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Diesel Exhaust Fluid (DEF)

DEF Home | What is DEF? | DEF Usage | What is SCR? | DEF FAQ's | DEF Equipment | DEF Truck Stop Prices

                        What is SCR?

Q: What’s SCR?
A: SCR stands for Selective Catalytic Reduction and is the most commonly used emission control technology used by diesel engine manufacturers to meet the new EPA 2010 emission standards effective as of January 1, 2010.A key component of the SCR system is Diesel Exhaust Fluid. The DEF is injected into the exhaust stream in front of the catalytic converter where it hydrolyses to ammonia and starts the chemical process of converting the nitrogen oxides (NOx) in the exhaust to nitrogen and water vapor.


Q: What does the driver have to do in order to operate SCR?
A: Practically nothing! The system is completely automatic and apart from making sure the operator tops up the DEF tank as he/she would top up the diesel and coolant tanks on the vehicle it requires no intervention. For added convenience, warning signals for low DEF levels are part of the instrument panel on the dashboard similar to the diesel gauge.
Q: What are the parts of the SCR system?
A: The SCR system is very straight forward: simplistically it consists of a DEF tank, fluid lines, injector, sensors and the catalytic convertor assembly.
Q: How does an SCR system work?
A: The purpose of the SCR system is to reduce levels of NOx (oxides of nitrogen emitted from engines) that are harmful to our health and the environment. SCR is the aftertreatment technology that treats exhaust gas downstream of the engine. Injected into the exhaust, DEF hydrolyses and forms ammonia which in turn triggers the chemical reaction that converts the NOx gases in the exhaust into harmless nitrogen and water.
Q: Why use SCR technology?
A: SCR technology allows the engine manufacturers to re-tune the engines for optimum performance to increase power and torque at the same time as reducing fuel consumption. Commonly cited benefits associated with SCR technology include:

Fuel economy 5% or more (dependent on application)

Reduced heat rejection

Increased power density

Reduced diesel particulate filter regenerations

Reduced total operating cost

Q: Does the SCR system affect the trucks weight?
A: Yes – for a Class 8 truck, the SCR system will add about 300 pounds depending on the configuration and application. Please note, additional weight is also the case with trucks using the alternative technology of exhaust gas recirculation.