First of all, let’s clear up the acronyms: SCR equals Selective Catalytic Reduction, while EGR stands for Exhaust Gas Recirculation. Although both are intended to do the same job – namely, clean up exhaust emissions to meet the new Euro 4 standard – the two systems work in entirely different ways.
Put simply, SCR is an exhaust after-treatment method which requires a catalytic converter and a urea-based additive known as AdBlue to meet the standard. EGR, on the other hand, is a fully-engine-integrated solution which attacks the gases at source and requires nothing but diesel to make it work.
Those are the facts. Now, here’s a myth: ‘EGR has clear disadvantages in terms of running costs, higher maintenance costs and potential reliability risks.’ Those words were used at a fleet management conference recently, unsurprisingly by a manufacturer which will produce only SCR-equipped Euro 4 vehicles. Equally unsurprisingly, commercial vehicle makers planning to offer both SCR and EGR (which includes Scania) are taking a more level stance.
For its part, Scania will utilise its own patented EGR system, known as Scania EGR, on the majority of its engines while SCR will feature, at least initially, on its V8 power unit range. More of that in a moment, but let’s first attend to the issue of EGR and its performance.
The truth of the matter is that EGR is an entirely tried and proven system that has been in use on passenger cars and heavy trucks in the United States for many years. In terms of reliability and maintenance costs, EGR is largely a matter of ‘fitted and forgotten’, ie: it is an integral part of the engine that has no routine maintenance requirement whatsoever – which therefore means no additional servicing or any changes to the established service procedures.
SCR, however, is a far more complex system involving many more parts.
“There are maintenance issues associated with SCR,” comments Scania (Great Britain) Limited’s Technical Manager, Clive Burnet. “While EGR is a largely maintenance-free system, SCR units will require regular servicing. Also, from time to time various SCR components will have to be replaced to ensure on-going Euro 4 compliance – and with catalytic converters likely to be priced in excess of £3,500 each, that could prove to be a costly exercise indeed.”
AdBlue
While it is fair to say that an EGR-equipped vehicle will use more diesel than its equivalent SCR counterpart, it must also be remembered that SCR vehicles will require AdBlue – and that will cost. “It is estimated that AdBlue will be consumed at a rate of approximately three to five percent of diesel fuel used, so that has to be taken into account in any cost-calculations,” says Clive Burnet. “Also, SCR trucks will have to carry a separate AdBlue tank, which will have to be heated in cold temperatures to prevent the AdBlue from crystallising and will add weight and take up space, thereby sacrificing either payload or fuel tank capacity – especially on short-wheelbase tractor units – or both. So that’s another factor to consider.”
There is another matter clouding the issue as far as SCR is concerned: How widespread the availability of AdBlue will prove to be remains to be seen. Many operators are already expressing concerns over this while others – notably rental companies – are concerned that drivers may allow the AdBlue tank to run dry. A NOx sensor due to be introduced in 2007 will seek to combat this by reducing the power output of the engine by 40 percent. But as that will only happen once the engine has been switched off and back on again, there may be a temptation for some drivers simply to leave the engine running all day to avoid that particular problem although this will be recorded by the vehicle's electronics and constitutes a criminal offence.
Future developments
So having said all that, why is Scania planning to offer SCR on its high-horsepower 16-litre engines? “The reason we have opted for SCR on our V8s at this time is that it provided us with the most rapid route to gaining certification under the German MAUT system, which offers tax advantages to operators running low-emissions vehicles,” says Clive Burnet.
“On the long-haul, high average cruising speed journeys typically undertaken by our V8 operators, SCR provides a perfectly acceptable solution. Many of the concerns related to SCR are less valid in such scenarios and we are therefore confident of the system’s ability to perform well in these circumstances.
“However, Scania ultimately intends to offer only EGR across its entire engine range and we are now in the process of developing Scania EGR versions of our V8 engines. These are scheduled to be introduced ahead of Euro 5 coming into force, which will happen in 2009.
“In short, our philosophy is that it is better to build a clean-burning engine in the first place rather than to rely on an after-treatment system. That’s why Scania EGR represents the future as far as we’re concerned.”
