ALUMINIUM versus CAST-IRON FOR PISTONS.
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Some Reasons for the Growing Popularity of the Lighter Metal, Despite the Fact that Certain Disadvantages Arise.
PARADOXICAL though it may seem, it is probably true that the progress of the aluminium piston for use in commercial-vehicle engines has suffered somewhat owing to the lightness of the material. The low density of aluminium has led people to suppose that this is its only claim to superiority over cast-iron, and, consequently, it has been thought that for a relatively low-speed engine, such as is fitted to most commercial vehicles, the aluminium piston was not worth while. However, -although aluminium pistons have made comparatively slow progress, it seems that, in the light of many experiments carried out during the past few years by manufacturers and by large users of commercial vehicles, the aluminium piston is at last to come into its own.
The aluminium alloy commonly used for pistons possesses a most important property in addition to lightness, namely, low resistance to the passage Of heat. In other words, it is a much better heat . conductor than its rival, cast-iron. Now, the flow of heat depends upon two factors, these being the sectional area of metal available and the conductivity of the material, so that not only does aluminium score in the second respect, but also, owing to its low density, at least twice the sectional area can be provided without making the piston as heavy as one bunt of cast-iron.
The greater rate at which heat is conducted away from the piston crown has far-reaching effects, and it is this factor rather than lightness alone which accounts for the increasing use of aluminium. In the first place the piston crown, other conditions being the same, will run at a considerably lower temperature in the case of aluminium, and this at once leads to a reduction of oil consumption and the rate of carbon deposit.
Reduction of Carbon Under the Crown of an Aluminium Piston.
Carbon, in addition to the nuisance of its periodic removal, represents a definite waste of lubricating oil, and is mainly due to the burning of the oil which comes in contact with the hot piston crown. Changing over from cast-iron to aluminium may have but a slight effect upon the amount of deposit on the top of the piston, caused by oil passing the rings, but it will materially reduce the amount of carbon on the underside of the crown, which is due to oil being flung up inside the piston.
Another Important point is that, owing to the cooler condition of the piston, Luocking or pinking is much less likely to occur. So marked is the influence of aluminium in this respect that in several cases engine builders have found it possible to increase the compression pressure materially simply by employing aluminium pistons in place of the cast-iron variety. Without entering into lengthy explanations it may be pointed but that an increase of compression pressure carries with it allround advantages, such as a greater power output and a reduced rate of petrol consumption.
The Disadvantage of Aluminium.
It is an old truism that one cannot get something for nothing, so that it is not surprising to find that the aluminium piston has certain disadvantages. Hitherto the most serious of these has been due to the fact that the alloys used expanded considerably more than cast-iron for a given rise of temperature, so that the piston clearance when c28 the engine was cold had to be made unduly large. This was the cause a the phenomenon known as " piston slap," but alloys have now been brought out with a coefficient of expansion practically the same as that of cast-iron. There are, of course, several designs on the market in which the piston skirt is split into sections by saw cuts, and matters are arranged so that Its diameter will automatically adjust itself according to the temperature conditions, but it would seem that a solid piston made of an alloy not subject to a high rate of expansion is the better solution.
Another disadvantage with which the alumininm piston is charged is that it gives rise to rapid cylinder wear. There is a definite reason for this; indeed, the forces at work are just the same as those which account for a hard-steel crankshaft becoming worn, while the soft bearings in which it runs remain untouched.
The Trouble of Embedded Particles of Hard Metal.
Briefly, the trouble is that any hard particles of grit or metal present in the lubricating oil become washed in between the working surfaces and embed themselves in the softer of the two. In other words. the piston tends to pick up hard particles until it literally bristles with them, and then proceeds to have an abrasive action on the cylinders, which is the same as that caused by lead laps.
This trouble is being overcome by using cast-iron for the cylinders of a. composition which has a high resistance to wear, and by using harder alloys for the pistons. In America the trouble is also being tackled bY the adoption of air cleaners, which remove grit particles from the air entering the carburetter, so preventing them from reaching the engine and mixing with the oil. Another point is that, in finishing aluminum pistons in the shops, they should be turned and not ground.
To sum up, while there are still some disadvantages to be overcome, the benefits which accrue from using aluminium in place of cast-iron for pistons are so important that it may be surmised that the lighter material will become more and more used in the future.