Aluminium Piston Developments.
Page 36
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WHILST THE use of aluminium for pistons, in place of cast-iron, • , has made great progress for pleagure cars, cast-iron is still preferred by many commercial-vehicle designers. Presumably, it is felt that aluminium is not required for a large slow-speed engine, but this view is hardly justified by the facts of the case.
The lightness of an aluminium piston is, of course' an important feature, and one which leads to reduced bearing wear, but the excellent heat conductivity of the lighter metal is of equal value. Heat is conducted away from the head very rapidly, with the result that carbonization is reduced and the consumption of oil thereby lessened.
The composition of the aluminium used for pistons requires considerable thought and , research, because, with many alloys, the co-efficient of expansion is so high that a large clearance is necessary. This leads to noisy running and loss of compression when the engme is cold. One of the most successful alloys is that used by Specialloid, Ltd.; the expansion of which is very nearly the same as that of cast-iron. Diirability is another important feature of this metal.
Specialloid, Ltd., have recently moved to new and enlarged, works at Friern Park, Finchley, N.I2, where the plant is being planned for an output of 10,000 pistons per week. A feature of the works is the fact that they are entirely self-contained, a suction. plant and gas engine supplying electricity for lighting, gas and compressed air for the foundry furr aces, and power for the machine shops.
The smooth finish of the pistons is obtained entirely by tooling, grinding being considered unadvisable because it is found impossible to prevent fine particles of abrasive from becoming em bedded in the aluminitam As a result, Fiston which has been ground serves as a tap when fitted to an engine and is liable to produce very rapid cylinder wear.
A very efficient design is used in these pistons. Eight ribs are formed in the casting between the head, skirt, and gudgeon-pin bosses, these serving to strengthen the piston and to provide an additional thickness of metal for the conductance of heat from the piston crown. A wide, shallow groove is formed below the lowest ring in which oil can collect, the surplus being scraped off and returned to the inside of the piston through holes drilled round the periphery.
Twn such holes lead to the gudgeonpin bosses, conveying oil to the pin, which floats both an the small end of the rod and in the piston bosses. It is located endwise by two aluminium caps. The hardened steel pins are also produced at the Friern Park Works, and every piston sent out is delivered complete with_ pin and rings.
In addition to having obtained a very enviable reputation in the hands of many private owners who have need them, Specialloid pistons have been sub mated to exhaustive tests by several concerns employing large numbers of commercial vehicles with very striking results. Some of the results of an irfvestigation now being carried out on a large scale will be available for publication in the course of a few months.