Oct 1, 2008 12:00 PM, By David A Kolman

Selective Catalytic Reduction (SCR)

In essence, SCR is a system that works by a chemical reaction triggered by heat. A fine mist of diesel exhaust fluid (DEF) is injected into the downstream exhaust. The exhaust's high temperature converts nitrogen oxide levels into harmless levels of nitrogen and water vapor and eliminates the diesel smell.

Diesel exhaust fluid, also known as urea, is an organic compound that is harmless to the environment. Urea already is widely distributed for many industrial and agricultural needs.

Urea consumption varies with duty cycle and other factors but is not expected to exceed 5% of fuel consumption.

SCR systems require a separate container for urea, along with extra wiring, hoses, and sensors to manage the injection flow of urea into the truck's exhaust stream.

Exhaust Gas Recirculation (EGR)

In simple terms, exhaust gas recirculation, also referred to as cooled EGR, captures a small proportion of exhaust gas and injects into the engine's combustion cycle, along with fresh air and fuel. This slows down the violence of the combustion itself, resulting in a lower level of nitrogen oxide emissions.

Unlike SCR, EGR does not require an additive.

Both EGR and SCR are proven approaches to emissions reduction and are used in a wide range of applications for diesel-powered commercial vehicles around the world.

Diesel particulate filters (DPF), installed in place of mufflers, typically contain a porous substance to “strain” and “catch” the microscopic-sized particulate matter from the exhaust stream and prevent these particles from reaching the atmosphere.

Regeneration

Over time, these traps “fill up” and need to be periodically cleaned by means of a regeneration process. Otherwise, the filter can plug up and adversely affect the engine's performance and fuel economy.

This regeneration process is typically achieved by burning off the trapped particulate matter.

The two types of regeneration are passive and active. With passive regeneration, particulate matter is continually burned off while a vehicle is driven. Active regeneration refers to a periodic burning of particulate matter by adding a small amount of diesel fuel into the exhaust gas.

Active regenerations typically won't be necessary for those applications where truck engines work hard enough to generate the heat necessary to continually burn off the trapped particulate matter, as in highway applications. They may, however, be required for applications such as city or suburban operations where vehicles do a lot of stop-and-go operation or prolonged idling, and engines don't generate enough heat for regeneration.

Drivers will not notice passive regeneration, as the engine continues to operate normally. The only sign of the regeneration is an indicator light on the dashboard.