As environmental legislators maintain their quest to reduce harmful exhaust emissions yet further, it is argued by many, including powertrain engineers, that attempts to make oxides of nitrogen (NOx) and particulate matter (PM) limits for diesel vehicles any more stringent, beyond Euro VI and EPA 2010 maxima, would be impractical.
At such ‘near zero’ levels, there are misgivings about achievable accuracy and repeatability in type approval testing which now, under Euro VI legislation, also includes a complex particle number (PN) measurement. For the time being, the law makers appear to have put any plans for a future ‘Euro VII’ raft of pollutant emission regulations to one side, preferring to concentrate, as far as commercial vehicles are concerned, on formulating enforceable greenhouse gas (GHG) limits.
It is argued by many, including powertrain engineers, that attempts to make oxides of nitrogen (NOx) and particulate matter (PM) limits for diesel vehicles any more stringent would be impractical
Such requirements ought logically to be based on some kind of ‘matrix’, relating allowable GHG – principally carbon dioxide – limits to a blend of factors including engine horsepower, vehicle weight and maybe even frontal area with, ideally, some allowance for the huge variation in real-life truck weights between the unladen and fully-laden condition. Clearly such complications will daunt those responsible for framing the relevant Directives, as EU lawyers and their counterparts in the US or elsewhere inevitably contemplate a host of potential loopholes.
Where next, then, to focus attention? There is pressure in some quarters for the kind of restrictions under which older, more polluting, vehicles are banned from those urban areas where air quality is a prime issue, to be embodied into EU-wide or US federal legislation. That would imply, over a phased statutory period of maybe five years, a demand for all older trucks and buses to be either taken off the road or upgraded to ‘cleaner’ but practically achievable standards, graded according to a vehicle’s age.
Retrofitting catalysed diesel particulate filters (DPFs) on older taxicabs and vans in cities such as London has proved to be fraught with problems
The main reduction in PM and NOx emissions from those older diesel vehicles would have to come from downstream aftertreatment of the exhaust gases. Retrofitting catalysed diesel particulate filters (DPFs) on older taxicabs and vans in cities such as London has proved to be fraught with problems, not least because stop-start operation in heavy traffic prevents the catalyst reaching the temperature needed to oxidise the PM deposited on the filter. Low exhaust temperature likewise inhibits the functioning of selective catalytic reduction (SCR) de-NOx aftertreatment systems.
Before future EU or US-wide legislation designed to cut PM and NOx levels in vehicles already in service could be implemented, the viability and availability of retrofitted emission control systems would need to be addressed. The low exhaust temperature issue is not insurmountable, though its solution would require a new exhaust manifold incorporating a downstream fuel injector, plus a sensor arrangement to detect DPF clogging or excessive NOx, in combination with electronically-controlled manifold fuel dosing. An option for PM control is to employ a fuel-borne catalyst, but an equivalent NOx reduction strategy has yet to be developed.
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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