In the wake of events like Dieselgate, many people want “green” cars. The concept of going green has gone beyond making environmentally friendly sedans. Some automakers are developing electric pickup trucks, sports cars, and more. This mindset affects cars that aren’t electric or hybrids, too. This is why diesel exhaust fluid (DEF) is so important.

In 2010, the Environmental Protection Agency began mandating that all future diesel engines must use selective catalytic reduction (SCR). And DEF is necessary for that process. This scared some drivers, who saw it as an extra cost for driving a diesel vehicle. But SCR and DEF actually became a net benefit. Here’s why.

What is DEF?

DEF is mostly just water and a chemical called urea. Yes, urea is also found in urine. But to be clear, urea is not pee. SCR is a process that cleans up the emissions from diesel engines extremely well.

In short, after diesel is burned, the exhaust goes through a particulate filter that catches all the large harmful particles. Then, DEF is injected into the resulting exhaust. Through a series of chemical reactions, it turns that exhaust into mostly harmless water and air. For SCR to work, the vehicle must have a supply of DEF.

Fortunately for diesel vehicle owners, DEF is very cheap — it’s mostly just water. Prices vary depending on the retailer and location. However, according to DiscoverDEF, the average price for a bulk order of DEF was about $5.85 a gallon in 2015. According to MSN, drivers only need to refuel their DEF supply as often as they change their oil. 

How effective is DEF?

The EPA mandated SCR in diesel vehicles due to its effectiveness, which depends on the automaker’s configurations. DieselForum estimates that SCR systems should reduce particulate matter (things like smoke and soot), as well as emissions from nitrogen oxides, to almost zero. Carbon emissions would also be reduced by upwards of 90%.

SCR is also beneficial to fuel economy. Commercial truck drivers get about 4% better fuel mileage when SCR is utilized, reports DieselForum. While 4% may not sound like much, it equates to big savings on fuel costs in the long run. For commercial trucker drivers, 4% means even more as they already spend a lot of money on fuel.

The future of DEF

Because every new vehicle with a diesel engine must have an SCR system, DEF will only become more important and expensive as more people buy new diesel vehicles. Automakers won’t stop making diesel cars anytime soon. So understanding DEF and why it’s a helpful, necessary commodity is an ideal path forward.

Of course, when completely green trucks and semi-trailer trucks become a practical, feasible way to haul goods, DEF will lose importance. However, the electric truck market is still in its infancy. For now, DEF is a useful and mandatory purchase for all diesel vehicle owners.