The Jaguar XKR | Part Two

Jaguar XKR

Jaguar XKR

Supercharged 4.2-litre engine

The ability to call upon impressive reserves of power and acceleration in an instant is the hallmark of every great Jaguar. The new XKR ensures this by using a remarkable 4.2-litre supercharged AJ-V8 engine. Producing 420bhp (SAE) at 6250rpm and 560Nm (EEC) of torque at 4000rpm it is capable of propelling the Coupe version to 60mph in 4.9 seconds (5.0 seconds for the Convertible) and onto an electronically limited top speed of 155mph (250km/h).

The addition of a supercharger and twin air intakes means the new XKR benefits from a 120bhp (SAE) power increase over the normally aspirated 4.2-litre XK with which it shares its engine configuration. Torque, so essential for instant acceleration at any engine speed, also increases significantly – by 36% over the XK, although the overall weight of the XKR is raised by just 70kg. Consequently the power-to-weight ratio compared to the normally aspirated 4.2-litre XK is an impressive 34% higher.

And in comparison to the previous generation XKR, the new engine produces 7.7% more power and 3.3% more torque. Combined with the significantly stiffer and lighter aluminium monocoque body structure of the new XKR, these performance improvements lead to a significant leap in the power-to-weight ratio of 12% over the previous XKR and an equally useful jump of 7.7% in the torque-to-weight ratio. To achieve the equivalent improvements in power and torque-to-weight in the outgoing XKR would have required nearly 50 more horsepower. Consequently, the 0-60mph sprint time of the new XKR Coupe is reduced by 0.3 seconds.

The heart of these significant performance improvements is the XKR’s lightweight eight-cylinder engine with an Eaton supercharger. The water-cooled cylinders are arranged in a 90-degree V configuration and the crankshaft is supported by five main bearings. Each of the two cylinder head assemblies incorporates twin camshafts operating four valves per cylinder. One of the reasons for the engine’s impressive output is the continuously variable valve timing which helps to deliver a wide spread of torque. The engine uses an Electronic Return-less Fuel System (ERFS) and a three-way catalyst exhaust system.

The increase in power and torque over the outgoing XKR model is down to two significant additions to the engine. The air intake supply to the engine has been significantly enhanced thanks to the use of twin air inlets and a Variable Inlet Camshaft Timing system is used for the first time on the XKR. By continuously adjusting the timing of the inlet camshaft on both banks of the V8 depending on the engine speed and load, Jaguar’s engineers have ensured large improvements in torque, particularly at lower revs.

The system is controlled by the Engine Management System (EMS) which receives engine speed, throttle position and oil temperature data from its sensors. The EMS determines the correct inlet camshaft timing by continuously referring to a digital three-dimensional map developed to provide optimum performance. The EMS then transmits the appropriate signals to two solenoids that control the degree of hydraulic force provided to the valve actuators.

The normally aspirated 4.2-litre XK has been praised for its acoustic qualities, remaining unobtrusive and relaxed at modest speeds but producing an unmistakeable V8 soundtrack when worked hard. On the XKR, the presence of supercharger whine threatened to dominate the acoustic character and mask the underlying sound quality. However, through enhancements to the vehicle acoustic pack, the supercharger noise has been reduced by 5dB compared to the previous XKR.

This approach to acoustics allowed the engineers to concentrate on using the exhaust system to deliver the best possible sound quality character. This was accomplished through the use of the XKR’s Active Exhaust system which varies the flow of exhaust gases through the main silencer box to ensure that the XKR remains quiet at cruising speeds but delivers a substantially more purposeful sound quality character under hard acceleration.

continues... | Part Three
Published 2 July 2006 Melanie Carter

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