Fuelled by government incentives to cut carbon, the fleet industry has adopted diesel fuel en masse since the latest tax system was introduced in 2002. But, as air pollution starts to come under focus and the real cost of moving to diesel engines comes to light, it seems the tide is starting to turn against the business fuel of choice.

Carbon-based taxation has had a profound effect on UK buying habits. In 2001, diesel engines comprised 13.8% of the vehicle parc, according to Department for Transport figures. By the end of last year, this had risen to 34.5%, and Brits’ appetite for heavy oil isn’t subsiding. During the first half of 2014, SMMT data shows diesel sales climbed 12.4%, or 100,000 units, year on year – almost twice the rate of petroleum growth, to take 50.3% of the market.

The downside of diesel engines is they produce significantly higher NOx emissions than an equivalent petrol engine. A collective term for nitric oxide and nitrogen dioxide, both of which are produced during the combustion cycle, NOx is linked to smog and acid rain, and in high concentration can cause lung damage, exacerbate heart disease and lead to early deaths.

It’s also a big problem in the UK. Defra studies showed 16 zones which would fail to meet 2010 NOx limits by 2025, with London, Birmingham and Leeds unable to do so until 2030. Air pollution, it said, causes 29,000 premature deaths in the country each year – more than smoking and traffic accidents combined. The Court of Justice of the European Union responded by launching legal action against the UK, for failing to protect inhabitants from poor air quality.

The good news on the horizon for diesel is Euro 6, the latest emissions standards set by the EU, which will set a 55% cut in NOx limits for diesel vehicles, bringing them in line with Euro 4 petrol engines and close to a modern petrol. In recognition, the 3% Benefit in Kind levy for diesel engines is to be scrapped from 2016, and that’s just the start.

Euro 6 is the lynchpin of London Mayor Boris Johnson’s air quality manifesto which, in July, laid out plans to double the Congestion Charge for diesels which don’t meet the latest standard. It’s a plan that Labour’s newly proposed city centre lowemission zones are likely to mimic, which makes the standard a key component of improving air quality in the UK. However, sceptics aren’t convinced that this will have the desired effect.

Among them is Emissions Anaytics, an independent company which carries out road tests on new vehicles to monitor real-world performance against manufacturer-issued figures. In a study it carried out with Imperial College London, a sample group of ten Euro 5 cars all failed to even meet Euro 4 NOx levels, while only three achieved Euro 3 compliance in realworld use.

Stop-start driving, particularly in the dense traffic of pollution-prone city centres, makes the difference more pronounced, the company said in its report, adding that it would be interesting to see how the more stringent requirements of Euro 6 would affect future results. But is it?

The technology behind Euro 6

Exhaust Gas Recirculation

This diverts exhaust gas back into the engine’s combustion chamber, reducing excess oxygen which could otherwise form NOx.

NOx Traps

These absorb NOx from exhaust gases and store it. Once full, the system is purged with fuel to form water and nitrogen, released as fumes.

Selective Catalytic Reduction

This injects a catalyst into the exhaust fumes (AdBlue is the best known), breaking NOx down to form nitrogen, water and carbon dioxide.

Putting Euro 6 to the test

From its headquarters in Teddington, the scale of the Emissions Analytics operation is vast.

The company provides real-world data on fuel economy and exhaust emissions for 250 vehicles in the UK each year, and another 250 in the United States, which has created a database covering around 90% of all new cars. This is made available to third parties to give a clearer, more detailed view of a set car’s environmental impact.

Testing takes place on a two and a half hour road route, rather than a laboratory as in factory homologation, and the data is gathered using a Portable Emissions Measurement System attached to the car’s tailpipe. This measures the flow of exhaust gases, revealing the carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide, hydrocarbons and particulate matter content, as well as real-world fuel consumption and additional data from the car’s on-board systems.

Jane Thomas, global sales manager at Emissions Analytics, says this sort of testing is vital: ‘The laboratory test used to assess vehicle emissions is not stringent enough for today’s cars and drivers. In particular it underestimates speed and acceleration which are key factors in the causes of emissions. Thus, when taken out of the lab and on to the road, the gap can be significant.’

With the standards coming into force from September for completely new vehicles, and all new registrations from January, we assembled four close-equivalent vehicles from mainstream fleet manufacturers and Emissions Analytics analysed the performance of Euro 6 petrol models with the Euro 6 and Euro 5 results for their equivalent diesel variants.

The results…

The results paint a bleak picture of the real-world performance of Euro 6 diesels. Against the 55% targeted reduction over Euro 5, Emissions Analytics found real-world improvements vary enormously. Only one of the four Euro 6 cars met the requirements, with two of the remainder failing to meet Euro 4 and one exceeding Euro 3 limits – set in 2000 – by more than three fold.

By comparison, Euro 6 hasn’t set stricter NOx limits for petrol engines, which means our group would have met 2009’s Euro 5 requirements under laboratory conditions. Yet three out of four of the models tested met Euro 6 requirements, two of which were 88% and 90% underneath the benchmark, and the fourth only failed by 12%. Average NOx emissions for the Euro 6 petrol group was 18 times lower than for their equivalent diesels.

Thomas says this is typical of the data gathered so far: ‘Based on the data we have collected so far, some Euro 6 diesels will be exceeding their predicted NOx emissions many times over,’ she explains. ‘This means that the capital may not see the reductions in air pollution that is required to meet the EU’s targets and this will continue to be the case until the test process incorporates real-world operation.’

Manufacturers have been quick to respond to political pressure on high-carbon vehicles, and have made great strides to bring down fleet-average CO2 emissions in this area as tax demands it. But for a clearer view of the real environmental impact of your fleet, real-world data suggests it might be best to take a broader view, and that even the newest diesel cars might not be as green as the government policy suggests.

The accuracy of real-world data

While laboratory results can have strictly standardised conditions, on-road testing takes a little more work to ensure data is collected fairly. Emissions Analytics conducts all tests on the same 2.5-hour route, with the same small pool of trained technicians, tyres inflated to the manufacturer’s recommendations and the air conditioning switched off. Results are abandoned if the driver runs into severe congestion, and tests are postponed to avoid extreme weather to keep the data constant.