D-DAY has arrived at premium brand Infiniti with the introduction of diesel engines to its line up.

The luxury arm of Nissan has moved swiftly to plug the gap in its armoury which so far has featured only high powered V6 and V8 gas guzzlers.

Now it is bringing three-litre diesels into play in the sharply-styled EX and FX 4x4s boosting fuel economy, in some cases, by almost 50 per cent.

Both models can now average more than 30 miles to the gallon although emissions remain excessive at more than 220g/km leaving them in high tax territory.

With diesels becoming ever popular in Britain, Infiniti sees the newcomers as the spearhead of its expansion plans.

A third dealership is due to open in Birmingham within days to be followed by another in Glasgow before the year is out, adding to the initial Infiniti centre in Reading and the recently opened prestige site in London's Piccadilly.

Other centres are already slated for Leeds and Stockport next year and two more in London.

The diesel is a torquey three-litre V6 developed under the Renault Nissan Alliance and in a slightly different guise also sees action in the French made Renault Laguna Coupe.

For use by Infiniti it is set up to deliver 235bhp which in the smaller EX30d results in a 0 to 60 acceleration time of 7.9 seconds and a top speed of 137mph.

The bigger FX30d tops out at 132 with acceleration slightly slower at 8.3 seconds.

The EX is an alternative to the likes of the Audi Q5 and BMW X3 while the FX is more in the territory of the German pair's Q7 and X5. It's also an alternative to the Range Rover Sport while the EX gives a hint of what Range Rover's new baby, the Evoque, will be up against.

Both Infiniti diesels are quite feisty. The engine has plenty of guts - it's got more pull than the whopping five-litre V8 petrol used in the flagship FX50 - yet can average a claimed 33.2mpg in the EX30d.

Emissions are 224g/km in the EX and 238 in the FX which has slightly lower fuel economy at 31.4mpg. Nevertheless that's hugely more acceptable than the 21 to the gallon expected from the V8 or the 25mpg the 3.7-litre V6 petrol engined EX will return.

Sound insulation is excellent in both Infinitis and the road ride is smooth and accommodating.

The engine is mated to a seven speed automatic gearbox which is equally slick and precise when used manually. The FX30d is fitted with paddle shifters making rapid fire changes easy.

Plenty of kit is an Infiniti hallmark and inside they are positively posh and high tech. The EX30d in top specification GT Premium trim comes with leather upholstery, hard disc sat nav, self repairing paint and power operated folding rear seats and costs from £41,220.

The rear seat arrangement is highly practical and increases luggage space from 340 to 1,175 litres at the touch of a couple of buttons - and the seat backs fold back again under power.

Both models also feature advanced safety systems which include a lane departure warning device which brings the vehicle back on track automatically - a first in the 4x4 market.

While the EX30d is impressive, the ride quality in the FX is even better.

That's largely down to the continuous damping control which makes it a large all-wheel-drive vehicle with near limo comfort.

The top specification FX30d is priced from £51,370 but is available from around £44,000 in GT trim.

The EX diesel can be had from £35,975 which makes it slightly cheaper than the equivalent petrol version which is priced at £36,275.