A new four-stroke carburetor motorcycle engine without any engine adjustments was used to study the impact of fuel aromatic content on exhaust emissions of criteria air pollutants (CO, THC, and NOx). Three aromatic fuels were tested, containing 15, 25, and 50% (vol) aromatics mixed with gasoline. A commercial unleaded gasoline was also tested as a reference case (RF). The experimental data indicated that a lower aromatic content (25 and 15 vol%) in gasoline reduced the amounts of THC and NOxby more than 10% compared to the reference fuel (aromatic content 30 vol%). CO emissions, on the other hand, did not appear to be related to the aromatic content of gasoline. The excess air ratio (λ) values for the aromatic test fuels were lower than 1.0, i.e., under fuel-rich conditions, and CO emissions increased due to lack of oxygen. In contrast, high NOxemissions appeared in a near stoichiometric air-fuel ratio, and decreased as the fuel mixture approached lean or rich conditions. The results also showed that decreasing the aromatic content from 50 to 25 and 15 vol% in gasoline may result in a reduction of benzene emissions from the motorcycles without a catalyst converter. This study shows that decreasing the aromatic content in gasoline may reduce the emissions of THC, NOx, and benzene, but not CO, from four-stroke carburetor motorcycles.
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