includes a total of three cleaning units 13 13 u2032 of these as in the configuration of fig1 and 2 described previously cleaning unit 13 is arranged opposite the aperture for exhaust gas inlet 11 but cleaning unit 13 is constructed as an integral part of housing 10 in this case housing 10 serves as floor 17 of reflection means 15 the bottom curvature of bowl shaped reflection means 15 is also replaced by the existing curvature of the housing see bottom left corner of housing 10 only the top curvature of reflection means 14 must be provided as an extra part in housing 10 this extra part may be permanently secured to the housing wall as a curved panel member this provision not only enables the amount of materials used to be reduced it also provides a means for improved heat dissipation over the cool surface of the housing to this end floor 17 may also be corrugated so that the surface and therewith the heat dissipation is increased additionally a corrugated or relief shaped floor 17 prevents reflected exhaust gases 22 from being forced through catalytic element 14 in varying volumes in addition the two further cleaning units 13 u2032 are arranged on the housing wall above and below the aperture for exhaust gas inlet 11 opposite cleaning unit 13 these further cleaning units 13 u2032 further increase the number of.