Selective catalytic reduction system

Abstract

A selective catalytic reduction system includes a urea aqueous solution tank to retain a urea aqueous solution to be injected into an exhaust pipe of an engine, a level sensor to detect a solution level height in the tank, a lowpass filter having a signal response characteristic in which a time constant is small when an engine rpm is low and the time constant is larger when the engine rpm is high, with respect to an output signal of the sensor, a remaining amount indicator to indicate a remaining amount of the solution based on an output signal of the sensor processed by the lowpass filter, and a unit to prohibit the engine from being started when the remaining amount of the solution is less than a lower limit value.

1 . An SCR system, comprising:
 a urea aqueous solution tank configured to retain a urea aqueous solution to be injected into an exhaust pipe of an engine;   a level sensor configured to detect a solution level height in the urea aqueous solution tank;   a lowpass filter having a signal response characteristic in which a time constant is small when an engine rpm is low and the time constant is larger when the engine rpm is high, with respect to an output signal of the level sensor;   a remaining amount indicator configured to indicate a remaining amount of the urea aqueous solution based on an output signal of the level sensor processed by the lowpass filter; and   a start prohibiting unit in deficiency of the urea aqueous solution configured to prohibit the engine from being started when the remaining amount of the urea aqueous solution is less than a lowerlimit value.   
     
     
         2 . The SCR system according to  claim 1 , wherein the time constant of the lowpass filter is 0 in the range of the engine rpm of 0 to a cranking rpm and increases with the increase in the engine rpm over the cranking rpm.