The New Generation Jaguar XK XK8 And XKR | Part Two

The new 4.2-litre V8 in detail

"We considered a number of bore and stroke configurations before deciding on the engine capacity we needed in order to deliver the performance we wanted. The increase to 4.2-litre may not seem that much, but it has allowed us to achieve all of our goals." Ron Lee, Group Chief Engineer

Jaguar’s new AJ34 family of engines was developed at the Jaguar Engineering Centre at Whitley in Coventry. Designed to deliver more performance, better economy and improved refinement when compared with the original AJ28 AJ-V8 from which it has evolved, the AJ34 retains the same compact, 90 degree V8 dimensions but is structurally even stiffer.

Capacity is increased to 4196cc, with bore and stroke now 86.0 x 90.3mm. With a ribbed block and cylinder heads, the engine is very light but extremely strong, its best-in-class stiffness minimising radiated engine noise and increasing mechanical reliability. The four-throw, six balance-weight crankshaft is supported in five main bearings and the connecting rods feature fracture-split big-end journals, for strength with light weight and perfect balance. Mechanical refinement is improved, reducing engine-generated noise and vibration even further.

The four chain-driven overhead camshafts are ‘rifle-drilled’ to save weight and improve performance. Revised camshaft drive sprockets and finer-pitch inverted-tooth chains further reduce the already-low radiated noise levels and mechanical load. The camshafts operate four valves in each pentroof combustion chamber, and direct-acting aluminium bucket tappets need no adjustment for the life of the engine. The unusually narrow 28 degree valve angle benefits both combustion chamber shape and overall packaging.

The new 4.2-litre V8 benefits from continuously Variable Camshaft Phasing. A hydraulic actuator on each inlet camshaft is operated by a pair of electronically-switched oil-pressure control valves, activated by the engine control module – which contains maps of engine speeds and loads. The system continuously adjusts the phasing of the camshafts, from maximum advance to maximum retard in less than 0.7 seconds. The inlet timing is therefore optimised at all times, according to load and other conditions, to produce faster engine response and optimum performance at all engine speeds – with more torque at low speeds and maximum power at high speeds. It has the added advantage of providing internal exhaust gas recirculation, reducing NOX emissions by slowing down the combustion rate and reducing HC emissions by reburning the exhaust gases.

With exactly 300bhp (224kW DIN) at 6000rev/min and 400bhp (298kW DIN) at 6100rev/min, the new engines are respectively 3.5 and 8.1 per cent more powerful than their predecessors.

The improvements in torque are equally significant. Maximum torque for the normally aspirated 4.2-litre V8 is now 420Nm (310lb ft) at 4100rev/min, while the supercharged engine peaks with 553Nm (408lb ft) at just 3500rev/min. That said, ‘peaks’ is hardly the way to describe the torque delivery from the supercharged 4.2-litre engine. The torque curve for the new V8 sits significantly above the equivalent curve for the previous 4.0-litre engine, but once again it is impressively flat across the whole of the engine’s usable rev range. That includes at least 86 per cent of its peak torque all the way from 2000rev/min through to maximum engine revolutions.

To match the increased output and the increased mechanical and thermal stresses that supercharging imposes, the new XKR V8 has stronger pistons, forged from single billets of aluminium alloy. It also features oil jet cooling on the underside of the piston crowns, for maximum cooling and minimum distortion.

High performance and high efficiency

Maximum road speed may be electronically limited to 155mph, but the combination of increased torque across the rev range and six speeds rather than five, make the XK’s mid-range performance even more flexible – and powerful – than ever. In both versions of the engine, a portfolio of changes beyond the increased capacity and revised architecture contribute to increased power output and increased efficiency. Branched exhaust manifolds are used to allow tuning of the primary length of the exhaust manifolds and pairing of the manifold branches of appropriate cylinders, which helps to improve volumetric efficiency and so engine performance.

The original AJ-V8 featured an innovative composite-moulded air intake system, with integral fuel rails for the ultra-accurate positioning of the injectors relative to the cylinder heads. This is improved with the introduction of new multi-hole injectors to refine the spray pattern in the combustion chamber, increasing power while aiding economy and therefore emissions.

Emissions are further reduced by increasing ignition retard during cold starts, minimising exhaust catalyst warm-up time and reducing the need for a richer fuel mixture. Close-coupled catalytic converters in the five-box stainless steel exhaust system begin to operate within 30 seconds of start-up. Exhaust manifolds with low thermal inertia further reduce warm-up time and the new V8 retains Jaguar’s patented low-volume, high-velocity cooling system. This uses minimum volumes of coolant in carefully designed cylinder head and block cooling passages, again helping the 4.2-litre engine achieve a very fast warm-up time (less than four minutes in a simulated urban test cycle), to the benefit of both economy and emissions.

The Electronic Management System controls all primary engine functions including fuel delivery, ignition timing and throttle control, plus secondary systems such as variable cam phasing, exhaust gas recirculation, fuel purging, knock control and cooling fans. Its ‘drive-by-wire’ function operates the engine’s throttles electronically in instant response to the driver’s positioning of the throttle pedal and incorporates a ‘limp home’ facility, which in the unlikely event of a system failure allows the car to be driven to safety or assistance, at a restricted speed. The hydraulically-tensioned camshaft chains and the seven-rib spring-tensioned poly-vee belt which drives the alternator, power steering, water and air conditioning pumps are maintenance free, with a service life in excess of 100,000 miles (160,000km).

The new 4.2-litre XKR engine uses a rotor-type Eaton supercharger, belt-driven from the front of the crankshaft. It also features two industry firsts on a production car supercharger – helical rotor gears for low noise, and coated rotors for increased efficiency. These innovations have contributed to a supercharger speed five per cent greater than that of the supercharger in the 4.0-litre engine, further benefiting volumetric efficiency.

The AJ-V8 Story

The AJ-V8 engine was Jaguar’s first ever V8, and only the fourth series production engine type in the company’s history. Following the classic twin-cam in-line sixes, the ground-breaking V12s and the second-generation all-alloy 24-valve AJ6 sixes, the engine made its public debut in 1996 in the XK8. In 1998, the new XKR adopted Jaguar’s first supercharged production V8, which was at that time newly introduced in the XJR sports saloon. The strengths of supercharging were already proven with the powerful and refined XJR6 saloon and the AJ-V8 engine, with its lightweight but extremely stiff cylinder block, provided the perfect platform for a forced induction engine that delivered a very special combination of power with refinement. As a compact, lightweight, all-aluminium, four-cam, 32-valve V8 with programmable variable valve phasing, the AJ-V8 quickly earned a reputation for three Jaguar signature characteristics – high performance, smooth power delivery and exceptional mechanical refinement. It offered a best-in-class specific power output of 72.5bhp/litre (54kW/litre) and a superb power-to-weight ratio. Even in its original form the AJ-V8 was known for its outstanding flexibility, with more than 80 per cent of peak torque available all the way from 1400rev/min to 6400rev/min. With its class-leading power peak of 290bhp, the AJ–V8 made the first XK8 not only a genuine performance car, but one with real depth. The new 4.2-litre V8 engines mark the first major change from the original AJ-V8. Like the 4.0-litre engine, the new AJ34 will be built at the Bridgend Engine Plant in Wales.

Lightweight construction, heavyweight performance

The original AJ-V8 weighed only 200kg and was the lightest engine in its class, as well as being rated best in class for performance feel and powertrain refinement. Both the naturally aspirated and supercharged versions of the new 4.2-litre V8 are even lighter (6kg and 14kg respectively) than their 4.0-litre predecessors, and with the additional benefit of the new transmission, offer even smoother performance feel. The cylinder block, cylinder heads and many other major components are, as before, in lightweight aluminium alloys. All the weight savings in the new engines are to the benefit of both responsiveness and outright acceleration.

The new-generation 4.2-litre AJ-V8s have excellent economy for engines giving this level of performance, and as with the original engine they are again among the best in class. The naturally-aspirated 300bhp XK8 coupe achieves 34.0mpg (8.3litres/100km) in the EU extra-urban cycle, and has a combined fuel consumption of 24.9mpg (11.3litres/100km). The supercharged 400bhp XKR coupe delivers 31.0mpg (9.1litres/100km) in the extra-urban cycle, with a combined figure of 22.9mpg (12.4litres/100km). Both engines comfortably meet the most stringent current emissions requirement worldwide.

The New 4.2-Litre AJ-V8 Engine: Key Features
  • Capacity increased from 3996cc to 4196cc, with bore and stroke of 86.0/90.3mm (previously 86.0/86.0mm)
  • Naturally aspirated XK8 engine produces 300bhp (DIN) and 420Nm (310lb ft) of torque. Supercharged XKR produces 400bhp (DIN) and 553Nm (408lb ft) of torque, with more than 86 per cent of peak torque available from just 2000rev/min. Comparable power and torque figures for the out-going 4.0-litre engine are respectively 290bhp (DIN)/393Nm and 370bhp (DIN)/525Nm.
  • Eight cylinders in 90 degree vee, five main bearings, in naturally aspirated and mechanically super charged versions
  • Aluminium-alloy block and cylinder heads for light weight in a compact package with maximum rigidity and strength
  • Redesigned block and bed-plate make new engine even lighter and stiffer than original, further improving refinement
  • Four chain-driven overhead camshafts operating four valves per cylinder with variable inlet and exhaust camshaft phasing
  • Revised camshaft drive further reduces loads and already extremely low levels of radiated noise
  • Valve clearances require no adjustment for life
  • XKR uses belt-driven Eaton rotor-type supercharger with intercooling, now with helical rotor gears for low noise and coated rotors for maximum efficiency – both industry firsts on a production engine
  • Supercharger speed increased, and volumetric efficiency further improved. Improvements to fuel-injection system further improves efficiency and reduces emissions
  • Electronic Engine Management System features ‘drive-by-wire’ throttle action with;get-you-home; facility
Supercharging: The Key To Serious Accessible Power Supercharging has been used as a way of extracting more power from the internal combustion engine or almost a century. The power an engine can produce is ultimately limited by the amount of fuel that it can burn in a specific time. And because fuel needs air in which to burn, the amount of fuel that can be burned is limited by the amount of air the engine can take in.

And that’s where supercharging comes in, as one way of increasing the mass of air flowing through the engine. By pumping air into he engine under pressure, rather than having it drawn in by suction alone, more air is delivered. More air means more fuel can be burned and more fuel burned means more power. The detail is far more complicated, but that is the essence of supercharging.

By increasing volumetric efficiency – the volume of fuel-air mixture an engine takes in during one complete cycle, expressed as a percentage compared to the capacity of the engine – supercharging increases power. A 4.2-litre engine taking in 4.2 litres of air-fuel mixture in each cycle would have a volumetric efficiency of 100 per cent – but in practice, with a naturally aspirated engine, that is not possible, because of the effects of friction and pumping losses in the intake system. A supercharged engine, however, forces mixture into the engine and can reach a volumetric efficiency figure of more than 100 per cent, with 130 per cent quite possible.
continues... | Part Three
Published 9 August 2002 Melanie Carter
 
 

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