NOx emission reduction from a CNG-fueled vehicle
Taylor, John



     Abstract: Based on the results of the study, it was clear that at ultra-low emission levels, very slight variances in engine operating parameters and test conditions can greatly affect the emission levels that can be achieved. It was also evident that high catalytic activity was essential for achieving very low emissions. With new catalysts in place, it was shown that extremely low (less than 40% of the ULEV values) regulated emission levels could be realized. The use of increased exhaust gas re-circulation rates also produced NOx emissions of less than one-fourth the ULEV specified values. Toxic exhaust emissions with the CNG fuel were also significantly less than those associated with the use of reformulated gasoline. The study also reinforced that careful control of mixture stoichiometry, spark timing and exhaust gas re-circulation rates is required to achieve low emissions. It is also important to reiterate that the use of a close-coupled catalyst in conjunction with the main catalyst formulated specifically for natural gas was required to achieve the reported results.

     At very low exhaust emission rates achieved, factors that may not play important roles in meeting less stringent emission standards start to become important. Since fuel rail pressure is an important variable affecting open-loop (start-up) emissions, it is important to have a pressure regulator that maintains consistent fuel rail pressure regardless of tank pressure. Variation in air and fuel distribution to the engine cylinders was also identified as significant variables.

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