Internal combustion engines, more especially diesel engines, produce considerable quantities of soot, especially when they are under load. This is particularly noticeable with motor vehicles. For this reason, various attempts have already been made to filter out the soot from the exhaust gas and to regenerate the filter by burning the soot. This burning process may be effected during the running of a diesel engine, since it is driven with an excess amount of oxygen, so that there is always sufficient oxygen still available in the exhaust gas for the burning process. Under particular operating conditions, it is also possible for the temperature of the exhaust gas to be so high that the burning of a layer of soot on the filter is automatically induced before the filter becomes congested. Such operating conditions are, however, often undesirable and are also dependent on chance.
As a consquence thereof, there has been a tendency to induce the burning of the layer of soot on the exhaust gas filter on purpose, especially when the flow resistance of the filter has risen to a predetermined level. In particular, reference is made to German patent publication No. 30 07 642 where such a filter is described; German patent publication No. 30 24 539 which is concerned with inducing burning by means of microwaves; and German patent publication No. 31 11 228 which discloses how to reduce the ignition temperature by using copper (I) chloride as a soot remover having catalytic properties. In addition, in U.K. Patent Application No. 2145349A which is not a prior publication, it is proposed to add ammonium nitrate as well as copper (I) chloride to the soot which has been trapped on the filter, so that the temperature for inducing the burning process can be reduced even further, i.e. to an exhaust gas temperature of approximately 280.degree. C. However, even this temperature is often still too high. It was also apparent that copper (I) chloride, which is precipitated as copper oxide on the fibrous filter, may result in a reduced permeability of the filter after a relatively long period of operation.