Selective catalytic reduction of nitrogen oxides is achieved through the combined use of a catalyst and a reductant. The most widely used reductant is a urea-water solution at a concentration of 32.5% or 40%. The urea is collected from a tank by a dosing pump triggered by an ‘engine run’ signal in combination with minimum and maximum exhaust gas temperatures. The temperature limits depend on the fuel quality and the chosen catalyst, and generally fall between 250 ºC and 525 ºC.