There can’t be anyone who is not fully aware of CO2 emissions – they impact on many aspects of fleet operations from VED rates, company car tax and writing down allowances.

The Government’s strategy of pinning fleet taxes to CO2 has resulted in diesel-engined vehicles achieving all-time high levels of market penetration as firms choose them for their economy and driver taxation savings.

But from September, another gas will become more prominent – NOx, or nitrogen oxide – and this will cause problems for diesels.

NOx affects local air quality and impacts directly on the air we breathe (think of London air smog), while CO2 impacts on climate change.

While diesel is good for low CO2, it produces higher quantities of NOx and PM (particulate matter), and with the advent in September of the Euro 5 emissions standard – the level set by Europe on what new cars must achieve to be certified – diesels may start to be at a disadvantage.

The reason is that diesel engines will require new technology to reduce their NOx emissions.

The present Euro 4 emissions standard requires diesels to emit no more than 250mg/km of NOx, but Euro 5 (which affects all new vehicles coming to market from September 1, and all registrations by January 1, 2011) will require that limit to drop to 180mg/km.

Much work has already been done by manufacturers, with particulate traps on diesels in particular helping to ensure Euro 5 is relatively easy to achieve.

However, these are costly units and will undoubtedly push up the price of diesel engines.

But manufacturers are now hard at work meeting the next emissions challenge – Euro 6, which expects NOx levels to be capped at 80mg/km.

This comes into effect in 2015 for all registrations, with all new models having to meet the earlier deadline as with Euro 5, although no specific date has yet been announced.

John Kingston, environment manager at Honda UK, said: “The anticipated drop in NOx values for Euro 5 is going to be pretty challenging.

“As it is diesel vehicles which are primarily challenged by NOx and PM reductions, it is probably fair to say that both Euro 5 and Euro 6 limits will require further engineering innovation and will lead to additional cost as manufacturers look at new technology to reduce these emissions.”

Honda already has a Euro 5-compliant diesel in the Accord range, but its main focus is in developing petrol-electric hybrid engines as they offer not only low CO2, but are also low on NOx and PM.

The same is true of Toyota and Lexus, with their hybrid environmental solution.

Leading Euro 6 charge

An unlikely vehicle is leading the charge to Euro 6 levels.

Audi’s gargantuan Q7 SUV, which has recently been revised and goes on sale in July, will feature the first Euro 6-compliant diesel engine.

The 3.0 TDI Clean Diesel uses a new exhaust recirculation system to ensure highly efficient combustion and a new catalytic converter to reduce NOx gases.

This works in parallel with a pump which injects an additive called AdBlue into the hot exhaust gases.

This substance decomposes into ammonia, which splits NOx gases into harmless nitrogen and water.

The AdBlue additive will be replenished by Audi dealerships during routine servicing.

The only snag is that the Clean Diesel costs £1,500 more than the standard 3.0 TDI, which will continue to be offered.