Filter for cleaning exhaust gases of diesel engines
This is a division of application Ser. No. 264,265 filed Oct. 28, 1988. 
    
    
     The present invention relates to a filter consisting of hollow cylindrical open-pore ceramic foam cylinder elements and an end plate made of gas-impermeable material, as an insert in a filter cup for cleaning exhaust gases of diesel engines, in which the exhaust gases flow through the active filter section essentially perpendicularly to the cylinder axis. 
     Filters of the aforementioned kind are well known. 
     When fitting the hollow cylindrical open-pore ceramic foam cylinder elements in these filters, it turned out that absolute surface evenness and plane-parallelism of the end faces of the ceramic elements is necessary, as otherwise any bending moments which occur lead to breaking of the ceramic foam before reaching the necessary initial axial mechanical tension. The initial mechanical tension is usually produced while the filter is in use by the thermal expansion of the expanding mats disposed between the ceramic foam cylinder elements, or further intensified by using conventional mechanical tensioning means. 
     The necessary surface evenness and plane parallelism of the two end faces of a ceramic foam cylinder element is not achieved by conventional sinter burning of the ceramic foam. Various possible solutions are available for meeting the requirements made of the ceramic foam cylinder elements, for example, regrinding or elastic post-deformation by afterburning with simultaneous plane-parallel weighting of the end faces of the ceramic foam cylinder element. 
     Regrinding has the disadvantage that the delicate ceramic webs break off at least partially, or completely break up, during the grinding process, so that filter efficiency in this region is impaired. Afterburning in the manner just mentioned is not acceptable on account of the high expenditure and associated costs. 
     In view of the circumstances, the inventors set themselves the object of providing a filter of the kind mentioned hereinbefore, in which ceramic foam cylinder elements with level and plane-parallel end faces are used, so that bending moments do not arise, and to specify a method which allows manufacture of filters of this kind. 
     According to the invention, the foregoing object is achieved by a filter consisting of hollow cylindrical open-pore ceramic foam cylinder elements and an end plate made of gas-impermeable material, as an insert in a filter cup for cleaning exhaust gases of diesel engines, in which the exhaust gases flow through the active filter section essentially perpendicularly to the cylinder axis, characterized in that on the end faces of the ceramic foam cylinder elements are located temperature-resistant and thermal shock-resistant layers of material which are plastic at room temperature. Advantageous developments of the filter according to the invention are characterised by the layers of material consisting of refractory cement mortar and the layers of material being impermeable to exhaust gases. 
     Furthermore, the partial object relating to manufacture of a filter of the kind according to the invention is achieved by a method characterized in that before assembly of the ceramic foam cylinder elements, plastic layers of material are applied to the end faces thereof, levelled until plane-parallel, dried and burned at 1000° to 1300° C. and thus bonded to the material of the ceramic foam cylinder elements. Further embodiments of the method according to the invention are characterised by the layers of material being reground after burning. In addition, the layers of material may be applied to the unburned ceramic foam cylinder elements. In addition, the layers of material may be applied to the end faces of the ceramic cylinder elements which have already been strenghtened with intrinsic ceramics. 
    
    
     Further advantages, characteristics and details of the filter according to the invention are apparent from the drawings; these show schematically: 
     FIG. 1 a longitudinal section through a filter cup with filter according to the invention, and 
     FIG. 2 a longitudinal section through a ceramic foam cylinder element. 
    
    
     The filter 10 consists of a system of several relatively short ceramic foam cylinder elements 11 which are arranged axially in a filter cup 20 having an end plate 17 made of gas-impermeable material, an inlet side E and an outlet side A. Exhaust gases enter the inlet, flow in the direction of the arrows through the ceramic foam elements and exit the outlet. A ceramic foam cylinder element 11 comprises on each end face 110 a temperature-resistant and thermal shock-resistant layer of material 120 which is plastic at room temperature. This layer of material preferably consists of a refractory cement mortar and is impermeable to exhaust gases of diesel engines. 
     The end faces 110 can be prestrengthened by intrinsic ceramics, which has the particular advantage that the compression which holds the elements 11 essentially in the axial direction can be applied to a particularly high extent. 
     The ceramic foam cylinder elements 11 can be assembled directly into a filter after application and plane-parallel levelling of the layers of material, if necessary after an initial drying process, wherein during initial use of the filter for cleaning the exhaust gases of diesel engines, on account of the elevated temperature the layers of material solidify themselves. But is more advantageous to manufacture the filter by the method characterized in that before assembly of the ceramic foam cylinder elements 11, plastic layers of material 120 are applied to the end faces 110 thereof, levelled until plane-parallel, dried and burned at 1000° to 1300° C. and thus bonded to the material of the ceramic foam cylinder elements. In this way each individual ceramic foam cylinder element 11 can be checked before fitting in the filter, and if necessary the layers of material can be adjusted to the required surface evenness and plane parallelism by regrinding.