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This article is reproduced here by kind permission of Roger Bywater.
 

The origins of the Jaguar V12 engine go right back to about 1954 when it was being thought about sufficiently seriously for drawings to have been produced even though no hardware was created. The motivation, of course, was to carry on from the success of the Le Mans winning XK which clearly could not long remain fully competitive in top league racing. The concept at that time was straightforward enough - more or less a handed pair of 2.5 litre XK engines joined at 60 degrees with a common crankcase and crankshaft.
Following the factory withdrawal from racing the V12 scheme languished until the early 1960s when it was resurrected with a view to taking on Ford and Ferrari, by then the rivals for glory at Le Mans. That such a race engine could also spawn a very impressive production engine, with great appeal to Jaguar enthusiasts all over the world, was a possibility which Sir William Lyons and his design team were very much aware of. Of course the XJ13 was the prototype race car but by the time it turned its wheels it was already obsolete and it was apparent that the opposition were moving ahead technically at a much faster pace.

Paradoxically, although it fell short of the expected power output, the engine was probably good enough to win at that time, Ford finding success with production based pushrod V8s. However, despite wonderful aesthetics the XJ13 was obviously a long way behind in terms of chassis and suspension design and aerodynamically had not moved on much from the low drag principles which had helped to make the D Type a success. It is easy to gain the impression that the project somehow lacked urgency but maybe there was just a lack of appreciation of how much racing had changed since the fifties. How else can one explain the car being built at all, for Lyons was not a man to allow his engineers to squander time and effort on a project with no future? In any event it was becoming clear that Ford were determined to dominate Le Mans at almost any cost and Jaguars chances of success against such commitment were thin indeed so it is probably for the best that the project was allowed to quietly die.

The fundamentals - combustion chamber and port design

By the time the first V12s were actually built the original concept had evolved somewhat and whilst the cylinder heads were clearly of XK parentage the inlet ports had shifted to a downdraft (Fig.1) position between the camshafts. This layout was not unique to Jaguar and almost seemed to be a fashion in the early 1960s with BRM and Ferrari using it in Formula 1 and also Ford going the same way for their advanced 4 valve, 4 cam, Indy V8. In those days the advantages of the modern, narrow angle, cylinder head design had not been recognised and relatively wide valve angles were common leaving little space between the heads of vee engines to find room for inlet porting. The downdraft layout with direct flow into the cylinder seemed to offer a better solution and whilst it was known that the complex Mercedes M196 engines which earlier did battle with the D Type were far from remarkable for their performance despite their high specification, this was generally considered to have been because the ports and valves were too large, rather than a failing of the downdraft head layout.
Jaguar had to do a lot of work on the inlet port geometry to get the same specific power from the twin cam V12 as had been achieved with relative ease a decade or so before from the XK. A contributory factor was that the pronounced hemispherical combustion chamber which had worked so well on the long stroke XK was not nearly as efficient for a short stroke design like the V12. Only after bringing the inlet ports to an angle barely 40 degrees away from the valve axis instead of the original 60 degrees (Fig. 1) could 500 b.h.p. be exceeded and even then the torque band was unimpressive. Harry Mundys 1972 paper to the Institute of Mechanical Engineers explained all this in detail and concluded that the downdraft layout gave poor flow characteristics and reduced combustion efficiency because of indifferent charge turbulence. Once Keith Duckworth demonstrated the superiority of the narrow angle 4 valve layout when he created the Cosworth DFV F1 engine in 1966-7 the matter was settled and the downdraft port was finally dead and buried. Interestingly, Duckworths well-reasoned and innovative 4 valve design was prompted by his experiences with an earlier cylinder head layout which was destined to appear on the production V12.

Returning for the moment to the twin cam V12, it simultaneously followed 2 development paths, one for the all-out racer and the other towards a more refined and gentle creature for a production car. The noisy partly geared cam drive of the high revving racer became a multiple chain drive: moderate cams and port sizes sacrificed power for drivability: multiple stack Lucas fuel injection was replaced by 6 SU carbs. Yet still it was not good enough, quite apart from being bulky and heavy. Now shortly before this time Coventry Climax, by then a subsidiary of Jaguar, had introduced a range of industrial engines which used a simple flat OHC cylinder head design with bowl in piston combustion chambers. Under the legendary Walter Hassan, Climax had found that this layout provided a very good balance of performance, economy and detonation resistance, and was compact and easy to manufacture. It was not long before it was realised that here could be the answer to the road going V12s problems - simple and compact cylinder heads with single camshafts, a simplified cam drive needing only one chain, and a substantial weight saving. Single cylinder test bed work showed that performance would be more than adequate, in fact, mid-range performance was better than the twin cam so the day came when V12s of both types were fitted into Mk10 saloons and compared on the road. It was no contest really and only the aura of the twin cam remained - but not for long.

This was a very significant point in the genesis of the V12 and one which is somewhat puzzling. The bad experience which prompted Keith Duckworth to arrive at the DFV design had been with his own Formula 2 SCA engine from 1964 - a flat head, single cam design with bowl in piston combustion chambers. Whilst it performed well enough to win a lot of races it always had a fundamental combustion problem and needed a lot of ignition advance to work properly, just as the twin cam V12 did. Certainly Duckworth was amazed to hear that Jaguar intended to proceed with the V12 as a flat head engine. Perhaps the conclusions reached by Climax were because of a fortunate combination of bore and stroke, valve sizes or whatever, but the usage pattern of an industrial engine may also be significant, spending long periods at about 75% load, rather than full load as in a race engine, or mostly light load as in a road car. It is not a criticism of Walter Hassan, who by now was deeply involved with the V12, or any of his team to point out this quandary. In his 1972 paper to the SAE Hassan admitted that at that time the knowledge of what happened to the charge in the cylinder of the flat head V12 was very much open to conjecture and that charge turbulence may well stagnate in some conditions. Nevertheless there can be no doubt that the flat head V12 (Fig. 2) was a very much more practical proposition than the twin cam version ever was.

In arriving at this point a great deal of experimental work had been done with different port layouts and spark plug locations but gradually the design evolved into the final cross-flow layout with a steeply inclined inlet port which remained to the end. Compression ratio was originally intended to be 10:1 for production, even 10.6:1 being considered for a time, but impending emission legislation and the disappearance of 5 star fuel decreed a change to 9:1 for European markets and a miserable 7.8:1 for 91 octane lead free fuel in the USA. Those who experienced driving them always maintained that the original 10:1 EFI engines were the best by far, but it was not until 1980 that such an engine was actually on sale for about a year prior to the arrival of the HE. The original 3.4 litre version of the XK was always regarded as the best of the line and the same can be said of the rather rare 10:1 V12 rated at 300 b.h.p.

At this point a slight diversion is necessary because in the early 1970s a couple of 4 valve V12s were built, one for road use, the other as a potential race unit. These were modern narrow valve angle designs and the race version was soon developed to produce a very impressive 630 b.h.p. from 5.3 litres using a standard crankshaft. The one and only prototype was loaned to TWR in the 1980s for them to study prior to producing their own 4 valver. Unbelievably, after they had stripped it and looked it over they threw it in a skip. One has to wonder if there is a scrap man somewhere who recognised what it was and saved it from the furnace. If so, perhaps it may yet turn up but what value would be placed on such an engine now?
As the 1970s progressed the thirst of the V12, even with electronic fuel injection, became a matter for serious concern and a number of ways to improve it were tried. Stratified charge pre-combustion chambers were in fashion and some low key experiments took place with devices which screwed into the spark plug hole of a single cylinder test engine. Another experiment which made no headway was a small inlet port concept intended to generate stronger turbulence. Ceramic coatings on heads and pistons to both reduce friction and cut heat loss showed some promise, but of course the real problem had been identified correctly by Keith Duckworth some years before - the flat head combustion chamber design was just not good enough. Only a substantial improvement would be worthwhile yet a major redesign was out of the question, so how could it be achieved? It so happened that around this time a Swiss engineer called Michael May was claiming some impressive results from a high turbulence combustion chamber based on the conventional 2 valve in-line configuration then in widespread use. Most manufacturers looked into it without finding any great advantage but it arrived just in time to save the Jaguar V12.

The rather unimaginatively named May "Fireball" combustion chamber (Fig. 3) consisted of a more or less circular pocket around and under the exhaust valve. The essential feature was that squish action created as the, now flat-topped, piston approached the cylinder head, was directed by a channel from around the inlet valve to impinge tangentially into the chamber to generate a strong swirl effect under the exhaust valve. With the spark plug relocated to the side of this chamber, the entire concept was a clever interpretation of accepted wisdom for burning lean mixtures at high compression ratios. One might easily argue that any combustion chamber design which generated some much needed turbulence in part throttle conditions would have done the trick but there can be no doubt that the May design, aided by a judicious raising of the differential gearing, transformed the fuel efficiency of the V12 and ensured its survival. Some prototype engines ran at diesel-like compression ratios of more than 14:1, but 12.5:1 was decided on for the introduction in 1981 of the "HE V12" as it was christened. The May combustion chamber design continued for the remainder of the production life of the V12 with the compression ratio lowered later to 11.5:1 for catalyst engines and finally to 11:1, lower compression ratios giving a vital benefit of quicker catalyst light up.

Before leaving the subject of combustion chamber design those of the race engines produced by Broadspeed and TWR cannot be ignored because they were different again.
The Broadspeed engines of the mid 1970s had shallow chambers formed in the cylinder head to various shapes, the most effective being yet another due to Cosworth, originally used on their F3 MAE engine of the 1960s, resembling an opened out version of the classic BMC / Weslake heart shape.

TWR used flat head engines with chambers formed in the piston crown from deep valve pockets merging into a central bowl almost exactly like the Cosworth SCA Formula 2 engine of a couple of decades or so earlier which had caused Keith Duckworth so much frustration. Like the SCA they were not ideal but were good enough to win races and it will be no surprise to learn that Cosworth played a significant part in their creation. TWR also built some 4 valve V12s but fuel consumption regulations put them at a disadvantage and the added weight at the top of the engine caused handling problems which negated the extra power, so they were never used in anger.

Fuel injection or carburettors?

Having decided on the flat head route back in the late 1960s the Jaguar team then had to decide what sort of fuel system should be used. The embryonic AE Brico electronic fuel injection system showed great promise and on the V12 gave a substantial power advantage over carburetters yet, strange as it now seems, was perceived as being more of a challenge than carbs to be able to satisfy impending legislation regarding exhaust emissions. Jaguar never had to decide between the two because the Brico board of directors, faced with a longer than expected development program, got cold feet and scrapped the project. Aston Martins DB6 and Ferraris 246 Dino were also left in the lurch by this decision. So now there was no choice, the V12 had to have carburetters - two to each bank on overhung manifolds (Fig. 2) to obtain a reasonable ram length for maintaining torque (ideally, had there been space, still more ram length would have been beneficial). The overhung manifold design had one very serious drawback - cold starting required vast overfuelling just to get enough combustible mixture up over the cam covers for firing up, earning the V12 the dubious honour of being the first UK home market engine to need air injection into the exhaust ports to burn off the excess fuel. Really, when one looks at a carburetter V12 it is hard to imagine that it was anything other than a stop-gap measure until a suitable EFI system became available. It has been suggested that if suitable downdraft carbs had been available they would have been used, but this must be doubtful if only because of the existing location of the distributor and spark plugs, and in any event the individual tracts would have been far too short for any useful ram effect. The team responsible were respected and experienced engineers and would surely not have compromised the installation of carburetters by placing the distributor and plugs in the centre of the vee unless they envisaged only using fuel injection from the outset.

Fortunately the same original work by Bendix on which the Brico system was based had already spawned the Bosch D Jetronic system which had been used successfully by Mercedes, VW and one or two others, so it was arranged that Lucas would develop a version of it to suit the V12. In fact the two systems only differed in detail except that D Jetronic could not drive 12 injectors. This was easily resolved by the addition of an amplifier which also changed the polarity of the drive circuit to take injector current to ground as is now accepted practice. The inlet manifolds and throttle assemblies (Fig. 2) designed for the original Brico engines were not a lot different even to those found on the last V12 to be built. A curious early feature was that the throttles were aerodynamically shaped castings mounted on a solid spindle, but they were soon replaced by conventional throttle discs in slotted spindles. It is interesting that despite virtually all other EFI Jaguar engines using airflow meters of one sort or another to measure engine load, all injected production V12s from the very first to the last relied on manifold pressure measurement.

By the time the EFI V12 was launched in 1975 it was clear that V12s with carbs were never going to meet ever tighter US emission standards because of the poor cold start performance. Of course D Jetronic was not sophisticated enough to run with Lambda (exhaust oxygen) sensors as modern systems do so these early emission V12s with low (7.8:1) compression ran with oxidising catalysts and air pumps just as carburetter engines had done. NOx emissions were dealt with by incorporating solenoid operated EGR (exhaust gas recirculation, an established method) valves into the EFI system mounted under the throttles, bringing the unexpected problem of exhaust noise being clearly audible from the air intakes, cured by relocating the EGR take off points. Performance of these engines, as with all emission engines of the period, was very stunted compared to the 9:1 compression European version which was very much better than the carburetter engines. The engine itself did not change at this time so the advantage of EFI, which should have been there right from the start, was very clear.

Electronic fuel injection evolves

Bosch D Jetronic, whilst a very important ancestor of all modern EFI systems, was in fact rather primitive, having a multiplicity of transistors and other components using obsolete analogue techniques based on voltages and simple timer circuits. The vaguely timed one-shot-per-cycle fuel delivery was not good enough for the forthcoming HE engine so something better was needed.
The definitive 10:1 compression V12 launched in 1980 used the new Lucas P Digital EFI system employing a main integrated circuit chip, consisting of a large number of transistor elements configured in manufacture to generate a map of the fuel requirement spread over hundreds of data points according to engine speed and load. Temperature and other corrections were still applied by analogue means but the important advance was that feedback correction from oxygen (Lambda) sensors in the two exhaust streams was now possible with the injectors for each bank corrected independently. This meant that modern three-way catalysts could be used to advantage in emission sensitive markets. An unusual feature of the Lucas systems which remained right through into the 1990s even as the ECUs evolved, was the complex dual output circuit for the injectors grouped for each bank. Basic injector pulses passed via a resistor pack whilst a current sensing bypass circuit produced direct additional short pulses to maintain the required overall injector current. Firing the injectors of each bank alternately twice per cycle satisfied the more critical HE mixture requirements when it arrived a year later

The Digital ECU type 6CU was superseded in 1986 by the pin compatible, microprocessor based 16CU type, from which in turn evolved the 26CU and 36CU with increasing levels of sophistication.

These were succeeded in 1994 by PECUS (Programmable Electronic Control Units System) an advanced management system incorporating ignition control and of much greater capability than its predecessors to meet ever more demanding emission legislation.

As an aside from all these mainstream production systems, during the 1990s Zytec, a respected manufacturer of racing engine management systems, provided quite a number of control units for low volume applications such as the XJR-S.

Electronic ignition - an essential ingredient

A 12 cylinder engine running at 6000 r.p.m. has just 1.666 thousandths of a second between sparks. It is asking rather a lot of a contact breaker to function at this rate and give acceptable life so it was obvious right at the outset that the V12 would need some sort of electronic ignition system. Fortunately Lucas had already developed their OPUS system for high revving racing engines so there was no need to look any further and with minor changes over the years it served the V12 reasonably well for its first decade, running with conventional centrifugal and vacuum control of ignition timing.
The HE with its 12.5:1 compression and lean mixtures was even more demanding but by this time constant energy ignition systems were available which could maintain a consistent charge current through a low resistance coil over a wide speed range. A clever trick was still needed to meet the abnormal energy requirement of the HE V12 - the coil had a second, non-firing, coil connected in parallel thereby doubling the rate at which energy built up in the system. A centrifugal advance mechanism was retained but because of the large amount of advance needed by the HE engine to burn lean part throttle mixtures the vacuum advance system became a complex mass of pipes, valves and solenoids that worked better than it looked. The long term reliability of such a system would always be questionable so the need for long term emission control durability meant something better would be needed, however this system remained in limited use on Series 3 V12 saloons into the early 1990s .

In 1988 the Marelli system appeared using modern programmed mapping techniques to control ignition timing precisely over a wide range of conditions. Two coils were still used but now one was allocated to each cylinder bank and they fired alternately through a dual level distributor. Timing advance was deduced from various sensors and triggered from a 3 toothed rotor behind the crank pulley. Introduced at the same time that compression was dropped across the board to 11.5:1 the sophistication of this ignition system prevented the power loss from being too significant.

In the final years, ignition functions fell under the control of Zytec or PECUS full engine management systems as noted earlier.

Mechanical tribulations

By and large the V12 was a very reliable engine, as one would expect, but it was not without problems although few were really serious. The crankshaft was pretty well "bomb proof" being a substantial forging from EN16T steel and Tuftrided to create a hard wear resistant surface. Overlap of the main and crank journals was sufficient to permit straight through oil drillings (Fig. 4), carefully worked out to deposit oil at the optimum position to lubricate the crank pins under load. The dreadful sludge traps used on the XK were pointedly avoided. The rope seal at the rear main bearing was never a very happy arrangement and if it dried out through prolonged standing would either leak or worse, rub on the crank and heat it sufficiently to cause failure of the rear bearing, but such problems were rare and later engines had a proper neoprene seal anyway. Of more concern was a tendency for crank pulleys to work loose, fret and cause damage to the locating keyway. This never seemed to happen on early engines yet those made during the 1980s were susceptible. Perhaps the compression pressures of the HE induced some peculiar torsional loading which was not present before.
It has been said that the open deck construction of the cylinder block, chosen to simplify the casting process, lacked rigidity. This is true but the multiplicity of studs to clamp the cylinder heads in place gave ample integrity to the completed structure. Main bearing shells would sometimes display witness marks indicating some movement but this never caused trouble in normal operation. On the other hand the abnormal loads generated by a bearing failure could certainly cause enough distortion to necessitate line-boring before fitting a new crank and bearings. Fortunately bearing failures were exceedingly rare and usually followed some sort of neglect. The generous spacing between cylinder bores is perhaps a matter for debate but then again the substantial structure and the volume of coolant therein may well have contributed to the refinement for which the engine was noted by absorbing vibrations. Certainly it is possible to open up the bore size from the standard 90mm to 98mm and to swing a crank throw in excess of 90mm instead of the original 70mm (98mm by 90mm gives 8.1 litres, but with a bit of work 9 litres would be feasible) but the bigger engines hardly ever seem as sweet as the original 5.3. Most observers would say that the final 6 litre (78.5mm stroke) was a bit rough but what is puzzling is that one or two experimental (84 mm stroke) 6.4s built in the 1970s ran like 5.3s. No modern engine would be designed with so much capability for stretching and ultimately this may have sealed the V12s fate. Rapid warm-up is essential for compliance with modern emissions legislation and an engine system that contains 5 or 6 gallons of coolant, as in the case of the V12 Jaguar, is struggling under a hopeless handicap.

At the front of the crank was mounted the epicyclic oil pump - an unusual choice which absorbed a fair amount of power, but apart from some noise problems in the early days it gave no trouble at all. Next to it was the timing sprocket from which the duplex timing chain had a long run round both camshaft sprockets and the central jack shaft. Some chain thrash was evident initially but with the aid of visual investigation via windows in the timing cover, damper pads were soon in place to get rid of it. In fact the timing chain of the V12 probably has an easier time than most with plenty of overlap of both the firing impulses from the crankshaft and the torsional loading from the camshafts. The design of the blade chain tensioner is more questionable. Attractively simple, it works well for most of the time with a clever one-way jamming arrangement which takes up any slack, with a simple means of disengagement if the chain needs to be relaxed for repair work. The trouble is that if it starts to slip it soon becomes useless and then a major engine strip is needed to replace it.

The slip fit cylinder liner arrangement showed strong Coventry Climax influence having a short "wet" section exposed directly to the coolant at the top located in an aluminium surround extending up out of the crankcase (Fig. 2). The dimensional changes with temperature were thereby maintained within reasonable limits so "nip" at the head gasket joint would not relax as the engine warmed up. This method had been well proven by the successful Climax V8 F1 engine of the early 1960s, after some early hiccups before the dimensions were right, so gasket sealing was never a problem on the V12.
Not surprisingly, Climax-like features are to be seen in the design of the valve gear, yet this was one area which was a source of niggling problems over many years. The complaint was excessive tappet noise, which was very puzzling because the cam profile, cam follower (tappet) and much of the valve gear differed little from the XK.

The root cause is that the cast iron followers run in an aluminium cam carrier rather than cast iron sleeves as on the XK so the running clearance varies with temperature. All engine components are manufactured within certain tolerance limits and in this case the largest permissible follower must be able to run in the smallest permissible carrier bore - BUT - this must be so down to minus 40 degrees in a severe North American winter. At the other extreme a low limit follower mated with a top limit carrier bore could be quite sloppy in a hot engine. The actions of these components are exceedingly difficult to analyse but it does seem that cam profile, tappet clearance, side movement, rock-over at peak lift, tappet rotation and valve concentricity with its seat, all play a part in the generation of what is perceived as tappet noise. The range of side clearances involved is not great, ranging from about 0.0005" to 0.002" at room temperature, so effort was concentrated on cam profiles with gentle take up which would be less likely to provoke the followers to rattle about. An acceptable cam was introduced in the early 1970s but it was always a good rule of thumb to set the inlet valve clearances to tight limit and exhausts to wide limit (being hotter the running clearance works out about the same). By 1993 the market required some further cam profile refinement although it should be noted that the fundamental valve motion hardly changed over the years.

Manufacturing and alternative configurations

Throughout its life the XK engine was made by what might be called knife and fork methods on out-of-date machinery. A complex engine like the V12 could not have been viable without using modern automated processes (Fig. 5). The cost for such a facility, reputed to have been about £3,000,000 in the late 1960s, seems like peanuts today yet the need to raise such funding prompted the sale of Jaguar into the BMC group although this also opened the way to much needed dealerships around the world. The need to replace the aging XK was an issue of obvious importance but surely it was not thought that the V12 would be the only engine for the future. Indeed, had it been known that the various alternative engine concepts based on using segments of the V12 would all prove to be unusable the V12 might never have got off the ground. These included a 60 degree 3.5 litre V8, rejected for lack of refinement, and a 2.65 litre slant six using just one cylinder bank which was too small. Increasing the stroke to 90mm would have resolved that problem but then the extra block height could not be accommodated on the V12 machinery. However a number of slant sixes using cut and welded up V12 crankcases were found very useful for testing 4 valve and May cylinder heads. An advantage of the original flat cylinder heads had been ease of manufacture but the May combustion chamber for the HE required some further operations which cost about £500,000 to implement. In these days of Formula 1 racing engines reputed to cost £1,000,000 apiece, the V12 facility looks like a bargain. It seems sad that it is now shut down for ever, but at least the V12 did not linger on like the XK waiting desperately for a successor. The V12s successor, the AJ-V8, is already here and carving its own place in Jaguar history, but the V12, despite being a little paunchy and lacking a real punch, was a class act and will always remain just that little bit special.

Roger Bywater
AJ6 Engineering