Tech Stuff

Diesel exhaust fluid: Fluid-injection for NOx reduction

A “typical” SCR (Selective Catalytic Reduction) system (the Delphi system is shown here). This illustrates the basic layout of an SCR system, where the heated DEF solution is injected (dosed) into the exhaust stream, based on engine operation.
<p>A “typical” SCR (Selective Catalytic Reduction) system (the Delphi system is shown here). This illustrates the basic layout of an SCR system, where the heated DEF solution is injected (dosed) into the exhaust stream, based on engine operation.</p>

If your shop doesn’t routinely service diesel-equipped vehicles, you may not be fully aware of the use of DEF (Diesel Exhaust Fluid). DEF consists of a mixture of high-purity urea and deionized water.

Controlled by a “dosing” module, DEF is injected into the diesel engine’s exhaust stream. DEF consists of a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water.

DEF dosing is controlled by the engine’s ECU. When heated, DEF splits into ammonia and carbon dioxide, which is then atomized and vaporized. Once DEF enters the exhaust, the water in the DEF vaporizes, leaving ammonia molecules to travel to the catalytic converter where it neutralizes NOx molecules. This reaction converts NOx to harmless nitrogen and water, substantially cleaning up diesel emissions.

The EPA has mandated that all on-road diesel-equipped vehicles manufactured after January 2010 must reduce NOx emissions. The most common method to achieve this reduction is by injecting DEF into the exhaust path. All diesel-equipped vehicles that feature an SCR (Selective Catalytic Reduction) system utilize DEF injection.

The use of SCR systems reportedly provides an additional benefit of a 3% to 5% increase in diesel fuel economy. This is a result of enhanced combustion as exhaust temperature rises, especially in the heavy-duty applications. Cummins reportedly has gained substantial mileage improvements with their SCR system.

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