DEF tank filler cap F-250

What is DEF for Diesel Trucks?

If you’re like me, you are curious about what goes into our trucks and how it works. That kind of knowledge makes decisions about upgrading, maintenance, and casual conversation go much smoother. So with that in mind, just what is DEF for diesel trucks?

Put simply, it’s a means to reduce air pollution. Diesel Exhaust Fluid, or DEF, is made up of 2/3 distilled water, and 1/3 urea. Yep, the naturally-occurring and easily reproduced compound that is widely used in fertilizer also helps keep your exhaust clean. Who’da thunk it?

DEF, or Diesel Exhaust Fluid

DEF came into regular use in 2010, with tighter emissions restrictions being mandated by the EPA. It is relatively inexpensive at 2-3 bucks a gallon, and most trucks burn through about 5 gallons per oil change. Consumption can range anywhere from 2-10% DEF/diesel, but it’s typically on the lower end. This works out to about 1/2 a penny per mile. You can find it at auto parts suppliers and gas stations in jugs or sometimes at the pump. The shelf life is about two years, and you don’t have to worry about separation, so stock up and just pour it in when you run low.

DEF does freeze, but your tank is heated so this is a minor concern. What you don’t want to do is get the DEF tank mixed up with the diesel fuel tank. DEF by itself is relatively harmless, but it has a nasty effect on your metal engine components. If you do accidentally confuse the two, don’t start the engine, and call the local garage for some advice on how to clean up the mix-up.

DEF gauge F-250
DEF gauge, F-250 XLT

Your truck is designed with several warnings to let you know when you’re running low. The EPA has restricted manufacturers to prevent newer diesel vehicles from operating without DEF- your truck will not start if you run out, and it will severely limit the maximum speed if you are already on the road. Keep an eye on that gauge and watch for the warnings to avoid an embarrassing situation.

Early Attempts at Cleaning Up Diesel Exhaust

Up until about 40 years ago, diesel trucks ran dirty. There were efforts to clean up the exhaust, to an extent, using systems such as exhaust gas recirculation, or EGR. Implemented in the 1970s, EGR re-routes exhaust gasses back into the manifold, effectively reducing combustion temperatures and excess oxygen. In turn, NOx is significantly reduced. NOx is a term used for nitrogen monoxide and nitrogen dioxide- instrumental in things such as acid rain, ozone depletion and lung damage. For a very detailed engineering article on EGR, check it out here.

EGR System,

The downside to this process is a significant drop in power and efficiency. It also added more complexity to an already expensive power plant. Finally, consider that this did nothing to remove particulates- the soot that blows out of the tailpipe.

SCR, or Selective Catalytic Reduction (DEF Used Here)

In 2010, the EPA mandated improvements be made regarding diesel-burning engines. To that end, selective catalytic reduction systems, or SCR, were introduced as a more effective and efficient way to remove NOx.

SCR has been around for roughly 60 years in coal-burning plants. These systems used ammonia in a gas form to reduce the NOx emissions. Check here for a collection of very technical articles if you want the details.

The DEF utilized in our trucks contains urea, which when sprayed into the hot exhaust system breaks down into ammonia, which in turn breaks down the NOx. As a result you get carbon dioxide (it’s what plants breathe), nitrogen (inert, harmless), and water.

Ford Diesel Emission System
Credit: Ford Motor Company

The beauty of this system is that it all happens after the engine has done its work. This means that manufacturers are free to improve and tweak their engines to maximize things like fuel economy and power without having to factor in the emissions system. Ever wonder why new diesel pickups are putting out more than a thousand pound-feet of torque, when just a few years ago the best you could find in a stock system was maybe half of that?

Furthermore, because SCR is so effective, manufacturers have been able to reduce the amount of EGR used, which means more power and better mileage.

Diesel Particulate Filter

In 2008, just two years before the EPA mandate for the SCR system, they came down with a requirement for reducing particulates in the exhaust. These particles are the smoke- or black soot that blows out of older engines. Some guys work hard to eliminate as much of the exhaust system as possible, in a bid to improve power, but the result is the “rolling coal” effect.

The Diesel Particulate Filter, or DPF, is designed to remove the black stuff.

Something to understand about the filter is that it’s designed to be a permanent part of the system. You don’t have to replace it at a regular interval. The reason for this is regeneration.

Regeneration comes in three different flavors, all intended to clear the metal filter.

  • Passive, or “spontaneous” regeneration happens when the truck is running normally. The exhaust system heats up to roughly 1100°F, burning off the particulates
  • Dynamic regeneration is more aggressive. The truck will pump extra fuel to increase the exhaust gas temperature. These burns often last for anywhere between 20-35 minutes, and require the truck to maintain a steady throttle and speed in excess of 30 mph. If you can’t meet these requirements every few hundred miles, you’ll get a warning light. If you’re in the forums and see someone talking about regen, this is probably it.
  • Maintenance regeneration is done at the shop. You’ll have to take the truck in if you get a light

Another Part in the System, the DOC

So far we’ve covered EGR, SCR and DEF, and DPFs. There’s another component to the madness though, and it is called DOC, or Diesel Oxidation Catalyst. This guy is actually the first component in the process, after EGR. The DOC oxidizes hydrocarbons (famous for organ damage) and carbon monoxide (prevents blood from carrying oxygen), converting them into carbon dioxide.

The DOC is your catalytic converter. It’s a simple component, and maintenance free. It works well when combined with modern low-sulfur diesel fuel. For more details, look here.

What I’ve Learned…

Honestly, before I dug into all of this I had no idea how bad engine exhaust can mess us up. As frustrating as government agencies such as the EPA can be, I have to admit I really don’t want to go into kidney failure because my truck was idling too long in a drive-thru. Ford and other manufacturers are doing an admirable job of cleaning up our rigs, without screwing up the power available.

I know I went beyond answering the base question at the beginning of this, but I hope I’ve given you some good information. If you have any other questions about the exhaust system on these trucks, or if you have any comments and experiences you’d like to share, feel free to add it below.

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