EFFECT OF FUEL STRUCTURE ON EMISSIONS FROM A SPARK-IGNITED ENGINE .2. NAPHTHENE AND AROMATIC FUELS

A single-cylinder, production-type engine has been run at four operating conditions on two naphthene fuels (cyclohexane and methylcyclohexane), an aromatic blend (containing xylenes and ethylbenzene), and a fully blended gasoline. Engine-out hydrocarbon (HC) emissions for the naphthenes contain 12% 1,3-butadiene and 5% benzene. This represents the first observation of substantial benzene emission from a nonaromatic fuel. Butadiene emissions are large in comparison to the gasoline blend, and caution should be exercised when adding large quantities of the cyclohexanes to a fully blended fuel if butadiene emission is a concern. With the exception of benzene, the HC species emitted can be rationalized on the basis of the known high-temperature chemistry of cyclohexane. The aromatic blend produces significant quantities of both benzene (2-5%) and toluene (3-6%) in the engine-out emissions. This contributes to the observed enrichment (relative to the fuel composition) of these species in the exhaust of engines operating on gasolines containing heavier alkylated benzenes.