Business owners managing vehicle fleets face a myriad of challenges, including how to stay compliant amid changing federal regulations. Oftentimes, it can be an expensive and time-consuming process. Happily, there is at least one area of compliance that’s not only making vehicles more efficient and cost-effective for businesses; it’s also aiding the environment in the process. We’re talking about cleaner diesel exhausts through the use of Diesel Exhaust Fluid, or DEF as it’s often called.
Some confusion still lingers about how the regulations impact fleets. But like diesel exhaust itself, once you break it down, it’s far easier to manage.
Why NOx is Bad
Inside a diesel engine, the air and fuel mixture—along with combustion—creates nitric oxide/nitrogen dioxide (NOx) that ends up as exhaust. The problem: when NOx emissions and volatile organic compounds react with sunlight, they produce smog and acid rain, photochemical pollutants that can be a source of harmful climate change. Thus, five years ago, the Environmental Protection Agency (EPA) enacted restrictions aimed at cleaning up diesel emissions.

How NOx is Managed—Diesel Exhaust Fluid
Since 2007, truck manufacturers have used a variety of methods to reduce NOx emissions. The most efficient way is through Selective Catalytic Reduction (SCR), an active emissions control technology. NOx emissions from the engine pass through the SCR, where they’re treated with a catalytic agent—ammonia—which converts NOx to simple, non-toxic nitrogen and water.
The secret ingredient in this process is a harmless solution of high-grade urea and de-ionized water, known to the trucking industry as DEF. This liquid is pumped, not unlike gas, into the truck’s distinctive blue DEF opening.
What Are the Most Important Facts About SCR and DEF?
Unlike other NOx reduction systems, SCR is an after-treatment technology. It removes NOx from the exhaust, rather than at the combustion stage. This effectively leaves the engine out of the process, allowing manufacturers to tune engines for maximum power and performance. In fact, fleets using SCR-equipped trucks have reported fuel economy increases of up to 5 percent since the technology was introduced. Compliance is rarely cost-effective, but in this case, it has been.
The rate at which the SCR consumes DEF—usually called the “dosing rate”—is a percentage of diesel consumption. According to, two to three gallons of DEF are typically consumed for every 100 gallons of diesel fuel. But, like any other fluid in a commercial truck, DEF usage varies according to road conditions, mileage, load factors, and other variables.
Never Empty
The most important thing to know about DEF is that your truck will never run empty of it. Trucks equipped with SCR technology (thus requiring DEF) are engineered to reduce power, and even suspend ignition, when the DEF tank is empty. Trucks have a staged warning system featuring a dashboard gauge similar to a fuel indicator. Typically, a blinking light shows when the tank is at 10%, followed by a solid light at 5% or below. At critically low DEF levels the truck will only operate at 5 MPH. If DEF isn’t replenished immediately, the engine will eventually shut down. At that point, it won’t restart until the DEF is refilled.
When refilling DEF, carefully note the DEF opening, which should have a blue cap. Putting diesel fuel (or any other automotive fluid) in the DEF tank will contaminate the system, triggering costly repairs.
Fortunately, DEF is now widely available from fleet rental locations and a variety of other sources. It’s even available at the pump at a number of truck stops. In its retail form, DEF is most often available in 2.5-gallon jugs, although some fleets buy in bulk.
Selective Catalytic Reduction and DEF have reduced the carbon footprint of the commercial trucking industry significantly since 2010. And the improved fuel efficiency of SCR-equipped trucks has more than offset the additional cost of SCR. As long as fleets and individual drivers pay attention to dashboard DEF indicators, then being compliant and environmentally friendly are as easy as pouring DEF in the correct opening whenever you’re running low.
This article was originally featured on

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Ana Hernandez

Marketing Manager, Digital Content Delivery Ryder System, Inc.

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