The Single O.H.C. Engine

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A Technical Appraisal of Current Production Practice
by Autolycus

(Continued from the June issue)

The three-bearing chilled cast-iron camshaft of the Hillman Imp engine is mounted in a separate die-cast aluminium tappet block, having split caps for the replaceable Babbitt-lined bearings.

Cast-iron valve guides have stem seals on the inlet valve to control oil being sucked into the port when there is a negative pressure in this region such as on over-run with a closed throttle. Valves seat on sintered iron inserts and clearance between the valves and tappets is obtained by selecting from a range of varying thickness biscuits retained in the upper part of the valve collar.

The simple roller chain drives the camshaft directly from the crank, the distributor and in-line oil pump being driven direct from the front of the crank by a pair of spiral gears. Chain tensioning is another example of the well-tried Weller design on the slack side with a nitrile-faced damper pad opposite to it. Generous water passages in the head eliminate the need for a separate water gallery, there being a single take-off on the rear face by means of a small casting containing the thermostat and the by-pass pipes.

Development of the Rover 2000 extended over a period of more than five years; in fact, there were some unkind stories doing the rounds in the Midlands that several prototypes had to be submitted for M.o.T. tests when the age limit was five years, before the new model was announced. Today, this is not an inordinately long time for such a wholly new vehicle, including the four-cylinder single o.h.c. engine.

Basic elements of the engine are a five main-bearing cast-iron cylinder block, aluminium flat head, Heron-type combustion chamber, with the chain-driven camshaft mounted in a cast-iron separate tappet block and operating the in-line valves through a classic Ballot-type inverted tappet. Cylinder proportions are exactly square with a bore and stroke of 85.7 mm. which contributes the rather deep combustion bowl in the piston.

A feature of the cylinder block is the open sides closed by bolted-on pressed steel plates. The advantages are more rigid cores when casting, ability to ensure complete freedom from sand and swarf in manufacture, easy inspection to ensure clear water passages, particularly between the bores, and to maintain constant wall thickness for the bores by locating for machining on the outside of the cylinders, obviously rigidity can be lost by the elimination of the rigid cast outside walls and a certain amount of trouble on the head joint was experienced on early models before additional stiffening webs were added to overcome the problem.

With such a deep combustion bowel—when compared with the British Ford, for instance—the piston is relatively heavy at 1 lb. 2.5 oz. Another difference between the Ford and Rover design is the deep top land of the latter and one wonders if it is really necessary to locate the top ring below the floor of the combustion chamber. Allowance has obviously been made for this relatively heavy piston as seen in the choice of 2.0 in. diameter copper lead big-ends, 1.0 in. diameter gudgeon pin and 12/32 in. diameter big-end bolts. One of the little believed advantages of the bowl in piston design is that it results in a lower piston crown temperature of some 50°C. than either the flat design as used with a wedge or bath-tube type of chamber or the dome of a hemispherical layout.

An unusual feature of the head is that part of the induction manifold is cast integrally with it. From the 1.65 in. valve head and 1.48 in. throat of the inlet port, this changes from circular to rectangular section where it merges into a rectangular through on top joint face of the cylinder head; this ensures good location for the one-piece port cores when casting, and permits easy fettling and polishing. The trough is closed by a die-cast aluminium plate which is the other half of the induction “pipe”. Each cast-iron valve guide has an internal “O” ring seating in a rectangular recess counterbored in the main stem bore to prevent oil reaching the valve heads. It is surprising how much “coke” can be formed on the neck of the inlet valve, with undesirable effect on filling capacity and added weight if such a precaution is not taken.

Whereas many manufacturers operate their tappets directly in aluminium and exploit fully the economic advantages of die-casting techniques when this is a separate component, Rovers use a cast-iron design with five replaceable Babbitt-lined bearings. There is an obvious cost and weight penalty, but the running clearances when hot are closer with a presumably lower noise level of operation. A most surprising feature of the valve gear is the use of two “biscuits” for obtaining necessary valve clearance. Their functions are hard to visualise and obviously add to valve gear reciprocating weight; perhaps some day the Rover designer will enlighten us on the advantages of the layout.

In single carburettor form the engine develops 91 b.h.p. at 5.000 r.p.m.—9 b.h.p./litre/1,000 r.p.m. which increases to 110 b.h.p. at 5,500 r.p.m. with twin 2.0. in. diameter S.U. carburettors—just 10 b.h.p./litre/1,000 r.p.m. Each of these figures are with all equipment as fitted in the car.

Of all the engines considered in this review the Rover is the only one using a two-stage camshaft drive which, of course, adds to the cost; it does, however, permit a smaller diameter sprocket to be fitted to the camshaft as the required 1-to-2 reduction can be achieved in two steps. The first stage from the crank drives a sprocket off the end of a shaft in the cylinder block and from which the oil pump and distributor are driven; the second stage is a straight run to the camshaft. On each duplex roller chain, which have almost identical sprocket centres, there is a Reynolds hydraulic tensioner on the slack side and a nitrile-faced damper pad on the outside of the chain on the tight side.

Another company which had a good look at the Coventry-Climax design before embarking on their design of their new leaning four-cylinder unit for Saab, known as type P.E.104S, was Standard-Triumph. Like the Vauxhall the ports for the wedge head are opposed, with the exhausts on the outside of the 45-degree cylinder angle and also similarly is an obvious advance guard for a V8 unit to be expected later. With ample water spaces between the cylinder bores—like any good new design should have—there is considerable inbuilt ability to produce different bores and hence engines of varying capacity to meet future needs.

As conceived for 1.5-litre unit originally the engine had a bore and stroke of 76 x 80 mm., surprisingly under-square. As supplied to Saab the proportions are 83.5 x 78 mm. for a capacity of 1,709 c.c.; even lower stroke-to-bore ratios are envisaged for the V8, the minimum possible with the basic dimension being 0.75 to 1. As designed for the Saab the flywheel and clutch are at the front end and the transmission is below the crank, although completely separated from it for lubrication purposes.

It is probably the parameters of this installation through lack of room to provide belt drives and feed pipes which dictated the unusual location of the coolant pump. This is mechanically driven with spiral gears, from a jack-shaft located vertically above the crank in a similar manner to the distributor and oil pup towards the flywheel end of the engine. It will be interesting to watch if this layout is retained on the four- and eight-cylinder units which Triumph will introduce for their own use later.

Crankshaft dimensions are not so massive as the Vauxhall Victor, the mains being 2.125 in. diameter and big-ends 1.75 in., but the webs are really substantial which will permit the use of side-by-side rods on the V8 without making the shaft too weak. The valve angle to cylinder axis is a wide 26 degrees, which has resulted in some shrouding of the inlet valves on the deep side of the chamber. This angle appears to have arisen from the need to provide easy access for the cylinder head nuts fitted on the inlet side, which again are unusual. Five of them on the inlet side are angled at approximately 74 degrees to the block and head joint faces. They are provided with saw-slots for insertion after the head has been roughly located with the set bolts on the exhaust side, and then the nut is applied. Considerable development work was necessary to prevent head-to-block movement with this stud arrangement and finally it was necessary to provide very close clearances between the stud and the hole and in the head.

Like the Aston Martin, Lotus twin-cam and several other designs, no separate tappet block is used to the Triumph, the tappets operating directly in bores machined in the head casting. A difficulty with this design arises from the danger of casting sand being trapped in the valve chest area and extremely careful and efficient cleaning methods during manufacture are necessary to remove it. Valve clearance is by the usual variable thickness biscuit method and there are no stem seats fitted to either valve. To meet the rigorous specification laid down by Saab the stems of each valve are chromium plated and the seats of the exhaust are satellite-faced. Seats inserted into the aluminium alloy head are a high chrome content iron alloy and like most similar designs are shrunk into position by heating the head to approximately 150°C.

A simple roller chain is used to drive the camshaft and jackshaft; between the sprockets of the latter on the upperside there is a curved nitrile-faced blade against which the chain is always loaded as it tries to “straighten-out”, and there is a straight damper on the lower side. There tensioner is located between the crank and jackshaft sprockets and is of the Reynolds hydraulic type. When the extra bank of cylinders is added for the V8 one can imagine this layout being retained and a straightforward two-stage drive in front of the present one, used for the additional camshaft.

In its present form when breathing through a single 1.75 in. diameter Stromberg C.D. carburettor the performance could not be called exciting. Maximum power is 80 b.h.p. at 5,000 r.p.m. and peak b.m.c.p. 144 p.s.i. at 3,000 r.p.m. But with later versions, particularly the V8 fitted with petrol injection on which Triumph now have considerable experience, the prospects could be quite exciting.

It is common knowledge that B.M.C. under the technical guidance of Alec Issigonis investigated and built examples of narrow angle V4 and V6 engines suitable for mounting transversely in a chassis but presumably there were problems, perhaps with balance and cost which led to their abandonment in favour of an in-line four-cylinder for the Austin Maxi. Some of these used cog belts for driving the camshafts and it is perhaps not without significance that an orthodox simple roller chain has been selected for the latest production design. Orthodox is probably not quite apt for there are unusual features in the layout. It is an endless chain and because the tunnel for it is formed integral with the cylinder block and the centrally mounted water pump feeds directly into the front of the block the chain has to pass round the circular inlet feed pipe.

The construction means that the chain has to be riveted in position on assembly for manufacturers always fight shy of the slip link—as used on bicycle chains—for timing gear applications. It has also led to the need for curved nitrile–faced guide plates on the tight and slack side. Tensioning is achieved by using a Renolds hydraulic unit; its position also contributes to improved chain lap on the relatively small crankshaft sprocket which has only 20 teeth.

It is somewhat surprising that a new power unit reputed to carry some £16 m. pounds tooling investment has been designed with no water between the bores. Moreover, the land between each bore is minimal for gasket sealing and this appears to place a limit on any further increases of engine capacity by bore enlargement; engine proportions are already undersquare with a bore and stroke of 76.2 x 81.3 mm. (1,485 c.c.). This undoubtedly imposed problems of adequate valve sizes for an engine producing its maximum power of adequate valve sizes for an engine producing its maximum power of 74 b.h.p. at 5,500 r.p.m. and no doubt contributed to the type of combustion chamber and valve layout used, which in fact is very reminiscent of the Wolseley Hornet, Morris Minor and M.G. of the late twenties. The valves are inclined away from each other at an included angle of 10 degrees and thus their adjacent diameters can overlap each other when viewed in plan. A disadvantage in this type of layout is that the transverse edge of the inlet valve head becomes very close to the cylinder wall on full lift. In fact, on the Maxi it would appear to give minimal clearance so that there is not only a limit on bore size but valve diameter also in terms of future development.

In plan form the combustion chamber is very similar to the modified kidney shape as used on the current A., B. and C. series engines with its claimed inlet swirl inducement and the plug adjacent to the commencement of the swirl path for positively picking up induction mixture to give infallible firing at idle and sharp throttle openings. Inlet and exhausts are on the same side of the head, the latter being the shorter to avoid too much heat being transferred to the coolant; it also permits the use of relatively long induction parts of the inlet valve.

A chilled iron camshaft is carried in a separate die-cast aluminium tappet block with three integral bearings meaning that the complete assembly must be removed for tappet clearance adjustment. It is certainly more difficult than if separate bearing caps were used, but is obviously cheaper. There should be no problems if the valves do not sink on their seating’s during running and this really depends on seat temperature controlled largely by the amount of coolant around them. An unusual feature of the tappet adjustment is that the biscuits are located in recesses on the underside of the tappet rather than extensions of the valve collars. This would appear to make machining more difficult as the chilled surface on the tappet face runs through the head thickness and there seems to be no technical advantages in the method adopted.

It is perhaps not without significance that sintered iron rings are used for the three compression type on the piston—the number is also unusual today as most manufacturers find two to be quite adequate. Sintered rings with their porous matrix have a natural ability to absorb oil and are thus excellent in anti-scuffing properties. Mercedes Benz, whose engines also have no cooling water between cylinder bores since they were enlarge to 2.5-ltires and above, have adopted molybdenum spraying on the surface of their two compression rings to combat scuffing which is basically a welding phenomenon resulting from high rubbing temperatures.

In 1957 Dr. Giacosa, director of vehicle engineering at Fiat, read a classical paper to the Institution of Mechanical Engineers entitled “Some problems concerning the small utility car”. This was before the introduction of the B.M.C. Mini, and in it he said: “It may be gainsaid that it would have been possible to reduce the length of the front-wheel-drive car if only the engine, contrary to the arrangements shown were fitted crosswise; that is parallel to the wheel axis. This perhaps is true, but as will be apparent further on the problem of power plant suspension would have been very difficult, if not impossible, to solve in a simple way. I believe that at present not even for a small economical car can an engine suspension likely to transmit severe vibrations to the bodywork, be regarded as acceptable.”

A most profound statement indeed and it outlines the difficult problems which B.M.C. were to encounter in their range of transverse engine car, and yet Fiat, first with the Autobianchi Primula and now with the Fiat 128, have followed this layout exactly. How, then, have the basic problems of transmitted vibrations been overcome? The answer is a clever three-point mounting. On the rear face at about mid-point along the cylinder block, just below crankshaft centre line and corresponding to the engine’s C of G is a perforated rubber pad to give controlled stiffness vertically and movement laterally. Mounted high up at each end of the engine is a large circular section rubber bushing, to one of which is attached a stay which controls reaction form engine torque.

This 1,116 c.c. engine with a bore and stroke of 83 x 55.5 mm. is extremely over-square—an s/b ratio of 0.625 which is the lowest of any production car today and exceeded only by the Ford Anglia. Obviously this allows the use of very generous size valves, the respective head diameters being, inlet 1.42 and exhaust 1.2 in. diameter and these should permit much higher rotational speeds than 6,000 r.p.m. at which the maximum installed power of 55 b.h.p. is now produced. With such a short stroke the crankshaft is obviously very stiff and no doubt contributes to the use of nodular iron for this component as a Fiat first. If the usual difficulties of technical translations are also accepted there is also another innovation in the use of Malleable cast-iron for the connecting rods; certainly assessing their dimensions form the arrangement drawing a material different from forged steel is used.

The combustion chamber in the die-cast aluminium head with seat inserts—almost universal practice with Continental manufacturers even on the cheapest engines—is an extremely compact design, but the valves appear to be somewhat shrouded on the deep side of the chamber rather like the Triumph-Saab engine. There is however a subtle difference from the orthodox. On the head face corresponding to each bore there is a shallow machined spherical depression which presumably does not completely quench the end gas in the rather large squish areas and is perhaps aimed at reducing the levels of unburned hydrocarbon in the exhaust—by far the most difficult problem to solve so far on emissions. A die-cast aluminium tappet block also encloses the valve gear and there is a clever arrangement for obtaining tappet clearance similar to that used on the 124S and 125. Clearances are obtained by inserted hardened steel discs of varying thickness into recesses formed in the top face of the bucket-type tappets. Thus no hardening is required on the tappet face and the operation can be performed readily by using a relatively simple special purpose tool to depress the tappets and which is inserted through the angled face top cover. Engineers may think that such a relatively thin biscuit could be subjected to deflection arising from bending loads when the cam is approaching and receding form the nose. Presumably the other Fiat engines mentioned have been satisfactory in this respect and Honda have used a similar design with a hardened disc spun into a light alloy body both on the competition and production engines.

A Cog-belt as on the 125 is used for driving the camshaft and a short jackshaft for the distributor, and oil pump housed in a deep pressed steel sump. It is tensioned by jockey pulley, having a preloaded spring and clamp adjustment and operates on the back of the belt on the slack sides. As located on the Fiat it increases wrap-over on the crank and camshaft pulleys which is important on cog belts. If the wrap is insufficient, particularly in conjunction with small pulley diameters, it is very easy to jump a cog and so slip the timing.

A further interesting feature of this engine is the location of the thermostat on the inlet side of the coolant pump. This practice is used also by B.M.W. on their current range of engines and who hold the patent rights for it. The claims made are that even temperatures are more easily maintained—which is difficult to visualise; furthermore, there would seem to be a restriction to the inlet flow with a possible increase of cavitation at the pump impellor eye.

Longest established advocates of the single o.h.c. principle are, of course, Daimler-Benz, who use it throughout their range of engines, including the automotive diesel. Furthermore, they have an unusual type of combustion chamber worthy of analysis, tracing its development from the now superseded 300-engine series. In that range the top face of the block was machined at an angle which formed a pocket in conjunction with special piston crowns. The valves were displaced to each other when viewed end on; thus the inlet opened directly into the cylinder bore and was completely free from any masking and the exhaust was directly above the pocket formed in the block. This construction allowed the injectors to be fitted into the cylinder block when direct fuel injection was used as on the 300 SL. Also the flame front during combustion spread from the hot exhaust area towards the extremely well-cooled region between the angled piston crown and ahead.

Similar principles apply with the head design used on the current 200 engine series, but the layout is simpler and provision is no longer made to inject fuel directly into the cylinders on those models fitted with petrol injection. Joint faces of the head and block are flat and horizontal and a combustion is formed to one side of the head and in which the exhaust valve is located. The light alloy head has shrunk inserts of very hard material.

In fact, the whole of the valve gear is designed to extremely high standards, probably arising from the fact that a relatively small engine powers a very large car at very high speeds. The valves are Stellite-faced on the seats and each is fitted with positive rotators—a device which imparts a slight and controlled spin to each valve when seating which presents the build-up of deposits which are mainly oxides of lead. Exhaust valves have hollow stems filled with sodium which melts at running temperatures and transfers heat from the head to the stem and thence through the guides to the coolant. It is claimed to reduce valve head temperatures by 50 C. and would thus lower it to below 600 C.—still a bright cherry red colour–at maximum power.

All Mercedes engines use a finger-type rocker between cam and valve which originally had a screwed adjustment on the stem end as shows in the cross-section of the 300 series engine, but now is adjusted by raising or lowering the spherically-ended pillar at the pivot end. Incidentally there is no locking device provided, the pitch of the two threads being fractionally different to give a controlled degree of interference. This arrangement was first used by Porsche, who could still hold the original patent for it.

Recently the most important change in the 200 engine range is the adoption of seven main bearings for the crankshaft. Although this tends to lower the natural frequency of the shaft—always a troublesome factor in a six-cylinder engine—it has undoubtedly been accommodated in the damper design. This bearing is related to the use of longer strokes which, in a four-main-bearing design, would increase the tendency to bend under load which in conjunction with the gyroscopic inertia of a heavy flywheel rotating at speeds in excess of 6,000 r.p.m. can lead to severe vibrations.

Duplex roller chains are used for driving the camshaft and other auxiliaries and it is a feature of all Mercedes designs that jockey and sometimes idler sprockets are positioned to give very good chain warp on all sprockets. An example is seen on the V8 type 600 which has a single run chain for the camshaft in each bank and uses to less than five camper pads and sprockets, one being an adjustable jockey hydraulically loaded.

The 600 engine uses a wedge-type head, but the valve-operating gear is to established practice. This difference is difficult to assess but may be related to the provision of better inlet ports, on the insides of the Vee.

With its increasing importance and rapid production rise the Japanese industry has been quick to adopt the single o.h.c. Examples are seen on the Nissan Cedric, Datsun Bluebird Toyota Crown and Mazda 1000. All are fairly orthodox in conception, but it was considered interesting to publish an example of the Nissan Cedric as an illustration of Japanese engineering, which is of a very high standard—equal to anything produced in Europe in its price range.