The California Low-Emission Vehicle (LEV) standards mandate a reduction in non-methane organic gases (NMOG). With the aim of analyzing NMOG emissions, a comparison was made of the hydrocarbon species found in the exhaust gas when different types of catalyst systems and fuel specifications were used. NMOG emissions are usually measured by removing methane from the total hydrocarbon (THC) emissions and adding aldehyde and ketone emissions. The NMOG level found in this way is thus influenced by the rate of methane in THC emissions. Another important factor in the LEV standards is specific reactivity (SR), indicating the formation potential of ozone, which is one cause of photochemical smog. Specific reactivity is expressed by the amount of ozone generated per unit weight of NMOG emissions, and is affected by the respective proportion of hydrocarbon species in the total NMOG emissions.
Focusing on these points, an examination was made of three catalyst systems: a three-way-catalyst (TWC), an electrically heated catalyst (EHC) and a hydrocarbon adsorption system. An analysis was made of the methane emission rate, specific reactivity and respective proportion of hydrocarbon species with each of these systems. Moreover, in order to examine the effects of fuel specifications, a comparison was made of reformulated gasoline, having reduced levels of aromatics and sulfur, a low T90 distillation property and containing a MTBE additive, with standard gasoline.
The experimental results indicate that each catalyst system had a different methane emission rate and specific reactivity, resulting from differences in the hydrocarbon species contained in the exhaust gases due to the different emission conversion mechanism involved in each case. The hydrocarbon adsorption system in particular tended to show higher specific reactivity, which is mainly attributed to the effect of the adsorption materials used. The reformulated gasoline tested resulted in lower NMOG emissions than the standard gasoline, but did not reduce specific reactivity. An analysis was also made of the reasons for these characteristics.