Above: Volvo Titan diesel badge (Wikimedia Commons/Herranderssvensson)

Diesel exhaust fluid (DEF) is an additive used mainly in trucks to reduce overall emissions from diesel engines. The term is somewhat misleading because the liquid actually contains no diesel. In fact, DEF is about one-third urea (CO(NH2)2) and two-thirds purified water (water with very low levels of minerals and other impurities). In Europe, DEF is also commonly known by the brand name AdBlue.

Did you know? Urea is a chemical commonly used in fertilizers as a source of nitrogen.But why would anyone want to inject diluted urea—a key ingredient of urine!—into a diesel engine? Basically, it helps diesel engines run cleaner. To find out more, let’s take a closer look at the workings of diesel-powered vehicles and how DEF can help reduce emissions.

Diesel engines and emissions

Like gasoline, diesel is one of the thousands of products obtained from oil refining. When diesel and air are compressed inside a small cylinder, the mixture of diesel and oxygen from the air self-ignites without the need for a spark (unlike a gasoline engine, which uses spark plugs). The resulting explosion drives the engine.

However, no matter the type of engine, these combustion reactions are never 100% efficient. Air contains 78% nitrogen. The energy created in an engine can cause this nitrogen to react with excess oxygen to produce harmful nitrogen oxides (NO and NO2, often abbreviated as NOx). Also, small quantities of sulfur compounds and benzene are almost always present in fuel. As a result, vehicle exhaust contains much more than just carbon dioxide and water. It also contains carbon monoxide, sulfurous oxides, nitrogen oxides, other volatile organic compounds, and small particulates (soot).

diesel + O2 (air) → carbon dioxide (CO2) + water (H2O) + energy (+ unburnt fuel)

↓+ excess O2 + N2 (air) → nitrogen oxides (NOx)

Unless they are removed by the vehicle's exhaust system, these emissions are released into the atmosphere. While some of them are harmful to human health, they are almost all harmful to the environment. In order to protect both people and the environment, all modern vehicle engines are fitted with two types of devices that remove pollutants from the exhaust stream: Particulate traps filter out soot and catalytic converters remove harmful gases—mostly nitrogen oxides, carbon monoxide, and unburnt fuel.

Emissions standards and DEF

The US Environmental Protection Agency and its counterparts around the globe are continually setting stricter emissions standards for vehicles in order to reduce health and environmental risks associated with air pollution. In particular, they set strict limits on nitrogen oxides, which contribute to smog in cities.

Did you know? Although toxic to humans, ammonia is a natural byproduct of protein digestion. The body gets rid of ammonia by producing urea, which is then expelled in urine.Gasoline engines, which run much cleaner than diesel engines, have little trouble meeting these standards with typical catalytic converters. Diesel engines also typically run cooler. This makes catalytic conversion, which depends on heat to produce chemical reactions, less effective. Still, many drivers prefer diesel engines, as they have better fuel economy, which means they can travel farther with less fuel and lower cost.

So the catalytic converter in a cooler-running diesel engine needs a little help. Diesel engines are actually equipped with “selective” catalytic converters, which are geared to reducing the emissions of one particular gas, such as nitrogen oxide. A selective catalytic converter can reduce nitrogen oxide emissions by more than 90 % by converting them into nitrogen and water.

DEF is used in selective catalytic convertors to help treat the exhaust after it leaves the engine and passes through the particulate trap. And this is where where the urea in DEF comes into play. The urea reacts with nitrogen oxides to produce nitrogen and water vapor by offering hydrogen to the nitrogen oxides. This converts more than 90% of the remaining nitrogen oxides into harmless gases.

Here's an overview of the entire process, from the engine to the exhaust pipe:

In short, by reducing the emissions of nitrogen oxide, DEF helps reduce air pollution. Think about that the next time you’re exercising outdoors and a diesel truck drives by, or your view is blocked by smog!


Ammonia & Liver Damage (Lyn Michaud, LiveStrong.com) Case Study of Diesel Particulate Exposure in the Canadian Railway Industry (Lorraine Shaw, McMaster University) Demineralized water FAQ (Lenntech) Diesel Exhaust Fluid (DEF) - How it works (YouTube.com/SunCoastResourcesInc) An Introduction to Indoor Air Quality (IAQ): Volatile Organic Compounds (VOCs) (US Environmental Protection Agency) Land Use and Environmental Change in the Thompson-Okanagan (Living Landscapes… Thompson-Okanagan: Past, Present & Future, Royal British Columbia Museum and Okanagan University College) The Mammalian Urea Cycle (Virtual Chembook, Elmhurst College) A partial list of products made from Petroleum (144 of 6000 items) (Raken Energy) Petrochemicals: Introduction and Aliphatic Compounds (James Richard Fromm) Selective Catalytic Reduction: Public Health Impact (FactsAboutSCR.com) Urea Dosing and Injection Systems (W. Addy Majewski, DieselNet Technology Guide) Volatile Organic Compounds (VOCs) (Clean Air Alliance) What is a urea fertilizer? (Leslie Rose, SFGate)

Lars Rose. Energy: Modern Energy Storage, Conversion, and Transmission in the 21st Century. Nova Science, 2013.

Lars Rose

Lars Rose is a PhD candidate in high temperature Solid Oxide Fuel Cell research (that is sustainable energies), at the Department of Materials Engineering in the Faculty of Applied Science at the University of British Columbia (UBC), and at the National Research Council Canada, Institute for Fuel Cell Innovation (NRC-IFCI). He enjoys teaching fun stuff and is the current Media Relations and Human Resources coordinator of the outreach program Let's Talk Science at UBC. He enjoys writing science in a fun way for CurioCity, UBC Terry, the Science Creative Quarterly, Fuel Cell Today and Ubyssey.

Comments are closed.