Solid particle filters for engines can be made of cellular ceramic such as cordierite extruded in the form of a honeycomb monolith, impregnated or not with a catalyst. Every other cell in such honeycomb monoliths is blocked at one end. The blocked ends alternate from one cell to the next contiguous cell so that the gas to be filtered cannot flow directly through a given cell, but rather is forced to go through the wall of the cell.
Other types of filters, using metal fibers, covered or not with alumina, or comprising balls, etc., are also known.
The result of the first of the above arrangements is that the solid particles in suspension in the gas arriving at the filter are trapped in the cells. The trapping of the solid particles in the cells finally clogs them, causing an increasing pressure drop across the filter and thus a significant loss of engine power.
Therefore it is periodically necessary to regenerate the filter by combustion of said trapped particles, essentially consisting of carbon, in the form of a thermal energy input sufficient to start their combustion. However, this input must be controlled. Otherwise, the combustion of the solid particles can lead to too fast a rise in the temperature of the ceramic and damage its structure by local fusion.
Let us point out that combustion of these particles begins to occur at around 550.degree. C. in the case of a filter not impregnated with a catalyst and on the order of 450.degree. C. in the case of a filter impregnated with a catalyst.
Regeneration devices of various types for such filters are already known.
In the case of catalytic filters, injection at a given moment of a certain amount of gas oil in one of the engine cylinders causes partial cracking of this fuel into gaseous hydrocarbons. The gaseous hydrocarbons, with the excess oxygen present in the fuel gases, burn on the catalytic surfaces, producing the amount of heat necessary to start combustion of the trapped particles.
For noncatalytic filters there are other devices such as gas oil burners whose outside combustion air intake is performed by the action of an additional pump placed in the vicinity of the container.
In the various cases cited, the drawbacks are many: momentary and bothersome disturbances of operation of the engine, risk of formation of carbon particles coming from incomplete cracking or nonignition of the fuel burner, need to have an air pump, etc.