SCR Basics

May 23, 2014

An off-road machine’s selective catalytic reduction (SCR) system converts toxic nitrogen oxides in a diesel’s exhaust—nitrogen monoxide (NO) and nitrogen dioxide (NO2)—into harmless nitrogen, water, and carbon dioxide. To accomplish this, the SCR system uses an electronically controlled injection system to spray diesel exhaust fluid (DEF) into the exhaust stream just ahead of a catalyst. DEF is 67.5 percent automotive-grade urea, a substance also used in fertilizer, and 32.5 percent de-ionized water. (The urea used in DEF is highly refined, compared with that of fertilizer-grade.)

Heat in the machine’s exhaust, says Adam Kotrba, director of research and applied sciences for the Tenneco Clean Air Division, helps breaks down the DEF mixture to form ammonia (NH3), which reacts with NOx over the catalyst and renders the toxic nitrogen gases harmless. An “ammonia slip” catalyst behind the SCR catalyst neutralizes any excess ammonia resulting from the process.

The spray of DEF into the exhaust stream is not continuous, says Kotrba, because certain operating conditions fall outside the capability of the system, such as engine idling or cold starts. In both of these instances, says Kotrba, DEF will not be injected. NOx sensors located both upstream and downstream of the catalyst track NOx reduction as it occurs, he says, to ensure that the system is reaching appropriate levels of emissions reduction. 

Because its high water content causes DEF to freeze at 12F, SCR system developers, says Kotrba, typically use lines from the engine-coolant system to heat the DEF reservoir and also electrically heat the lines that carry DEF from the reservoir to the injector. Some systems, he says, are designed to purge DEF from the injector lines during engine shutdown to avoid potential damage from DEF freezing. (Equipment manufacturers caution not to flip the battery-disconnect switch too soon after machine shutdown to allow power for the DEF system to purge.)

These resources provide detailed information about DEF and SCR