Oxides of nitrogen (NOx) are the most difficult gaseous pollutant emissions to control on a diesel-engined truck or bus. Progressively more stringent NOx limits imposed by legislators have brought formidable challenges, requiring engine manufacturers’ research and development teams to collaborate with outside specialists, most notably in the field of catalytic chemistry. With few exceptions, heavy-duty automotive diesels engineered for the most up-to-date legislation are being equipped with SCR (selective catalytic reduction) deNOx aftertreatment.
Even in the United States where, two or three years ago, the so-called lean NOx trap (LNT) was seen as a viable alternative technology, the adoption of SCR to achieve full compliance with EPA 2010 requirements is now almost universal. In each application SCR is an addition to the EGR (exhaust gas recirculation) ‘in cylinder’ NOx reduction method chosen by all North American market contenders to meet the tougher regulations which came into force back in 2007. In a truck and bus context the LNT, as a stand-alone deNOx technology, has been quietly forgotten.
Opposition to SCR, in the form it is applied to today’s production vehicles, has been proclaimed most vocally by Navistar and relates primarily to the need for a further consumable. The urea-water solution, required to generate the ammonia (NH3) which activates the SCR catalyst, is known in Europe as AdBlue and in North America as diesel exhaust fluid (DEF). One complaint centres on the questionable coast-to-coast availability of approved-quality fluid, of the correct (32.5% urea) strength and purity – effectively requiring distilled water.
More objections to SCR come from those who point out that an engine will function quite satisfactorily with the AdBlue tank either empty or containing just water or a sub-standard urea solution. Under such conditions the level of NOx leaving the tailpipe could be many times greater than the statutory limit – though with no visible indication.
Despite their operational shortcomings, there is no doubt that AdBlue/DEF reliant SCR systems are currently the ‘least worst’ method of neutralising NOx diesel emissions downstream from the engine
In Europe, as a safeguard against such abuses, wilful or otherwise, OBD (on-board diagnostic) systems, designed to monitor in-service tailpipe NOx levels, are now mandatory on new vehicles. However, the accuracy of today’s OBD sensors leaves a lot to be desired. In consequence, a generous error margin has had to be allowed, so that driver warnings of excessive NOx, and the subsequent cutting back of engine torque if those warnings are ignored, are not activated until the sensor detects a NOx level up to 100% above the statutory limit.
Such OBD provisions have yet to be incorporated into US legislation, though similar requirements are scheduled to come into force in 2013, when the EPA looks likely also to adopt world-harmonised engine/vehicle certification test cycles (WHCs), close to the time that Euro 6 legislation – also based on WHCs – is introduced on the other side of the Atlantic.
Despite their operational shortcomings, there is no doubt that AdBlue/DEF reliant SCR systems are currently the ‘least worst’ method of neutralising NOx diesel emissions downstream from the engine. However, the situation could change during the coming decade. A number of alternative strategies for generating the ammonia, on which SCR systems ultimately rely, have been mooted. Eaton continues to work on its self-contained SCR system which borrows some LNT technology and a fuel reformer catalyst to produce the necessary NH3. Honda has progressed further with similar technology to Eaton’s, but only for light-duty diesels. But because their LNT catalysts use expensive precious metal coatings, both systems are intolerant of high fuel sulphur levels.
As legislated NOx limits for trucks and buses are tightened, it follows that greater demands are being made of SCR systems, implying ever higher urea/ammonia consumption. There are doubts as to whether the Eaton/Honda approach can keep pace with those demands. The same applies to the Danish Amminex development, in which Navistar has invested. It derives ammonia for SCR use from a solid material dubbed AdAmmine, which is held in the form of a renewable cartridge.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
