But is it possible for an Alfa Romeo to have a diesel engine without blowing its aura? Stranger things have happened at Alfa: there have been Alfa Romeo vans and lorries, even an Alfa Romeo Dauphine (a licence-built version of the ancient rear-engined Renault). And there have, in mainland Europe, been plenty of diesel Alfas, which have not caused the Alfa image to go up in a puff of particulate smoke.
Too risky for the UK, though. Alfa Romeo hasn't dared sell a diesel-drinking Alfa here for fear of undermining the brand's essence. If an Alfa is partly defined, more than with most cars, by its charismatic engine, then fitting a diesel from someone else (usually VM, which has also supplied Chrysler, Ford and Rover) seems an act of sabotage.
Now, though, Alfa Romeo has the 156. It looks gorgeous, it's built properly, it's a delight to drive and it sells here like no other Alfa before. This has put Alfa in confident mood. Now, too, Alfa has its JTD engines, its own turbodiesels (well, its own and parent company Fiat's, anyway). These have hi-tech, electronically controlled, common-rail injection - the very first European engines to do so.
The 156s so equipped were the first common-rail cars to go on sale in Europe, with lower-powered Fiats following. And now the larger of the two JTDs, a five-cylinder, 2.4-litre unit, joins the UK line-up of 156 engines. The UK's first diesel Alfa. Does it work? Or is the whole concept oxymoronic?
An Alfa engine should be tuneful. That's half the appeal. Diesels don't usually have a voice, they just mumble and grumble. But this one does: it's not a normal Alfa voice, but it is sporty and suggestive of spirit. The five-cylinder layout's deep, harmonic throb helps; so does the lack of diesely clatter, except just after the engine has fired up.
An Alfa engine should also feel crisp and eager, a tricky task for a diesel which, if powerful, usually delivers its power in a soft-edged, elastic way. Again, Alfa attributes win through. A drive will disclose all.
The first impression is of a smoothness remarkable for a diesel. A balancer shaft helps here, as does this common-rail injection system's ability to squirt a tiny amount of fuel into each cylinder ahead of the main deluge. This gets combustion going, so that when the main bang happens there's less of a noise-inducing "temperature gradient" within the cylinder.
The next impression is of a delay in response to the accelerator at very low engine speeds, typical of a turbodiesel. This happens because there aren't enough exhaust gases to get the turbocharger spinning and boost pressure building up. What happens next, though, makes the "turbo lag" easy to forgive. Boost, and therefore power, comes quickly and smoothly, and from 2,000rpm through to the engine's 5,000rpm ceiling the accelerator responds as crisply and quickly as any good petrol engine's.
You want to change down a gear? Blip the accelerator for a smooth shift, and the JTD zips up to speed as readily as any Alfa engine. The difference is that you'll be doing far fewer downshifts, because the engine's 224lb ft of torque gives this 156 a thrustability in high gears far beyond that of any other 156. Even the 2.5-litre petrol V6 feels feeble next to the mighty turbodiesel; you'll expend half the effort in the JTD, but get along just as quickly.
You'll also expend a fair amount less fuel, which is, after all, the main point of a diesel. More than 40mpg should be easy to achieve, whereas the V6 might manage 25. Curiously, the JTD feels better balanced in corners, too, less nose-heavy, more willing to change direction and better at telling you what it is doing in the process.
I have driven two JTDs, one with standard suspension, the other with optional Sport supension, which, hitherto, has been a bad choice for a 156 unless you drive only on perfect roads. Now, though, the suspension has revised rubber bushes, a minor change which has had a big effect. The tautness remains, but the fidget, bang and shudder have gone. The Sport option, then, is now the one to have.
A Sport-pack JTD is a fine and desirable car, which just happens to require diesel fuel to feed it, and it's possibly my favourite 156. See? A diesel Alfa Romeo isn't such a daft idea after all.
Model: Alfa 156 2.4 JTD
Price: from pounds 20,335
Engine: 2,387cc, five cylinders, 10 valves, 136bhp at 4,200rpm
Transmission: five-speed gearbox, front-wheel drive
Performance: 126mph, 0-60 in 9.2sec, 37-42mpg
Audi A4 2.5 TDI: pounds 24,257. Powerful, smooth-sounding V6 makes the A4 a sophisticated diesel, but the driving experience is less sharp than Alfa's.
BMW 320d: pounds 21,350 approx. Poised for UK launch, the common-rail turbodiesel 3-series has just four cylinders but surprising power.
Mercedes-Benz C220 CDI: pounds 22,240. Another common-rail four-cylinder, the C-class has less power than the BMW. All-new C-class comes soon.
Saab 9-3 2.2 TiD: pounds 18,025. First-ever diesel Saab uses a Vauxhall-based, direct-injection (but not common-rail), 16-valve engine. Not exciting.