exit 58 in fig1 is a catalytic conversion element 68 surrounded by an insulator 70 element 68 is a cylindrical porous material designed to promote combustion of combustible exhaust gasses such materials are well known in the art of exhaust system design and a suitable material is available from allied signal as their part number 38972 element 68 has a reasonable porosity and size at 600 cells per square inch 3 0 inches in diameter and 2 6 inches in length to perform its intended function without creating excessive exhaust back pressure insulator 70 is a rolled sheet of vermiculite having nominal uncrushed thickness of about 5 millimeters together catalytic conversion element 68 and insulator 70 completely span exhaust exit 58 such that all exhaust gas entering manifold 54 is forced to flow through element 68 before exiting the manifold by disposing the conversion catalyst within the manifold itself relatively close to the exhaust source the high temperatures developed by secondary combustion are safely contained within a liquid cooled housing so as to not present any exposed high temperature surfaces as shown in fig1 a major length of catalytic element 68 is substantially surrounded by coolant passage 64 although not specifically illustrated it should be understood from the above disclosure that another advantageous arrangement is to house an appropriately sized catalytic conversion element such as element 68 within a manifold not adapted to circulate cooling fluid and then surrounding the manifold with a secondary cooling jacket such as that shown in fig1 6 it should also be understood that manifold 54 may be modified to provide the coolant exit coaxially with the exhaust exit such that the exiting coolant flows directly into an injection elbow or.