HIGH EFFICIENCY PISTON DESIGN.
Including a brief review of the leading proprietary makes.
By W. S. B.
THE essential feature of any high efficiency engine is its high compression ratio. Many other points exist, such as v alv es, operation methods, 2-stroke or 4-stroke, etc., on which various iews are held, and on which every designer has his own ideas ; but the efficiency of an engine definitely increases (up to a point, limited by practical considerations, such as detonation of the fuel) with an increase in the compression ratio. High output entails high temperatures, and unless the beat which is not converted into work can be quickly got rid of, the high temperature will cause failure of the pistons by melting, or seizing from excessive expansion.
From this it will be seen that the first requirement of a racing piston is that it should run cool. The only way this can be done is to get the heat from the piston head to the skirt, as, owing to the large area of the skirt, the heat will get to the cylinder walls easily enough. This means that the material of which the piston is constructed must have a high co-efficient of heat conductivity so as to reduce as much as possible the volume of metal, and thus the weight required. Next in importance is the question of mechanical strength, for, in view of the high engine speeds now common, the forces on the reciprocating parts are very large and will cause cracked pistons, etc., unless steps are taken in the design to provide the required strength. Aluminium alloy is almost exclusively used for racing pistons to-day, owing to its high thermal conductivity and its lightness, as compared with iron or steel. Owing to its much lovc er melting point however, it must be run much cooler than either of these in order to avoid burning, while its better heat conductivity is insufficient alone to ensure this. To offset this, many designers increase the thickness, and so the weight of the conventional designs, and this, while cooling the piston, fails to produce the lightness which is so essential for
high engine speeds. It should be noted that lightness is of secondary importance to cool running and strength.
As a result of the efforts of metallurgists taken in conjunction with the experience gained in the last few years in racing, aluminium alloys have been produced in which practically all the disadvantages of the earlier alloy pistons have been eliminated. New alloys and accurate heat treatment have rendered possible the production of pistons which do not grow, and which can be run with clearances similar to iron and steel, thus eliminating piston slap. Research has also produced alloys with the very important property of retaining their strength at working temperatures, and it should here be mentioned that figures of tests carried out when cold, are of little value as an indication of how the metal will behave at the temperature attained in the engine.
Having set out roughly the requirements of a ” fast ” piston, it will be of interest to consider the methods used by leading manufacturers, who specialise in this work to overcome the difficulties.
The principal feature of the well-known Specialloid design is the internal ribbed construction, the object of the ribs being to serve as a means of transferring the heat from the head to the skirt, and thence to the cylinder walls. By this method, a piston is produced which combines good cooling, strength and lightness. Other advantages claimed by the makers are the elimination of growth, and the comparatively small clearances with which they can be run without having to use split skirts, expanders or other of the numerous devices so frequently resorted to. This latter point, in conjunction with the special design of recessed waist and oil return holes, effectually
prevents oil reaching the combustion chamber when running at small throttle openings.
Specialloid successes have not been confined to road and track work, and it is interesting to note the world’s record for r I litre motor boats was recently established by a boat built by J. W. Brooke and Co., Limited, of Lowestoft, and fitted with a Brooke supercharged marine engine using Specialloid Pistons. Incidentally, we had the pleasure of examining some pistons out of this engine which, from the point of view of piston design, formed an extremely difficult problem. Various proprietary designs and makes of piston (including cast iron and steel pistons) entirely failed to withstand the enormous temperature due to the high supercharger pressures used in this engine. The Specialloid Racing Pistons finally fitted have been in use in this engine for nearly eighteen months without any ill effects. Messrs. Specialloid Limited make no claim to have had no failures, and showed us some of their pistons which they had succeeded in burning out by tests of abnormal severity, realising that this is the only way to make sure that the final design which they supply shall be completely reliable. These pistons are the sole product of the firm of Specialloid Limited, Priem Park, North Finchley, N.12, and the range includes every size of engine, a point of interest being the enormous number of pistons supplied to leading Motor Omnibus and Transport Companies, the engines of which get anything but gentle treatment.
The latest Hepolite piston produced by Hepworth and Graddage, Ltd., St. John’s Works, Wakefield Rd., Bradford, is of very ingenious construction, and it is claimed that it can be fitted with finer clearances than cast iron, and yet can be put under load without any ” nursing ” as soon as it is installed.
This is achieved by actually separating the piston skirt from the head. The piston casting is formed of two concentric cylinders, the inner extending from the head to the gudgeon pin bosses which join the two. Thus the head is at the same time supported and heat got away from it by the inner cylinder. In addition, two transverse webs from the bosses support the walls. During machining, the walls are cut through above the gudgeon pin, so separating the skirt completely from the head and leaving it supported by the bosses
and webs above mentioned. The skirt is then slotted in six places, so that any expansion within the cylinder is compensated for by the closing of these slots.
Although all the advantages of alloy pistons—lightness, heat conductivity, etc., are obtained, it is claimed that the Hepolite compensating piston can be fitted with clearances as fine as cast iron, so eliminating slap, at the same time avoiding possibility of seizure.
The many successful users of these pistons include Mr. B. H. Davenport, whose amazing little V-twin Frazer-Nash continues to collect such a remarkable pile of ” Firsts ” on Southport sands and elsewhere.
A piston which is outstandingly successful at Brooklands, but which does not appear to be as widely known as it deserves, is the ” Martlet ” made by Messrs. Brewster and Johnson, who have a depot at the track, and are thus in constant touch with the users of their products, and so lose no time in finding out any alterations that are necessary. The great feature of the Martlet piston is the central trunk in the head from which webs come down to the gudgeon pin bosses. This gives very great strength, and at the same time provides two separate paths in parallel by which the heat can flow from the bead to the skirt. This means that the portion of the head between the central trunk and the edge can be made considerably lighter than with the conventional design without risk of failure. They are made of a special alloy which it is claimed does not grow and can be fitted with smaller clearances than ordinary alloy pistons. An argument in favour of these pistons which cannot lightly be overlooked is the fact that, apart from some records in the ” baby” classes under 175 c.c. made by a 2-stroke, every motor-cycle record made at Brooklands this season has been made on Martlet pistons. They of course supply pistons for any engine, air or water
cooled, but there is little doubt that the modern aircooled racing motor-cycle engine with its very high r.p.m., and terrific compression ratios, combined with the silencer regulations now in force at Brooklands, provides a test for pistons which it would be hard to surpass for severity.
As the Zephyr piston has been on the market since 1910 the views of its inventor are bound to be of considerable interest, as in 1913 many records were broken on a Vauxhall, fitted with Zephyr steel pistons, and they have been widely used ever since.
When aluminium alloy came into use for pistons the inventor at once realized its advantages for cooling purposes, but did not consider it good enough as a bearing metal owing to its liability to wear the cylinder, and the latest composite Zephyr piston is an endeavour to make full use of aluminium as a heat dissipater at the same time retaining the advantages of steel as a hard wearing bearing surface.
The head is of the usual aluminium alloy down to the gudgeon pin, the feature being that the aluminium does not touch the cylinder walls except at the time of maximum expansion. A special alloy steel skirt is bolted to the head and performs all the duties of a bearing. The steel skirt is so resistant to wear that earlier types of Zephyr pistons made from the same material are still in use in the original cylinders after periods of 12 to 14 years. This type of Zephyr piston can be and is made lighter than an all aluminium piston of conventional design as the steel skirt can be made extremely thin.
All Zephyr pistons are fitted with ” Zephoil ” rings which are claimed to be much more effective as scrapers than the ordinary ring.
Miralite Ltd., 87-91, High St., Mortlake, S.W. 14, specialise in castings of special alloy for any pistons, and have the enviable distinction of having supplied the piston castings for the fastest car in the world, J. G. Parry-Thomas’ famous ” Babs.” We understand that finished pistons to any design in Miralite alloy can be obtained from Mr. Thomas, or Miralite will supply
castings direct to any design if they are sent the patterns and core-boxes, but they do not themselves supply finished pistons, being alloy casting specialists only.
No article on pistons would be complete without some reference to the firm of Laystall whose standard De Luxe piston, while not used for racing, is much favoured by the owners of sports cars. These De Luxe pistons are remarkably fine iron castings and are so designed to provide equal strength to an aluminium piston of the same weight with a claim to greater longevity. The principal points of design are the internal ribs radiating from the centre of the crown and extending to the skirt, thus allowing very thin walls and consequent lightness without losing strength. From personal experience of these pistons in a Meadows engine we can state that they definitely improve speed and acceleration, wear well, eliminate vibration, and do not seem to overheat any more than an aluminium piston.
Messrs. Laystall of course do not confine themselves to these, their standard product, but in the course of the many racing jobs that they undertake they have to turn out many different designs of aluminium pistons. For this class of special work they have a reputation second to none.