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You are currently viewing A Guide to Diesel Exhaust Fluid (DEF) – All You Need To Know

A Guide to Diesel Exhaust Fluid (DEF) – All You Need To Know

As the signs of global warming became more apparent, in 2010, EPA revised their standards of lowering NOx (nitrogen oxide) levels in trucks to reduce pollution by improving diesel emissions and making them cleaner. Standards for both one-ton trucks and three-quarter trucks were changed to make them increasingly stringent. As a result, manufacturers had to come up with ways of reducing emissions without compromising on the vehicle’s power. 

One solution that OE trucking fleet manufacturers came up with was SCR technology – Selective Catalytic Reduction. This technology reduced NOx emissions and increased both fuel economy and reliability without changing the design of an engine. It also gave rise to diesel exhaust fluid – a key component of the technology. 

Initially, the new regulations became problematic for both drivers and operators. However, with increased growth of both SCR and DEF technologies, using diesel exhaust fluid is less burdening and gives drivers the chance to save on fuel while also reducing emissions. 

But what exactly is DEF, and what are their benefits? Here’s what you need to know.

What is DEF, and how does it work?

DEF stands for diesel exhaust fluid and is a mixture of 32.5% urea and 67.5% deionized water. While N2 is quite inert and safe, NOx can lead to a number of problems. For instance, if it is inhaled, it can cause health issues. It can also convert into nitric acid, which can destroy ozone and contribute to climate change. 

Given the harmful effects of NOx in the atmosphere, the need for lowering the emissions, particularly in areas where people work and live, grew. This is where DEF comes in; it works with diesel engines to reduce NOx emissions.    

While petrol and gasoline engines tend to have a higher temperature, producing a relatively lower amount of NOx, their diesel counterparts run on a lower temperature and contribute more to the amount of NOx emission in the air. Since petrol and gasoline engines have a lower operating temperature, fewer NOx compounds are released, and they don’t need SCR. With diesel, the case is a little different.

When it comes to diesel engines, a standard catalytic converter proves to be ineffective at minimizing diesel fuel emissions. For this reason, they are equipped with a selective catalytic converter that helps to lower harmful emissions like nitrogen oxide. 

Diesel exhaust fluid is crucial for this whole process. The urea present in it reacts with the NOx once it leaves the engine, and turns up to 90% of the emission into water vapor and nitrogen. As a result, it nearly eliminates all the harmful emissions present in the vehicle’s exhaust.

Uses of diesel exhaust fluid

While SCR was initially introduced for trucks, nearly every vehicle manufactured since 2010 uses SCR and needs DEF. Vehicles include trucks such as heavy-duty delivery vans and trucks, diesel pickup trucks, and several European luxury cars as well as SUVs. 

Since 2014, off-road vehicles running on diesel, like those used for agriculture and construction, are also required to use SCR. Let’s have an in-depth look into uses of diesel exhaust fluid:

  • In commercial vehicles

Commercial vehicles today can be categorized as the following:

  • Passenger carrying vehicles like mini-buses, coaches, and buses
  • Light commercial vehicles like small trucks and vans
  • Heavy or large vehicles for transporting goods, like trucks

As mentioned earlier, diesel trucks and other vehicles must have SRC to reduce their NOx emissions. For this reason, DEF is used in commercial vehicles. It is a safe fluid that is filled in a separate tank.  

  • Non-road mobile machinery

Limits to NOx emissions have been implemented over the last few years, and the requirements are only getting stricter with time. More and more vehicles now have to meet these requirements, and non-road vehicles with diesel engines are no different. Vehicles such as dump trucks, portside vehicles, tractors, and cranes will have to follow the strict NOx emissions standards set by the EPA.  

In fact, SCR for reducing NOx is also now common in different heavy-duty vehicle types, such as mining machinery, construction equipment, mobile and static cranes, farm machinery, and dockside and airport vehicles.  

  • Passenger vehicles 

Passenger vehicles have not been left behind when it comes to SCR technology and diesel exhaust fluid. These include cars, campers, and SUVs. Manufacturers like Mercedes Benz, BMW, and Volkswagen are frontrunners in bringing the system into the passenger vehicle market. Big names like Mazda, Toyota, GM, and Ford have also introduced models that make use of diesel exhaust fluid. Other manufacturers are also expected to follow suit.  

  • Train

Just like trucks and cars, trains also use diesel exhaust fluid to reduce the NOx emissions emitted from the exhaust of the train’s diesel engines. Train engines also have SCR technology, and DEF is put into a separate tank.

Benefits of diesel exhaust fluid

While diesel exhaust fluid was introduced to meet EPA requirements, it provides multiple benefits. Let’s have a detailed look into these:

  • Lowers pollution

As mentioned earlier, diesel exhaust fluid is primarily used to break down NOx gases into water and nitrogen. Nitrogen oxide emissions are quite poisonous and are one of the major causes of air pollution. These emissions are also often responsible for acid rain and accelerating the greenhouse effect. However, once it is broken down into water and nitrogen, it no longer causes harm. Both water and nitrogen are harmless on their own, so they can be safely released into the atmosphere.  

  • Meets the regulations 

This is perhaps the greatest benefit of diesel exhaust fluid. Using it means you comply with EPA regulations, plus there’s no way around it too. Today, most trucks include SCR systems, and these can only run when the tank has diesel exhaust fluid. If you let it run empty, you might just find yourself stranded on the road. 

  • Improves efficiency 

While many truck owners worried about their vehicle’s performance with DEF, trucks with the fluid are comparatively more efficient than those without it. Due to the SCR, manufacturers can design the engines to provide the optimal performance while the DEF takes care of the emissions. However, trucks without SCR systems must redesign engines such that they reduce NOx emissions, which reduces the performance.

  • Demands less maintenance 

Engines fitted with SCR systems have a superior design. As a result, over the course of its life, the engine experiences less wear and tear. This not only reduces the need for frequent maintenance but also reduces the chance of your engine breaking down on you. 


While driving on diesel comes with a number of undeniable benefits, it doesn’t change the fact that it produces quite a lot of NOx emissions. Despite the low price per litre of diesel that can help you save up a lot, the cool temperatures that the diesel engine runs at causes it to release harmful gases. Plus, a catalytic converter isn’t enough to purify harmful gases and isn’t as effective as it is in a petrol, or gasoline engine.

All of these are just proof that DEF is essential for vehicles today. At Birdfuels, you can also get bulk or mini-bulk DEF containers put at a given point to refill the fluid in vehicles, given that storage guidelines are followed.   

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