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Quite simply...
Page 30
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SCR is better
MIKE DAVIES' letter ( CM 16 June) asks "Why does anyone go the SCR route to Euro-4?" I am happy to list the major reasons for him.
Compared to Euro-3, there will be a fuel penalty for running EGR and a benefit with SCR.This will give a significant gap in fuel consumption. EGR also has higher levels of oil contamination and shorter service intervals.
In addition, EGR's thermal values are higher and this will have an impact on R&M costs and engine life due to the greater stresses.This could also raise issues with residual values. And. for those who care, there will be environmental benefits with SCR.
Yes, there have been challenging engineering issues to overcome with SCR, but the responsible manufacturer's perspective has led us, and others, to the SCR solution. EGR does make sense with smaller capacity engines, in vans for example, where SCR's fuel advantage has a limited effect and the cost of the extra equipment represents too high a percentage of the vehicle. At the larger capacities appropriate for heavy trucks,it is not an effective solution, And looking to the future. EGR will not get you to Euro-5.
Ian Norwell PR Manager Mercedes-Benz CVs Milton Keynes