‘If you can’t beat them, join them’ is a popular adage which could well now be applied to the rival protagonists in the long-running controversy on the preferred way to cut oxides of nitrogen (NOx) diesel exhaust emissions. When current Euro 5 legislation and the roughly equivalent statutory requirements of EPA 2007 in North America came into force, heavy-duty truck and bus manufacturers fell into two camps.
There were those who thought it better to prevent the formation of excessive NOx inside the engine, through exhaust gas recirculation (EGR). On the other side were the champions of selective catalytic reduction (SCR), allowing high levels of NOx but which could be neutralised downstream, before reaching the tailpipe. The pros and cons of the two technologies have been well aired.
Adding SCR to an existing EGR engine translates into a US$5000 or more increase in a diesel truck’s list price. No wonder there has been a significant increase in enquiries for natural gas fuelled vehicles in recent months
But with the advent of much more stringent EPA 2010 and Euro 6 permitted NOx levels, the hatchet has been buried, so to speak. Nearly all parties are agreed that to achieve the ultra-low limits – on particulate matter (PM) as well as NOx – now being demanded by the legislators, those two methods of NOx reduction must work in tandem. Huge additional costs are involved for both sides. Adding SCR to an existing EGR engine translates into a US$5000 or more increase in a diesel truck’s list price. No wonder there has been a significant increase in enquiries for natural gas fuelled vehicles in recent months.
It is also unsurprising that Navistar, as the ultimate backer of EGR, held out so long against the adoption of SCR, with its accompanying up-front vehicle price implications. Without SCR, Navistar remained extremely price competitive. Its recent U-turn in favour of adding SCR will inevitably weaken that competitiveness.
That leaves only Fiat-Iveco continuing to eschew the SCR-plus-EGR combination. At their Arbon, Switzerland, R&D centre, its diesel engineers say they can meet Euro 6 standards with SCR alone. As one of the rumoured future bidders for Navistar – alongside Volkswagen and Caterpillar – Iveco’s deNOx strategy is a neat, or perhaps ironical, mirror image of Navistar’s erstwhile stance.
None of the EGR-plus-SCR protagonists are forthcoming about the percentage NOx reduction contributed by each of the two technologies
Iveco’s competitors claim that the Euro 6 NOx limit of 0.4g/kWh is so demanding that SCR aftertreatment alone requires unacceptably high dosing of the AdBlue (or diesel exhaust fluid, as it’s known in the US) reductant necessary to activate the system’s vital (non precious metal) catalyst and probably a larger catalyst to boot. The alleged unacceptability comes mainly from the cost of the reductant, per kilometre, which must be factored into fuel consumption costings.
None of the EGR-plus-SCR protagonists are forthcoming about the percentage NOx reduction contributed by each of the two technologies. It is likely that those who started from an ‘EGR baseline’ – viz all the US OEMs plus Scania and MAN in Europe – are letting EGR do the lion’s share, while the remainder, who went with SCR for Euro 5, have added EGR as a ‘supporting measure’.
In both cases it’s a balancing act, weighing AdBlue cost against the engine cooling and extra soot formation (and hence more frequent oil drain intervals) associated with EGR.
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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