1998 Highlights

Source and Control

Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives Joseph W. Bozzelli, Chemical Engineering and Chemistry, New Jersey Institute of Technology

The dimethyl ether radical CH3OC.H2 reaction with oxygen, O2, is shown to be chain propagating through reactions with barriers that are below initial energies of reactants. Reaction paths and kinetics are determined by ab initio calculations (MP2/CBSq and G2) and compared with experiment. Calculated reaction paths explain the explosive nature of ethers relative to hydrocarbons.

Variable pressure reaction mechanisms for use in a NJIT modified pressure dependent Chemkin Code are developed for MTBE and for dimethyl ether combustion.

Origin and Characteristics of Fine Soot Particles in Combustion Jack B. Howard, Massachusetts Institute of Technology

Preliminary SMPS results comparing high (2.8) and low (2.1) equivalence ratio frames show that low equivalence ratios produce less soot overall, but have a larger number of small particles for a given height above the burner (HAB). The SMPS analysis shows good reproducibility for measuring soot particle sizes. The tracer method of determining dilution ratio promises to improve the reproducibility of the soot concentration measurements. Results of the TEM size analysis show good agreement with the normalized SMPS size distribution for the same regions in the flame. The agreement implies that the SMPS sampling technique inhibits soot agglomeration and size dependent losses during soot extraction, transportation, and analysis. Additionally, TEM results clearly reveal when there is agglomeration of soot particles. Although some agglomerates are found at the highest HAB, it can be seen that agglomeration is not a major factor for soot particle growth in methane/oxygen flames at an equivalence ratio of 2.5.

Investigation of the Formation of Particulate Matter in Spark-Ignited Engines Simone Hochgreb, Arthur Lafleur, Massachusetts Institute of Technology

Measurements of PM emissions from SI engines as a function of operating conditions, fuel and lubricant type reveals the overwhelming role played by liquid fuel presence in the cylinder, as well as an unexpected high amount of PM under lean as well as rich conditions.

Fuel sensitivity studies show that aromatic fuels produce substantially more PM than paraffinic fuels of similar volatility. The studies also showed that addition of oxygenated fuels (MTBE) to indolene does not significantly affect PM emissions.

Lubricant oil was found to contribute negligible amounts to PM formation, based on experiments with gaseous propane.

A physically-based model for PM emissions was formulated to interpret the experimental results. Analysis of the combined PM results showed the relative contribution of the different nucleation (gas and liquid phase), growth and oxidation phenomena.

Transient measurements of PM during load and fuel/air transients showed that the characteristic time for PM change is much longer than that for the change in engine conditions, and that particle growth dynamics continues for a long time. No evidence of a second order dynamic effect leading to a spike in PM concentrations was found during transients.

Combustion Chemistry of Polycyclic Aromatic Compounds Jack B. Howard, Massachusetts Institute of Technology

The data incorporation technique has been used to develop a soot nucleation model, cited in the previous annual report as an area of future work. An automated optimization algorithm has been developed and implemented that works in conjunction with the DIT to explore parameter sub-spaces for optimal solutions. The error of rate coefficients obtained through these methods is compared to values that could be obtained without the DIT, and found to be at least an order of magnitude less uncertain, in the range of order of magnitude estimates.

Chemical Kinetic Modeling of Products of Incomplete Combustion in Spark Ignited Engines Simone Hochgreb, Massachusetts Institute of Technology

Development of a one-dimensional reactive diffusive code for calculations of the post-flame oxidation of fuel in spark ignited engines. The model uses an adaptive gridding method with stiff time integration to resolve the spatial and time scales involved. The model is now also capable of treating two-phase (liquid) oxidation.

Understanding of the role of chemistry and diffusion on the process of post-flame oxidation in spark ignited engines. Interpretation of the simulation results shows under what conditions diffusion or reaction limits the oxidation of the in-cylinder hydrocarbons, resulting in emissions. Sensitivity analysis allowed quantification of the factors limiting oxidation, and the temperatures over which oxidation is reaction limited vs. diffusion limited, for the fuels investigated (propane, ethane, ethene and isooctane). A detailed analysis of the pathways for reaction was also performed, to understand the underlying chemical reasons for the lower reaction rate in the case of short-chain fuels relative to long chemical reaction chain fuels.

Integration of the results for cases where experimental results are available showed that the simulations can well capture the effects of operating conditions as well as fuel on the overall rate of in-cylinder oxidation. In addition, the results showed that the product distribution is also reasonably well captured with the existing models.

Transport and Transformation

Modeling and Analysis of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols Glen R. Cass, California Institute of Technology

A photochemical trajectory model has been applied to study the formation and transport of particulate matter in the atmosphere along air parcel trajectories crossing the Los Angeles basin. Model predictions are in good agreement with detailed measurements of particle size and chemical composition and reveal the major emissions source types that contribute to the Los Angeles fine particle problem.

Laboratory Studies of Intermediate Steps in the Atmospheric Oxidation of Organic Compounds Mario J. Molina, Massachusetts Institute of Technology

Several intermediates in the atmospheric oxidation of toluene have been successfully generated and detected in the flow-tube CIMS apparatus using O2+, NO+ and O2- as the ionization reagents. Our results corroborate the formation of a bicyclic peroxy radical whose existence had been postulated previously on the basis of theoretical calculations.

Kinetics, Reaction Path Analysis and Elementary Reaction Mechanism Development for Atmospheric Photochemical Oxidation of Aromatics and Oxygenated Aromatics Joseph W. Bozzelli and Tsan H. Lay, Chemical Engineering and Chemistry, New Jersey Institute of Technology

A detailed reaction mechanism with microscopic reversibility and the associate thermodynamic database has been established for toluene atmospheric oxidation. Predictions of our kinetic modeling are in good agreement with the experimental results. An initial mechanism for styrene is developed. Bond energies for C--H bonds on styrene, fulvene, and other " aromatic" like systems and on hydrocarbon esters are calculated to be slightly (2-5 kcal/mole) higher than literature estimates. Rate constants for HO2 addition to olefins increase (barriers decrease) with increased hydrocarbon substitution on the olefinic carbon.

Atmospheric Transformations of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion John H. Seinfeld and Richard C. Flagan, California Institute of Technology

We have obtained a comprehensive data set on the atmospheric aerosol-forcing potential of biogenic hydrocarbons. Results of prior smog-chamber studies on secondary organic aerosol formation have been integrated into the three-dimensional gas/aerosol model.

Monitoring and Source Attribution

Markers for Emissions from Combustion Sources John B. Vander Sande, Massachusetts Institute of Technology

As a means for a simple and decisive source assignment of unknown samples the concept of using a " Truth table" has been introduced and developed. This logical process would, in the future, enable sample identification and source assignment without relying on subjective judgements. This is a breakthrough in assignment determination that should be employed by the general environmental monitoring community.

Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols Glen R. Cass, California Institute of Technology

A field experiment has been conducted in which the chemical evolution of air pollutant particles has been observed as air parcels pass over the Los Angeles urban area. Gas-to-particle conversion processes have been observed at the single particle level using aerosol time of flight mass spectrometers that have been calibrated to read in terms of absolute particle number concentrations and chemical species concentrations.