During the period when exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) were ‘rival’ technologies for the control of tailpipe oxides of nitrogen (NOx) diesel emissions, the trend through the previous decade or so, towards engine downsizing, was put on hold or even reversed by the champions of EGR. Diverting exhaust gases back through the engine could, they said, deprive the combustion process of oxygen under certain conditions, something which could be countered by increasing cylinder capacity.
But now, with the need to cut both NOx and particulate matter (PM) even further, to meet Euro VI and EPA 2010 emissions legislation in Europe and North America respectively, a combination of EGR and SCR has become almost universal. So too has the requirement for a diesel particulate filter (DPF), made necessary at Euro VI by the particle number limit in the regulations.
It would be better, says Scania, to specify a lower rating, causing the engine to work harder and thereby maintain a higher exhaust – and aftertreatment – temperature
That shift has, almost unwittingly, refocussed the attention of truck and bus diesel engineers on the attractions of either downsizing, whose original attributes of cost and weight reduction, along with lower friction losses, remain as valid as ever; or specifying lower power and/or torque ratings where offered from the same sized engine.
Scania, one of the key protagonists of EGR at Euro V, is offering, at Euro VI, its ‘mainstream’ 12.7-litre in-line-six engine [enlarged earlier from 11.7 litres to help ‘breathing’ under EGR-only NOx control] in output ratings from 410 to 490hp. But it is advising its truck customers to steer clear of the higher ratings, unless the increased performance is really needed at consistently high gross weights or in consistently challenging terrain.
The reason? There would be a risk of the engines running too cool to allow the SCR and DPF catalysts to work effectively. That in turn, within today’s electronically-managed diesel control scenario, would trigger more frequent ‘post injection’ fuel system events, burning extra fuel simply to bring those catalysts up to their light-off temperatures – to the inevitable detriment of overall fuel consumption. It would be better, says Scania, to specify a lower rating, causing the engine to work harder and thereby maintain a higher exhaust – and aftertreatment – temperature.
There is nevertheless a certain irony in the need for diesels to run as hot as reasonably feasible
It is clearly a philosophy which also holds good for the handful of heavy-duty diesels devoid of EGR which are being offered at Euro VI, notably by Iveco and indeed by Scania at selected 9.3 and 12.7-litre engine ratings. Any extra fuel burned to heat their catalysts, either by in-cylinder post injection, or sprayed directly into the manifold downstream, represents a further cost to be added to that of their considerably higher AdBlue (liquid urea) usage.
There is nevertheless a certain irony in the need for diesels to run as hot as reasonably feasible. It implies high heat-rejection levels, with heat – for which read energy and fuel – being dissipated wastefully into the cooling system. It is now recognised however that the high, 2000bar plus, injection pressures and accompanying high peak cylinder pressures now unavoidable in maximising an engine’s fuel efficiency, cannot be realised without correspondingly high combustion – and overall engine – temperatures.
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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