System and method of purifying exhaust gas

A system of purifying exhaust gas comprising:

• an engine including an injector for injecting fuel thereinto, generating power by burning mixture of air and the fuel, and exhausting the exhaust gas generated at combustion process to an exterior thereof through an exhaust pipe;
• a lean NOx trap (LNT) mounted on the exhaust pipe, and adapted to absorb nitrogen oxide (NOx) contained in the exhaust gas at a lean air/fuel ratio, to release the absorbed nitrogen oxide at a rich air/fuel ratio, and to reduce the nitrogen oxide contained in the exhaust gas or the released nitrogen oxide;
• a dosing module mounted on the exhaust pipe and adapted to inject a reducing agent into the exhaust gas;
• a selective catalytic reduction catalyst on a diesel particulate filter (SDPF) mounted on the exhaust pipe downstream of the dosing module and adapted to trap particulate matter contained in the exhaust gas and to reduce the nitrogen oxide contained in the exhaust gas using the reducing agent injected through the dosing module;
• and
• a controller performing denitrification (DeNOx) using the LNT when temperature of the exhaust gas is lower than transient temperature, and performing denitrification using the SDPF when the temperature of the exhaust gas is higher than or equal to the transient temperature.