Methyl tertiary--butyl ether, evaluation as a high octane blending component for unleaded gasoline

A study was conducted to evaluate methyl tertiary-butyl ether (MTBE) as a high octane number blending component for automotive gasoline. Road octane ratings with a 1976 model year (MY) Buick confirmed the octane increases indicated by Research and Motor octane number determinations. The blend of 15% MTBE with 86 RON gasoline had a road octane number equivalent to that of a 91 RON commercial gasoline. Again the octane improving effect of the MTBE was most pronounced for low octane base fuels. Exhaust emissions and fuel economy were measured with gasoline (clear Indolene) and blends of Indolene and 5, 10, and 15% MTBE. A 1978 MY Pontiac LeMans calibrated for operation with gasoline was tested according to the 1975 Federal Test Procedure (FTP). As the fraction of MTBE in the blend was increased, the carbon monoxide emissions decreased substantially; and the emissions of oxides of nitrogen increased somewhat, but not enough to cause the vehicle to exceed the emission standards for which it was designed. Furthermore, poorer driveability was experienced during the FTP with the MTBE-gasoline blends. Most of the changes were attributed to leaner engine operation caused by the altered stoichiometry of the MTBE blends. Hydrocarbon emissions and volumetric fuelmore » economy remained essentially constant despite the stoichiometry differences and the reduced energy content of the blends. The results of this study indicate that MTBE is a high octane blending component that will improve the road octane rating of unleaded gasolines. Although no substantial advantage or disadvantage of 15% MTBE-gasoline blends was found in single-cylinder engine comparisons at the same equivalence ratio, the slightly altered fuel stoichiometry and energy content of blends can cause moderate changes in the behavior of vehicles calibrated for gasolines. Generaly, the changes were those associated with leaner operation of the engine.« less