Dirty diesel cars tested

Emissions tests on 32 Euro 6 diesel passenger cars from ten different manufacturers and with different abatement technologies show that most cars fail to meet the NOx emissions standards under more realistic driving conditions.

As of September 2015, all new diesel passenger cars will have to meet the Euro 6 NOx emission limit of 80 mg/km. While all diesel car manufacturers have managed to meet this requirement during the regulated laboratory test, which is done using an outdated emissions certification driving cycle (the New European Driving Cycle, NEDC), many studies have shown that the “real-world” NOx emissions of diesel passenger cars are substantially higher than the certified limit.

From 2017, the NEDC will be replaced by the more realistic Worldwide Harmonized Light Vehicles Test Cycle (WLTC) that better represents actual on-road emissions. But according to the study, the biggest challenge for diesel passenger car manufacturers arises not from the certification cycle but from the real-driving emissions (RDE) test, which is scheduled to become a mandatory step for the type approval of passenger cars in the EU in January 2016. Under this new testing framework, diesel passenger cars will have to prove that they can keep NOx emissions at reasonably low levels during a test that more closely represents real-world driving situations.

The study analyzes the results of emissions tests on 32 Euro 6 diesel passenger cars from ten different manufacturers, equipped with different types of exhaust after-treatment technologies, tested over both the NEDC and WLTC driving cycles. Three main technologies are available for controlling NOx emissions from modern diesel passenger cars: inner-engine modifications coupled with exhaust gas recirculation (EGR), lean-burn NOx adsorbers (also called lean NOx traps, or LNT), and selective catalytic reduction (SCR). Of the Euro 6 diesel cars sold last year in the EU, about 55 per cent were equipped with LNT technology, 40 per cent with SCR, and 5 per cent with EGR.

It should be noted that in the US market, combined after-treatment systems are featured in some models that otherwise use a single NOx control technology in their European market versions. This is explained by different regulatory frameworks, since the US has a tougher NOx emission limit value, a more demanding test cycle, and a robust enforcement and compliance programme that the EU lacks.

Of the 12.5 million passenger cars sold in the EU in 2014, 6.6 million (53%) were powered by diesel. In the US, 16.4 million passenger vehicles were sold in 2014, but only 138,000 (0.84%) were diesel-driven. The test results showed that 31 out of the 32 vehicles met the limit of 80 mg/km over the less demanding NEDC cycle. Most EGR- and SCR-equipped vehicles performed better than LNT-equipped vehicles over the WLTC cycle, but their average emissions were still far higher than those over the NEDC (by a factor of 2.3 for EGR-equipped vehicles and 2.8 for SCR-equipped vehicles). The same factor was 8.0 for the average of all LNT-equipped vehicles.

Three LNT-equipped vehicles showed very poor performance over the WLTC, with one car emitting up to 1,167 mg/km of NOx, i.e. 15 times higher than the limit. The authors conclude that this casts a shadow of doubt over the real-world performance of all current NOx control approaches, especially those relying on LNT, and underscores the importance of engine and after-treatment calibration to realize the full potential of available technologies and achieve satisfactory real-world performance. This autumn, the European Commission is about to propose emissions limit multipliers that will apply to the new on-road vehicle emissions tests. These “conformity factors” will have a large impact on the deployment of emissions control technologies, and thus on the real-world emissions performance of new diesel cars.

The European Commission will phase in RDE testing in two subsequent steps with increasing levels of stringency. The report says that it is widely expected that the initial step of conformity factors (applicable from September 2017 onward) will lie around a value of 2 for NOx emissions from diesel cars, which would mean that these vehicles will still be allowed to emit about twice as much as the limit of 80 mg/km during the on-road test, effectively making this the first time that the Euro standards will be changed to raise an emission limit instead of lowering it. Moreover, since RDE does not include cold-start emissions, the allowed increase will be substantially higher than is indicated by the conformity factor.

The second step of RDE is expected to apply from 2019 onward and bring conformity factors close to 1, thus making Euro 6 diesel cars come closer to delivering on their promise, albeit seven years after their initial market introduction. The ICCT points out that since the RDE testing cannot apply retroactively to existing Euro 6 type approval certificates, it is essential to act fast and ensure that high emitters of NOx are prevented from entering the market.

Christer Ågren

The report “NOx control technologies for Euro 6 diesel passenger cars – Market penetration and experimental performance assessment” (September 2015) can be downloaded from: www.theicct.org

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