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ICL study of 39 new Euro-6 diesels finds huge variability in NOx emissions with an average 4.5x the type approval limit

A new study by researchers from Imperial College London(ICL) of 39 new Euro 6 diesel passenger cars has foundhuge variability” in the on-road NOx emissions of Euro 6 diesel passenger cars, with results ranging from 1 to 22 times the type approval limit. All but 2 exhibited higher NOx than the limit.

The average NOx emission from the test cars of 0.36 g km-1 equates to 4.5 times the type approval limit; this rose to 5.4 times for urban driving. They attributed the increase in urban cycle NOx emissions in part to more frequent acceleration events. (Urban driving emissions could be reduced by more effective management of traffic flows (e.g., earlier post), easing of congestion and promotion of eco-driving, though further work is required to confirm this, they suggested.)

The Imperial College London team examined nitrogen dioxide (NO2 ) and nitric oxide (NO) emissions, collectively known as NOx, because of their direct link to health problems such as childhood asthma. The UK’s air pollution is thought to contribute to 40,000 premature deaths each year, and NOx (particularly NO2 ) plays a large part in this. Air pollution in general contributes to strokes, asthma, heart disease, chronic obstructive pulmonary disease, lung cancer, and lung infections. Across the EU, it is thought to reduce life expectancy by nine months.

Unfortunately the EURO-6 diesel cars seem to be following the pattern of EURO-4 and 5 by far exceeding type approval in real world driving. These results are concerning, particularly for people living in urban areas, as NO2 is linked to childhood asthma among other health issues.

Higher levels of NO2 are particularly problematic in urban areas, and we found that many of the diesel cars emitted large proportions of NO2, directly enhancing concentrations of this toxic component at road-side locations.

—Lead author Rosalind O’Driscoll, Centre for Environmental Policy, ICL

The UK and several other European member states consistently breach the EU air quality limit value for ambient concentrations of NO2 . This has resulted in the current court battle between the Department of Environment Food and Rural Affairs and the environmental law NGO ClientEarth.

Using portable emissions measurement systems (PEMS) attached to vehicle exhausts, the researchers collaborated with Emissions Analytics to measure the amount of NOx produced by the 39 Euro 6 diesel cars.

The researchers compared their results to estimates of vehicle emissions using COPERT (Computer Program to Calculate Emissions from Road Transport), an air quality emissions model developed by the European Environment Agency. The average NO2 emission in the new study was 2.5 times the COPERT model estimate, rising to 2.8 for urban driving.

Comparison of urban and motorway trip average NOx emissions (note: y axis varies). Vehicle ID at top of each plot. O’Driscoll et al. Click to enlarge.

The researchers also found no clear best between the after-treatment technologies. Both SCR and LNT were able to meet the Euro 6 standard and NTE limit while EGR alone was not.

Since Euro 6 vehicles are already in use, the researchers argue that it is important to identify the worst performing cars and ban them, rather than just relying on generic EURO-6 standards for regulation. This would enhance the effectiveness of schemes like Clean Air Zones and allow for consumers to make informed decisions when purchasing a new car. The researchers found that removing the five most polluting vehicles reduced average emissions considerably.

At present, manufacturers must adhere to emissions limits tested only in a lab setting, but next year, type approval testing will include real-world driving environments to tackle real-world emissions levels. The authors also suggest that rather than using generic guidelines for all cars, each car model should be tested on its own merit with regard to emissions.


  • Rosalind O’Driscoll, Helen M. ApSimon, Tim Oxley, Nick Molden, Marc E.J. Stettler, Aravinth Thiyagarajah (2016) “A Portable Emissions Measurement System (PEMS) study of NOx and primary NO2 emissions from Euro 6 diesel passenger cars and comparison with COPERT emission factors,” Atmospheric Environment, Volume 145, Pages 81-91 doi: 10.1016/j.atmosenv.2016.09.021



Should diesels be the first ICEVs to be banned in city centers? BEV/FCEV buses are ready to take over? The use of diesel trucks could be restricted?

Dr. Strange Love

Harvey. Are you implying that modern diesel emissions equipment (SCR/Urea +) pollute more in terms of Well-to-Wheel than a Modern Gasser or an Electric that receives energy from a Pre-dominantly Dirty source.

Please Elaborate.


Of course diesel cars are polluting much more than any stationary power source in terms of NOx or solid particles. From that perspective Stationary power sources has no effect on living environment except China or several other countries.


@Darius - that's not true in most regional grids in the U.S. according to the latest version of Argonne National Laboratory's GREET model (

In fact, even diesel cars with no after-treatment NOx control (e.g., SCR) produce less damage to public health and the environment than comparable electric cars in most grids in the U.S.


In reality, GHG & pollution from diesel ICE vehicles is much higher than reported and much higher than the average gasoline ICEV.

Driving behind a city bus and/or a heavy diesel truck, with black smoke from their exhaust, confirms that fact.

Most diesel units, including tractors, heavy machinery, locomotives, ships etc have been over polluting the atmosphere for 130+ years. We are overdue for a change.


This - - tends to refute your claims, Harvey.


There will always be a pro-diesel and/or pro coal/NG fan around to try to demonstrate that diesel ICEs and CPPs/NGPPs are clean, which is not true.

That's how we got to 400+ ppm CO2 and rising fast.

The world will have to stop burning coal and other fossil fuels if we want to return to under 300 ppm CO2.

Electrification (with REs) and affordable??? NPPs may be the solution?


@Harvey - what about the ACES reports demonstrating very low emissions from current production diesel truck engines?

You may live in an area that has no CPPs/NGPPs, but most don't.


Reality is that most vehicles using diesel ICE pollute a lot more (4 to 5 times) than they are supposed to do.

They should be the first ICEVs to be replaced with BEVs or FCEVs. Us and our kids would be healthier.


I repeat - in MOST grids in the U.S., BEVs and FCVs produce more damage to public health and the environment than the cheating TDIs, never mind the diesel ICEVs that do well in real-world conditions/driving styles.

Even in the subject study, several diesels do fine, e.g., L2.0a, L2.0b, S2.0e. Why should the entire technology be eliminated because some manufacturers take short-cuts?

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