NOx: a new challenge for diesel light duty vehicles.

While focusing on CO2: NOx emissions are major challenges for diesel car manufacturers.

Liuhanzi YANG, EnvIM 2014 student, realized her professional thesis on the “Investigation of real-world emissions from Euro 6 diesel passenger cars”, at the International Council for Cleaner transportation ICCT in Berlin. ICCT purpose was to study the real-world NOX, in order to provide insights into the emission performance of modern diesel passenger cars and to support European policy makers in the development of real-world emission regulations.

Hanzi had the opportunity to contribute to the ICCT sept 2015 White Paper, “NOx control technologies for Euro 6 diesel passenger cars” here

and an article in Environmental Science & Technology here

Context : diesel and clean air

Vehicle emission is one of the major sources of air pollutants which threaten the public health. The four pollutants in the European emission standards are, hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOX) and particles. Among them, NOX is the main focus of diesel emission control.

The Euro norms for light-duty diesel vehicles

For Light Duty Vehicle standards the toxic emission stages are typically referred to as Euro 1 in 1993 to Euro 6 since Sept 2015. Different standards apply for each vehicle type – emission limits - and compliance is determined by running the engine at a standardized test cycle.

If advanced technologies to reduce emissions do exist, the limits adopted by EU are gradually more restrictive, in order to increase pressure for clean air requirements as well as energy conservation. But NOX emissions seem not properly controlled under the current test cycle framework! In response to this problem, the EU is planning to replace the test procedure with a more realistic and transient driving cycle test.

EU NOx norms

Investigation of real-world emissions from

Euro 6 diesel passenger cars

As of the full phase-in of the Euro 6 standard in September 2015, all newly registered diesel passenger cars in the European Union have to meet a nitrogen oxides (NOX) emission limit of 80 mg/km over the European light-duty vehicle emissions certification cycle (New European Driving Cycle, NEDC).

While all diesel manufacturers have managed to meet this limit during the regulatory test, it is widely recognized that the real-world NOX emissions of diesel passenger cars are substantially higher than the certified limit. This was one of the main drivers behind the recent amendment of the Euro 6 standard to require an on-road, real-driving emissions (RDE) test using portable emission measurement systems (PEMS) for the type approval of passenger cars in the EU.

NOx measurement methods

Once RDE testing is legally enforced in 2017, passenger cars will have to demonstrate reasonably low emissions under conditions that resemble real-world use more closely than laboratory cycles. RDE testing will therefore pose additional challenges for diesel passenger car manufacturers in the EU.

The laboratory results indicated that the NOX emissions over the certification cycle (NEDC) were significantly lower than over the more realistic Worldwide Harmonized Light Vehicles Test Cycle (WLTC), and the on-road PEMS tests showed that the average NOX emissions were 7.7 times the certified emission limit, with some vehicles emitting up to 20 times the limit. These results add to the overwhelming amount of empirical evidence that the NOX emissions from diesel passenger cars are not properly controlled under the current, NEDC-based testing framework.

NOx control technologies

More stringent policies and regulations are urgently needed to better control the emissions from passenger cars, especially NOX emissions from diesel vehicles. For this purpose, a replacement of NEDC by the more realistic WLTC cycle is much needed, but likely not sufficient. The mandatory on-road testing of RDE regulation is a step in the right direction to incentivize vehicle manufacturers to effectively deploy the already-existing NOX control technologies that work efficiently during real-world driving. In the short run, this should lead to more robust implementations of existing NOX control technologies, and it could have a significant impact upon the hardware choices made by diesel car manufacturers. In the long term, RDE should also deliver substantial improvements in urban air quality in Europe.

Interview with Liuhanzi Yang

Liuhanzi during her defense

Liuhanzi during the defense of her thesis

(from left to right : Vicente Franco, Liuhanzi Yang, Cathy Descamps-Large, Christophe Coquelet)

Why did you choose this professional mission at ICCT? Are you especially interested by transportation emissions and air pollution?

Yes, I am very interested in air pollution control and I have been learning about this field for 4 years now. At Tsinghua, I learnt the technical and scientific basis of air pollution, such as the formation and control of vehicle emissions, or the physics and chemistry of air pollutants. At the EnvIM program at Mines ParisTech, I learnt more about the environmental management, and realized how regulations can play an important role in air pollution control. ICCT is an independent non-profit research organization that provides technical and scientific analysis to environmental regulators around the world, so this professional mission fits my interests and background very well.

Are there many differences between countries concerning the NOX control technologies and the measurement methods used?

Yes. Based on our research results, we found that the main technology for the control of NOX emissions from new diesel cars in the EU, the so-called lean NOX trap (LNT) grew from 5% to 55% in the past 3 years for new Euro 6 diesel passenger cars. However, in the US, selective catalytic reduction (SCR) and combined systems of LNT and SCR captured the majority (67%) of the diesel vehicle market in 2014. SCR and SCR+LNTs are more robust technology solutions, and we think these differences are related to the fact that emission standards in the US are more stringent than European ones.

For vehicle emission measurement methods, chassis dynamometer measurement (on a “roller bench” in a laboratory) is widely used for emissions certification (type-approval) around the world, but on-road testing using portable emission measurement systems (PEMS) has been developing rapidly in recent years. Last May, the real-driving emission (RDE) regulation was approved in the EU. This means that Europe will be the first world region to require PEMS testing for the emissions certification of passenger cars. The emission limits for this test will be enforced from 2017.

What technology is better, according to you, to reduce NOX emissions? and why?

From the experimental results we have seen that SCR is the better technology on the market to reduce NOX emissions. In a SCR system, urea is injected before a special catalyst to convert NOX into N2. The current commercial SCR systems can approach 95% of NOX conversion efficiency. However, SCR requires additional infrastructure for the distribution of urea solutions (commercially known as AdBlue) and an on-board storage tank, which makes it the most expensive technology to reduce NOX emissions.

What do you think about the future of diesel?

Diesel accounts for 53% of new car registrations in the EU (66% in France), so it is probably here to stay. However, especially after the VW scandal, diesel will be under a lot of scrutiny from the general public and from regulators in Europe. From my work at the ICCT I have learnt that the technologies that enable clean diesel cars already exist. If the right regulations are put in place and the manufacturers apply the best technologies, we could see NOX concentration levels in cities go down significantly in the next few years.

What about the problems of air pollution due to passenger cars emissions in China?

In some megacities in China, vehicle emissions are one of the most important contributors that affect air quality. From the latest research results of Beijing Municipal Environmental Protection Bureau, vehicle emissions contributed 31% of the PM2.5 emissions in Beijing (PM2.5 is the main pollutant that leads to the formation of smog). I am glad that more stringent emission standards and emission control policies are being implemented increasingly in China. I hope the air quality will become better with an effective implementation of these regulations.