What Is DEF And How Does It Work?

Diesel exhaust fluid, or DEF, is part of the selective catalytic reduction system present in all diesel vehicles made since 2010. The following article will explain what diesel exhaust fluid consists of and how it works to clean a vehicle’s exhaust.

What Is Diesel Exhaust Fluid?

Diesel exhaust fluid is a clear, non-hazardous liquid that cleans the exhaust from a diesel-powered vehicle, removing pollutants and improving air quality. It is added directly to the exhaust stream, not to the fuel tank, as it is not a fuel additive. In fact, if diesel exhaust fluid is added to the diesel tank by mistake, the vehicle will stop running instantly.

What Does Diesel Exhaust Fluid Contain?

According to ISO 22241 international quality standards, diesel exhaust fluid must contain two components: 32.5% urea and 67.5% de-ionized water. Urea is the nitrogen-based chemical compound present in urine, hence the similarity of the words. When it is heated, urea breaks down into ammonia. This concentration of urea in the water gives the solution the lowest freezing point. De-ionized water is a pure form of water that has had mineral ions (for example, calcium and fluoride) removed. Even trace amounts of these materials can contaminate the selective catalytic reduction system.

How Does Diesel Exhaust Fluid Work?

When diesel exhaust fluid is injected into the exhaust stream of a diesel truck, it reacts with the nitrogen oxides in the exhaust. Nitrogen oxides are pollutants produced by diesel engines, and they greatly decrease air quality. Emissions standards require a minimal level of these chemicals in diesel exhaust. The fluid works to neutralize these particles by breaking them down into harmless nitrogen and water molecules.

What Are Some Benefits Of The Selective Catalytic Reduction System?

Once this system was added to vehicles, manufacturers didn’t have to worry so much about emissions. This allowed them to tune engines to improve fuel efficiency as well as engine performance. Fleet owners report a five percent savings on fuel costs when comparing vehicles manufactured before and after the selective catalytic reduction system was introduced.

Drivers can expect to refill a 15 to 20-gallon diesel exhaust fluid tank approximately every 6,000 miles. In most vehicles, there is be a gauge on the dashboard that indicates when the fluid level is getting low.