A farmer’s guide to diesel exhaust fluid

A farmer’s guide to diesel exhaust fluid

June 26, 2018 2 By Ray Bohacz

The government mandate for clean air has arrived at the farm. The modern diesel engine differs greatly from its predecessors that were devoid of any emission reduction systems. As of now all new on-road and more powerful agricultural (100+ hp) engines will have a host of emission control systems based upon the use of diesel exhaust fluid (DEF).


The two pollutants from the engine’s exhaust that the EPA is concerned with are oxides of nitrogen (NOx) and particulate matter (PM).


Pressure, heat and exposure time are the three things required to create NOx during combustion. A diesel by nature of its combustion process and slow burn rate of the fuel produces a high level of NOx. This pollutant contributes to acid rain and photochemical smog.


PM mainly consists of tiny carbon particles and other poisonous substances that are created when all of the fuel introduced to a diesel engine’s cylinder is not consumed during combustion.


DEF is employed to reduce NOx while a trap or filter (DPF) placed in the exhaust controls PM. When DEF is used on an engine it is then equipped with a system identified as SCR for Selective Catalytic Reduction.

For our purpose the word catalyst can be defined as something that speeds up or creates a chemical reaction without itself being consumed. Both the DPF and SCR are integral to the engine’s exhaust system.


DEF is held in a separate tank with its own pump, controller, injector and sensors. By industry standard the fill cap is always blue. DEF should never be mixed with diesel fuel or vice versa.


Chemistry lesson


DEF is a mixture of 32.5% automotive grade very pure urea (a more refined version of urea fertilizer) and 67.5% de-ionized and also very pure water.


DEF is produced under strict guidelines identified by the American Petroleum Institute (API) and ISO 22241-1. Only DEF meeting this standard should be used. Poor quality DEF will ruin an entire SCR system and the repairs will be in the tens of thousands of dollars. Always look for the API logo on the container.


An engine with an SCR system will have DEF automatically injected into the SCR catalyst. It then reacts with the materials in the unit to set up a chemical reaction that converts NOx to harmless nitrogen and water, both which occur naturally in the environment. DEF allows the engine to function at higher and more optimal combustion temperatures while controlling the NOx as an after-treatment.


DEF is offered in many different brands and as long as the product meets the API standard it can be used in any SCR equipped engine. Thus, if you have a 2.5 -gallon jug of DEF labeled from John Deere it can be used without concern in your SCR equipped pick-up truck.


DEF Facts


Most engine manufacturers have concluded that DEF is consumed at the rate of about 3 to 4 percent of the fuel used. Consuming 100 gallons of diesel fuel will use about three to four gallons of DEF.


A NOx sensor measures the amount of pollution and through a controller injects or doses the proper amount of DEF. Thus, under certain operating conditions the amount of DEF being dosed will vary. For example, a tractor will use more DEF while pulling a chisel plow then when hooked to an empty grain wagon. In theory, during certain operating states the NOx production may be low enough that little to no DEF will be used.


Under temperatures below 12 degrees F, DEF will freeze. For this reason, all SCR systems employ an integral heating system and temperature sensor to keep the DEF liquid while the engine is being operated.


It expands at a rate of 7% when frozen and there is no concern of failure of the plastic tank or lines during idle time in the winter. Regardless of the ambient temperature when the engine is shut off, the DEF is automatically pumped from the lines and put back into the storage tank.


DEF weighs approximately nine pounds per gallon.


If it is stored at a temperature of between 10 degrees F and 90 degrees F it has a shelf life of about one year. If the temperature never goes above 75 degrees F, then its life extends to two years.


DEF labels have a date code so you know that you are not getting stale and thus, less effective product. This is important if you buy DEF in quantity.


DEF is packaged in 1.0 and 2.5gallon jugs, 55 -gallon drums, 275 and 330-gallon totes, and through bulk delivery.


It is extremely corrosive to copper and brass and only API approved for DEF containers, pumps, filters and nozzles can be used.


If DEF is spilled on the ground it is environmentally safe (in small quantity) and when exposed to oxygen will dry and leave behind a crystal-like substance. This will normally be found around the DEF tank fill tube since it may get moist while being replenished. DEF is not harmful to humans or animals. It is not considered flammable or combustible. If you do get it on your skin simply rinse off with plenty of water. No protective clothing is required when working with it.


At first blush the SCR system and the need to inventory another fluid may seem burdensome but in actual practice the benefits out weigh the effort. Most if not all manufacturers state that their SCR equipped engines are more powerful and fuel efficient than the previous versions they replace. The theory of reducing NOx as an after treatment instead of in the cylinder allows the engineers to provide the farmer with power and fuel economy to get the job done.