Abstract: A method of operating a combustion apparatus such as an internal combustion engine is described, in which the apparatus includes at least one combustion chamber with an inlet port for primary combustion air, an apparatus to introduce into the combustion chamber primary fuel for combustion with the primary air, an exhaust port for combustion products, and an exhaust system for exhausting the combustion products to atmosphere, the method including introducing into the exhaust system secondary air, mechanically acting upon the secondary air and products of combustion in the exhaust system in the presence of a catalyst, to produce a reformed fuel, introducing the reformed fuel into the combustion chamber for combustion with primary fuel and primary air.