Welcome to the alternative Audi: a four-door saloon. OK, it doesn't sound very radical but the German company has put great emphasis in recent times on what the industry calls niche models.

These are the relatively small-selling variants such as big 4x4s, cabriolets, 4x4 estates and sports cars.

In fact, Audi was once best-known for its A4 saloon but the company now turns out 18 mainstream models, from the A2 right through to the ambassadorial A8 and the huge Q7 off-roader.

But it hasn't forgotten the A4. In fact, a whole new version was launched in 2005 and the range has been expanded but by bit ever since. It remains a viable alternative to BMW 3 Series and Mercedes-Benz C-Class with an emphasis on performance and under-stated style.

Such is the rise in importance of diesel that Audi are rightly proud of their new 2.0 litre diesel A4 which came on stream in July.

They point out that far from being a cumbersome or noisy car, it actually has the sort of power and refinement that you might expect in a V6 car. It sounds good and it seems to whip along from 0 to 60mph far quicker than the official 8.6 seconds indicate.

The secret, I must confess, is beyond me. I'll let Audi explain.

It has piezo power'. These are essentially engines with a fuel injection system containing piezo crystals.

Quite what they do is beyond me except they allow the engine to operate far more efficiently. Hence, it feels most unlike a traditional diesel and more like a powerful sports saloon. It also has an exquisite sound which I've not come across in a diesel before.

You can choose the old-style 140 PS diesel unit for a couple of thousand pounds less but Audi are confident that you'll feel the benefit is worth the extra cash.

In fact, it sounds ridiculous to say it but there is very little argument in choosing a petrol version when you can have a diesel as smooth and responsive as this.

What's more the 170PS is available at 4,200 revs per minute, which is low eaning you don't have to rev the engine to get to the power.

Registering impressively low C02 outputs of 154g/km, the new more powerful front-wheel-drive A4 helps make financial sense for tax-conscious company car drivers. A 40 per cent tax payer can expect a competitive tax liability of £1,823.36 per year for the A4 2.0 TDI.

The enhancement of the four-cylinder TDI range -which is also available in A3 models - closely follows improvements to the six-cylinder A4 TDI range, which earlier this year adopted a more powerful 233PS version of the 3.0-litre TDI engine, along with the 2.7-litre V6 TDI with 180PS from the A6, replacing the 2.5-litre TDI.

In terms of build quality, Audi really matches its rivals. Not only that but Audi cabins are among the best in the business.

Some say they look bland but I love the understated quality and the modern feel.

Some argue that the C-Class is more comfortable and the BMW 3 Series better to drive but it's a fine line.

  • Audi's sporty A4 saloons and estates are now a sound proposition - the firm has added a BOSE sound system as standard for SE, S line and S4 variants.
The premium audio system comes at no extra cost until December 31, 2006 - see dealerships for full details.

PRICE: £23,305. What Car? says £21,410.

ENGINE: A 2.0 litre unit generating 170PS.

PERFORMANCE: 0 to 60mph in 8.6 seconds.

COSTS: Average 34.9mpg.

EMISSIONS: 154g/km.

WARRANTY: Three years, 60,000 miles.

INSURANCE: Group 12.

All T&A road test vehicles are independently selected and assessments are carried out impartially by members of our own staff who base their opinions only on the vehicle supplied for these purposes by the manufacturer.