermediate species keeps the overall concentrations of intermediate species approximately constant after the intermediate species initially increase.
3. Burned gas temperature effects : The burned gas temperature is the critical factor determining the oxidation level and the switch between diffusion and reaction control. Fast energy release raises the local temperatures near the walls, simulating the oxidation rate of unburned hydrocarbons in the cases of high initial core temperatures. In contrast, at low initial core temperatures, radical generation near the burned gas is not significant and the whole conversion process is very slow.
4. Turbulent diffusion effects : Molecular diffusivity dominates in the regions close to the wall (within 0.1 mm) and throughout the reaction zone. Sensitivity analysis of the effect of diffusion on the oxidation process has been conducted by increasing turbulent diffusivity by a factor of two. The effect of diffusion strongly depends on the initial burned gas temperature : a large increase in oxidation is possible by increasing diffusion rates at high initial temperatures. The overall concentrations of non-fuel species is little influenced by the change of turbulent diffusivity, so that the remaining fuel/nonfuel ratio decreases with increasing diffusivity.
5. Fuel effects: The effect of reactant chemistry on the oxidation process has been investigated by comparing the oxidation results between the cases of propane, ethene, ethane, and isooctane.
a) Ethane vs. ethene : These two compounds have very similar molecular diffusivity, yet the resulting extents of oxidation are different. Comparisons of the reaction fluxes shows that the oxidation of ethane requires initial conversion into ethene, which is then either oxidized in the high temperature region or diffuses towards the cold wall. Ethene undergoes a unidirectional flux from the walls towards the hot gases, facing a shorter oxidation chain towards carbon monoxide. Since the production of hydrogen atoms and chain branching at high temperatures depends on the last step in the chain of fuel oxidation through formyl radical decomposition (see conclusion VI), the radical production and the overall oxidation rate is higher for ethene than ethane.
b) Propane and Isooctane: The very long radical chain involved in the oxidation of isooctane, and the large fraction of intermediate products that diffuse towards the cold wall reduce the rate of radical production in the case of isooctane in comparison to the other fuels tested. The production of low reactivity intermediates (such as isobutene) may also contribute to the slower reaction as compared to propane, but this hypothesis would be better tested when comparing isooctane and normal octane. Propane oxidation is slower than either ethane and ethene, owing to the same reasons as stated above, related to the reaction chain length, and the diffusion of intermediates towards the cold walls.

6. Factors determining the oxidation rate: Reaction path analysis indicates most of hydrocarbons decompose by the attack of radicals (OH, O, and H). The drastic difference in the oxidation rate of unburned hydrocarbons between the cases of propane and isooctane are mainly due to the difference in the reactant concentrations (especially radicals) in the reaction zone. Hydrogen atom is the key radical for triggering most of the important chain-branching reactions. The main pathways of hydrogen atom production are through HCO decomposition and the reaction of carbon monoxide and hydroxyl. The rate of hydrocarbon conversion (to HCO) determines the rates of radical generation. The slow rate of conversion of fuel into hydrogen atoms due to long chain length and the diffusion of intermediate hydrocarbons into the cold zone is responsible for the lower concentration of radicals and lower oxidation levels of different fuels for the same initial conditions.

References
1. Green, R. G., and Cloutman, L. D., "Planar LIF Observations of Unburned Fuel Escaping the Upper Ring-Land Crevice in an SI Engine," Soc. Autom. Eng. Paper 970823 (1997).
2. Wu, K. and Hochgreb, S. "The Roles of Chemistry and Diffusion on Hydrocarbon Post-Flame Oxidation", Combust. Sci. Tech., in press, 1997b.
3. Wu, K. and Hochgreb, S. "Numerical Simulation of Post-Flame Oxidation of Hydrocarbons in Spark Ignition Engines", Soc. Autom. Eng. Paper 970886 (1997a).

Key Personnel

Graduate Students: Kuochun Wu and Ivan Oliveira

Future Activities:

Future Plans: Plans include implementing an adaptive gridding for code acceleration, concluding simulations using aromatic and oxygenated fuels. We shall then compare the simulations and experimental data, and analyze the models and simplify them.

Supplemental Keywords:

RFA, Air, Scientific Discipline, Waste, particulate matter, Environmental Chemistry, Incineration/Combustion, Atmospheric Sciences, spark ignition engines, combustion contaminants, hydrocarbons, incineration, engine deposits, ambient aerosol, hydrocarbon, emissions, kinetc models, products of incomplete combustion (PIC), atmospheric chemistry, chemical contaminants, combustion byproducts, modeling studies, combustion, particulates, chemical kinetics, modeling, oxidation, ambient air quality

Progress and Final Reports:

Original Abstract

Main Center Abstract and Reports:

R824970 HSRC (1989) - Northeast HSRC

Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R824970C001 Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines
R824970C002 Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions
R824970C003 Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion
R824970C004 Mathematical Models of the Transport and Fate of Airborne Organics
R824970C005 Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene
R824970C006 Simultaneous Removal of Soot and NOx from the Exhaust of Diesel Powered Vehicles
R824970C007 Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds
R824970C008 Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives
R824970C009 Markers for Emissions from Combustion Sources
R824970C010 Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
R824970C011 Microengineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants

Top of Page

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.