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    Exhaust gas recirculation (EGR) lowers the engine's combustion temperature, thus lowering nitrogen oxide (NOx) levels.

Complete Review of the Dodge Ram 1500 and the Ford F-150 Lariate Edition.

There's one silver lining in our current downturn. With so many trucks operating at less-than-full capacity, fleet managers have the time to study what their purchasing options will be for the next engine update. Luckily, the effect of the engine emission upgrade will hit about the time our economy will be rising to full throttle.

The crux of the choice is the type of emission reduction strategy that is best for your fleet. To understand the differences, one needs a short review of diesel exhaust pollutants and how each emissions reduction strategy helps achieve EPA limits.

In October 2002, when, due to penalties for earlier violations, EPA forced most enginemakers to reduce nitrogen oxides (NOx) to meet the 2004 standard of 2.5 grams per horsepower-hour (g/hp-hr), exhaust gas recirculation (EGR) was the universally accepted strategy.

EGR drew exhaust gas from the tailpipe, cooled it in an under-hood heat exchanger using engine coolant, and introduced the exhaust gas back into the cylinders through the intake air. This diluted the intake air with inert exhaust, slowing the process and lowering the engines' combustion temperatures. That reduced NOx.

But EGR raised under-hood temperatures and put added stress on cooling systems. Components had to be redesigned to withstand higher temperatures.

The next emissions restrictions in 2007 required 90% less particulate matter (PM), including soot. The limits, which were 0.10 g/hp-hr, dropped to 0.01 g/hp-hr.

Control of PM is done in a diesel particulate filter (DPF). Exhaust flows into a diesel oxidation catalyst (DOC) that starts the chemical conversion of soot to inert carbon dioxide (CO2). Any remaining solids are then trapped in an ultra-fine ceramic filter. When the filter starts to clog, raw diesel fuel is sprayed over another catalyst, raising the internal temperature enough to oxidize the carbon soot and convert it to CO2 in an active regeneration process. The ongoing catalytic conversion is passive regeneration.

In Europe, where NOx has been targeted in recent years, truck manufacturers have used selective catalytic reduction (SCR). This process injects diluted urea into an exhaust after-treatment device. The urea is a catalyst to help chemically change the oxides back to pure nitrogen and oxygen, the two major components of air.

As urea levels drop, warnings are issued to the driver. If the urea is not replenished in a timely manner, engine power is reduced until more is added. In North America, the urea is called diesel emission fluid, or DEF.

Preferring EGR over SCR

In a survey conducted by Braun Research, of more than 300 class 5 to 8 truck operators with purchasing authority, 51% preferred EGR only; 24% preferred SCR. Their reasons were higher engine prices, reliability, added maintenance costs and new maintenance requirements, and added weight. Navistar commissioned the study.