Diesel Exhaust Fluid
I decided to write this because my 2018 Colorado ran low on DEF today and threatened to limit my speed if I didn’t add some. I had 3 gallons on the shelf in my garage and one in the truck as I knew the level was getting low. I wanted to wait to add some so I could monitor what 1 gallon equates to on the gauge since I don’t actually have a gauge. The truck just tells me the level is “OK” until it gets down to 30%. Today I found out that it shows you the percentage remaining until you get to 5% - then it says “LOW” and threatens to limit your speed. So I added DEF for the first time at 9792 miles - 1 gallon took me from “LOW” (roughly 5%) to 10%, and it went down to 9% on my 35 mile drive home, where I added the remaining 3 gallons.
So what is DEF?
DEF (Diesel Exhaust Fluid) is a solution of about 1/3 urea and 2/3 water (32.5/67.5% according to specifications). It is used as a catalyst in the exhaust after-treatment system of modern diesel engines to reduce NOx emissions. This is referred to as SCR (selective catalytic reduction).
I’ll spare you a big chemistry lesson, but basically the DEF evaporates as it’s injected into the exhaust stream - the water vapor passes through the exhaust system and the urea breaks down into carbon dioxide (which passes through the exhaust as well) and ammonia. The ammonia and NOx combine with a catalyst and create nitrogen and water which are then expelled through the exhaust.
The computer which controls the engine injects the appropriate amount of DEF to neutralize the majority fo the NOx which is created. As I’ve mentioned in previous posts, NOx is a byproduct of high combustion temperatures. It is created both in gasoline and diesel engines. However, diesel engines tend to produce larger amounts as they are “lean burn engines.”
A lean burn engine has more air in the combustion chamber than is necessary to burn all the fuel. Specifically, modern diesels run about 16-18:1 air:fuel ratio. If you’re familiar with gasoline engines and especially carbureted engines, you’ll probably know that a lean running gasoline engine results in hotter combustion. Because gasoline engines aren’t designed to run this lean, more fuel is added and/or airflow is restricted by closing the throttle to bring the air:fuel ratio closer to 14.7:1 and even a little below at idle, when cold, or at full throttle. A lean running gasoline engine produces NOx just like a diesel does, but in the case of gasoline, this would be considered a malfunction. Typically, there is not enough NOx produced in a gasoline engine to warrant exhaust after-treatment to address it specifically.
Anyway, back to diesel - because a diesel naturally runs lean, combustion temperatures are on the higher end. As more fuel is added, engine speed increases and (assuming modern turbocharged diesel engines) turbo boost increases, so it’s still a lean air:fuel mixture, but there’s more fuel and so even more heat and more NOx.
In a gasoline engine, you would “fatten” or “richen” the mixture by adding fuel or closing the throttle while injecting the same amount of fuel. This doesn’t work with a diesel engine because air intake is not restricted - the accelerator controls fuel flow on a diesel engine while the accelerator controls air flow on a gasoline engine.
The more fuel you add, the faster the engine runs and more power (and heat) are generated so fattening it up isn’t really effective in reducing combustion temperatures. Further, if you add enough fuel so you start to get below 16:1 air/fuel, the engine begins to produce soot because diesel fuel is slow burning compared to gasoline and can’t burn completely without the all the excess air.
The other popular (and controversial) method of reducing combustion temperatures and NOx emissions is by using EGR (exhaust gas recirculation). Basically, this system recycles some of the burned exhaust gasses, sending it through the intake to be mixed with fresh, incoming air and back into the combustion chamber. This exhaust is inert because the fuel was burned previously and its oxygen was used to do so. So while diesel exhaust is hot, it actually has the effect of reducing combustion temperature because it displaces oxygen, allowing a smaller amount of diesel fuel to be burned and it still absorbs some heat from the combustion of the fresh air and fuel which is injected.
EGR is problematic because particulate (soot) being a byproduct of diesel combustion is recirculated through the intake tract, including the turbocharger. Excessive amounts of soot in the exhaust stream can cause deposits to form in the intake and in the turbocharger and decrease efficiency by retaining heat and reducing airflow. In extreme situations, if enough deposits form they could eventually break free and then you have solid objects passing through the turbocharger, which could cause catastrophic damage to the turbocharger and if it were to make it past the turbocharger, through the intercooler and into the engine, could damage the valves as well.
So… this is why DEF exists - so we can run a diesel engine lean enough to produce minimal amounts of soot and clean up the NOx emissions which is a major component in things like smog and acid rain and reacts with other things to create ozone. Ozone is good in the upper atmosphere, but bad at ground level where inhaled by people and animals, causing respiratory issues and lung damage.
Stay tuned for an article about soot next!
Originally published on MidMichigan Oil LLC, independent AMSOIL dealer, restored from archive.