Exhaust purification device and method of calculating NOx mass adsorbed in lean NOx trap of exhaust purification device

A method of calculating a nitrogen oxide (NOx) mass adsorbed in a lean NOx trap (LNT) of an exhaust purification device, wherein the LNT is mounted at an exhaust pipe, and adsorbs NOx contained in an exhaust gas at a lean air/fuel ratio, releases the adsorbed NOx at a rich air/fuel ratio, and reduces the NOx contained in the exhaust gas or the released NOx using reductant including carbon or hydrogen contained in the exhaust gas, and wherein the method is performed by a controller to control adsorption and release of the NOx by controlling an air/fuel ratio according to the NOx mass adsorbed in the LNT and a temperature of the exhaust gas, the method comprising:

• calculating a NOx mass flow stored in the LNT;
• calculating a NOx mass flow thermally released from the LNT;
• calculating a NOx mass flow released from the LNT at the rich air/fuel ratio;
• calculating a NOx mass flow chemically reacting with the reductant at the LNT;
• and
• integrating a value obtained by subtracting the NOx mass flow thermally released from the LNT, the NOx mass flow released from the LNT at the rich air/fuel ratio, and the NOx mass flow chemically reacting with the reductant at the LNT from the NOx mass flow stored in the LNT,
• wherein the step of calculating a NOx mass flow stored in the LNT comprises:
• calculating a NOx mass flow flowing into the LNT;
• calculating a NOx storing efficiency of the LNT;
• and
• calculating a NOx mass flow adsorbed in the LNT by multiplying the NOx mass flow flowing into the LNT and the NOx storing efficiency of the LNT, and wherein a NOx storing efficiency at the rich air/fuel ratio and a NOx storing efficiency at the lean air/fuel ratio are calculated respectively at the step of calculating a NOx storing efficiency of the LNT.