Patent ID: 8074444

Claim:
An exhaust emission control device for decreasing nitrogen oxide contained in exhaust gas, comprising: an additive valve that discharges a reducing agent into the exhaust gas; and a control unit that sets a valve opening period of time and a valve closing period of time so as to alternately elapse in a repetition cycle of the valve opening periods and controls the additive valve to discharge the reducing agent during each valve opening period and to stop discharging the reducing agent during each valve closing period, wherein the control unit fixes the valve opening period at a first value when a flow rate of the exhaust gas is lower than a threshold value, the control unit sets the valve opening period at a second value, which is higher than the first value and depends on the flow rate of the exhaust gas and flow rate of the nitrogen oxide so as to shorten the valve opening period as the flow rate of the exhaust gas is lowered, when the flow rate of the exhaust gas is higher than the threshold value, the control unit sets the repetition cycle at a third value depending on the flow rate of the nitrogen oxide when the flow rate of the exhaust gas is lower than the threshold value, and the control unit fixes the repetition cycle at a fourth value when the flow rate of the exhaust gas is higher than the threshold value.