Exhaust gas purification apparatus for engine

Abstract

There is provided an exhaust gas purification apparatus for an engine capable of effectively removing NOx in exhaust gas even when the temperature of exhaust gas is relatively low. The exhaust gas purification apparatus includes a selective reduction catalyst provided in an exhaust pipe, a liquid injection nozzle provided on the upstream side of the selective reduction catalyst, a liquid injecting means capable of injecting a urea-based liquid via the liquid injection nozzle, a controller, and a mixer. The apparatus further includes an ammonia purification catalyst provided on the downstream side of the selective reduction catalyst, a diesel particulate filter provided on the upstream side of the nozzle, an oxidation catalyst provided on the upstream side of the diesel particulate filter, and a temperature sensor for detecting the temperature of the selective reduction catalyst. A fuel injection device is configured so as to be capable of injecting fuel into a cylinder after the top dead center of piston, and when the temperature sensor detects that the selective reduction catalyst has a temperature not higher than a predetermined temperature, the controller controls the fuel injection device so that fuel is injected into the cylinder after the top dead center of piston.

1 . An exhaust gas purification apparatus comprising:
 a selective reduction catalyst provided in an exhaust pipe of a diesel engine driven by burning a fuel, which is injected from a fuel injection device, in a cylinder;   a liquid injection nozzle provided in the exhaust pipe on the exhaust gas upstream side of the selective reduction catalysts;   a liquid injecting means capable of injecting a urea-based liquid toward the selective reduction catalyst via the liquid injection nozzle;   a controller for controlling the liquid injecting means; and   a mixer which is provided in the exhaust pipe between the liquid injection nozzle and the selective reduction catalyst and is configured so that the liquid injected from the liquid injection nozzle is mixed with exhaust gas, characterized in that   the exhaust gas purification apparatus further comprises:   an ammonia purification catalyst provided in the exhaust pipe on the exhaust gas downstream side of the selective reduction catalyst;   a diesel particulate filter provided in the exhaust pipe on the exhaust gas upstream side of the liquid injection nozzle;   an oxidation catalyst provided in the exhaust pipe on the exhaust gas upstream side of the diesel particulate filter; and   a temperature sensor for detecting the temperature of the selective reduction catalyst and sending the detected temperature to the controllers.   
   
   
       2 . The exhaust gas purification apparatus according to  claim 1 , wherein
 the fuel injection device is configured so as to be capable of injecting the fuel into the cylinder after the top dead center of piston;   the temperature sensor is provided at the inlet of the selective reduction catalyst and is configured so as to be capable of measuring the temperature of exhaust gas flowing into the selective reduction catalyst; and   when the temperature sensor detects a temperature not higher than a predetermined temperature, the controller controls the fuel injection device so that the fuel is injected into the cylinder after the top dead center of piston.   
   
   
       3 . The exhaust gas purification apparatus according to  claim 2 , wherein a turbocharger is provided on the engine, and the oxidation catalyst consists of a first oxidation catalyst provided in the exhaust pipe just in rear of the turbocharger and a second oxidation catalyst provided in the exhaust pipe just in front of the diesel particulate filter. 
   
   
       4 . The exhaust gas purification apparatus according to  claim 2 , wherein
 the fuel injection device is configured so as to be capable of accomplishing both of after injection for injecting the fuel when the crank angle is in the range of 30 to 50 degrees after the top dead center of piston and post injection for injecting the fuel when the crank angle is in the range of 90 to 130 degrees after the top dead center of piston; and   when the temperature sensor detects that the selective reduction catalyst has a temperature not higher than the predetermined temperature, the controller controls the fuel injection device so that both of the after injection and the post injection are accomplished.