Even some quite new diesels, cars that are pleasant to drive and engender an almost evangelical applied logic among their owners, can prove surprisingly stinky to those stuck behind them in traffic. But there is hope for the diesel's social rehabilitation - or maybe just habilitation - so that it can take its place on equal terms with today's squeaky-clean petrol engines.
The current buzz-phrase is "common-rail direct injection", which refers to the way the fuel is supplied to and squirted into the engine with a precision hitherto unknown when such high injection pressures are involved. It's claimed to make diesels cleaner, quieter, more responsive, more petrol- engine-like and yet more economical. The Fiat Group is the first to put such an engine into production - so far, in a version of the new Alfa 156 that isn't coming to the UK; the Peugeot-Citroen Group follows later in the year. Others aren't far behind.
In the meantime, try this. The Audi A6 you see here has the most advanced diesel engine you can buy in Britain. It's a V6, for a start, which is currently unique among diesels. It has four valves per cylinder like a good modern petrol engine, it has direct injection and electronic management (to overcome a direct injection engine's usual percussive racket), and delivers a hefty 150bhp from its 2.5 litres. That's only 15bhp down from the 2.4-litre petrol V6 also offered in the A6 range, and at 229lb ft its pulling ability exceeds even that of the grandest A6's 2.8-litre petrol engine. It has catalytic converter; it smells and smokes hardly at all; all it lacks, for now, is this common-rail stuff.
Audi doesn't label this car a V6 on its tail, merely a 2.5 TDI, as it did the old-shape A6 with its five-cylinder engine. This modesty is excessive, because the engine delivers a whole new experience. Start it up, and it sounds at first like a miniaturised luxury coach: deep and six-cylindered, but still with an overlay of diesel "cackle". Drive off, let it warm up; soon you'll be aware merely of a distant V6 hum, deeper than in a petrol engine but still a creamy voice.
And that's the point. This diesel, unlike most, has a voice, a sound that connects with the brain's pleasure centres in a way a good petrol engine can. That, and the way the engine blips readily up to speed, together make this the most undiesel-like diesel I have ever tried.
It does all the usual turbo-diesel things - pulls vigorously from low speeds, uses amazingly little fuel, encourages a low-revs, low-stress approach to driving - yet it also spins smoothly and speeds to petrol- like velocities. Like most diesels its usable engine-speed range is relatively narrow, but the monstrous torque makes possible very long-legged gear ratios. And there are six of them.
If all diesels could be as sweet and effortless as this, I could be a convert. One day they probably will be. For now, it's a good reason to buy an Audi A6, along with the futuristically sculptural good looks, the comfort, the inviting interior and the feeling of high-quality permanence. It's not perfect - the steering is too light at speed, the doors spring too violently into their held-open positions, the photochromic anti-dazzle rear-view mirror wipes out the detail of what's behind you at night - but it's thoroughly desirable. Here's a diesel with all the advantages and, apparently, without the snags, wrapped up in the best big saloon you can currently buy.
Prices: pounds 26,320, or pounds 28,284 for SE version.
Engine: 2,496 cc V6, 24 valves, direct-injection turbo-diesel, 150bhp at 4,000rpm.
Transmission: five-speed gearbox, front-wheel drive.
Performance: top speed 135mph, 0-60 in 9.5sec, 36-41mpg.
BMW 525tds: pounds 26,200. Slightly slower and thirstier than Audi, sounds smooth but lacks Audi's eagerness.
Citroen XM 2.5TD Exclusive: pounds 28,555. Only four cylinders but colossal pulling power. Engagingly odd.
Mercedes-Benz E300D Classic: pounds 28,040. Base-trim version of desirable diesel Benz, not so fast but will last for ever.
Vauxhall Omega 2.5TD Elite: pounds 27,495. Plushest and priciest of BMW diesel- powered Omegas.
A BMW is better.
Volvo S70 TDI CD: pounds 27,775. Grandest diesel S70, uses old but tuneful Audi five-cylinder. Pleasant, but lacks German build quality.