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Although the control of nitrogen oxides from automobile exhaust is not yet required by law, several methods have been investigated to determine how much control is possible and what penalties result. These methods involved spark timing, carburetion, and exhaust recirculation. On one test car, a control system was developed which reduced nitrogen oxides 74% while maintaining hydrocarbon and carbon monoxide control at current (1968) federal levels. However, a 13% loss in fuel economy was incurred, and driveability problems were observed. Application to other car models, deposit accumulation, durability, and performance under extreme weather conditions are all open questions at this time.

A manganese fuel additive, MMT, is now being used in many unleaded gasolines to improve their octane quality. Use of MMT at concentrations up to 0.033 g Mn/ℓ (0.125 g Mn/gal) is expected to increase. To determine the effect of MMT on exhaust emission control systems, five cars were tested for 80 000 km (50,000 miles) using a driving schedule which included 113 km/h (70 mph) steady speed driving. In this type of operation, use of MMT caused: plugging of monolithic converters located close to the exhaust manifold; partial plugging of an underfloor bead converter; an increase of hydrocarbon emissions from the engines; and excessive spark plug deposits. However, use of MMT apparently enhanced catalytic converter oxidizing activity and did not substantially affect octane requirement increase. These preliminary data suggest that use of MMT in commercial gasolines may cause problems with exhaust emission control systems now in use.