Exhaust emission control system of diesel engine

In an exhaust emission control system of a diesel engine, comprising an exhaust gas purifying device arranged in an exhaust gas passage extending from the engine, the exhaust gas purifying device having at least one of two functions, one function being a function of trapping particulate matter (PM) of the exhaust gas flowing in the exhaust gas passage and the other function being a function of trapping nitrogen oxides (NOx) in the exhaust gas when the exhaust gas shows a higher excess air ratio and reducing the trapped nitrogen oxides (NOx) to purify the same when the exhaust gas shows a lower excess air ratio;

• a three-way catalytic converter arranged in the exhaust gas passage upstream of the exhaust gas purifying device, the converter purifying hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas when the excess air ratio of the exhaust gas is approximately 1 (one);
• and a hydrocarbon adsorbing/releasing member arranged in the exhaust gas passage upstream of the three-way catalytic converter, the member selectively adsorbing thereon hydrocarbon in the exhaust gas and releasing the adsorbed hydrocarbon into the exhaust gas in accordance with a temperature of a catalyst bed of the three-way catalytic converter,
• a method of controlling the exhaust emission control system, upon need of reactivation of the exhaust gas purifying device under cold operation of the engine, comprising:
• lowering the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is lower than or equal to a first predetermined temperature;
• raising the excess air ratio of the exhaust gas when the temperature of the catalyst bed of the three-way catalytic converter is higher than the first predetermined temperature but lower than or equal to a second predetermined temperature;
• and
• lowering the excess air ratio of the exhaust gas until the temperature of the exhaust gas purifying device becomes to a third predetermined temperature when the temperature of the catalyst bed of the three-way catalytic converter is higher than the second predetermined temperature.