Selective Catalytic Reduction

Using Urea Injection for Lower Emissions



Urea or DEF (diesel exhaust fluid) injection is a key component of the SCR (selective catalyst reduction) emissions system for the 6.7L Power Stroke & 6.6L Duramax. SCR is one way that manufacturers are combating diesel emissions in order to meet/exceed current regulations. SCR is a system that reduces NOx (nitrous oxide) emissions by injecting diesel exhaust fluid into the exhaust stream. DEF, also referred to as "reductant", is a solution of 32.5% urea and 67.5% water. Its moderate urea content is where it gets the slang term "urea injection". DEF is injected via a dosing module (injector) into the SCR portion of the exhaust. It is atomized with the exhaust stream by means of a mixer, which resembles an auger welded inside a small section of exhaust pipe. The heat of the exhaust causes urea to split into carbon dioxide and ammonia. Exhaust then travels through a ceramic catalyst where the reduction reaction occurs, converting ammonia and NOx into nitrogen gas (N2) and water (H2O).


Nitrous oxides emissions are regulated in diesels. They are formed when a diesel runs on the lean side, so automakers have combatted them by richening the air-to-fuel mixture. By reducing NOx emissions, the SCR system allows automakers to develop engines that run leaner, therefore increasing the efficiency potential. SCR systems eliminate NOx emissions, even under extremely lean operating conditions. One of the huge benefits of a diesel engine is the ability to safely operate in a broad spectrum of air-to-fuel ratios, but NOx emissions regulated just how lean they could run the engine. Obviously, a leaner running diesel is using less fuel than a rich one. As a result, 2011 urea injected trucks are seeing huge improvements in fuel economy, which was previously robbed by the introduction of the diesel particulate filter.


More Information About DEF & SCR:
- Expect more and more fuel stations to start selling DEF by the gallon, making the process of keeping the DEF tank more convenient.
- Under normal operating conditions, the DEF tank should only need to be filled at every oil change interval. Higher load conditions, such as towing or a heavy right foot, may require the tank to be filled more frequently.
- Reductant is corrosive, so care must be taken when filling DEF tank as not to spill any on paint, wiring, or electrical connections.
- Some trucks may go into “limp mode” if the DEF tank goes empty. This will prevent normal operation. All SCR equipped trucks have multiple warning indicators to let you know when your DEF tank is in need of a refill.
- 2011 Ford (6.7L Power Stroke) and GM (6.6L Duramax) diesel powered trucks currently use SCR systems. Dodge’s 6.7L Cummins equipped trucks currently do not use SCR.
- Names for DEF: DEF, diesel exhaust fluid, urea injection, reductant.


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