The Jaguar XJ range has never had a diesel option - the rattles just didn't fit with its sumptuous reputation. But the newest collection does - and it really purrs, says John Simister

We live in interesting times. During the last year, Jaguar has sold more diesel cars in the UK than petrol cars. Jaguar - yes, Jaguar, which has never made anything utilitarian in its life - now has a diesel majority.

It has also massively increased its total sales of late, precisely because the X-type and the S-type have been made available with excellent diesel engines, and mainland European buyers, even more than British ones, have leapt on that fact. But a crucial gap has remained: no diesel for the biggest, most prestigious Jaguar saloons, the XJs.

Despite that omission, in the UK the XJ has still outsold all its luxury-car rivals - cars like the Mercedes-Benz S-class, the Audi A8, the BMW 7-series. But clearly there have been sales opportunities missed through the lack of a diesel. And that is something Jaguar has known for a long time: I remember a visit to the company's Whitley engineering centre, near Coventry, in the early 1990s, just as the final incarnation of the previous XJ range, known as X300, was being readied for sale.

I was handed a pair of headphones and asked to identify an engine sound, recorded from a test prototype. "It's a six-cylinder," I said, "but it sounds different. It wouldn't be a..." "A diesel? Exactly," replied the engineer. "We know we need one, but we can't afford to develop our own so we're trying a BMW unit."

It came to nothing, but collaboration has indeed proved the solution. The X-type diesel is an improved version of Ford's Mondeo unit (and is now available in 2.2-litre as well as entry-level 2.0-litre versions), while the S-type diesel uses a twin-turbo V6, which is built in Ford's Dagenham factory to a design jointly developed by Ford and Peugeot. A single-turbo version powers the Land Rover Discovery to good effect.

And it's the twin-turbo unit, with 207bhp and 321lb ft of torque, which propels the new Jaguar XJ6 TDVi. You might think that the XJ would be upstaged by the diesel S-type, which is smaller and presumably more rapid as a result. But you'd reckon without the XJ's lightweight aluminium construction, which makes this bigger car lighter than the S-type. So honour is upheld. It also matters little that the XJ's engine is smaller than those of its rivals, as its outputs are sufficient for competitive pace and its fuel economy, and emissions, are the best in the class.

This is all very fine, but the notion of a diesel XJ, a rattly engine in a car known for its refinement, still seems all wrong. But clever things can be done with acoustics and electronics nowadays, and this XJ is the proof. Stand outside when it's idling and you would barely know a diesel lurks within; there's just a metallic edge to an otherwise typical V6 hum. Highly sound-absorbent foam under the bonnet and airtight seals between bonnet and engine compartment are the killer tricks here.

Inside, there's similar tranquillity. The bulkhead between engine and occupants is double-skinned, and all the glass is laminated with an acoustically absorbent interlayer. The killer feature, though, is the "active" control of the engine mountings, a first in Europe.

Engine mountings filled with silicone fluid are not new, and are a good way of tailoring the mounting to absorb particular vibration frequencies. But the Jaguar mountings go a stage further, by using a solenoid-controlled diaphragm inside the mount to expand or contract the fluid space inside the mountings, many times a second, in tune with the engine's vibrations.

The result is that the vibration is soaked up before it gets to the XJ6's structure, and the occupants feel nothing. When driving, the only clue that this is a diesel engine is a slight metallic ticking at about 3,000rpm. What you're actually hearing is the last vestige of diesel combustion noise, but of clatter and "cackle" there are no traces.

So this is as quiet a Jaguar as any. It's also a rapid Jaguar, able to accelerate just as quickly as the 3.0-litre petrol XJ6, and almost able to match it for top speed. Yet it can average 35mpg, against 27mpg for that petrol version. It's also a more pleasing engine to drive behind, because it pulls very strongly across its speed range and is matched to an excellent ZF six-speed automatic. This shifts gears with no vague, power-sapping slurring, and it's always smooth.

This diesel engine, now calibrated to Euro 4 emission standards and fitted with a maintenance-free particle filter so you never see any smoke, is quite a light unit. This makes the XJ feel wonderfully agile. The example I tried was a Sport Premium - there is also a luxury Sovereign for the same price (£49,995), plus a cheaper Executive at £43,995 - which has firmer suspension and fatter, lower-profile tyres, but still it rides with typical Jaguar serenity even though it grips hard, has a beautifully natural balance and steering with excellent accuracy and firm weighting. I've never enjoyed a big, luxury saloon more on challenging roads.

For me, this Sport Premium version is definitely the one to have. The current XJ range has justifiably drawn criticism for its obviously retro looks, and it's true that unless you see them together, it's far from obvious whether you're looking at the current car or the old one. But there's a tidier, cleaner, purer look, achieved by simple actions such as deleting the side rubbing strakes and depriving the windscreen and rear window of their finishing strips.

Combine that with the Sport Premium's lack of chrome and its mesh front grille, and suddenly the shape looks classically clean rather than steeped in sepia. Inside, too, the Sport Premium abandons its wood-trimmed past in favour of embossed aluminium and dark trim colours. The effect is to create a car of today, not a revitalised relic.

So there it is. The best and most cleverly conceived Jaguar of all is powered by a diesel engine. And it is probably, overall, the best luxury car you can buy. Interesting times indeed.

The rivals

AUDI A8 3.0 TDI QUATTRO £48,365
Four-wheel drive is a benefit in poor weather, and the A8 shares Jaguar's weight advantage thanks to aluminium construction. It looks ultra-modern, and is beautifully built, but lacks Jaguar's serenity and ultimate comfort.

BMW 730d £47,505
Just as we got used to the weird looks of the 7-series, BMW promptly restyled it to calm down the oddness, and in doing so they took away some of its character. Infamous for bringing us iDrive, the 7-series is still a roomy car with pace and poise to match the price-tag.

Up to now this has been the best luxury-car all-rounder of all, and it still looks classically beautiful. Across Europe it's the best seller in the class, so it will be interesting to see how the imminent new one fares. It's not handsome.

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