II. BACKGROUND
Catalytic emission control systems have been used on U.S. gasoline passenger cars since the 1975 model year - first oxidation catalysts (OC) and then, since the early 1980s, three-way conversion (TWC) catalysts. For over 30 years, the closed loop TWC system has dominated as the most efficient automobile exhaust emission control system. This system consists of front and rear oxygen sensors, a three-way catalytic converter, an electronic control unit (ECU), and a controllable fuel metering unit. The system provides very good fuel economy and engine performance, as well as the simultaneous control of hydrocarbons (HC), carbon monoxide (CO)