Engine exhaust emission control

A control device for an engine, the engine comprising a fuel supply mechanism which supplies main fuel to generate an air-fuel mixture for combustion, an exhaust passage which discharges exhaust gas generated by the combustion of the air-fuel mixture, a first catalyst disposed in the exhaust passage which stores oxygen contained in the exhaust gas and releases stored oxygen for oxidizing a reducing agent component contained in the exhaust gas, and a second catalyst disposed downstream of the first catalyst in the exhaust passage which stores nitrogen oxides in the exhaust gas and releases stored nitrogen oxides by reducing the stored nitrogen oxides by the reducing agent component contained in the exhaust gas, the control device comprising:

• a first means for detecting a running condition of the engine;
• a second means for calculating a storage amount of nitrogen oxides by the second catalyst based on the running condition;
• a third means for determining whether or not the storage amount is greater than a predetermined value;
• a fourth means for controlling the fuel supply mechanism to supply additional fuel to the engine after the combustion of the air-fuel mixture generated by the main fuel when the storage amount exceeds the predetermined value;
• a fifth means for determining, after a supply of the additional fuel to the engine has started, whether or not a predetermined condition is satisfied;
• and
• a sixth means controlling the fuel supply mechanism to stop supply of the additional fuel and to increase an amount of the main fuel when the predetermined condition is satisfied.

A control method for an engine, the engine comprising a fuel supply mechanism which supplies main fuel to generate an air-fuel mixture for combustion, an exhaust passage which discharges exhaust gas generated by the combustion of the air-fuel mixture, a first catalyst disposed in the exhaust passage which stores oxygen contained in the exhaust gas and releases stored oxygen for oxidizing a reducing agent component contained in the exhaust gas, and a second catalyst disposed downstream of the first catalyst in the exhaust passage which stores nitrogen oxides in the exhaust gas and releases stored nitrogen oxides by reducing the stored nitrogen oxides by the reducing agent component contained in the exhaust gas, the control method comprising:

• detecting a running condition of the engine;
• calculating a storage amount of nitrogen oxides by the second catalyst based on the running condition;
• determining whether or not the storage amount is greater than a predetermined value;
• controlling the fuel supply mechanism to supply additional fuel to the engine after the combustion of the air-fuel mixture generated by the main fuel when the storage amount exceeds the predetermined value;
• determining, after a supply of the additional fuel to the engine has started, whether or not a predetermined condition is satisfied;
• and
• controlling the fuel supply mechanism to stop supply of the additional fuel and to increase an amount of the main fuel when the predetermined condition is satisfied.