An Overview of Lean Exhaust deNOx Aftertreatment Technologies and NOx Emission Regulations in the European Union

Abstract

This paper reviews the recent advances in the management of nitrogen oxide (NOx) emissions from the internal combustion engine of light-duty and heavy-duty vehicles, addressing both technical and legal aspects. Particular focus is devoted to the often-virtuous interaction between new legislation imposing more restrictions on the permitted pollutant emission levels and new technologies developed in order to meet these restrictions. The review begins first with the American and then European directives promulgated in the 1970s, aimed at limiting emissions of pollutants from road transport vehicles. Particular attention is paid to the introduction of the Euro standards in the European Union for light- and heavy-duty vehicles, used as a legal and time frame reference for the evolution of emission aftertreatment systems (ATSs). The paper also describes governmental approaches implemented for the control of pollutant emissions in circulating vehicles, such as market surveillance and in-service conformity. In parallel, it is explained how the gradual introduction of small-scale devices aimed at the NOx control, such as lean NOx traps (LNTs) systems, and, most of all, the selective catalytic reduction (SCR) of NOx, permitted the application to road-transport vehicles of this ATS, originally designed in larger sizes for industrial usage. The paper reviews chemical processes occurring in SCR systems and their advantages and drawbacks with respect to the pollutant emission limits imposed by the legislation. Their potential side effects are also addressed, such as the emission of extra, not-yet regulated pollutants such as, for example, NH3 and N2O. The NOx, N2O, and NH3 emission level evolution with the various Euro standards for both light- and heavy-duty vehicles are reported in the light of experimental data obtained at the European Commission’s Joint Research Centre. It is observed that the new technologies, boosted by increasingly stricter legal limits, have led in the last two decades to a clear decrease of over one order of magnitude of NOx emissions in Diesel light-duty vehicles, bringing them to the same level as Euro 6 gasoline vehicles (10 mg/km to 20 mg/km in average). On the other hand, an obvious increase in the emissions of both NH3 and N2O is observed in both Diesel and gasoline light-duty vehicles, whereby NH3 emissions in spark-ignition vehicles are mainly linked to two-reaction mechanisms occurring in three-way catalysts after the catalyst light-off and during engine rich-operation. NH3 emissions measured in recent Euro 6 light-duty vehicles amount to a few mg/km for both gasoline and Diesel engines, whereby N2O emissions exceeding a dozen mg/km have been observed in Diesel vehicles only. The present paper can be regarded as part of a general assessment in view of the next EU emission standards, and a discussion on the role the SCR technology may serve as a NOx emission control strategy from lean-burn vehicles.

Measuring Emissions from a Demonstrator Heavy-Duty Diesel Vehicle under Real-World Conditions—Moving Forward to Euro VII

“…This indicates fairly continuous and low emissions, without significantly different events in the cycle such as cold start vs. stabilised hot emissions. This is in line with what was already reported in the literature [2,15] and consistent with possible pathways for N 2 O production in Diesel emission control systems. Indeed, these are nowadays characterised by the requirement for very low NOx tailpipe emissions, requiring active DOCs, and an increase in the DEF injection rates together with the necessity of limiting any NH 3 slip at the tailpipe.…”

“…Indeed, since the early 1990s, the EU has introduced a series of directives and regulations, commonly referred as Euro standards (Euro I to Euro VI step E for heavy-duty vehicles (HDVs)), aimed at curbing the contribution to air pollutants from the transport sector in Europe. This has resulted in a significant reduction in the emissions of a number of noxious compounds [1] and has contributed to the achievement of 2020 targets for the main air pollutants for EU as a whole [2]. However, despite this positive development, additional efforts are being discussed to counteract the effects of certain trends (such as increases in population and in economic output) currently eroding the beneficial effects of the aforementioned regulations [1].…”

“…In addition, among unregulated pollutants, also HCHO represents a concern. An extensive literature review on the topic has been recently published and is available to the interested reader [2].…”

NH3 and N2O Real World Emissions Measurement from a CNG Heavy Duty Vehicle Using On-Board Measurement Systems

“…The implementation of the vehicle emission regulations in Europe-commonly known as Euro standards-has resulted in a reduction in the emissions of a number of air pollutants from the road transport sector [1][2][3]. However, despite this positive development, additional efforts are being discussed to counteract the effects of certain trends (such as increases in population and in economic output) currently eroding the beneficial effects of the aforementioned regulations [1].…”

Effect of Extreme Temperatures and Driving Conditions on Gaseous Pollutants of a Euro 6d-Temp Gasoline Vehicle

“…NH 3 formation over the catalyst is enhanced at low air/fuel (lambda) ratios (rich operating conditions). The mechanisms that take place under these conditions are the water-gas (CO) shift reaction, producing H 2 and the reaction of NO and H 2 [58]. The availability of H 2 and CO, the exhaust gas temperature, and the lambda also affects small differences in the slopes of the cumulative NH 3 functions between different temperatures [59].…”