Jaguar XK Engine and Transmission

Engine

A key element in the character of a sports car is its engine. The new XK was launched with a powerful four-cam naturally aspirated 4.2-litre AJ-V8 powerplant. This compact, lightweight engine is based on that fitted to the latest generation XJ saloon and has undergone significant development compared with the engine used in the previous XK, including new fuel-injection technology.

The naturally aspirated 4.2-litre V8 engine produces 300bhp SAE and develops maximum torque of 303 lb-ft at 4,100rpm. Again, the spread of torque is an important ingredient in the XK’s effortlessly sporty character, and this engine delivers more than 85 percent of torque all the way from 2,000 to 6,000rpm. Yet it still offers fine fuel economy and low emissions figures, with a drop in CO 2 emissions of five percent.

The naturally aspirated 4.2-litre V8 XK Coupe has an electronically limited maximum speed of 155mph (250 km/h) and a 0-60mph time of 5.9 seconds (0-100 km/h time of 6.2 seconds), plus instant throttle response and broad flexibility for punchy performance across the range. With the new XK’s weight savings, the naturally aspirated 4.2’s standing quarter-mile time of 14.4 seconds is less than half a second off the pace of the previous supercharged 4.2 XKR.

The compact AJ-V8 engine has very stiff but lightweight all-aluminium construction, with eight cylinders in a 90-degree ‘V’. The combination of strength and lightness begins with a ribbed cylinder block and cylinder heads. The 4.2-litre version has bore and stroke of 3.39 x 3.56 inches for a capacity of 4196cc. The fully balanced four-throw, six-counterweight crankshaft is supported in five main bearings. The connecting rods use split-fractured big-end journals for strength with light weight and perfect balance. Each cylinder head carries two chain-driven overhead camshafts, which are hollow, to save weight and improve performance by allowing higher engine speeds. The camshafts operate four valves (two inlet and two exhaust) in each pent roof combustion chamber, around a central spark plug. An unusually narrow 28 degree included valve angle allows a compact combustion chamber shape and narrower heads, which benefits overall packaging.

The inlet camshafts allow variable inlet valve timing, with VCP Variable Camshaft Phasing, which is controlled by three-dimensional digital maps stored in the Engine Management System, on the basis of engine speed, throttle position and oil temperature data gathered from a series of sensors in the engine. The phase of the inlet camshafts is varied hydraulically and continuously (advancing the opening of the inlet valves at high engine speeds to allow the combustion process to start earlier). That delivers faster throttle response at all engine speeds, and gives optimum performance at all speeds and under all loads - with more torque at low speeds and maximum power at high speeds, while optimising fuel consumption. The VCP system has the added advantage of providing a degree of internal exhaust gas recirculation – which reduces emissions of NOx by slowing the combustion rate, and of hydrocarbons by re-burning some of the exhaust gases.

The major difference between this engine and the previous generation 4.2-litre XK engine is in the fuel injection technology. This latest V8 now uses multi-hole injectors, which improve the fuel spray pattern in the combustion chambers, improving both power and fuel efficiency.

Optimum throttle response (a crucial ingredient in confirming the new XK’s sports car character) is delivered by full ‘drive-by-wire’ electronic throttle control, with no mechanical connection between the accelerator pedal and the throttle body. The response is based on the torque demand for every instantaneous driving situation. That is calculated by the electronic engine management control, based on parameters including the driver’s accelerator input, and other vehicle factors such as road speed, engine speed and gear selection. The electronic controls then call up the required torque at any instant by adjusting throttle position, variable cam phasing, fuel flow and exhaust gas recirculation settings.

Equally important for its new role in Jaguar’s sportiest cars, the 4.2-litre engine has been engineered to give the sound expected from a real sports car engine – especially under acceleration – but without being undesirably noisy. The new XK’s Semi-Active Exhaust system varies the flow of exhaust gases through the main, large silencer box depending on the pressure in the system, and features acoustically tuned tailpipes that eliminate low speed boom. There is also an underfloor resonator with two chambers (one for each cylinder bank) which balances the sound from the two banks. By tuning the sounds from the air-induction system and the exhaust system, Jaguar concentrated on both the solid, powerful low-frequency sounds and more technically ‘sophisticated’ higher frequency sounds, to give a feeling of power and performance.

Transmission

The XK uses the latest version of Jaguar’s six-speed epicyclic automatic transmission, which is widely regarded as one of the best automatic transmissions in the premium, sports car market. It features Bosch Mechatronic shift – an electro-hydraulic shift mechanism whose adaptive shift strategy responds to both road conditions and driving style, to give the smoothest shifts with optimum performance.

The XK’s transmission introduces a new generation of automatic gearshift for Jaguar, replacing the familiar ‘J’ gate with the Jaguar Sequential Shift system with Park, Reverse, Neutral, Drive and Sport modes. The fully automatic Drive mode adapts to individual driving styles, while a Sport Auto mode can also be selected. This offers an even more responsive fully automatic shift strategy.

For the first time in a Jaguar, drivers will be able to use steering wheel-mounted paddles to change gears. In either Drive or Sport modes instant access to manual operation is achieved via the shift paddles. In manual mode, the transmission controller uses an alternative parameter set to control gear shifts, enabling delivery of extremely rapid and responsive manual shifts, whilst maintaining class-leading Jaguar shift quality in automatic modes. Interaction with the torque-based engine management system allows for precise torque control during shifts – engine inertia is used to enhance acceleration during upshifts, and an engine torque increase (‘throttle blip’) is used to significantly shorten over-run downshifts.

The epicyclic geartrain utilises clutch-to-clutch synchronous shifting to ensure that a controlled amount of torque is always being transferred during power on upshifts. This makes the shift much smoother than in the automated manual gearboxes adopted by some of the new XK’s competitors, where the use of an automated clutch completely interrupts the flow of torque during shifts.

The extremely rapid shift times often quoted for automated manual transmissions relate solely to the duration of this torque interrupt. The true shift time is significantly longer, since the clutch must be disengaged prior to the ratio change, and re-engaged after. In contrast, the Jaguar Sequential Shift suffers no torque interrupt resulting in a smoother more powerful shift feel, and a very short total shift time of approximately 600 milliseconds from the driver touching the shift paddle to the completion of the shift event.

In fact, during development, comparison tests between Jaguar Sequential Shift and rival automatic transmissions in the class showed the new XK’s transmission to be the fastest system of all, changing gear at least 400 milliseconds faster than a standard automatic and 100 milliseconds faster than the best automated manual system.