Diesels far dirtier than we thought

While the rush to buy diesel-engined cars has been fuelled by national vehicle taxation regimes that encourage low-carbon motoring, it's now looking more and more as if diesels are far more polluting than was previously though.

New research by the International Council on Clean Transportation (ICCT), a not-for-profit research organisation, shows that in real-world conditions, the nitrogen-oxide (NOx) emissions of modern diesel engines are as much as seven-times higher than the limit allowed for in the current Euro6 emissions regulations.

Euro6, which came into force last month, has long been held as a landmark set of regulations within the car industry. So stringent are the limits on NOx that many car makers have taken the route of fitting AdBlue systems to the exhausts of their diesel cars. AdBlue squirts a spray of urea into the exhaust gases, and that helps to cut down on the NOx being emitted. Because of the expense of this and other clean-diesel systems, Euro6 was often held to be the point where diesel engines would become so expensive to make we'd all turn back to petrol.

That hasn’t happened yet. Indeed, in the Irish market, diesel engines have taken 1 per cent more market share so far this year than last. It’s these booming diesel sales that have national and local governments worried, because of the impact of NOx emissions, higher by far from diesel than from petrol, on health.

NOx can have serious adverse health effects. In particular, nitrogen dioxide is the main precursor to ground-level ozone, which causes significant respiratory problems that can lead to premature death.

Air quality

Ambient NOx concentrations in

Europe

regularly exceed current EU air quality standards (200 µg/m3 one-hour average for nitrogen dioxide, 40 µg/m3 one-year average), especially in urban areas. Europe’s transport sector, with its 50 per cent passenger car diesel share, is the single largest contributor (about 40 per cent) to total ambient NOx concentrations.

The study carried out by ICCT, which it claims is the most comprehensive report on the real-world behaviour of the latest generation of diesel cars published to date, found remarkable differences among individual vehicle models, indicating that technologies for real-world clean diesels already exist but are not being employed consistently by different vehicle manufacturers.

The findings come at a time when the European Commission is preparing to propose an improved car emissions testing procedure, including on-road measurement, that could take effect in 2017.

The study is based on on-road tests of 15 vehicles using portable emissions measurement systems. It covers more than 140 hours and 6,400km of driving, and provides, says ICCT, the largest collection of public data on Euro6 equivalent diesel cars available.

Real-world emissions “On average, we found that real-world nitrogen oxides emissions for modern diesel cars were about seven times higher than required by the Euro 6 standard”, says Dr

Peter Mock, managing director of ICCT Europe.

The ICCT researchers demonstrate that the majority of exceedances found could not be attributed to extreme or untypical driving. Instead, they were due to driving situations that are part of the normal operation of diesel cars, such as mild uphill driving and regeneration events of the diesel exhaust after-treatment system.

The ICCT experts also found significant differences among the performance of all the vehicles tested, with a few vehicles performing substantially better than the others.

“This supports the notion that the technologies for real-world clean diesels already exist,” says Dr Vicente Franco, researcher at the ICCT and lead author of the study.