The functional principle of the metering unit (metering module): a urea flow of a certain pressure is supplied by the urea pump, sensed by the temperature and pressure sensor, and calculated and driven by the controller, the metering valve is opened at a certain frequency and pulse width, and the The command flow rate is precisely injected into the exhaust pipe and mixed with the engine exhaust. Under the action of the catalyst, a catalytic reduction reaction is produced to reduce the concentration of nitrogen oxides.