Tech Stuff

Diesel exhaust fluid: Fluid-injection for NOx reduction

DEF usage rate

A “typical” DEF capacity on a full-size light truck should provide a driving range of somewhere in the neighborhood of 5,000 to 7,000 miles or so (depending on the make, model, size of the DEF tank, driving habits, etc.). The SCR system in Chevy and GMC light trucks uses DEF at a reported rate of about 1% to 1.25% of the vehicle’s diesel fuel usage. Citing the Chevy Silverado HD as an example, a full tank of 5.3 gallons of DEF provides a driving range of about 4,970 miles.

An example of Ford’s SCR system. After the exhaust stream leaves the diesel oxidation catalyst, the selective catalytic reduction stage kicks in (on demand), at which point the exhaust stream is further purified by a diesel particulate filter.
<p>An example of Ford’s SCR system. After the exhaust stream leaves the diesel oxidation catalyst, the selective catalytic reduction stage kicks in (on demand), at which point the exhaust stream is further purified by a diesel particulate filter.</p>

When the DEF on-board tank is approaching empty, the vehicle’s onboard warning system will alert the driver with enough lead-time to replenish the DEF tank. Using the Chevy Silverado as an example, the first warning will occur at a remaining range of about 1,000 miles with additional warnings at the 300 mile and 0 range. Below a range of 300 miles, the warning (possibly along with a speed limiting warning) will appear every time the engine is re-started.

Again, using the GM vehicle as an example, if the DEF tank is allowed to run empty, at the next engine start, vehicle speed will be reduced to a maximum of 55 mph and eventually to a miserable 3 mph to 5 mph. If the DEF tank is not refilled, no damage will result from running the DEF tank empty.

If the DEF tank is run dry, according to GM, when refilling, more than one gallon of DEF must be added to release the speed limitation. You must then wait about 30 seconds with the engine at idle for the exhaust fluid message to clear.

Ford F-Series trucks feature the DEF fill point alongside the diesel fuel fill point (green cap for diesel and blue cap for DEF).
<p>Ford F-Series trucks feature the DEF fill point alongside the diesel fuel fill point (green cap for diesel and blue cap for DEF).</p>

If the vehicle is driven before the message clears, the speed limiting may remain in effect. If the exhaust fluid empty message clears while driving, the vehicle must be brought to a complete stop in order to release the speed limitation.

Ford diesel vehicle SCR design is slightly different. An alert is illuminated when the system determines an 800-mile-to-go status (Ford claims that their system requires a refill every 7,500 miles). If the DEF tank runs dry, about a 50% loss of power will be realized during that drive cycle (not just after the next ignition-on cycle).

Instead of (or in addition to) refilling DEF tanks when an early warning is provided, it’s wise to top-off a DEF tank at the same time that the engine oil is changed (for example, every 3,000 to 4,000 miles). In other words, you need to keep on top of the DEF level status. Running the system dry will result in severe driver inconvenience, in terms of entering a limp mode (either loss of power or severely limited vehicle speed).

Dodge Ram diesel light trucks feature an 8.2-gallon DEF tank. According to Chrysler, the anticipated usage rate is about one gallon of DEF for every 50 gallons of diesel fuel used.

Ford F-Series trucks (model 2010 and later) share the same quarter panels in both gas and diesel applications. Shown here is a gasoline-powered Ford F-Series fuel fill point. Notice the plugged-off (unused) location to the right of the fuel cap.
<p>Ford F-Series trucks (model 2010 and later) share the same quarter panels in both gas and diesel applications. Shown here is a gasoline-powered Ford F-Series fuel fill point. Notice the plugged-off (unused) location to the right of the fuel cap.</p>

The Ram early-warning system initiates when there is about two gallons of DEF remaining in the tank. A “Low DEF Refill Soon” warning is displayed, along with an audible chime. When only one gallon of DEF remains, a message will be displayed that says “Refill DEF — Engine Will not Start in XXX Miles” (the number of miles displayed is calculated by the system’s controller). If the warnings are ignored and the tank runs dry, a “Refill DEF — Engine Will Not Start” message appears. This indicates that the engine will not start on the next key-on cycle. At least two gallons of DEF must be added in order to re-start the engine.

Tags: ACDelco  OTC 
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