How DEF Works: Understanding Selective Catalytic Reduction (SCR) Systems

If you operate a diesel-powered truck, tractor, or piece of heavy equipment made after 2010, chances are it’s equipped with a Selective Catalytic Reduction (SCR) system—and it runs on Diesel Exhaust Fluid (DEF). But what exactly is SCR? And how does DEF help your engine meet emissions standards while running efficiently?

In this blog, we’ll break down how SCR works, why DEF is essential, and what it means for diesel operators who want performance and compliance.

What Is Selective Catalytic Reduction (SCR)?

Selective Catalytic Reduction (SCR) is an advanced emissions control technology designed to reduce harmful nitrogen oxide (NOx) emissions from diesel engines. NOx is a major contributor to smog and air pollution, which is why federal and global regulations now require engines to meet strict emissions standards.

Here’s how SCR works in simple terms:

1. Exhaust gas exits the diesel engine.

2. Diesel Exhaust Fluid (DEF) is injected into the exhaust stream.

3. Inside the SCR catalyst, DEF breaks down into ammonia.

4. The ammonia reacts with NOx in the exhaust.

5. The result is harmless nitrogen and water vapor—released through the tailpipe.

What Is DEF Made Of?

DEF is a non-toxic, colorless fluid made of:

• 32.5% urea (a compound made from natural gas)

• 67.5% deionized water

This precise blend allows it to perform reliably across a wide range of engine temperatures while ensuring compliance with ISO 22241 standards. It’s not a fuel additive—it never enters the fuel system or engine. Instead, it's used strictly for exhaust treatment.

Why SCR and DEF Are Used Together

SCR systems and DEF were introduced to meet the EPA’s 2010 emissions standards and have since become standard in most diesel engines for:

• Class 8 trucks

• Construction equipment

• Agricultural machinery

• Generators and marine diesel systems

Together, SCR + DEF allow diesel engines to:

• Run more efficiently (since engine tuning focuses on performance, not emissions)

• Reduce NOx emissions by up to 90%

• Comply with federal and state clean air laws

How the SCR System Works in Real Time

Let’s walk through the process once your diesel engine is running:

1. As combustion occurs, hot exhaust gases (containing NOx) move through the exhaust system.

2. The system’s onboard computer signals the DEF injector to spray a precise amount of fluid.

3. In the SCR catalyst, DEF converts into ammonia.

4. The ammonia chemically reacts with NOx and neutralizes it.

5. Nitrogen (N₂) and water (H₂O) exit the tailpipe—cleaner than what you breathe in most cities.

What Happens If You Run Out of DEF?

Modern diesel vehicles are equipped with DEF sensors. If you run low:

• You’ll receive dashboard warnings.

• Power may be reduced to encourage refilling.

• If you run out completely, many systems will not restart until DEF is added.

Benefits of SCR + DEF for Fleet Operators

• Better fuel efficiency than older EGR-only systems

• Lower emissions = fewer compliance headaches

• Proven, reliable technology used by all major manufacturers

• Less engine wear due to optimized combustion

Whether you operate a long-haul truck, a combine harvester, or construction equipment, DEF and SCR systems are not just a regulation—they’re a performance advantage. Understanding how this system works empowers you to maintain it correctly and avoid costly downtime.

We supply DEF in bulk, totes, and jugs, with expert guidance to ensure your equipment stays running clean and compliant.

Have questions about DEF or SCR systems?We’re here to help. Contact us for supply solutions, technical advice, or to schedule a delivery.