It is widely acknowledged that, in Europe as well as North America, legislation regulating pollutant exhaust emissions, from diesel engines especially, has reached a ‘plateau’. Any further tightening of particulate matter (PM) and oxides of nitrogen (NOx) limits, as currently defined, is deemed unrealistic. Euro VI and EPA 2010 requirements, specifying maximum permissible PM and NOx levels, are now so demanding that any lower limits would be impossible to measure with guaranteed accuracy and repeatability, even under official type approval conditions, let alone during subsequent in-service testing.
There is also growing recognition that, in real life air quality terms, especially in congested urban traffic conditions, diesel exhaust particulates – both their mass and their number – are often less of a health hazard than particles from a myriad other sources, including brake and tyre wear dust.
Any further tightening of PM and NOx limits, as currently defined, is deemed unrealistic. Euro VI and EPA 2010 requirements are now so demanding that any lower limits would be impossible to measure with guaranteed accuracy and repeatability
On both sides of the Atlantic, the environmental focus is switching to greenhouse (GHG) gas emissions, principally carbon dioxide, where the direct link to fuel consumption is providing a further legislative incentive, through the universal imperative of energy conservation.
That does not mean, however, that diesel engineers and aftertreatment specialists are being allowed to rest on their Euro VI or EPA 2010 laurels. There is growing anticipation that, although the key PM and NOx limits may remain unchanged for the forseeable future, legislators might well start looking at those pollutants more analytically. It has, arguably, already happened, as far as particulates are concerned, with the inclusion at Euro VI for the first time of a particle number requirement, which has compelled some truck and bus manufacturers to include a diesel particulate filter (DPF) in the aftertreatment package which they contend would not otherwise have been needed.
Similarly, the NOx limit currently written into legislation is thought by some researchers to be an unnecessarily crude determinant of environmental harmfulness; ‘NOx’ is, after all, a convenient composite emission classification of several different oxides of nitrogen, including nitrogen dioxide (NO2), nitric oxide (NO) and nitrous oxide (N2O). Within a total NOx mass, the proportion of these NOx ‘species’ (the term used by researchers) can vary.
On both sides of the Atlantic, the environmental focus is switching to greenhouse (GHG) gas emissions, principally carbon dioxide, where the direct link to fuel consumption is providing a further legislative incentive
Suggestions have accordingly been made by researchers at Ricardo Consulting Engineers in the UK and elsewhere that a separate limit for the more harmful NO2 emission species could be beneficially added to the Euro VI NOx limit, as a safeguard against the observed effect of some SCR (selective catalytic reduction) de-NOx aftertreatment systems which deal more with the less-harmful NO exhaust constituent.
Engine designers might then be forced to augment their SCR technology with other strategies to limit NO2 emissions, the most obvious being fuel system reprogramming, especially injection retardation. That would, in the view of many, be a retrograde step – no pun intended – because of its negative effect on fuel consumption and GHG emissions, when those factors are now at the forefront of global environmental deliberations. It would also tend to jeopardise particulate mass and number compliance.
In consequence, legislative as well as technological complexities lie ahead, with accompanying challenges for those framing future type approval and in-service emissions testing procedures.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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