The ABCs of DEF
Everything you need to know about diesel exhaust fluid
By Paul Abelson
A new set of initials has entered trucking’s vocabulary. DEF (Diesel Exhaust Fluid) is being sold at truckstops and dealerships all across the country. Little has generated so many questions as this new product, so here are some answers.
Why do we need DEF?
Actually, most of us don’t, at least not immediately. DEF is a chemical consumed during selective catalytic reduction (SCR), one of two alternative strategies to achieve 2010 EPA emissions regulations. If you don’t drive a 2010 or newer truck, you won’t need DEF. And if your 2010 is from Navistar, you won’t need it either. Their alternative for achieving EPA 2010 is to use Advanced EGR (exhaust gas recirculation), an enhanced version of EGR, the strategy all engine makers used to reduce oxides of nitrogen (NOx) for the 2004 regulations. All other engine makers combine EGR and SCR.
The SCR process adds a urea solution to the exhaust stream. The urea is converted to ammonia and passed over precious metal catalysts that convert more than 90 percent of harmful nitrogen oxides to pure nitrogen and water vapor.
Where do I find it? How do I buy it?
Your favorite travel center and all truck dealers except International have supplies on hand. In a pinch, you can also find it at some car dealerships (that sell diesel cars), but only in small containers, usually one-half or one gallon. At truckstops, you can buy in large quantity containers, typically 2.5 gallons, or pump DEF from dedicated equipment.
I heard DEF freezes. Will my truck still run if it does?
DEF freezes at 11 degrees F, but thaws with no degradation of the fluid. The DEF tank is warmed by engine coolant, so while your engine idles during warm-up, the fluid will start to thaw. Engines will idle without DEF.
What if I run out of DEF? Will my engine stop?
You will have ample warning before running out of DEF. Most trucks have a DEF gauge that uses lights and audio alerts to tell you you’re running low, usually at one-quarter tank. That will give at least 100 miles to empty. If you do run out completely, you’ll be limited to 5 mph. It probably wouldn’t hurt to keep a spare gallon in your truck. That should get you as far down the road as 50 gallons of diesel would.
How much DEF do I need?
Since exhaust emissions are directly related to the amount of fuel burned, SCR systems use DEF based on diesel used. The standard calculation is 2 percent, or a gallon of DEF for every 50 gallons of fuel. (Though that can increase depending on driver performance and road conditions.) If you use 100 gallons a day and run 5 days a week, you’ll need to replenish at least 10 gallons a week.
Why can’t I just get urea at my local fertilizer store?
There’s a huge difference between urea for fertilizer and DEF. Because the reaction that reduces harmful NOx to benign N2 and H2O takes place with precious metal catalysts, impurities in fertilizer grade urea will contaminate the catalyst and degrade the SCR system’s function. Sensors will detect changes in the exhaust and reduce engine power. The fix is to replace the catalyst. Replacement can be quite costly. The new SCR systems add up to $9,000 to 2010 truck list prices.
With fertilizer, ratios are not critical, and impurities get dumped onto the ground. As long as the impurities are non-toxic, precision is not critical. In a scientific process, it is. Also, the water used to make the DEF solution must be pure.
DEF is a 32.5 percent aqueous urea solution in de-mineralized water. There are strict standards that limit impurities and metals, most to less than one part per million.
How long can I keep DEF before it deteriorates?
DEF remains stable for a year or more at temperatures between 10 and 90 degrees F. Bulk dispensers kept outdoors at travel centers have their own climate control mechanisms.
What brands of DEF are available?
Since DEF is a highly controlled product, there is not much differentiation in brands. To be DEF, urea must be made according to International Standards Organization standard ISO 22241-1. For assurance of that quality look for the API Certification label on the container.