Background and methodology

MTBE (methyl tertiary-butyl ether) is an oxygenate that comprises 11% of nearly all reformulated gasoline (RFG) in California; it is used in about 20% of gasoline nationwide. MTBE is used as a component of gasoline primarily for its contribution to reduced automotive carbon monoxide emissions. In addition, RFG is intended to decrease toxic emissions (especially benzene) from motor vehicles and to reduce ozone formation. During cold start or malfunctioning conditions, not all of the fuel burns in the engine�s high temperature flame zone, producing significant amounts of pure MTBE and its combustion byproducts in the emissions. The experimental component of this study uses a combustion-driven flow reactor with post-flame injection to determine the key combustion byproducts of different MTBE fuel mixtures: pure (chemical grade) MTBE, refinery grade MTBE (containing other oxygenates and contaminants), and a California Phase 2 RFG containing MTBE. Thus, this experimental work bridges existing data gaps by evaluating the combustion byproducts of pure MTBE and MTBE as a component of reformulated gasoline in a controlled laboratory setting. The formation of MTBE and reformulated fuel combustion byproducts are detected using Fourier Transform Infrared (FTIR) spectrographic analysis. The experimental reactor conditions are intended to simulate temperatures and fuel stoichiometry in an engine under "worst-case" operating conditions when the catalytic converter is not operating and complete combustion is not achieved, as in cases of cold engine starts or malfunctioning catalysts. This experimental work investigates the formation of MTBE byproduct compounds most likely to be formed from oxygenated fuels, particularly aldehydes, ketones, and alcohols.

Ongoing research

We are currently conducting experimental work in the laboratory. We have configured our flow reactor to provide the range of temperatures we would like to observe (roughly 500 to 800(C), and we are calibrating the FTIR for MTBE and its key byproducts. We have procured chemical grade MTBE as well as real-world samples of refinery grade MTBE, a refinery sample of California Phase 2 Reformulated gasoline with MTBE and a refinery sample of gasoline meeting the standards for California Phase 2 Reformulated gasoline that does not contain MTBE.

Literature research results

To date, limited studies have been conducted to determine the emission characteristics of MTBE and MTBE byproducts as a component of reformulated gasoline. This study examines existing data of MTBE combustion byproducts from three types of studies: reaction of pure MTBE under atmospheric conditions; reaction of pure MTBE at elevated temperatures; and vehicle studies using reformulated gasoline containing MTBE, including both dynamometer tests and actual on-road measurements. In particular, the extensive Auto/Oil collaborative research program examined the effects of MTBE in gasoline on speciated hydrocarbon emissions using dynamometer tests for different vehicle technologies. Analysis of these studies reveals that different key products are expected for MTBE reaction under atmospheric conditions than at elevated temperatures. Under ambient conditions, MTBE forms predominantly tert-butyl formate (TBF) and formaldehyde, with lesser amounts of methyl acetate and acetone as well as some trace species formed. However, at elevated temperatures, isobutene, methanol, and formaldehyde are the key reaction products expected to form. Laboratory studies indicate that catalysts are very effective in converting MTBE, and that no "surprise" compounds are formed over the catalyst. Vehicle and on-road studies with MTBE in gasoline confirm increased formaldehyde and unburned MTBE emissions from gasoline containing MTBE.