AERO ENGINES - MATERIALS With the cutting down of weight, and the ever more severe conditions as regards temperature and compression ratio, the problem of design is bound up with that of materials.
The introduction of the light alloys of aluminium was the first great advance in regard to materials. It is now possible to make extremely complex castings and drop forgings for a complete crank-case in this material (see Pl. 1.-4), and, even before this had been successfully achieved, the intro duction of the aluminium piston had largely reduced the mass of reciprocating parts and so brought relief in crank-pin bearing loads due to inertia. The high conductivity, too, of aluminium as compared with cast-iron has made for cooler pistons and less tendency to carbonization of the lubricating oil. The same property has led to extensive use of aluminium for making the cylinder heads of air-cooled engines, and, in combination with a steel liner, for complete cylinder construction of water-cooled types. It has even been tried to run a steel or iron piston direct in an aluminium cylinder. The two critical points in regard to high temperatures in a high-duty engine are the centre of the piston, and the exhaust valve, if this is of the poppet type.
Alloys of magnesium with copper and zinc or aluminium are lighter than the aluminium alloys. They cast well and machine easily and have been used to some extent for crank-cases with a saving of weight of about 35%. Their disadvantages are their high cost, excessive liability to corrosion and poor elastic prop erties.
Exhaust valves are called upon to run continuously at a bright red heat and to resist scaling or burning under these severe conditions. The only steels which can successfully do this are certain of the nickel and chromium alloys, and these in their turn introduce new problems: they are much more troublesome to machine than carbon steels and also they develop what is known as "temper-brittleness," a condition brought about by the high temperatures at which they are used, and their subsequent slow cooling after use.
