Wednesday, 30 September 2015

NOx emissions on VW and other manufacturer's vehicles

Diesel engines run more efficiently at high compression and temperature - but the high temperature leads to more NOx emissions that are bad for our health - apparently causing 6000 early deaths a year in London alone [1]. There are several different ways to reduce these emissions and the selective catalytic reduction method VW was using is one of them but not necessarily the best. In particular it performs less well at low speeds such as in urban traffic. The chart below compares the emissions from cars from different manufacturers under the current NEDC test (New European Driving Cycle test, actually 18 years old) and the somewhat more demanding WLTP (Worldwide harmonised Light vehicles Procedure) test which is expected to come into force in the EU sometime in 2017 [2].

Comparison of the NOx emission from cars of differnt manufacturers under the current NEDC test and the more demanding WLTP test. Both are rolling road tests. CF is conformance factor - values less than one meet the current regulations and values > one do not [2].
VW and BMW both do well on both WLTP and NEDC tests - both are on a rolling road so presumably the VW test detecting software cuts in on the WLTP too. However according to the ICCT, BMW does OK in the real world too [3]. Apart from those two manufacturers very few of the cars tested meet the standard under the WLTP test, which just goes to show that cars do less well in the real world than under the NEDC. Of course we knew that anyway from when I looked at these tests from the point of view of fuel efficiency - see Why your car does not achieve the fuel economy it is supposed to.

The WLTP is more demanding than the NEDC as it has higher acceleration and somewhat higher average speed. However, it is still a rolling road test. The EU has plans to bring in Real Driving Emissions (RDE) tests that involve running a car outside with random acceleration and deceleration (within limits to be agreed) and measuring the tailpipe emissions. VW and any other manufacturers wanting to cheat will have a harder time defeating those tests. The RDE procedure is supposed to start running early 2016 but the results won't be enforced for new cars until 2018. Can we speed this up a bit please? [4] Unfortunately, the RDE will test emissions but not efficiency so we will still be using the WLTP for that.

I am pleased to see that the EU emissions testing regulations require tests on vehicles that have been in use for a while as well as new ones. Tests are required to ensure that vehicles still meet the standard after 5 years or 100,000 km [5]. However, these 5 years are generally simulated with accelerated aging on a test bench so that isn't a real world test either. Sigh.

[1] Air pollution in London caused early deaths of nearly 9,500 people in a single year, study finds (Independent) July 2015
[2] NOx control technologies for EURO 6 diesel passenger cars (ICCT) Sep 2015
[3] EPA's notice of violation of the clean air act to Volkswagen (ICCT) Sep 2015
[4] EU commission fact sheet: Air pollution emission standards (European Commission) Sep 2015


  1. You can only have a real 5-year old vehicle test if you wait 5 years. Which is obviously inconvenient. What do you do if they fail after 5 years? The same model is probably no longer on sale so you can't stop selling it. Confiscate the ones people already have?

    1. The right thing to do would be to service/upgrade the car so that it did meet the emission tests, with a recall if necessary. However, in practice the in-use tests are done with accelerated ageing so this question does not arise.