Drawing on the experience gained in designing the original, acclaimed 2.7-litre engine, the new 3.0-litre AJ-V6D Gen III diesels demonstrate that it is possible to deliver improved performance, while reducing CO2 emissions and fuel economy. In the XF, the new engine produces 10 per cent less CO2 than the 2.7-litre, while power has increased by 33 per cent in Diesel S guise. As well as tackling CO2, the new 3.0-litre engines meet the forthcoming EU5 regulations, due to come into force at the start of 2011. And these great performance leaps are achieved with combined average fuel consumption in both models of 42.0mpg – an improvement on the 2.7-litre engine of 12 per cent.
Twin-turbos – maximum efficiency, instant response
A key feature of the new engine is the unique, parallel sequential turbocharger system, the first of its type to be fitted to a V-engine anywhere in the world. Delivering high torque throughout the entire engine rev range, improved throttle response and low CO2 emissions, the twin-turbochargers work sequentially to deliver unrivalled response and best-in-class torque – an impressive 61 per cent more than the 2.7-litre diesel from 1500rpm – while packing a huge punch at higher engine speeds.
For most day-to-day driving, including motorway cruising, a responsive, variable-geometry primary turbocharger does all the work, while the smaller, fixed-geometry, secondary turbo is dormant, saving energy and improving efficiency. When the engine revs climb above 2800rpm, the secondary turbo is brought on line within 300 milliseconds, smoothly and seamlessly boosting the engine output with no discernible turbo-lag or power-step.
Driving a turbocharger requires pressure from the exhaust, creating pumping losses in the engine and increasing fuel consumption. To alleviate this, valves under the control of the engine management system isolate the secondary turbocharger both from the exhaust stream and the engine inlet tract when it is not required.
Some twin-turbo systems rely on a smaller turbo for primary use, only using a larger turbo when higher power is required. Though effective, this has the disadvantage of raised exhaust pressure and increased pumping losses. The Jaguar system uses a larger, variable-geometry turbocharger more of the time, which not only reduces pumping losses, but also improves fuel consumption and CO2 emissions.
Jaguar engineers particularly focussed on the issue of turbocharger 'lag' at low engine speeds. The new AJ-V6D Gen III 3.0-litre diesels significantly out-perform their rivals by delivering 500Nm of torque in only 500 milliseconds from idle.
A new commonrail fuel-injection system delivers up to five injections on each cycle at a pressure of 2000bar. Each injector tip is perforated by seven holes through which finely atomised fuel is sprayed into the cylinders. The high-pressure injection increases power, improves economy and reduces both CO2 and particulate emissions. New, third-generation high-speed piezo injectors allow up to five precise injection events during each combustion cycle, minimising engine combustion noise.
Piezo crystal 'packs' operate each injector by expanding when an electric current is passed through them. They react virtually instantaneously but can make a distinctive click when fired, which can add to diesel engine noise at idle. The crystals in Jaguar's new injectors are fitted nearer the tip, meaning they are mounted deeper inside the engine providing better sound insulation and quieter operation.
Another new feature of the third-generation fuel-injection system is the metering mode. Traditional diesel commonrail fuel pumps oversupply the injectors, with the surplus being returned to the fuel tank. During this process, fuel temperature increases and cooling it again consumes considerable amounts of energy. In metering mode, the pump delivers fuel to the injectors only at the rate required. Consequentially, there is no rise in fuel temperature and no wasted energy.
Compact, light and clean
The two cylinder heads, with four valves per cylinder, are made from aluminium and the cylinder block is made from compact graphite iron (CGI). The higher tensile strength of CGI makes it possible to cast a smaller block; some 80mm shorter than a conventional 'grey' cast iron equivalent.
The new, water-cooled, exhaust gas recirculation system (EGR), important for reducing pumping losses and emissions of NOx in a diesel engine, is more efficient and consumes less power than the 2.7-litre unit. The valves that allow exhaust gas into the system are located on the 'hot side' of the engine nearest the exhaust manifolds; these valves never cool while the engine is running, so there is no condensation of combustion deposits which occurs on engines fitted with 'cold side' valves, hence the EGR system always works at maximum efficiency. Since the EGR cooling is so effective, exhaust gasses can bypass the system and return to the exhaust pipes, allowing faster engine warm-up from start-up and reducing emissions still further.
EU5 emissions regulations have been achieved ahead of the 2011 legislative timetable using conventional diesel oxidation catalysts and diesel particulate filters (DPFs). NOx levels are reduced at source through the combustion system design, the addition of the new commonrail injection system and the new EGR system with by-pass. As a result, specialised NOx exhaust after-treatment is unnecessary, avoiding a potential cost and the need to use additional precious metals in the exhaust system.
Remarkably quiet for a diesel
The CGI cylinder block and new piezo injectors reduce combustion noise in the engine. Multiple, precise injections of fuel on the combustion stroke also reduce combustion noise and all engine covers including camshaft covers, front covers and the sump have been optimised to subdue radiated noise. Engine enclosures have been ribbed to minimise radiated noise and the sump pan is manufactured from sound deadening steel (SDS), comprising a polymer layer sandwiched between two layers of steel.
Internal friction, a major contributor to unnecessary fuel consumption, has been addressed by careful optimisation of the crankshaft, valves and pistons. All these features combine to make the new Jaguar AJ-V6D Gen III engines amongst the quietest premium diesels on the market.
A major step forward
With its parallel sequential turbocharger system, third-generation commonrail fuel injection system and fully optimised EGR system, the new 3.0-litre AJ-V6D Gen III diesel sets new class standards when it comes to power, response and refinement in the premium diesel segment.
"The new XF challenges the rules and redefines Jaguar sporting luxury. Our designers and engineers have worked together to develop elegant, inspired solutions to complex technical challenges. It's a simple but very effective philosophy and the result is great new products like the new 3.0-litre diesel XF." - Mike O'Driscoll, Managing Director, Jaguar CarsPublished 30 December 2008