Proponents of selective catalytic reduction (SCR) technology for reducing NOx (oxides of nitrogen) diesel emissions – and they now include all heavy-duty truck and bus manufacturers in Europe and North America with the notable exception of Navistar – place improved fuel economy at the top of its proclaimed merits. By neutralising the NOx downstream, using an ammonia-activated catalyst, rather than using exhaust gas recirculation (EGR) to inhibit its formation inside the engine, the engine management system can be optimised to reduce both fuel usage and particulate matter (PM) emissions.
As Navistar has been quick to point out, users of SCR-equipped vehicles must set off their outlay on AdBlue/DEF against the fuel cost savings delivered by the system.
Specifically, it enables net injection timing to be advanced, which allows more complete burning of the fuel, albeit accompanied by an increase in peak cylinder pressures and temperatures, which results in higher NOx levels. It follows that the greater the level of ‘engine out’ NOx, the more reductant the SCR catalyst requires to bring tailpipe NOx down to the legally prescribed limit.
Those champions of SCR are apt to quote fuel economy improvements, compared with either EGR engines, or with older engines relying wholly on retardation of injection timing to control NOx, in straight percentage terms. Although such assessments are quite reasonable from a diesel technology perspective, they take no account of the financial cost, to the truck or bus operator, of the urea/water solution, known as AdBlue in Europe and DEF (diesel exhaust fluid) in the US. As Navistar has been quick to point out, users of SCR-equipped vehicles must set off their outlay on AdBlue/DEF against the fuel cost savings delivered by the system.
A large fleet which is able to justify having its own AdBlue storage tanks at its depots, replenished periodically by tanker, clearly pays a lower price than the small commercial vehicle operator buying the fluid in convenient small containers, typically from a dealer parts counter.
When AdBlue first became available in Europe, some four years ago, its price fluctuated quite wildly, according to supply and demand. The price per litre also varies according to the way in which it is obtained. A large fleet which is able to justify having its own AdBlue storage tanks at its depots, replenished periodically by tanker, clearly pays a lower price than the small commercial vehicle operator buying the fluid in convenient small containers, typically from a dealer parts counter.
In North America, where DEF is only now starting to become generally available, prices have come down with greater coast-to-coast availability of the fluid. Truckers are tending to make a direct per-gallon comparison between DEF and fuel prices. Because diesel (as well as gasoline) prices are so much lower in the US than in Europe, while AdBlue/DEF is now similarly priced on both sides of the Atlantic, its cost is much more of an issue for American operators.
In North America, where DEF is only now starting to become generally available, prices have come down with greater coast-to-coast availability of the fluid.
Clearly, if DEF usage is say 3% of fuel usage and SCR brings just a 3% fuel saving, and the two consumables are similarly priced, then Navistar’s generally derided ‘anti SCR’ propaganda has some credibility. It was recently suggested by Tim Cheyne of London-based Integer Research that, against the background of a rising fuel price and a relatively declining AdBlue/DEF price, the programming of SCR systems could be refined to selectively increase reductant dosing rates, allowing an additional net injection timing advance, with consequent fuel economy benefits.
Dr Steve Charlton, vice-president, heavy-duty engineering at Cummins, has made the same point, albeit more cautiously, telling AutomotiveWorld.com that “there is potential to change the DEF-to-fuel ratio with a re-optimised engine calibration”. There have been no hints of such a move in Europe, though with fuel prices already so high, and rising, such SCR dosing re-calibration could well come on to future engineering agendas.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
