Independent, Expert EV Reviews & Advice Since 2006

Discrepancies between diesel cars reaches record high



Diesel emissions have been looked at again by Emissions Analytics, and some data suggests that diesel cars can be cleaner than petrol models; the message is, don’t generalise when talking about diesel v petrol.

The first diesel vehicle that met the regulated Euro 6 limit for nitrogen oxides (NOx) on Emissions Analytics’ real-world EQUA Index ( test using a Portable Emissions Measurement System (PEMS) was in May 2013. Of the vehicles tested in that year, the cleanest 10% of diesels emitted 265 mg/km and the dirtiest 10% emitted 1777 mg/km – a ratio of 7 to 1. In 2017, the cleanest 10% achieved an impressive 32 mg/km, but the dirtiest 10% were 1020 mg/km, a ratio of 32 to 1.

On average, progress has certainly been made, with average diesel NOx emissions having fallen from 812 mg/km to 364 mg/km from Euro 5 to Euro 6, or a 55% reduction, driven by the prospect of the new Real Driving Emissions (RDE) regulations together with the aftermath of dieselgate. The very worst vehicles have now disappeared from the new car market. It is also true that in around 10 years’ time, the majority of diesels on the road are likely to be of the cleaner variety, through natural turnover of the fleet.

Emissions Analytics has now tested six of the latest RDE-compliant diesel vehicles, also known as ‘Euro 6d-temp’. Their average NOx emissions were 48 mg/km, 40% below the regulated limit itself, and 71% below the effective limit once the Conformity Factor of 2.1 is taken into account (as ever, it should be noted that while the EQUA Index test is broadly similar to an RDE test, it is not strictly compliant). However, it should be noted that there are many cleaner diesels even before RDE, with 30 prior models achieving real-world emissions of 80 mg/km or less.

While this sounds like good news, the elongated transition to RDE, and growing spread from the best to the worst, are creating a growing policy and consumer choice problem in the meantime. A vehicle in the highest-emitting decile today will likely be a significant contributor to urban NO2 pollution. Yet, the cleanest diesels are getting close to the average NOx emissions from new gasoline vehicles, which is 36 mg/km. Without the contemporary data to show this, policy makers would be forgiven for simply banning all diesels from urban locations.

The lowest NOx emission recorded so far this year is the 2017 model year Mercedes CLS, with selective catalytic reduction after-treatment and type-approved for 6d-temp, which recorded 15 mg of NOx per km.

But with some clean diesels identified, what is the right approach? With the ratio from best to worst now higher than at any time since Emissions Analytics has been testing (covering diesels back to Euro 3), the importance of good real-world performance data is greater than ever. It is not enough to know that clean diesels exist, it is necessary to know which are which, model year by model year.

Beware also the danger of incomplete information. Early results from Emissions Analytics’ testing particle number (PN) emissions on the latest vehicles shows diesels emitting 71% fewer particles than the latest gasoline vehicles (albeit before the deployment of gasoline particulate filters on direct injection versions). In addition, the average carbon dioxide (CO2) emissions per kilometre from these diesels is approximately 18% less than the gasoline. Therefore, one may conclude that any switch from one of the cleaner breed of diesels to an equivalent petrol is a Pareto worsening.

That said, as we discussed in our previous newsletter, after-treatment is complex and the consequences for emissions of malfunction are great. Therefore, enhanced inspection and maintenance must accompany any perpetuation of the internal combustion engine powertrain.

In summary, consumers are at the forefront of the changing complexion of the car fleet, as they are making purchasing decisions now, and need detailed, specific information of competitor models. Policy makers need to be careful not to chase the past, but rather understand the models – with their emissions and fuel economy characteristics – that are already coming through, whether they be diesel, gasoline, hybrid or electric.

From Emissions Analytics