SCR: Some Care Required

By Jim Galligan

Most light-duty and medium-duty commercial trucks will use selective catalytic reduction to control diesel emissions beginning in January. Here is a glimpse of what to expect.

Diesel emission standards change again in January 2010, but unlike the previous changes when the emission-controlling technology was essentially the same, truck buyers will have a choice this time. The buyer’s decision will affect the vehicle’s operation and costs throughout its life.

The choice next year will be between an expanded version of the current exhaust gas recirculation process, which will be used on International trucks and MaxxForce-brand engines built by Navistar Inc., and selective catalytic reduction, which will be used by all other commercial truck and diesel engine suppliers.

The Environmental Protection Agency’s goal for 2010 is to reduce nitrogen oxide emissions to 0.2 grams per brake horsepower-hour, or about 90% of pre-2004 levels.

All current emission-compliant diesel truck models reduce NOx in the combustion chamber with exhaust gas recirculation. EGR cools the exhaust gas and recirculates a portion back into the chamber. This process lowers the combustion temperature and, as a result, reduces NOx levels in the exhaust.

Executives from Navistar and truck makers planning to use SCR in 2010, most notably Freightliner Trucks, have engaged in a war of words over the past year. Representatives from each side accused the other of promoting misleading information while also seeking to explain why its technological strategy was best.

SCR proponents contend it is better and easier to reach the EPA’s new levels by treating NOx in the exhaust pipe rather than adding larger EGR systems to the engine. SCR proponents have said that the lower combustion temperature in an EGR system robs the engine of power and, in some cases, fuel economy. However, this is likely more of a concern for the big-bore, high mileage over-the-road engines than it is for lower-mileage diesels used in light and medium trucks.

The 2010-compliant trucks using SCR will still have EGR systems, but they may have slight changes in the hardware and EGR performance.

The SCR Process

SCR systems work by injecting a urea/water mixture, which truck and engine makers are calling diesel exhaust fluid, into the exhaust stream just before it enters a catalytic converter.

SCR systems need the DEF to clean the NOx, so the EPA is requiring that 2010 models include a consequence for running out of the urea/water mixture. T

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