Monitoring system for selective catalyst reduction system

A selective catalyst reduction system, comprising:

• a selective catalyst reduction apparatus that reacts with ammonia and reduces NOx;
• a first NOx sensor that is disposed upstream of the selective catalyst reduction apparatus and measures the amount of the NOx that flows into the selective catalyst reduction apparatus;
• a second NOx sensor that is disposed downstream of the selective catalyst reduction apparatus and measures the reduced amount of the NOx that flows out of the selective catalyst reduction apparatus;
• an injector that is disposed between the first NOx sensor and the selective catalyst reduction apparatus and injects a urea solution;
• and
• a control portion that calculates reduction efficiency based on NOx signals detected from the first NOx sensor and the second NOx sensor, determines the necessary amount of ammonia according to the reduction efficiency, and controls the determined injection amount of the urea solution through the injector;
• wherein the first NOx sensor and the second NOx sensor are mean value sensors respectively detecting a mean value of the NOx signals, wherein the difference between the amount of NOx measured by said second NOx sensor and a threshold amount of NOx for said system is calculated according to the following formula:
• Δyrear—Mean={(1−α)θthr−(1+α)θnor}xo−{θthr+θnor}β>0,
• in which
• α is the slope offset of the NOx sensor;
• β is the drift offset of the NOx sensor;
• θ is the slip rate of the NOx;
• xo is the NOx amount that is detected at the first sensor;
• yo:
• the NOx amount that is detected at the second sensor;
• ynor is the NOx amount that is measured at downstream of the normal selective catalyst reduction system;
• ythr is the threshold value of the NOx amount
• θnor is the slip rate of the NOx at normal condition;
• and
• θthr is the slip rate of the NOx at threshold condition,
• wherein a possible deviation of the first and second NOx sensors is within a predetermined range of ±6.7%.