The research by the International Council on Clean Transportation (ICCT) explored NOx control technologies for Euro 6 diesel passenger cars, analysing results of emissions tests on 32 Euro 6 diesel passenger cars from 10 different manufacturers over both the NEDC and WLTC driving cycles.

The results show that some automakers meeting diesel NOx emissions standards under more realistic driving conditions, while others lag badly.

The findings indicate that the implementation of exhaust control technologies by a few manufacturers is delivering acceptable results over both cycles, whereas other manufacturers are mostly focusing on meeting the limit over the NEDC while neglecting real-world operating conditions, even on the relatively low-load WLTC.

All vehicles tested except one met the legislative limit of 80mg/km of NOx over the less demanding NEDC cycle. Most EGR- and SCR-equipped vehicles performed better than LNT-equipped vehicles over the WLTC, 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 exhibited very poor performance over the WLTC, with one car emitting up to 1,167 mg/km of NOx (15 times the regulated limit).

The ICCT said this casts a shadow of doubt over the real-world performance of all current (pre-RDE) NOx control approaches, especially those relying on LNTs, and underscores the importance of engine and aftertreatment calibration to realise the full potential of available technologies and achieve satisfactory real-world performance.

With the industry set to move on from the current NEDC regime to the more realistic WLTC from 2017 on, the ICCT added that 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.

Environmental NGO T&E said the new RDE test must be quickly introduced to bring an end to ‘cycle beating’ techniques used by car manufacturers to attain emissions levels typically five times lower than actual air pollution emissions on the road for modern Euro 6 cars. Meanwhile, the car industry is trying to reopen the discussions through introducing new ‘transfer functions’ to normalise test results.