Johnson Matthey is a leading supplier of retrofit diesel emissions control systems for non-road machinery.

Passive Systems

Any filter has a finite capacity. Diesel particulate filters must be cleaned out, intermittently or continuously, if they are not to block. This is very important, as an overfilled filter can damage the engine through excessive exhaust backpressure and can itself be damaged or destroyed.

The material trapped in the filter is mostly carbon (C) particles with some absorbed hydrocarbons. There are two principle techniques for removing the particles:

1. combustion with oxygen (O2) - [C] + O2 CO2

2. combustion with nitrogen dioxide (NO2) - [C] + NO2 CO2 + NO

 The merits of the two different techniques can be summarised as follows:


O2-based regeneration

NO2-based regeneration

Temperature required
for regeneration

Requires temperatures of around 600C
(or 400C with a fuel-borne catalyst).

Reaction occurs from 250C.

Gas used in regeneration

O2 abundant in exhaust gas stream

NO2 needs to be made from
NO in exhaust stream.


NO2-based systems are commonly used because the reaction takes place at temperatures seen in most diesel exhausts. The leading DPF system around the world is Johnson Mattheys CRT® (Continuously Regenerating Trap) technology, which uses an oxidation catalyst in front of the filter to generate the NO2 required to keep it clean.
DPF systems that are able to regenerate themselves using only the exhaust gas stream, without additional energy inputs, are known as passive systems. The CRT® system is able to function in a wider range of conditions than any other passive system and this is the basis of its success. Its advantage is most clearly seen in applications with low exhaust temperatures, an advantage further enhanced in the CCRT® system.

Passive systems are favoured, particularly for retrofit applications, because they require no integration with the engine, no source of energy other than the exhaust gases themselves, and no complicated control systems.

Applications with variable or unreliable duty cycles, or with exhaust conditions not suitable for passive systems, require active regeneration. These are discussed in the active systems section.

© Johnson Matthey 2016