Independent testing by consumer advocates Which? magazine shows that the vast majority of modern diesel-engined cars still cannot pass their emissions tests. Which? claims to have tested 61 diesel-engined cars since the beginning of 2017, and has found that 47 of them have nitrogen oxide emissions (NOx) above the mandated limit of 0.08g/km.
On average the cars emitted 0.27g/km of NOx, around three-and-a-half times the legal limit. The worst-performing car, according to Which? was the 2013-on Subaru Forester 2.0 diesel, which emitted 2.0g/km of NOx, 25 times the official limit.
The cars which perform worst are not the compact, family-friendly models that you might expect. Which? is especially pointing out that Renault's diesel engines seem to be especially poor performers, with the 1.6-litre dCi Grand Scenic emitting 0.896g/km of NOx, while the Captur 1.5 dCi 90 scored 0.725g/km. Even the current mild-hybrid Scenic diesel emitted 0.478g/km of NOx.
Oddly, larger more luxurious cars seem to perform better. Mercedes' E220d, when fitted with the nine-speed automatic gearbox, recorded 0.023g/km of NOx, while the BMW 730d luxury car managed an on-the-button 0.08g/km. Other cars which Which? recorded as being below the limit for NOx were the Opel (Vauxhall) Zafira Tourer 2.0 CDTI; the Seat Ateca 2.0 TDI 4WD DSG; The Land Rover Discovery TDV6 SE; the BMW 520d; the Mercedes-Benz CLS400d; the Peugeot 308 SW 2.0 HDI 180hp GT; the Mercedes-Benz S400d; the Opel (Vauxhall) Grandland X 2.0d; the BMW X2 20d M-Sport xDrive; and the BMW 2 Series Active Tourer 218d.
Which? says that it 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.
Alex Neill, Which? managing director of home products and services, told The Irish Times that: "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."
Renault has responded to the Which? tests by saying 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 has invested about €3 billion 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."
