Exhaust gas purification system for a lean burn engine

An exhaust gas purification system for an engine equipped with a turbocharger (20) and an exhaust pipe having a NOx conversion catalyst (40) therein. The system comprises a reservoir (30) for storing pressurised air received from the turbocharger, a reductant injector (42) in fluid communication with a reductant supply (44), and a mixing chamber (36) in fluid communication with the reservoir (30) and connected to the reductant injector (42). The reductant injector (42) is responsive to an injection signal for atomising and injecting a quantity of reductant into the mixing chamber (36). The mixing chamber (36) is in fluid communication with the exhaust pipe for introducing a quantity of air and reductant mixture into the exhaust gas flow upstream of the NOx conversion catalyst (40).