The Single O.H.C. Engine
A technical appraisal of current Production Practice by Autolycis
A survey taken ten years ago would have shown that there were only 14 makes of cars in series production throughout the world with engines having camshafts located in the cylinder head. Of these, many were specialised, highly-priced vehicles often produced in relatively small quantities. Invariably they had twin overhead camshafts operating—either directly with inverted tappets based on the classic Ballot design, or indirectly through rockers or fingers—valves substantially opposed to each other in the combustion chamber.
The purpose of this article is to survey the single overhead camshaft engine operating valves which are substantially in line—an arrangement which has become very popular for mass-produced engines over the past four years and which will become increasingly so in future. Ten years ago there were only four such engines used in European production cars: Elva and Lotus, each of whom fitted automotive versions of the now legendary Coventry Climax "fire-pump" engine, the Lloyd twin-cylinder air-cooled unit, and the range of Mercedes four- and six-cylinder engines. Only the last-named could be considered as a serious mass-produced example.
Today there are at least 14 basic engines of this type produced in U.K., Europe, U.S.A. and Japan, so it is obvious that manufacturers are recognising the virtues of such a scheme and presumably have overcome their previously held views—that the production disadvantages in respect of cost and complexity did not really vast, have been overcome, or swallowed in the interests of improved performance and life with decreased noise levels.
Let us establish the type of valve gear in question. Firstly the camshaft must be located in the cylinder head. Secondly the included angle between the axes of the inlet and exhaust valves are either zero or the intersection of these two axes occurs at the stem as distinct from the head end of the valves.
The examples of single o.h.c. engines emanating from the vast General Motor organisation in the U.S.A., U.K. and Germany as produced by Pontiac, Vauxhall and Opel, make interesting study. General Motors appear to lay down broad lines of policy for their member firms and allow a great deal of local latitude on their interpretation. This is in direct contrast to Ford, in which organisation there seems to be extremely rigid control exercised from the Detroit headquarters; even to the extent of designs being prepared and handed over to their foreign vassals for manufacture which do not always appear to be ideally suited to local market needs or management ideas.
Cog-belt drive for the camshaft is a feature shared by the four-cylinder Vauxhall Victor and six-cylinder Pontiac Tempest: the latter and the four- and six-cylinder Opels use a basically similar wedge-head combustion chamber with in-line valves inclined at an angle of 20 degrees from the cylinder axis, but the method of operating the valves is different. The Victor has a part-spherical shallow combustion chamber with in-line valves, the axes of which are inclined at six degrees to the cylinder centre line: and the method of valve operation is entirely different from either the Opel or Pontiac; a basic Ballot-type inverted tappet being used having a unique method of adjustment for valve clearance.
When the Vauxhall Victor was first released in 1967 the company went to a great deal of trouble to point out in their publicity material that they had changed to the part-spherical combustion chamber after commencing development of the new engine in 1964 with the Heron or bowl-in-piston type using a flat head. The reasons given for the change were that the chosen design had a smaller surface area to volume ratio than the Heron type and that this is an advantage when meeting the extremely difficult American exhaust emission standards for 1970 onwards. Investigation will show that for the same compression ratio on an oversquare engine—and the Victor 2000 with its stroke-to-bore ratio of 0.73 is a very oversquare design—the bowl depth in the piston of a Heron layout can be extremely shallow and that this scheme provides the lowest surface area to volume ratio of any design; it must be emphasised that this statement only holds good on the oversquare engine to which the Heron design is best suited. Could it be that the change of design was motivated by the fact that Ford was first in the production field with the flat head and that the great General Motors could not give the impression of following on behind?
If Vauxhall had anticipated the future needs of exhaust emissions as early as 1964 one wonders why their engineers chose such over-square cylinder proportions for the Victor. As further knowledge is accumulated on this very complex subject the opinion of most engineers on both sides of the Atlantic is that an engine of around square proportions is the best compromise to maintain the balance between the minimum surface area to volume ratio and the ability to house valves of adequate size in an engine operating in the 5,500/6,000 r.p.m. range.
Maximum power of the 1,599 c.c. Victor occurs at 5,600 r.p.m. and at 5,500 r.p.m. for the 1,975 c.c. version. This represents a mean piston speed of 2,570 ft. per minute for the latter, which is very low and does not exploit fully the relatively short stroke. It has been left to Lotus to do this with their four-valve head version of the engine which was designed by Ron Burr, formerly with Coventry Climax. As developed for its Class win in the B.O.A.C. 500 on April 12th at Brands Hatch the maximum power output was 240 b.h.p. at 8,300 r.p.m. (3,800 ft./min. mean piston speed) and had a b.m.e.p. of 206 psi. at 6,000/6,500 r.p.m.—an extremely creditable standard.
The cylinder proportions of the Vauxhall Victor must surely have been fixed with a V8 in mind as a future development, for in a relatively small engine compartment—and one cannot visualise Vauxhall entering the big car market—a very short stroke provides the necessary low overall height and width. Also the jack-shaft which is used for driving the centrally disposed distributor and oil pump is mounted vertically above the main bearings in a position necessary without tooling changes to be common should another bank of cylinders be added to give a 90º included angle. The only components to be re-sited would be the oil filter, fuel pump and starter motor.
So much for conjecture—let us now assess the detail design. Like most modern four-cylinder engines there are five main bearings and these, like the big-ends, are fairly massive, the dimensions being 2.5 and 2.0 in. diameter respectively; this provides considerable bearing overlap and therefore desirable stiffness for the shell-moulded nodular iron crankshaft.
The real engineering interest in the Victor engine lies in its combustion chamber, valve gear and cog-belt driven camshaft. A shallow part-spherical combustion chamber is desirable even on a twin-camshaft design if it can be used in conjunction with a substantially flat piston, for it provides a smooth path for any swirl or other gas movement which is introduced into the burning process. With in-line valves, however, there is considerable masking and pocketing of the heads adjacent to the cylinder walls which can affect filling; this is perhaps reflected in the moderate net output of 88 b.h.p. at 5,500 r.p.m., even though the inlet valves with a head diameter of 1.70 in. and a cylinder head port throat of 1.50 in. are quite generous for the capacity.
To site the sparking plug, which is located on the exhaust side of the head as near the centre of the combustion chamber as possible, Vauxhall use the small-bodied A.C. type 42TS, which dispenses with the usual sealing washer; it screws down to a conical seating machined in the head. However, unlike the wedge-type of cylinder head there is no quench area for the "end-gas" on the far sides of the combustion chamber with a resulting long flame path in the large bore. In the past engineers would have expected this to be prone to detonation and run-on, but presumably the Vauxhall engineers have overcome this problem with inlet port swirl or other means.
The camshaft is mounted in a die-cast aluminium housing bolted on top of the cast-iron cylinder head. It runs directly in the aluminium and there are no separate bearing caps, the camshaft being inserted from the front end, and to make assembly easy the five bearings are progressively smaller by .032 in. on diameter from front to rear. In fact, the centre three bearings are not complete diameters, contact with the Camshaft being maintained for only 30% of the circumference on the top. This is perfectly satisfactory as the camshaft loads are always in an upward direction and the only problem is one of lubrication, which is arranged by providing a permanent controlled level in a trough formed in the valve chest. The centre bearings dip into this oil which is replenished by spill from the front and rear bearings, which are pressure-fed.
If the normal "biscuit" had been used for obtaining valve clearance with the bucket-type tappets it would have been necessary to remove the complete valve chest with this threaded-in camshaft arrangement. This could be costly in production and service; it is referred to later when discussing the new Austin Maxi. To overcome the problem Vauxhall have devised an ingenious angled screw type of adjustment which must add considerably to the tappet weight and therefore the reciprocating masses and hence spring loads. However, it is presumably satisfactory and still lighter than a push-rod and rocker arrangement for the valve crash speed is 7,000 r.p.m. This case-hardened adjusting screw operated with an Allen-type key through a hole in the tappet and a slot in the tappet block is inclined at 5º 30' to the tappet face and has a flat of the same angle milled on it which bears square on the top of the valve. One complete turn of the screw provides .003 in. of tappet adjustment and, of course, there is no need to provide any locking mechanism for it. It must be assumed that the surface finish of the threads on the screw and at the tappet must be maintained to a very high standard to prevent excessive bedding-in and hence loss of valve clearance in the early life of the engine.
Cog-tooth belts have been used for driving auxiliaries such as fuel injection pumps, magnetos or distributors and even oil pumps of competition engines for quite a long time now. The first production example of its use for camshaft drives was the Glas S-1004, which had a 992 c.c. four-cylinder single o.h.e. engine when introduced at the Frankfurt Show in 1961. The Vauxhall belt is made by UniRoyal, the basic material being nylon-neoprene with tension members of glass-fibre cord. It drives the camshaft and jacket in a single stage. The manufacturers claim that this type of belt has negligible stretch but all users of them seem to provide some means of adjustment: that on the Victor being a jockey pulley contacting the outside of the belt between the crankshaft and jackshaft, which also helps to increase belt lap on the pulleys of these two components.
A surprising feature of this belt drive is that no attempt has been made to provide a protective shield so one must presume there has been no trouble with damage from flying stones or lost finger-ends from inquisitive tinkerers. Incidentally, these belts are unsatisfactory in the presence of oil and this is why they are always external to the main engine compartment.
The Pontiac Tempest (followed later by the Firebird) was the first General Motors engine to have an overhead camshaft and have it driven with cog-toothed belt. Unlike the Vauxhall Victor it is protected from possible damage with quickly detachable cover plates and there is a most ingenious method of setting and altering belt adjustment. On the side of the crankcase there is a machined face on an inclined plane to which is bolted a die-cast aluminium housing containing a short shaft with spiral gears for driving the distributor and the external oil pump in tandem with it; the oil filter and fuel pump are also attached to this auxiliary housing. A pulley on the forward end of the shaft forms a three-point drive with the crankshaft and camshaft for the cogged belt. Thus by slackening the attachment bolts for the auxiliary housing and moving it up or down its mounting face the required degree of belt tension can be obtained and thus dispenses with the need for an auxiliary idler sprocket.
In its latest form this six-cylinder seven main-bearing engine has a bore and stroke of 98.4 x 82.5 for an s/b ratio of 0.84 to 1—not very excessive by American standards, although the general trend there is to less oversquare units. Whether this is in the search for improved exhaust emission or the fact that increases in bore sizes have reached a limit on existing designs and in the search for ever greater engine capacities, this can be achieved only by increasing the stroke, is a matter for conjecture.
Like most American engines the Pontiac uses a wedge-type combustion chamber with the in-line valves inclined at 15 degrees to the cylinder axis. This type of chamber is unquestionably a good production compromise. Adequate size inlet valves can be accommodated without too much restriction on filling from masking by the chamber walls. Squish turbulence necessary for good combustion can be controlled and adjusted by varying the relative degree of squish area on either side of the transverse section and a reasonably central plug position relative to the bore and having a short flame front can be achieved; also it can easily be provided with good water passages around it. Like all American engines there are no separate replaceable valve guides, the stems operating directly in bores machined in the cast-iron head. With a relatively short support length and the side thrust resulting from the use of fingers (as in the Pontiac) or rockers in a push-rod layout, one is always surprised how the wear problem is overcome. One reason is undoubtedly the use of a two-piece valve in which stems and heads of different specifications are welded together before machining. Thus a steel for the head end can be chosen to give good resistance to burning in the case of an exhaust valve and the stem to provide superior wear and anti-scuffing properties.
Interposed between the camshaft and valve stem is a finger which could be either cast and chilled on the cam-rubbing face and stem operating pad or a steel forging with welded-on hard-facing metal such as Stellite on these two contact areas. The camshaft is offset by some .75 in. and provides a cam-to-valve lift ratio of 1.5 to 1. By providing a hydraulically load cylinder beneath a ball pivot on the outboard and underside end of the valve-operating finger as a reaction member, zero clearance is obtained automatically and the need for adjustment eliminated; in fact, this hydraulic cylinder works on the same principle as hydraulic tappets used almost universally on American engines. Light contact is obtained between the finger and valve stem when the engine valves are on their seats. At the commencement of lift a ball check valve in the hydraulic cylinder is shut and the reaction from the load imposed by the valve springs is taken on a column of engine oil.
Pontiac must be very confident of the reliability of this valve gear as it would be relatively costly for routine inspection. A die-cast aluminium top cover looking like an orthodox valve cover contains the camshaft in seven complete circular bearings—threaded in from the front with the camshaft running directly in the base material as on the Vauxhall Victor. Furthermore, it contains the water outlet riser pipes and thermostat of the coolant system.
When Opel introduced their new four- and six-cylinder in-line engine in 1966 with the camshaft mounted to one side of the head using orthodox tappet and rocker geometry, it was difficult to see what the advantages were over the orthodox push-rod layout. The latter components were certainly eliminated, but at the expense of a much larger chain layout requiring sophisticated dampers and tensioners compared with the short centre side camshaft location.
The latest 2.8-litre version introduced in March this year included the use of a hydraulic tappet of the American type (and probably imported front one of the G.M. American companies) which eliminates the need for adjustment and are satisfactory for engine speeds up to 6,000 r.p.m.
The wedge-type combustion chamber of the Opel is very similar to that of the Pontiac, except that valve inclination is 20 degrees to cylinder axis with inlet and exhaust on the same side. Having a bore and stroke of 92 x 69.8 r.p.m. for the six-cylinder unit the necessary valve sizes and adequate water passages can be accommodated with this layout; furthermore, it permits the provision of convenient carburetter hot spots (necessary for part-throttle acceleration and essential as part of the techniques to reduce exhaust emissions) with the exhaust manifold.
There is a main bearing between each cylinder on each of the engines, the cylinder blocks and heads being made of cast-iron. Valves run directly in the head, as with the Opel, but the supported stem length is much greater. Somewhat surprisingly white-metal shell bearings support the camshaft, which is housed in a tunnel high up in the head on the side remote from the ports: yet the tappets, both of the old adjustable and the new hydraulic type, operate in holes bored direct in the casting. There would appear to be room for economy here for so many firms now dispense with thin-wall bearings whether using cast-iron or aluminium; and, of course, Volkswagen and Porsche have run their camshaft bearings direct in their magnesium alloy crankcases for years. After all, should a rare seizure occur bearing shells could be introduced as a salvage scheme.
A similar valve rocker geometry is used on the old and new engines. The rockers are deep-section steel pressing pivoting on a sintered iron spherical seating surrounding a near centrally disposed stud between the tappet and valve. A self-locking type of nut was the means of adjustment on the early mechanical tappet and is retained—presumably for initial setting—on the hydraulic design. The distributor and oil pump in tandem with it are driven from spiral gears on the crankshaft nose and thus there is only a single-stage double-roller chain drive direct from the crank to camshaft. Tensioning is by means of a spring-loaded Weller spring steel blade, having a plastic covering to reduce wear and noise, on the slack side of the blade. On long-chain centres damping is also necessary on the tight side; it is achieved by arranging a nitrile face fixed position blade to have approximately .100/.125 inches of constant contact with the chain on the outside of its run.
When the Hillman Imp was first conceived under the guidance of B. B. Winter, then Technical Director of Rootes, it was towards the end of the post-war economic squeeze in the U.K. and Europe. The idea was a really cheap car for four adults, economical to run and requiring minimum servicing. It had a horizontally opposed twin-cylinder air-cooled engine and non-synchromesh four-speed gearbox which was designed by and planned to be manufactured by Villiers of Wolverhampton. As development proceeded Peter Ware took over after Winters's retirement and this coincided with increasing demand for more sophistication in the minimal vehicle. It also so happened that there was a young development engineer on the Imp project by the name of Mike Parkes, whose weekends were spent helping David Fry with his competition cars, one of which was powered by an 1,100 c.c. Coventry Climax single o.h.c. four-cylinder engine. Parkes proposed to the management that this was the type of engine needed and there was a smaller 741 cc. version of this unit which had been successful at Le Mans. With its all-aluminium construction and installed weight of only 160 lb. for an output of 65 b.h.p. it could be mounted in-line at the rear without affecting weight distribution and would obviously have considerably enhanced performance. An engine was obtained from Coventry Climax, a new synchromesh gearbox designed and this is how the new Imp started life. It is therefore not surprising that the Imp power unit is still remarkably similar to the famous Coventry Climax "fire pump engine which wins motor races".
When first introduced the Imp engine had a capacity of 875 c.c and a bore and stroke of 68 x 60.4 mm. For the Rally Imp the bore is increased to 72.5 mm (998 c.c.) and in this form when fitted with twin 1.50 in. diameter Stromberg C.D. carburetters produces an installed power of 60 b.h.p. at 6,200 r.p.m. To achieve 10 b.h.p./litre/1,000 r.p.m. when fitted with car exhausts, air cleaners and driving all auxiliaries is a very creditable standard; a figure barely achieved by the Fiat 124 Sport Spider, for example.
Complete engine weight of the 998 c.c. version is 190 lb., equal to around 3 lb. per horse-power—achieved by only a handful of high-production engines. Much of the weight-saving comes from the use of die-cast aluminium alloys for the block, head, tappet block and valve cover. Designed as a high-pressure die-casting but still produced on gravity dies with all-metal cores, the interior of the crankcase is very smooth without any webbing, the necessary rigidity being obtained by external webs and "U" section external channels at each main hearing panel; this also permits the use of near-constant metal thickness throughout—most important when die-casting. Also for one-piece core withdrawal the block has an open top deck; centrifugal cast-iron liners are mechanically bonded in during the casting process. The aluminium overlaps the liners top and bottom to ensure rigid location and also eases final machining of the top joint fact. In its fully-machined state this three main-bearing component weighs a mere 22 lb.
Head design is almost pure Coventry Climax practice; the valves are angled at 20 degrees to the cylinder axis in a fully-machined wedge-type combustion without overlapping the cylinder bores. Efficiency of the combustion can be judged from the fact that both versions of the engine operate satisfactorily on premium fuel with a compression ratio of 10.0 to 1. Individual ports on the same side are used for exhaust and inlet, the latter having long constant diameter tracts for good ram filling before they join up with similar length tracts in the straight-rake intake manifold.
(To be continued)