Page 21 - Commercial Vehicle Engineer - November 2018
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Soot accumulated in a DPF’s pores increases  ltration ef ciency, minimising PN count. But as the DPF becomes soot-laden its walls become coated with
a “soot cake.” This prevents particles entering the DPF’s pores, so some pass straight through and into the tailpipe. Even routine passive regeneration of the DPF leaves some soot-cake on the DPF’s wall, explained Petersson, so PN  ltration remains impaired. This means the chances of passing an ISC PN check may depend crucially on DPF soot-loading.
All in all, is the Euro 6e in-service particulate number check set to throw another spanner into the diesel engine’s works? Probably not, reckons NGK Europe’s Adolph Schaefer-Sindlinger. Work with different DPF pore structures is looking promising, he told the symposium. Reducing mean pore-
size improves  ltration ef ciency. The challenge is to  nd the pore-size “sweet- spot” offering the best trade-off between optimum  ltration and an unwanted rise in exhaust back-pressure.
A couple of mentions of Euro 7 legislation in Gothenburg would have set alarm bells ringing among truck and bus operators. Is this really in the pipeline and if so, what will it mean for diesel engines in future? We put the question to the European Commission’s Bart Thedinga. “There is no talk of Euro 7 at the commission,” he says, adding cryptically: “We just talk about post-Euro-6.”
Some engineers use the term Euro 7
as shorthand for a possible new ultra- low NOx limit, perhaps following the example of the US state of California. Authorities there want to slash NOx emissions to below 10% of the current US federal EPA (Environmental Protection Agency) 2010 limit. That would mean
a NOx limit equivalent to only 26mg/ kWh, compared with Euro 6’s 460mg/ kWh. If exhaust after-treatment systems were developed to satisfy limits that
low in California it would be hard for European legislators to ignore them. And just such development work is already well advanced, according to several
Getting warmer: exhaust after-treatment could soon start at the turbo.
speakers at the Gothenburg symposium. A two-stage SCR system would appear
to be the most popular response to the ultra-low-NOx challenge. The  rst stage comprising a close-coupled SCR system (AdBlue dosing and NOx catalyst) close to the engine, perhaps at the turbocharger. One bene t of this location is that the  rst catalyst warms up fast, allowing de-NOx to start earlier and continue for longer when exhaust temperatures are dropping with low engine-loads. An additional NOx sensor and a sophisticated new controller apportion de-NOx work between the two stages, according to operating conditions. A two-stage close-coupled SCR system
is capable of eliminating “close to 100%” of engine-out NOx, according to Mario
de Monte of AVL, a big Graz, Austria- based powertrain development company. But the  rst-stage catalyst is exposed to high engine temperatures, he points out, so particular attention has to be paid to thermal ageing. AVL’s work so far on this has been con ned to computer simulation.
It will be move on to test beds next year. German powertrain research
company IAV also sees potential for two-stage SCR systems. IAV’s Vadim Strots said simulation studies suggest
this architecture can remove more than 99% of NOx, meeting the toughest limits envisaged anywhere. But IAV believes some form of “thermal management”
such as temporarily increasing exhaust back-pressure will be needed, making the engine work harder and heat-up quicker. The fuel consumption penalty resulting from this is put at around 3%. “We are not happy with this,” said Strots. “It de nes the next challenge.”
Another snag with two-stage SCR systems, namely that the  rst catalyst is exposed to a risk of poisoning by sulphur and hydrocarbons as a result of its proximity to the engine, was highlighted by Johnson Matthey’s Andrew Newman. An upstream diesel oxidation catalyst offers a solution but this slows warm-
up and the start of AdBlue dosing.

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