Page 22 - Commercial Vehicle Engineer - November 2018
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Alternatives to two-stage SCR identi ed by Newman include a lean-NOx trap (LNT) and passive NOx adsorber (PNA) immediately after the engine, holding NOx until the SCR catalyst is hot enough to deal with it. There are options aplenty for truck and engine manufacturers addressing tougher NOx limits, Newman concludes. But two-stage SCR seems to be the front-runner at present, with Daimler among several truck-makers looking closely at it. Nobody is yet ready to estimate what additional cost is involved, but given all the extra hardware and control equipment involved it seems sure to be signi cant.
Cutting NOx emissions from truck
and bus diesel engines invariably involves a trade-off against carbon dioxide (CO2) emissions: fuel economy, in other words. A couple of possible ways to avoid this surfaced at Gothenburg last month.
Tim Johnson, consultant to  lter manufacturer Corning, thinks that GPS (global positioning system) satellite- based geo-fencing could be used to modify engine calibration on the move. On trunk roads the engine would major on fuel ef ciency but in urban areas geo-fencing would automatically switch engine management to tighter NOx control. Cylinder de-activation offers similar potential, according to Johnson. In undemanding road conditions
several cylinders are taken out of commission by shutting off their
fuel supply and closing intake and exhaust valves.
Current thinking: lightweight battery-electric trucks are beginning to look feasible.
A US study of cylinder de-activation
that identi ed a 3.4% fuel-saving on the US federal test procedure, was cited by
the Corning consultant. Crucially, NOx reduction is not undermined because
heat is preserved for SCR catalysts under low-load conditions. Active cylinders work harder, so their exhaust gases stay hotter. “Everyone’s looking at it,” said Scania’s head of research and technology performance Per Stålhammar.
Andy Walker is technical marketing director at catalyst manufacturer Johnson Matthey (JM). Future buses looks certain to be battery-powered, he told the symposium, pointing to data showing that the total number of electric buses in service globally reached 385,000 last year.
Johnson Matthey’s Andy Walker: beware the impact of shifting fuel duty.
But over 380,000 of these are in China, where they are heavily subsidised, he stressed. Parts of Europe nevertheless are moving swiftly in the same direction, said Walker. The Netherlands, for instance, had committed to all its new buses
being zero-emission electric by 2025. Battery buses can already match or beat diesel equivalents in total ownership costs, maintains Walker. “Opportunistic charging”, at a bus stop for example, was helping pave the way for smaller, lighter battery packs.
Turning to electric trucks, Walker noted that ever-improving battery energy-density and falling prices were making battery power increasingly
Take it easy: cylinder de-activation could save fuel and cut NOx emissions.

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