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.
Highlights
Nitrogen oxides are a group of highly reactive gaseous compounds containing nitrogen and oxygen in different proportions
It is clear that, in parallel, the development of targeted technologies for a drastic reduction of nitrogen oxide (NOx) pollution derived from internal combustion engine (ICE) emissions has led to the production and commercialisation of vehicles able to face the conflicting exigencies of increasing mobility by the public on one Catalysts 2021, 11, 404 hand, and the increasingly stringent air pollution limits required by the environmental policies on the other
Typical modern Diesel aftertreatment systems (ATSs) are conventionally composed of a Diesel oxidation catalyst (DOC), a Diesel particulate filter (DPF) or a catalyzed DPF to trap soot particles, a Diesel exhaust fluid (DEF) injection point with a mixer in order to achieve uniform NH3, the selective catalytic reduction (SCR) unit, and, an Ammonia Slip Catalyst (ASC) catalyst to limit the slip of unreacted NH3 in the atmosphere
Summary
Nitrogen oxides are a group of highly reactive gaseous compounds containing nitrogen and oxygen in different proportions. VI-E for heavy-duty vehicles (HDVs)), that prescribe limits for the emissions of a series of air pollutants including NOx. To meet the requirements laid down by the procedures present in these emission standards, vehicle manufacturers have equipped their products with different emission control systems. The introduction of new legal and technical procedures for the control of pollutants and greenhouse gas emissions will be more extensively discussed in this paper. It is clear that, in parallel, the development of targeted technologies for a drastic reduction of NOx pollution derived from internal combustion engine (ICE) emissions has led to the production and commercialisation of vehicles able to face the conflicting exigencies of increasing mobility by the public on one Catalysts 2021, 11, 404 hand, and the increasingly stringent air pollution limits required by the environmental policies on the other. The present paper aims at summarising the policies and technological aspects of the challenge set by the reduction of nitrogen oxide emissions from the transport section in the last few years when the selective catalytic reduction systems appear to have become crucial
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
