The Design and Tuning of Competition Engines
~ 1963 ~
By Philip H. Smith, A.M.A.Mech.E.
Chartered Mechanical Engineer, Chartered Automobile Engineer, Member of The Guild of Motoring Writers, Technical Editor “Sports Car” and “Motor Racing” “Calculus” of “The Motor World”
While case-hardened steel camshafts are still to be found high-duty cast-iron is very frequently used, actually to a greater extent than is the case with crankshafts. A typical material (used for the Midland Motor Cylinder Co.'s shafts) is " Monikrom ", containing nickel, chromium and molybdenum. Providing the material is suitable it is quite feasible to incorporate one or more gearwheels on the shaft in any position, such as those required for oil-pump and distributor drives. This is a simple solution to an otherwise difficult problem, and it is to be noted that gears incorporated in this manner do not show wearing qualities in any way inferior to those of separate gears.
As the cam surfaces are very heavily loaded, a good depth of hardening is necessary. Using a suitable alloy iron, the " chill-casting " method is often used. In this, iron chills or insertions of the required shape are situated adjacent to the concerned surfaces. These result in rapid cooling of the surfaces, and the formation of a very hard carbide-of-iron layer, which is then finish-ground. On alloy-steel shafts the induction-hardening process is widely employed. The principle is that of rapidly heating the surface layer of the material to a high temperature, and then quenching it before the main body of metal has had time to absorb too much heat. This is done by passing a high-frequency current through a muff surrounding the surface concerned; clearance between the muff and the " work " allows quenching water-jets to be introduced. Electric current induced in the material surface as a result of its proximity to the muff causes rapid heating over a period of a few seconds, this being followed by a waterquench. The process is repeated as often as required to obtain the necessary hardness depth. Obviously this process can be controlled to a nicety, and it is thus coming to the fore not only for camshafts, but also for general alloy steel parts made in quantity.
The power imparted to the crankshaft is, of course, in the form of impulses, and thus the turning thereof is not uniform, the degree of impulsiveness depending on the number of power strokes, that is, on the number of cylinders. Similarly, the camshaft loading is again not uniform; the fewer cams there are on the shaft the more irregular will be the turning effort. Thus even such a simple looking matter as a camshaft drive can be quite a problem, involving as it does the coupling of two shafts, one to run at half the speed of the other, and neither of which rotate with a uniform effort.
The popular location for the camshaft is, of course, in the crankcase, where it is not very far away from the crankshaft, and where its lubrication arrangements present no difficulty, it being appreciated that, apart from the bearings, a good flow of oil is necessary to the highly-loaded faces of the cams. With this position for the camshaft, its drive becomes the simplest possible, either a pair of gearwheels or a roller chain being used. In the case of the former, extreme accuracy is essential if noise is to be avoided, as the shaft centres will undergo some slight variation as a result of heat expansion of the engine casting, while the impulsive drive means variation of gear tooth wear around the wheels. Gears are also difficult to silence, and the use of a camshaft wheel of non-metallic material is sometimes resorted to on this account. However, gears do provide a very positive form of drive, which is an important consideration since a slight variation in the relative positions of crank and cam, due to flexibility in the drive, may effect the timing.
The difficulty of silencing gears comes mainly from the necessity, due to space limitations, of using small wheels with fine tooth pitching. These in consequence are extremely sensitive to any variation in the shaft centre distance, which naturally affects the meshing of the teeth. Apart from the minor differences in dimensions, which are inseparable from quantity production, the fact that the engine operates over a widely varying temperature range is inimical to the maintenance of a constant centre distance. However many gear teeth there are on a wheel, the inescapable fact remains that only one tooth at a time transmits the load. The impact of transferring load from one tooth to the next in rotation, is liable to set up a whine, which is increased in impulsive conditions by load reversals across the backlash between the teeth.
These features of the drive, so unfavourable to gears, are of course by no means easy for a chain. The latter however starts with two important advantages; in wrapping around the two sprockets it engages a large proportion of the teeth simultaneously, so spreading the load; and it possesses a degree of weight, which acts to some extent as a damping medium on the impulsiveness of the drive.
The roller chain is by no means silent-running, but so long as sprockets of reasonable size are used (as is generally the case in modern engines) and the amount of chain slack is kept within limits, little or no noise will be heard above the general noise level of the engine. The use of excessively small wheels will increase the impact force between the first tooth and the chain; this is accentuated by chain-whip caused by excessive slack, and in extreme cases where there is not much clearance to the timing-cover a worn chain will rattle against the latter.
When the drive is at very short centres, as on the majority of engines having the camshaft located in the crankcase, the chain tension does not vary much with wear; thus, providing the initial amount of slack on assembly is the minimum specified (to allow a free-running drive and to compensate for centredistance variation due to temperature changes) it is quite possible to design a satisfactory drive having no provision whatever for chain adjustment. It will be realized that normal wear at the chain joints has little or no effect on the gearing action of the chain on the wheel teeth in spreading the load; the chain simply takes up automatically a larger pitch circle, higher up the teeth. Between the wheels, however, wear inescapably means more slack.
As long ago as 1912, chain drive pioneer Hans Renold, in a treatise to motor manufacturers, stressed the desirability of incorporating some means of adjusting the timing chain so as to maintain the tension absolutely correct. As the drive in those days frequently took in a magneto, it was not too difficult to arrange this on an adjustable mounting for the purpose. With the passage of time, the general simplification of engine auxiliary drives, and the advent of quantity production, nonadjustable chains became accepted so long as the centres were kept short. With longer centres, where excessive whipping could cause actual chain breakage, spring-blade tensioners of automatic type, or manually-set jockey sprockets were incorporated; there was a danger in the latter, however, in that too tight a setting, as by inexpert hands, could be even more detrimental to the drive than too much slack.
Hence, apart from those engine makers who, fairly successfully, took the view that no adjustment at all was better than further complication and the possibility of faulty servicing, there are others who have attempted to find a solution. This often takes the form of a flat spring-steel strip, bearing on the back of the chain on the non-driving or slack, side, and of sufficient strength to maintain a slight tension in the chain at all times. Superficially, such a device,might pass muster as an aid to quietening a worn drive, but it falls short of what is really required in several respects. If the spring strength is sufficient to exercise any real control over chain whip, the frictional loss is considerable, and wear takes place very quickly on both the spring face and the chain-plate edges. If too light, the spring is quite capable of uncontrolled oscillations in unison with the chain; in fact this phenomenon can take place at certain engine speeds however strong a spring is used.
Attempts to incorporate in such a device some method of damping the spring oscillations, while often successful, are liable to result in a bulky and relatively costly assembly. Further, there is the inescapable fact that the more the chain wears, the greater is the permissible movement of the tensioner spring. However, the automatic adjuster illustrated in Fig- 4 : 2 is now fitted extensively to a large variety of power units; one reason being that it was originally designed for incorporation in the simplest possible manner with shortcentre drives of non-tensioned type, and thus little structural modification is necessary for it to be fitted to existing as well as new designs.
The principle is that of a rubber-faced slipper-head, carried on a plunger which protrudes from a small cylinder bolted to the wall of the timing chest, in close proximity to the outside. of the non-driving strand of the chain. The slipper head contacts the chain, and is urged against it by the pressure of a light spring which, when the engine is running, is augmented by lubricating oil pressure; the oil emerges finally from a hole in the slipper, direct on to the chain.
The essential feature which enables control of the chain slack to be retained at a constant value irrespective of the amount of chain wear, is embodied in what is termed the restraint mechanism. This consists of a ratchet system, which whilst allowing the slipper to move against the chain without restriction, prevents any return movement beyond the small amount necessary for free chain operation at all times. A ratchet motion is of course the obvious method of obtaining this feature, and has been used on tensioning gear in the past, but the resulting arrangements have tended to be unwieldy. In the device shown, it will be noted that a large number of teeth are accommodated in the small space available inside the cylinder, by arranging them in a spiral slot, the carrier of which rotates to accommodate the linear motion of the plunger.
Due to the close control of backward movement, the pressure of the slipper head on the chain is quite light. The passage of lubricating oil through the whole of the mechanism, and its direct flow on to the slipper surface ensures thorough chain lubrication which is in itself an aid to efficient running.
While enthusiasts naturally expound the virtues of twincamshaft engines, duplication of shafts brings in a few mechanical problems, apart from any question of cost. In a four-cylinder with two shafts, there are only four cams on each shaft, which means an extremely uneven turning effort. In an attempt to make things better, some engines of this type have had the camshaft fitted with an additional cam providing four extra 'lobes and actuating a spring-loaded tappet, to no purpose beyond smoothing the turning effort. This sort of expedient can hardly be classed as good engineering, but shows the difficulties encountered.
With all the cams on one shaft, things are, of course, much better, and a six-cylinder (having twelve cams) provides quite a smooth turning effort. It is understandable therefore that in cars where silence and smoothness of operation are desirable features, but at the same time where expense in manufacture is to some extent a secondary consideration, a single camshaft in the crankcase is standard practice, in spite of the considerable number of reciprocating components necessary to operate the valves from this position.
The overhead camshaft mounted on the cylinder head, and operating the valves either through short, plungers or rocking followers, has considerable virtues. Rotary motion, wherever possible, is to be preferred to reciprocating, and the overhead camshaft carries this motion right up to the valve. In place of the tappet, push-rod, and rocker required for each valve on the crankcase-mounted shaft, the overhead shaft calls for only one item between cam and valve-stem.
At one time, there were quite a number of overheadcamshaft engines on the market, powering vehicles of the quality class. The fact that push-rod valve operation is the rule to-day calls for some explanation. Forgetting the sports types for the moment, it is evident that the push-rod engine fulfils most requirements as regards power output and silence of operation. In spite of the large number of valve-actuating parts, it can be regarded as simple in design, since so many of the parts are alike and not liable to cause confusion during overhauls. The head can be lifted without disturbing the valve timing, and there are no parts on the head requiring high-pressure and copious lubrication, with the attendant risk of oil-leaks. The type can be regarded as a sort of " standard " all over the world, so that personnel engaged in repairs and overhauls are quite familiar with its general construction.
Champions of the ohc engine claim as one virtue its silence of running due to the small number of reciprocating parts. In theory this may have some basis: in fact many ohc engines of the past and recent past have been disappointingly noisy, and this is an important point, since peculiar sounds from this quarter, even when doing no harm, have a distracting effect on the driver.
Untoward noise from the ohc engine is caused by several things. First is the difficulty of incorporating a method of valve clearance adjustment of comparable simplicity to the push-rod engine. This leads to hit-and-miss and incorrect adjustment. Then there is the position of the shaft, away from the sound-absorbing crankcase, and dependent on its own casing for silence. And finally there is the considerable length of drive. The first point, that of adjustment, is no condemnation of the ohc principle.
Most of the usual well-tried methods, such as eccentric follower-bushes, adjusting shims, and threaded tappets, work extremely well, but they do demand more care on the job than tackling the relatively simple row of rocker-end adjusters on a push-rod engine.
Enclosing the shaft in a sound-absorbing casing is a matter of design. There is nothing impossible about the task, but, of course, it has to be done on its own: there is no assistance from the main engine casting. This part of the job really comes down to one of what can be afforded.
As regards the drive, this is invariably by roller chain, a method of drive which has been proved at Le Mans and elsewhere. The system appears to be perfectly satisfactory, always providing proper attention is paid to tensioning arrangements as already described. Comments are sometimes made to the effect that one or two current chain-drive layouts look complicated in comparison with, for example, a vertical shaft and bevel or skew gears, but a little thought will show that this is quite unfounded, all the components being of simple and standard pattern. The main difficulty when using chain-drive is that of ensuring that the camshaft timing is not disturbed when the drive is uncoupled. Obviously it is almost impossible to guarantee that the timing cannot be lost, but makers go to considerable trouble to incorporate a method of retention of the timing during overhaul.
The necessity for high-pressure oil, and plenty of it, means that properly made faced joints must be used on the camshaft casings of ohc engines, and adequate oil returns provided to the crankcase. This inevitably means a somewhat more complicated lubrication system than that of the push-rod engine, there being, when the camshaft drive is taken into consideration also, a good many more points to which oil must be fed. In regard to lubrication of the valve gear, a point greatly in favour of ohc operation is the direct thrust given to the valve in contrast to the sideways movement however slight, that is inherent with rocker operation.
Ohc engines are usually free from the feeding of excessive oil down the valve stems even when no precautions are taken to prevent this. Some push-rod engines, however, often incorporate elaborate and in some cases rather makeshift oil shrouds and sealing devices at the top of the valve stems. Such items would hardly be necessary if there were no sideways thrust on the stem to cause wear and excessive