Nearly 80% of diesel cars exceed emissions limit in latest Which? tests

Most modern diesel cars are much dirtier than official limits, according to ongoing Which? tests that show that the majority exceed official emissions limits when put through more rigorous real-world tests.

Which? has tested 61 modern diesel cars since the start of 2017, and found 47 of them (77%) exceeded the Euro 6 limit of 0.08g/km of NOx (oxides of nitrogen), despite conforming to the official standards.

On average, the diesel cars tested under Which?’s current test programme produce 0.27g/km of NOx – nearly three and a half times the existing official Euro 6 limit.

The worst culprit among these was the Subaru Forester (2013-present) that produced 2.0g/km of NOx, a shocking 25 times the Euro 6 limit that it met in official tests.

All five diesel Renault cars tested by Which? since the start of 2017, including the mild diesel hybrid, appear in the top ten most NOx emitting cars. The two Renault vehicles that come second and third in our table of diesel cars that emit the most NOx are the Grand Scenic (Energy DCI 160 Bose Edition EDC)  which recorded 0.896g/km of NOx in Which? tests and the Captur (dCi 90 Intens) which produced 0.725g/km.

In stark contrast, Mercedes has produced some cars with extremely low NOx emissions. The current E-Class diesel that Which? tested (specifically the 2.0-litre four-cylinder E220d 9G-Tronic diesel engine) produced just 0.023g/km of NOx. The result was so surprising that Which? experts took it out of the lab and onto the road to see how the figures compared, but the figures shown on the Portable Emission Measuring System nearly matched the lab figures.

Additionally, Which? found that the BMW 2 Series Active Tourer (specifically the 218d Active Tourer Steptronic diesel engine) produced just 0.014g/km of NOx.

Which? car testing is more rigorous than the official tests, with vehicles tested in the default setting they start up in, rather than switching to a more economical driving mode (see notes to editors.)

Alex Neill, Which? Managing Director of Home Products and Services, said:

“The current official tests fail to measure the actual level of emissions that cars are producing on our roads.

“Most diesels we assess are producing far more NOx in our tests than official limits allow. The new official tests* should help reduce harmful emissions – but we will continue to penalise any car we find that produces excessive levels of pollutants in our tests.”

*See notes to editors

Advice from Which? for buying your next car

• We recommend you think carefully if you’re planning to buy a diesel car. Our latest tests have shown some hybrids are now as economical as some diesel cars on the motorway, proving that diesel isn’t always the default economical choice. See https://www.which.co.uk/reviews/new-and-used-cars/article/petrol-vs-diesel-cars-which-is-better

• More hybrids are appearing and as the technology continues to evolve, there continue to be more reasons to choose one. If electric doesn’t work for you, perhaps due to distance travelled or ability to charge, now is the time to go hybrid.

• Go to www.which.co.uk/cars to read our expert reviews.

Table of NOx emissions from the diesel vehicles tested

Notes to editors:

• For more information about diesel, petrol and hybrid cars, visit: https://www.which.co.uk/reviews/new-and-used-cars/article/petrol-vs-diesel-cars-which-is-better

• Which? aims to test all the relevant cars from the car makers that matter to our readership. We currently have 803 car reviews online from 43 car brands.

• NOx (oxides of nitrogen) is comprised of NO (Nitrogen Oxide) and NO2 (Nitrogen Dioxide). NOx is harmful and has been linked to tens of thousands of premature deaths around the world. It comes out of all diesel, petrol and hybrid cars but in our latest tests diesel engines still produce an average of 11.5 times the amount of NOx compared with petrol cars.

• Official emission tests and limits as we know them started back in 1992 with Euro 1, setting limits for various tailpipe emissions including NOx and CO. Today we are on the sixth iteration, Euro 6.  New limits apply in two phases. There is an initial date that all new generations of cars (new type approvals) have to comply with. The second date, which typically comes a year later, affects all other remaining new cars on sale.

• The existing Euro 6 limit of 0.08g/km is currently being replaced by the new Euro 6-temp RDE step 1 limit of 0.168g/km of NOx. The new tests and limits has been introduced for all new generations of cars as of September 2017, and will be in effect for all remaining new cars by September 2019. In our tests, 28 out of 61 diesel cars would meet this new limit. Find out more about the new official tests and limits here: https://www.which.co.uk/reviews/new-and-used-cars/article/how-we-test/how-we-test-mpg-and-emissions

• Which? tests cars in the default setting they start up in, rather than switching to a more economical driving mode. All test cycles are conducted with the air conditioning on, lights on dipped beam and the radio working. Crucially, our tests also include a unique motorway cycle, which official tests do not have, which measures both fuel economy and emission output in motorway conditions. All tests are lab based, but, if the results are suspicious the car is driven on real roads and emissions are measured using a PEMS (Portable Emission Measuring System). All cars are tested with a 200kg payload.

• By contrast, the outgoing type approval tests based on the outdated NEDC have been widely discredited. For emissions, these tests are currently being replaced with RDE tests, with all new cars having to conform by September 2019.

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Responses from vehicle manufacturers

• Groupe Renault said that while all its vehicles conform to applicable laws and standards, it’s aware that there remains significant potential for improving the level of NOx in real-use conditions.

• A Mercedes spokesperson said: “For premium vehicles, from our perspective, the Diesel has definitely a future. Therefore, Daimler AG [owner of Mercedes] has invested about 3 billion Euros into the development and production of a completely new diesel engine family… Our new engines are both highly efficient and produce low levels of NOx. It is a fact that it is worth improving the modern diesel instead of banning it.”

• Subaru has not replied to our request for comment.

Press Release