Urea injection system for an internal combustion engine

A urea injection system (10) for an internal combustion engine (25), comprising an injection nozzle (1) for injecting urea and air into an exhaust duct (8) of the internal combustion engine (25), a urea inlet (11) connectable to a urea source (12), a urea line (2) for supplying urea from the urea inlet (11) to the injection nozzle (1) and an air line (3) for supplying air to the injection nozzle (1). The system (10) further comprises a purging line (14) for supplying purging air to the urea line (2), which purging line (14) is provided with a purging valve (4) for controlling the flow of purging air from the purging line (14) to the urea line (2).