Diesel Exhaust Fluid

What is Diesel Fluid (DEF)?

Diesel Exhaust Fluid (DEF) is one of the key elements of the Selective Catalytic Reduction systems process used on Diesel Engines to reduce emissions. DEF is a nontoxic solution of 67.5% purified water and 32.5% urea that is injected into hot exhaust as a fine mist and passed over a catalyst, DEF helps convert NOx into nitrogen gas and water vapor. DEF is stable, colorless and odorless.

Before a truck’s DEF/ Urea tank empties, drivers see a series of dashboard alerts, warning them to add more fluid. When the DEF tank level drops below 10 percent, an amber warning lamp comes on. At five percent, it starts flashing. If DEF drops below 2.5 percent, drivers see a solid amber warning light. If drivers ignore the signs and run out of DEF, engine power is reduced and a solid red warning light displays.  If drivers continue to ignore warnings, vehicle speed is eventually restricted to five mph until drivers refill the DEF tank.

Selective Catalytic Reduction System (SCR) The Urea based fluid is produced synthetically from ammonia and carbon dioxide (CO2). Urea is found in every day US products such as hair conditioner, cosmetics, adhesives. It is not a fuel, nor a fuel additive and needs to be filled into a Dedicated Diesel Exhaust Fluid (DEF) tank located on your truck. It is replenished in a similar way to refueling diesel. Should you spill Diesel Exhaust Fluid (DEF) on your hands, simply wash it off with water.

Diesel Exhaust Fluid is an API certified fluid meeting ISO22241 specifications. If the truck were to be run without a supply of DEF, the truck will enter into a ‘Limp Mode’ which allows the truck to be safely driven to the nearest location to replenish the DEF.

While in the ‘Limp Mode’, the diesel engine will not produce full-power and by limiting power and the vehicle will be limited to 5-mph with inducement notification.

The DEF is consumed at a rate, approximately equivalent to, 2% to 3% of diesel fuel consumption and contributes to approximately 4% fuel economy savings of the vehicle.

Diesel Exhaust Fluid begins to freeze at 12° F (-11° C).

To meet Government Emissions standards in North America all on-road diesel vehicles manufactured in 2010 or later must be equipped with technology to reduce NOx emissions.

Trucks using the Selective Catalytic Reduction use instrumentation and gauges or warning lights as to alert the driver to the level of Diesel Exhaust Fluid in the vehicle.

 Trucks equipped with an SCR system uses sensors to determine that, in fact, Diesel Exhaust Fluid is in the system and not a liquid such as water.

Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia/urea in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another part of the tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.

Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or catalyst has degraded (see above).

SCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many of these contaminants can be removed while the unit is working. The unit can also be cleaned during a turnaround or by raising the exhaust temperature.

Leading trucking professionals on using SCR technology. 
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