| 摘要 |
PROBLEM TO BE SOLVED: To provide a technology of injecting urea water to exhaust gas to form suitable ammonia distribution in a NOx catalyst.SOLUTION: An exhaust emission control system for an internal combustion engine includes: a selective reduction type NOx catalyst; an injection valve; a tank; a supply route formed to supply urea water from the tank to an urea water supply valve; a pump provided in the supply route to pressure-feed urea water to the urea water supply valve; a return route formed to return part of the urea water pressure-fed by the pump from a predetermined portion of the supply route downstream of the pump before the urea water is made to flow into the urea water supply valve to the tank; a return control valve for controlling a flow rate of the returned urea water in the return route; and control means for controlling an opening of the return control valve. Based on an exhaust flow rate in an exhaust passage and a temperature of the selective reduction type NOx catalyst, the control means controls an opening of the return control valve so that injection pressure of the injection valve reaches optimal injection pressure.SELECTED DRAWING: Figure 1 |
