In gentleman's clubs across the land there may still be a few old buffers who splutter over their whiskies and soda when some fool raises the topic of a Jaguar fitted with a diesel engine. It's still, after all, quite a recent phenomenon. Just ten years ago the very idea would have been enough to give many people the hiccups. But a few miles behind the wheel should be enough to convince all but the most bigoted that the XJ 2.7 TDVi is in every respect a Good Thing.I've said many times in the past that I think the XJ is the best Jaguar, the model that is most securely in the company's comfort zone, the car that Jaguar people really want to build. All the versions fitted with petrol engines (supercharged or otherwise) are impressive, but the TDVi is just possibly the best car in the range.The 2.7-litre V6 turbo diesel unit has been discussed here often. It's the engine co-developed by Ford - mostly for its Premier Automotive Group products - and PSA Peugeot Citroen, and both conglomerates have been busily inserting it into any engine bay large enough to accept it. And so it can be found (in more or less the same form, though with some detail differences) in cars as diverse as the Citroen C6, the Jaguar S-Type, the Land Rover Discovery, the Peugeot 407 and the Range Rover Sport.The common thread here is that the engine is almost invariably the best thing about the car. It's powerful (around the 200bhp mark depending on application), it's economical (35mpg combined in the XJ), it's flexible, it's smooth and it's quiet. Noise levels in the XJ TDVi are about the same as those for the V8 petrol models in gentle motoring, and even under pressure the diesel provides nothing more offensive than an unusually throaty purr.Compared with the 3.0 V6, the TDVi offers various pros and cons. For cars with the same trim level, the diesel is £1980 more expensive and comes one insurance group higher (though since all XJs these days will set you back more than £40,000 this is perhaps trivial detail).The diesel produces less power but a full 100lb/ft more torque, and does it at much lower revs. The result is that the standard performance figures are almost identical; the TDVi has inferior top speed and 0-62mph stats to the piffling extent of 4mph and 0.1 seconds respectively.What makes this even more impressive is the fact that the TDVi is 120kg heavier thanks to the weight of the diesel engine (though it's also worth pointing out that, thanks to the aluminium body construction, its total avoirdupois of 1659kg makes it outstandingly lighter than the Audi, BMW and Mercedes-Benz opposition).More important than the car's ultimate straightline ability is the fact that strong performance is available from very low engine speeds simply by tickling the accelerator pedal. In conjunction with the low sound level, the makes the TDVi a most relaxing car to drive - and what, pray, would be the point of a TDVi that wasn't relaxing?The use of the diesel engine has done nothing to harm what I think of as one of the XJ range's most endearing qualities. Every model I have driven in the present range (including, most dramatically, the long-wheelbase V8) deals with corners in the same way.While the opposition, as a general rule, sucks in its breath and does what it can through a series of tight, twisting bends, relying on the grip provided by very large rubber, the XJ seems to shrink around you and dance through the same sequence as if it had been reared by a family of hot hatches and had never been told that it was really a large saloon.The TDVi is just the same as the other cars in the range, and the way it will dig itself out of a corner even with only a few revs on the clock adds yet another factor to the experience. For this reason among so many others, I'm tempted to say that the diesel XJ - despite the mighty attractions elsewhere in the range - is my favourite of them all. Engine 2722cc, 6 cylinders Power 204bhp Transmission 5-speed automatic Fuel/CO2 35.0mpg / 214g/km Acceleration 0-62mph: 8.2 seconds Top speed 141mph Price £45,020 Details correct at publication date