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Diesel Exhaust Fluid Guide

Understanding DEF & the SCR Emissions System




Diesel Exhaust Fluid

DEF (diesel exhaust fluid) is 67.5 percent distilled water and 32.5 percent dissolved urea. It is usually referred to simply as "urea", since that is its active compound. Urea injection is part of the SCR (selective catalytic reduction) emissions components. The SCR emissions system became standard on Ford's Power Stroke and GM's Duramax diesels for the 2011 model years. At this time, Ford was launching the 6.7L Power Stroke and General Motors was releasing a more potent version of their 6.6L Duramax. Both manufacturer's implemented the SCR system in order to meet Federal emissions regulations, but the SCR system proved to reverse many of the reliability concerns/issues surrounding DPF (diesel particulate filter) equipped trucks as well as boost fuel economy considerably. Ram Trucks began implementing the SCR system on their Cummins Turbodiesel for the 2013 model year when they concurrently introduced the H.O. (high output) version of the 6.7L Cummins. As with Ford and General Motors, the technology was required to meet current and future emissions regulations following performance improvements to their engine.

SCR - Selective Catalytic Reduction

The SCR emissions system is designed to combat NOx (read nitrous oxides) emissions. A diesel engine tends to produce larger quantities of nitrous oxides when the engine is operating lean (the higher the air-to-fuel ratio, the leaner the mixture), as NOx is produced in oxygen rich environments and at high temperature. You may or may not already be aware that EGR (exhaust gas recirculation) was introduced to battle NOx emissions. By introducing a metered amount of exhaust gas into the intake stream, less oxygen is present during combustion and the combustion temperatures are reduced. SCR is an aftertreatment; it does not reduce the amount of nitrous oxides produced during combustion, but rather converts these harmful emissions before they reach the atmosphere.


DEF is injected directly into the exhaust via the dosing module, where it is atomized with the exhaust stream by means of a specially designed mixer (which greatly resembles an auger welded inside a section of exhaust tubing). The heat of the exhaust causes urea in the DEF to split into carbon dioxide and ammonia before the compounds travel through a ceramic catalyst. In the SCR catalyst, a reduction reaction occurs, converting the ammonia and NOx into nitrogen gas (N2) and water (H2O).


In essence, SCR technology allows manufacturers to reduce NOx emissions after they are created by means of converting them into safer compounds. While it is seemingly inconvenient and costly to keep the DEF tank filled, SCR technology has many benefits.


• Increased fuel economy when compared to non-SCR equipped trucks with diesel particulate filters. SCR equipped trucks are able to operate under leaner conditions and take advantage of a diesel's ability to operate within a broad spectrum of air-to-fuel ratios. Under light load conditions, and SCR equipped truck can run much leaner while adhering to emissions regulations. This ability had been previously shrouded, as NOx emissions are greatest when an engine is operated on the leaner side of the acceptable air-to-fuel ratio spectrum.
• Reduced strain on the EGR system, since the EGR system is no longer the sole combatant against NOx emissions. This promotes a more reliable, problem free EGR system.
• Reduced strain on DPF, since the engine's are able to run leaner and an engine produces less particulate matter at such operating conditions.
• Cleaner emissions. Nitrous oxides can cause or increase the effects of a number of respiratory problems, as well as contribute to the formation of acid rain.

Ford, GM, & Ram SCR/DEF Specifics

The Ford Super Duty's DEF fill is located next to the fuel cap. The DEF tank is 5.2 gallons for the inner frame mounted tank and 5.73 gallons for the outer frame mounted tank. GMC & Chevrolet pickups' 5.3 gallon DEF tank fill is located under the hood, on the passenger side. The Cummins powered Ram's 8 gallon DEF tank fill is also located behind the fuel door. The system's are designed so that, ideally, DEF only needs to be added at oil change intervals. However, the rate at which DEF is dispensed will greatly depend on the driving conditions. Frequent towing and/or stop-and-go traffic conditions may increase the DEF usage rate, and owners should become familiar with checking and adding DEF as necessary. DEF is corrosive, so it is important to clean any spills immediately. Diesel exhaust fluid also has a shelf life, so be sure to check with the fluid's manufacturer so that it is not stored beyond its usable limit.


Punitive actions, including limiting speed and reducing performance, may occur if the DEF fluid level drops below a predetermined level; check with your truck's manufacturer for additional information and avoid such actions by maintaining proper fluid level and quality.