Abstract: A fuel composition, and a method and an apparatus for combusting it in a two-stroke engine, in which molybdenum introduced from the fuel will interact with alkaline earth metal originating from a detergent or other co-ingredient of the fuel in the combustion products to increase detergency without increasing the level of alkaline earth metal or compounds present in the fuel or combustion products thereof. In this manner, the engine operates more cleanly and efficiently without increasing the risk of harmful alkaline earth materials and compounds thereof blocking and poisoning catalysts, sensors and/or automotive on-board diagnostic devices, and it can lead to improved durability of exhaust after treatment systems.