Composite pistons consist of:
1) An alloy steel crown containing chromium and molybdenum to maintain strength and resist wear at the high temperatures reached especially when burning residual fuels.
2) A skirt of either cast iron or aluminium alloy. Cast iron has self lubricating properties and an expansion rate similar to that of the liner, thus it can be used through a variable load range. However, due to its mass, it has a greater inertia effect than aluminium alloy, which is light, and surface hardens to give good wear resistance. However aluminium alloy has a much larger coefficient of expansion therefore clearances must be made larger. This can lead to piston slap at low loads.
Neither Cast Iron or aluminium alloy can be used for the whole piston - ie skirt and crown - when residual fuels are to be burnt. This is because residual fuel burns at a much higher temperature and the material would become considerably weaker at these high temperatures leading to erosion by the hot gases and corrosion from deposits of vanadium and sodium.
