Mechanical Construction of High-Power Engines
(From The Design and Tuning of Competition Engines by Philip H. Smith, 1963)
Basic materials in construction
The first requirement for ensuring reliability under conditions of continuous high-speed operation, is that the materials chosen shall be of the correct type, and adequate for the duty, of any particular task. The advances made in foundry and metallurgical technique of recent years have ensured that under normal conditions, failure of a component is comparatively rare. It is still possible, of course, to encounter cases of fracture of such items as connecting rods or crankshafts under abnormal stresses, as in racing, but these can arise from many causes other than faulty material or dimensional errors.
There are two main classes of materials used in engine construction-ferrous and non-ferrous. The former comprise those with an iron base, such as cast-iron, mild steel, and alloy steel. The last-named can be case-hardened or otherwise specially treated as required for particular duties. A fairly recent newcomer to the ferrous range is high-duty or hightensile iron, in which various compounds are mixed to achieve a high degree of toughness and strength. This material is coming to the fore as an alternative to steel for crankshafts, Meehanite being a typical and familiar name in this connection.
Present-day foundry technique allows castings to be manufactured, the intricacy of which was unknown thirty years ago. This allows several " units " to be embodied in the same casting, which is all to the good from the point of view of rigidity and strength, though perhaps less desirable from the angle of accessibility when overhauls are contemplated. For instance, it is the custom nowadays to make the cylinder block, crankcase, and main bearing scantlings in one piece, with the cylinder head detachable above the top of the swept bore. This undoubtedly makes for a more accurately aligned and rigid assembly than the use of a separate crankcase with detachable cylinder block. The latter, on the other hand, when combined with its head, had many virtues, notably in the absence of a complicated gas-and water joint with its attendant studs, bosses and sealing gasket.
Aluminium alloy is, of course, the most used of the nonferrous range. It combines adequate strength with lightness, and is on the whole easier to cast and machine than iron. Its high thermal conductivity is also a very desirable feature in certain applications.
For large castings carrying little stress, ordinary cast aluminium is excellent. Thus it is widely used for oilcontainers, valve and timing gear covers, and so on. As far as oil-containers are concerned, the virtues of aluminium from the point of view of heat conductivity in comparison with, say, a pressed steel sump, are probably over-rated, since oil temperature should be kept within bounds by other aspects of design. However, the aluminium sump does combine strength with lightness, and this is a requirement in sumps of large capacity which are desirable for high-efficiency engines.
The thermal conductivity of aluminium really shows to advantage in the use of this material for cylinder heads typified by Plates 59 and 62. It will be apparent that the temperature range of the cycle has an important bearing on thermal efficiency, and that, whilst rapid heat dissipation at certain high-temperature phases in the cycle is essential to prevent overheating, retention of heat at other phases is desirable. Thus, a material which will rapidly transfer the heat between the mixture and the cooling water, in whichever direction is required, will make for high thermal efficiency as well as reliability under sustained high loading. A material of lower thermal conductivity characteristics, on the other hand, will tend to retain the heat within itself, leading to local superheated areas in conditions of high-temperature operation. For very high pressures, heads of aluminium-bronze alloy are sometimes used, as the mechanical strength in this case is equal to that of cast-iron, and valve-seat inserts (required with aluminium heads) may be dispensed with. Barronia, a copper-tin base alloy, is another successful material which can be used without valve-seat inserts.
Light-alloy crankcases, at one time common when separate cylinder blocks were the rule, are found occasionally, in which case the casting also incorporates the water-jacket, special iron cylinder liners being used.
These are frequently of the wet type, seating on suitable sealing rings at top and bottom to form a water joint with the aluminiurn casting shown on Fig- 4 : I. A form of construction used with dry liners is to cast the aluminium around the iron liners in the mould; the liners in this case have a specially finished exterior to form a mechanical interlock, such as threading or roughened " sandpaper " pockmarking. Both forms of construction make for commendably light power units and, although the manufacturing operations are to some extent increased in complexity, large scale production has a habit of overcoming such drawbacks.
Aluminium alloy is used without exception for pistons, usually in die-cast form, though forged pistons may be preferable for very high-speed work. The metal is chosen as much for its heat-conducting properties as for its lightness, since for adequate strength, the pistons are sufficiently robust in section to have considerable weight. No other metal would, however, be suitable, as ultra-rapid heat conduction from the piston crown to the cylinder walls is of outstanding importance.
Connecting rods have at times been made of duralumin or similar alloys, but forged steel is generally favoured, while as far as crankshafts are concerned, steel is still mostly used, though high-duty iron, notably Meehanite, has recently come to the fore, with cast construction instead of the usual forging.
Failure of highly stressed parts was at one time frequently caused by an actual fault in the metal. Nowadays, such failures are very exceptional, modern methods of production and inspection being almost foolproof. Breakages in modern engines almost invariably arise from fatigue, or tiring of the metal under abnormal stresses, resulting in a crack developing. Once such a fracture has started, it will, of course, rapidly spread until complete breakage occurs. The greater the factor of safety in the component, the less liability there is to fatigue, but where reciprocating parts are concerned superfluous weight of metal is undesirable. Unfortunately, these parts are the very ones in which fractures are most serious and frequent. Old age also leads to changes in the metal structure which lessen its resistance to fatigue, a point to watch where "vintage" engines are under consideration.
Elimination of places which are liable to encourage the start of cracks is an important part of correct design. Such places may be found at sudden changes of section, at the bottoms of screw-threads, junctions of bolt-heads with their shanks, and so on. Accidental scratches or file-marks can lead to early fracture, and, conversely, a high polish is a distinct discouragement to breakage. In this connection, it has been established by test that an accidental scratch on a polished surface causes a reduction in fatigue resistance of 15 per cent., while the finishing of a normal-smooth surface with fine carborundum to a high polish will increase it by 2 per cent.
Examination of a fracture can often provide useful information. The final breakage point is usually discernible by the rough portion at the break, the remaining part being almost polished in appearance, with curved lines back to their starting point. This semi-polished surface is caused by the working together of the surfaces before the final parting, and the start of the trouble is sometimes traceable to the commencement of the curved lines aforementioned. Investigation by an expert metallurgist can often give a clue to the direction of the force causing the breakage, and thus help in determining whether an abnormal load in the normal direction was responsible or whether some unexpected additional stress made its presence felt.
In very many respects the design of the competition car power unit follows closely its more sober counterpart. In fact, many of the components used on the latter types can be employed with equal success in engines of greater power output, providing they can cope with the extra stresses involved. A study of the chapters dealing with specifications of typical engines will help to indicate how various manufacturers deal with particular aspects of design. Here, we will consider briefly some general points of particular interest.