Road Test: Audi Q7 3.0 TDI 204 Quattro 5dr Tip Auto

The Q7 is not a greenie's favourite. Too big and brash and dirty, it's the sort of car they'll hang you by the eyelids for driving.

Except that this new face-lifted 3.0-litre TDI Q7 has cleaned up its act. It's the first car available in Britain with an ‘AdBlue' system, which adds urea to exhaust gases to neutralise smoggy Nitrous Oxide (NOx) emissions. Urea? As any doctor will tell you, that's the same stuff you get in urine. Hmm.

It's been added to lorry exhausts for a while now - truckers can buy it on a forecourt and top it up like screenwash. And the Americans are familiar with the tech as it's on some diesel-powered Audis, Mercs and VWs over there (due to strict Yanky ‘clean air' laws designed to reduce smog caused by diesels). But what relevance does it have over here?

On the face of things, not much. There's no financial incentive to run it, and you won't be taxed less like you are with a low-CO2 car. It comes down to the fact that CO2 is a greenhouse gas, and is directly proportional to mpg. To reduceit overnight would cause the death of big cars like the Q7, and kill off large chunks of the car industry. The government's solution is to tax it.

NOx is a poisonous air pollutant, which is horribly toxic, but has no relation to mpg and can be instantly reduced with things like AdBlue, even in performance cars. Therefore, it is controlled by EU standards, which are much less stringent than those of America.

Or at least they are at the moment. This 3.0-litre TDI complies with future ‘EU6' regs, which don't come into force until 2015. But there's no tax break for cars like this, which hit the standards ahead of schedule. If politicians weren't so busy expensing new croquet lawns, they would realise that's a missed opportunity to help the eco-aware driver. Or the SUV driver with a conscience.

So, Audi is six years ahead of the game. Thing is, with no financial incentive to buy one, it'll be left up to early adopters to crow about the benefits and get the market moving.

All of which means we've almost overlooked the Q7's subtle face-lift. But we can do that in one sentence: the changes are limited to some new rear LED lights, some wraparound running lights at the front, a new grille, new bumpers and a few dashboard inserts. The usual stuff.

But forget the design tweaks, the super-clean diesel is the big story here. The bigger V8 4.2-litre TDI is worth a mention too. It's an absolute sledgehammer of an engine and its 560lb ft of torque feels better suited to the Q7's 2.5 tonnes. But to choose it would be to risk the wrath of the greenies and surely play on your own conscience too.

Best stick with the new 3.0-litre TDI Q7 and watch your halo glow brighter than ever.