NOTES ON ALUMINIUM ALLOY PISTONS.
By a TECHNICAL CORRESPONDENT.
NOTWITHSTANDING the fact that sporting motorists, devoted to car or motor cycle speed work respectively, are unanimous as to the theoretical advantages offered by the employment of light pistons constructed in aluminium alloys, it may be said that the application of such alloys to high speed internal engines presents several problems to the designer and manufacturer alike, to say nothing of the racing enthusiast, who sometimes unintentionally undertakes the duties of experimental research.
Scientists tell us that aluminium is the most abundant metal in the crust of the earth, but it was not until the early eighties methods were devised for purifying the natural ore in sufficient quantities and at such a cost as to permit the use of aluminium for commercial and industrial purposes.
Immediately the latter possibilities became generally recognised, two groups of potential users developed, one crediting aluminium with properties it does not possess, and the other, disappointed as the result of the reckless use of the material, was afraid to continue research operations, save on an extremely limited basis.
One of the main objects of the designer of sporting machines is to combine lightness with adequate strength and therefore recent researches include the study of old and new alloys, improved methods of production, and especially careful investigations concerning the parts lending themselves to construction in aluminium alloys.
Physical Properties of Aluminium.
When first produced commercially, aluminium suffered the great disadvantage of its lack of strength, for in the pure state the tensile strength of aluminium is only 13,000 lbs. per square inch. This factor naturally led to the introduction of other metals as alloys, copper being used extensively for the purpose, but in the early stages of use, the tensile strength was a more or less uncertain factor. According to the S.A.E. specification No. 30, the total impurities shall not exceed 1.7 per cent., of which not over 0.2 per cent. shall be zinc and no other impurities than carbon, silicon, zinc, iron or manganese are permitted. The actual specification is given as follows : Aluminium, not less than … 90.00% Copper … 8.50 to 7.00%1
The above is one of the lightest aluminium alloys, possessing a high degree of strength and can be used where a light tough alloy of these characteristics is required in automobile construction. For a number of years the tensile strength of alloys similar to the above was given as 20,000 lbs. per square inch, but though obtained in standard test bars, the figure had to be lowered to 18,000 lbs. per square inch, to allow a reasonable margin for commercial production. Tensile strength, however, is not the only physical property of moment in automobile calculations ; and, • • • • •
when used constructionally, aluminium must have rigidity. This depends upon the modulus of elasticity which in the case of most aluminium alloys is approximately io,000,000 as compared with 20,000,000 in the case of cast iron. To continue the comparison, it follows that if two beams were constructed in cast iron and aluminium respectively, both having the same dimensions, it would take twice the load to deflect the former than would . be necessary to affect the latter in an equal degree. At first sight, it would appear futile to compare the two materials for construction purposes, but we must recollect that our iron beam weighs approximately two and a half times more than the one made of aluminium, so that actually the latter can be made two and
a half deeper than the former.
In automobile practice this feature becomes extremely valuable ; for with correct distribution of metal and suitable webbing aluminium parts have a decided advantage. A further advantage of aluminium consists in its ductility, which in cast iron is very low, being roughly I per cent. The ordinary casting aluminium alloys have an elongation factor of about per cent., whilst some run as high as 5 per cent, without any reduction in tensile strength.
For such components as automobile pistons, strength, rigidity and ductility are not the only requirements in a material of construction, for one of the duties of the piston is to waste as easily and efficiently as possible the heat in combustion chambers which cannot be converted into power. It might be supposed, therefore, that with a piston made from an alloy with a high heat conductivity factor, considerable loss of power would result. As a matter of fact the surface resistance to heat is approximately the same for all metals and the heat flow into the piston is practically determined by the exposed area of the metal. As soon as the piston has absorbed a proportion of the combustion heat, high conductivity helps to dissipate or waste the unwanted degree of the heat as we have already explained. This is one of the arguments in favour of aluminium alloy pistons, which as a rule work cooler than those of cast iron, as appears to be emphasised by the decreased tendency to carbonising, both above and below the piston, when aluminium is used for its construction.
To meet the special needs for strength at high engine temperatures, special alloys are used for piston work, the one given above being more suitable for castings called upon to withstand ordinary stressing alone.
Practical Piston Problems.
From a thermal point of view aluminium pistons may be divided into two classes : firstly, those designed in such a way as to permit the dissipation of heat from the head to the skirt and thence to the cylinder walls ;
and secondly, those intended to partially insulate the skirt from the heat of the head.
In the former type the designs are usually of a conventional character except that thicker walls are used and internal ribs are adopted on the inside to assist heat conductivity. An aluminium piston of this class is shown in Fig. 1. The second type of piston is exemplified by the Zephyr and Ricardo slipper designs shown in Figs. 2 and 3 respectively, which are well-known among racing men.
The Zephyr, originally a steel piston, may be supplied as a combination of aluminium and steel or cast iron. Modified forms of the same design are made entirely in aluminium by other people.
The ” Specialloid ” Piston shown in Fig. 4 is claimed to be the only alloy piston which is able to conduct the heat fully to the skirt and still give no slap, and although for racing purposes slap is immaterial, the ordinary touring car of these days must not slap. For racing purposes with aluminium alloys of a fairly low melting point it is essential that the thermal conductivity is not curbed in any way by designs in which the heat is prevented from running freely to the lowest
extremity of the piston, otherwise burning of the head is inevitable on account of the extreme heat caused by the high compression ratios now used.
The melting point of the alloy used in the ” Specialloid ” Racing Piston is just over 700° cent. whereas the usual alloy of most other pistons is about 6300 cent.
Piston and Cylinder Wear. Wear is not so important in racing circles as in touring and commercial propositions. In the touring class there are distinctly two types of motor manufacturer: (I) the manufacturer who builds his car to sell and price is the main point, irrespective of quality ; (2) the manufacturer who builds his car to last.
Unfortunately neither the manufacturer nor the expert motorist fully realises that the piston is really the most important part of his car, and has, in fact, greater hardships to put up with in cases of ill treatment. Neither do these gentlemen realise that the making of an efficient, economical and sweet running engine depends to a great extent on the piston.
Certain objections have been raised from time to time in connection with the use of aluminium pistons, but, generally speaking, such objections are commonly encountered when cast iron is used and comprise : wear, tendency to piston slap, heavy oil consumption, and fuel dilution of crankcase oil. 1. Piston and Cylinder Wear.—The amount of wear taking place on the skirts of aluminium pistons al d the cylinder bores is largely a matter of original smoothness of finish. If aluminium pistons are fitted to an engine of which the cylinder bores are rough, one naturally expects a considerable amount of wear to follow. Alloys of this description will provide good wearing surfaces, but the character of the surfaces in contact determines the amount of wear that will subsequently occur. A smooth alinninitun piston fitted to a relatively rough cylinder bore is equivalent to adjusting a crankshaft bearing to an imperfect journal. As a matter of fact many cylinders taken direct from the grinding machine have decidedly rough bores ; which, though unimportant when cast iron pistons are used, becomes a very serious matter if aluminium alloys are employed. This accounts for the fact of the increased durability and efficiency
obtained from engines which, after having been used for some time with cast iron pistons, have subsequently had special aluminium ones made and fitted.
Whilst the general public hears little of the manufacturer’s difficulties, authentic instances could be quoted where lack of experience and imperfect production methods have led to considerable trouble with aluminium pistons, but usually the pistons are not to blame. The writer knows of. a case at the present time where experiments are being conducted with steel pistons, simply because the conditions necessary for the successful adoption of a good aluminium alloy cannot be reached.
Dust drawn into the engine with the mixture has a great influence on piston wear and in course of time racing men will probably realise the advantages of some efficient form of air-filtering device. It is at present receiving very considerable attention, and it is anticipated that the trouble will be entirely removed in the near future. 2. Piston Slap.—Piston slap is largely a matterof determining the correct amount of working clearance for the piston, though the practice of ” off-setting ” the gudgeon pin helps very considerably in this respect. Provided the piston and cylinder are of correct design and the piston is made of a suitable alloy, properly well balanced and free from local hot-spots, there should be no difficulty with piston slap. Many forms of piston with split skirt have been introduced from time to
time, but in the opinion of the writer this appears to
be tackling the problem from the wrong end. In the case of the split skirts, off-setting the gudgeon pin and sloping heads, considerable wear is experienced, and also considerable loss of power through friction at high revolutions. 3. Excessive Oil Consumption.—In cases where excessive quantities of oil are thrown up into the cylinder bores, light fitting pistons and special rings will not always cure the trouble entirely, but the follow
ing results from some tests carried out by the Chief Engineer of the Packard Company, Detroit, U.S.A., provide interesting data.
(I) With no control on the oil being thrown into the cylinder, rings which seal the top and bottom edge of the groove reduced the oil consumption.
(2) When the oil is properly controlled, the oil consumption is very low, even with rings having an up and down clearance of 0.004 in.
(3) With the oil controlled and hot water circulating through the engine the volume of liquid in the base chamber increased, indicating the dilution with fuel which passed the piston and rings. This was independent of the kind of rings used.
Subsequent experiments, with the fuel mixture heated to about 160 degrees Fahr., the dilution in the crankcase was diminished and the viscosity of the lubricant was not impaired.