Leaping Jaguar

Page under construction, my apologies for the inconvenience, I will finish it as soon I'm done with my garden brick wall!

Article bellow is extracted from Jaguar World, written by Roger Bywater, from well known Jaguar tuning company, AJ6 Engineering.

V12 Story II

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 carburetters?
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.

Leaping Jaguar 2004 Copyright.

All rights reserved.

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 ingrediant
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.

You are viewing the text version of this site.

To view the full version please install the Adobe Flash Player and ensure your web browser has JavaScript enabled.

Need help? check the requirements page.

Get Flash Player