Continuously Regenerating Trap (CRT) System

The principle of CRT is based on the fact that is a much superior oxidizing agent for soot than the molecular oxygen.   oxidizes the dry carbon soot trapped in the filter below 300º C by the following reactions:

(6.12)

Although the trap substrate can be coated with a catalyst material to reduce soot oxidation temperatures to as low as 200º C, but  installation of  an oxidation catalyst upstream of particulate filter where NO is preferentially converted to  which then oxidizes the soot has been found more effective.  The catalysts used are noble metals. The oxidation catalyst is a flow through ceramic monolith using Pt-Pd catalyst impregnated on Al₂O₃ washcoat.
The schematic of a CRT is shown in Fig. 6.18.  is produced by oxidation of NO upstream of DPF. The soot trapped in the downstream DPF is continuously oxidized on the filter substrate by thus keeping the particulate filter essentially clean and the exhaust backpressure remains nearly unchanged.