What Is Diesel Exhaust Fluid: Explained

Though it sounds like a close cousin of blinker fluid, DEF is an important part of regulating the emissions of modern diesel engines.

byChris Rosales|
What Is Diesel Exhaust Fluid: Explained

Diesel engines are disappearing from smaller cars and SUVs in the U.S. thanks to a series of emissions scandals since Dieselgate. But there are still plenty of heavy-duty pickups and larger trucks running them. And of course, there are plenty of used diesel cars still on the road and on the market today. Keeping them burning clean is difficult. But one of the secrets to mitigating their emissions is the usage of diesel exhaust fluid (DEF), also known as AdBlue.

The DEF injector on a 2015 Volkswagen Jetta TDI. Chris Rosales

Yes, it’s a real thing—not a nerdy prank potion like “blinker fluid,” though it does sound like it might be made up. DEF serves an incredibly important purpose in modern diesel engines by acting as a sort of chemical catalytic converter. Modern diesel engines are bursting at the belt with emissions gear, including catalytic converters, particulate filters (DPF), exhaust gas recirculation, and triple the amount of sensors to monitor everything compared to a gas engine. 

The reason for this is that diesel engines emit things that gasoline engines don’t. Diesels have real issues with particulates (seen as soot or “rolling coal”) and oxides of nitrogen, known as NOx. Diesel engines lean burn during combustion, causing more NOx emissions. The particulates are handled by the DPF, but the management of NOx is handled by DEF. This is called selective catalytic reduction (SCR).

The DEF tank on a 2015 Volkswagen Jetta TDI. Chris Rosales

In SCR, the DEF works in conjunction with the catalytic converter to create a very specific chemical reaction that reduces the NOx in the exhaust. The system works by injecting DEF into the exhaust before the catalytic converter in specific quantities in a carefully controlled interaction. The formulas for the reaction are complex and beyond me, but it works. In fact, one of the main reasons for Dieselgate was the absurdly high concentration of NOx in the exhaust stream on VW TDI engines. The last generation of TDIs added SCR to combat this, though its unclear if it was a response to the internal cheating or growing pressure from emissions regs. 

DEF itself is a solution of 32.5 percent urea and 67.5 percent deionized water. The heat of the exhaust breaks this solution down into ammonia, which reacts with the NOx turning it into nitrogen and water. It’s fairly ingenious, but it's expensive to implement and requires a second tank to be filled. On diesel VWs it was pretty infrequent, but trucks will require more DEF.

Adobe Stock

Luckily the stuff is readily available. Many gas stations stock DEF in 2.5 gallon jugs, and you’ll find it at just about any auto parts store. As of this writing, price on a gallon ranges from $3 to $6. And you won’t be going through it nearly as rapidly as fuel. A heavy diesel pickup truck like a Ram Ecodiesel should burn about a gallon per 1,500 miles while something like a Jetta TDI only uses a gallon for every 3,000 miles of driving. But it all heavily depends on driving conditions like temperature and throttle usage, with no real range figure given by automakers. It’s a fill-as-needed sort of thing.

So yes, it is a real thing. It’s also important to know because this is something more recent diesel cars need on a regular basis. So if you’re looking for a used VW TDI or diesel light truck, keep this in mind when purchasing used. It isn’t expensive but it's something to think about. More importantly, it's another system to maintain in the future.