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Diesel Exhaust Fluid Not So Bad


I often hear comments from people about a strange and mysterious fluid called DEF (Diesel Exhaust Fluid) that is used in some late-model diesel trucks. Many owners of these newer diesel-powered vehicles don’t know what DEF is or why they have a separate tank or even how often to add the stuff. And what exactly is it made of (Hint: it’s not urine!)?

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The use of DEF in diesel trucks is based soley on its ability to help manufacturers meet today’s tougher diesel emissions standards. With the emission devices of the ’70 and ’80s still a bad memory for many, these devices robbed power, made vehicles more difficult and costly to repair, and ultimately led to the increased prices of vehicles.

One thing you have to remember, however, is that back then diesel engines were exempt from these rules and regulations. Diesel engines that were found in automobiles were never really in demand, and they didn’t make much power either. One positive thing about them was the fact that they generally got great fuel mileage and were in service a very long time.


As time has passed and we’ve learned a lot more about emissions devices, it really isn’t a sore subject among consumers anymore. In fact, with the power and reliability along with fuel mileage of today’s automobiles, no one really complains.

However, as diesel engines become more regulated there tends to be some flashbacks to the past. With this being said, I want to explain the use of DEF in diesel applications to help ease some concerns that may be holding your customers back from their next diesel truck purchase.

DEF is a mixture of urea, not a mixture of urine. I realize this is not science class, but it’s nice to understand what urea really is. Natural urea is waste excreted by humans and other mammals from metabolizing protein.  In humans, the liver breaks down protein and ammonia that forms the waste urea. The kidneys then transfer the urea from the blood to the urine.


The average person can excrete 30 grams of urea a day, mostly from urine and some through perspiration. DEF, however, is made from a synthetic urea. It is produced from a compound of ammonia and carbon dioxide manufactured for uses such as animal feed and fertilizer.

DEF is used to reduce the levels of nitrogen oxides, know as NOx. Nitrogen oxide is formed in an internal combustion engine from the reaction of nitrogen and oxygen during the combustion process. The NOx causes air pollution and in bigger cities is a major contributor of greenhouse gas.

NOx gas has been one of the major concerns for the EPA, which has been tightening emission standards for diesel engines in the last few years. Because a diesel engine has a leaner stoichiometry (air/fuel ratio), it tends to produce more NOx than other fuels. The use of DEF to lower NOx gas is known as SCR (Selective Catalytic Reduction). DEF is one way manufacturers have reacted to new emission requirements to lower NOx.


How the System Works

The DEF fluid is a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water. The solution is added to a tank on the vehicle that is located generally near the fuel tank for convenience. The DEF tank has a blue lid where the fuel tank has a green lid. The DEF fluid is transferred from the tank to an injector via a pump.

The injector for the DEF fluid is usually placed downstream (after turbo) in the exhaust system. The injector is operated electronically by a controller that will open it to allow a low dose of DEF into the exhaust stream. The controller for the injection of DEF is programmed to inject the right amount of fluid based on inputs from the engine controller.


As engine demands change, the amount of DEF will change depending on engine load, rpm, speed, temperature, etc. When the DEF is injected into the exhaust stream, it becomes a catalyst for the NOx. The NOx level is reduced between 70% and 90% based on application.

Here’s the beauty of SCR: by using urea, more NOx gas can be reduced than other treatments, which has always been a major issue when trying to reduce diesel emissions. This is why the EGR valve system was placed on diesel engines.

The EGR?was used to revert exhaust gas back into the intake in order to lower the oxygen content of the the incoming charge of air into the engine. When the oxygen level is reduced, the combustion temperature is also lowered.


In a diesel, when the combustion temperature is reduced, you begin to form soot instead. Then soot becomes an issue in the intake manifold along with the rest of the exhaust system. When the soot particulate had to be dealt with, manufacturers began incorporating an expensive solution known as a DPF (Diesel Particulate Filter).

This is known as a regeneration process where the soot is collected in a filter in the exhaust system and then later burned off by injecting fuel to clean the filter. But naturally, this creates higher fuel consumption.

By incorporating the use of SCR, NOx levels can be maintained while producing more power. Then the manufacturers rely less on other emission devices such as the EGR and DPF. So, more power can be made with less pollution. This is one of the reason today’s diesel engines can make almost twice the power on the same platform.

Injection timing can be tailored to make power instead of being altered to produce less NOx and deal with cooler combustion temperatures faced with EGR. Reflecting back, EGR systems have caused their share of problems. Look at the Ford 6.0L diesel and the problems the EGR coolers impose on those engines. So the use of DEF has proven to be a positive, low-cost solution to lower emissions while offering more power and reliability.
A sample can be taken from the DEF tank where the disc in the tester will measure the concentration of urea. If the concentration of urea is off by as little as 0.7%, replace the urea and determine why the urea concentrations were low.


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