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Throughout the world, governments are promulgating regulations that are intended to improve air quality. Some of these regulations affect the physical and chemical properties of gasoline. Consequently, refiners are under increasing pressure to reformulate their gasoline to be lower emitting when handled and combusted. These regulatory actions have also greatly reduced flexibility in the fuel formulation process. In many cases, refiners are attempting to reduce gasoline vapor pressure, sulfur, aromatic, and olefin content while simultaneously tightening distillation characteristics by removing butane and reducing the use of heavy reformate and FCC fractions. Because butane, aromatics and olefins can contribute substantially to pool octane levels, blending clean-burning gasoline with the required octane rating for acceptable vehicle performance can be difficult.

The Clean Air Act of 1990 requires on-board diagnostics (OED) capabilities on all new vehicles. These diagnostic systems monitor the performance of engine and emission system components and inform the vehicle operator when component or system degradation could significantly impact emissions. Acceptable operation of the monitor requires proper treatment of system variables. Fuel composition is one of many possible variables that must be considered for monitoring components directly in the exhaust stream. Recently, the octane enhancing, emissions reducing additive methylcyclopentadienyl manganese tricarbonyl (MMT) was reintroduced into unleaded gasoline in the U.S. Prior to reintroduction, the additive underwent extensive testing to demonstrate that use of MMT does not adversely affect vehicle emissions or the operation of emission systems such as OBD. However, questions have been raised about the influence of the additive on OBD systems.

Extensive vehicle fleet testing has demonstrated that use of MMT can reduce net tailpipe out emissions. The use of fuel containing the octane-enhancing, emission-reducing fuel additive leads to manganese oxide deposits in the vehicle exhaust system. Studies of the physical and chemical effects of manganese oxide deposits on the performance of catalytic converters conclusively demonstrated that MMT does not adversely affect catalytic converters and, in fact, protected the converters from phosphorus and zinc. Despite the overwhelming evidence that MMT is compatible with catalytic converters and vehicle emission control systems, concerns have recently been raised about the effect of manganese oxides on OBD-II catalytic converter monitoring.