After Treatment System

• Diesel Exhaust Fluid (DEF) – A solution made of urea and deionized water. DEF is introduced into the exhaust stream to reduce NOx emissions.
• DEF pump – A part of the DEF delivery system that vacuums DEF from the tank, through the pump, filters the DEF and primes and maintains pressure between the DEF pump and doser.
• DEF doser – The doser injects DEF into the exhaust system.
• Decomposition chamber – This assists in mixing the exhaust stream with DEF prior to the SCR.
• SCR – This is the catalyst for the chemical reaction that transforms the combination of the exhaust stream and decomposed DEF to meet regulations for NOx emissions.

The DOC and DPF work together at the front end of the system to reduce carbon monoxide and hydrocarbons, but more importantly, capture soot and ash in the exhaust system to reduce the risk of those particles reaching the atmosphere.

From there, the exhaust moves through a mixing chamber that introduces DEF … needed for the chemical reaction to reduce NOx in the SCR. The resulting ‘exhaust’ that gets reintroduced into the atmosphere is comprised of harmless nitrogen gas and water vapor.

In addition to the primary components of the aftertreatment system, there are also sensors. A differential pressure sensor estimates captured particles in the DPF.

Exhaust gas temperature sensors monitor the system temperatures. NOx sensors measure the NOx conversion. PM (particulate matter) sensors, also known as soot sensors, measure particulates in the exhaust (mainly ash and soot). All of these sensors indicate when the after-treatment system needs repair or maintenance.