The fourth national emission standard is the fourth national emission standard for motor vehicles. The pollutants emitted by automobiles mainly include HC (hydrocarbons), nitrogen oxides (nitrogen oxides), CO (carbon monoxide) and PM (particulate matter). Through the application of better catalytic converter active layer, secondary air injection and exhaust gas recirculation system with cooling device, the pollutants emitted by vehicles can be controlled and reduced to below the specified value.