Urea injection system for reducing emission of nitrogen oxide (NOx) of engine

The utility model discloses a urea injection system for reducing emission of nitrogen oxide (NOx) of an engine. The urea injection system mainly comprises a urea supply device, an injection device, an unfreezing device and a control device. The urea supply device comprises a urea straightway pump, a secondary filter, a first solenoid valve and a gas-liquid mixer. The urea straightway pump, the secondary filter, the first solenoid valve and the gas-liquid mixer are connected with a urea tank in sequence through first pipelines. Sprayer components of the injection device are used for injecting gas-liquid mixture to exhaust pipelines communicated with the upstream of a selective catalytic reduction (SCR) catalyst. Urea is extracted out from the urea tank and forms a certain pressure, and injection dose of the urea is automatically adjusted according to an operating condition of the engine. After atomized, the high pressured air is injected to the exhaust pipes of the engine from the sprayer to be reacted with the NOx exhausted from the engine on the SCR catalyst, and therefore the purpose of reducing emission of the NOx is achieved. The control device is in communication with a controller of the engine to coordinate normal operation of the whole system, and monitor working condition of the system and components.