The automotive urea injector operates precisely in stages, relying on the SCR system to ultimately achieve exhaust gas purification. The specific workflow is as follows:

1. Pre-injection Pressurization Phase
After the vehicle starts, the system first opens the injection valve to purge air; subsequently, the urea pump draws urea solution to fill the entire pipeline. Once the pipeline is filled, the injection valve closes to begin building pressure. When the pressure reaches the preset value, the return valve opens to maintain constant pressure. Finally, a pre-injection test is performed to prepare for the actual injection.

2. Precise Injection Phase
Acting as the "control brain," the Urea Injection ECU calculates the volume of nitrogen oxides (NOx) generated based on real-time engine operating data. Simultaneously, the system verifies the injection conditions: the nozzle is controlled to inject atomized urea solution into the exhaust pipe in a pulsed manner only when the temperature at the inlet of the exhaust aftertreatment unit exceeds 180°C, no fault codes are present, the urea tank fluid level is above 6%, and the temperature inside the tank exceeds -3°C.

3. Exhaust Purification Reaction
Upon entering the high-temperature exhaust pipe, the atomized urea solution rapidly decomposes into ammonia gas and carbon dioxide. Inside the SCR catalytic converter, the ammonia gas undergoes a redox reaction with the nitrogen oxides present in the exhaust gas, ultimately converting them into harmless nitrogen gas and water, which are then discharged, thereby completing the exhaust purification process.

4. Purging and Shutdown Phase
After the engine is shut off and the ignition key is switched off, the system initiates a purging process: the DEF injection unit adjusts the position of the directional valve, keeps the pump running, and opens the injection valve to draw any residual urea solution remaining in the pipeline back into the urea tank. This entire process lasts approximately 60 seconds; it serves to prevent residual urea from crystallizing and clogging the pipelines, ensuring the system is ready for its next operation.