Things I Learn: DEF

March 8, 2018

A short stop just east of Lincoln, Nebraska. Palmyra, at Casey’s General Store. The tall sign by the highway advertized DEF. What is that? The Trucker explained.

DEF stands for Diesel Exhaust Fluid. The newer trucks on the road today are equipped with a DPF, a Diesel Particulate Filter. It catches particles in the exhaust. Regulations require the new diesel motors not release into the atmosphere particles above a certain size. These particles must be caught in a filter. The filter, then, becomes clogged. The DEF is sprayed into the DPF, and ignited by an piece called an igniter, which is similar to a spark plug. It burns at a very high heat, reducing the particles to a fine ash, which is then released with the exhaust.

On the engine side of the DPF, a pressure switch monitors the level of back pressure on the engine caused by the amount of particles clogging the DPF. When it reaches a certain level, the process of burning out the filter begins. Some trucks have a manual control operated at will by the driver, but all do this automatically.

If you are ever around a newer truck, which suddenly begins pouring out clouds of black smoke, with a sharp, acrid, oily smell, the particulate filter is being burned off.

Just another little addition dreamed up by someone at a desk!

An interesting side note: DEF is known to farmers by another name, urea. Urea is spread on fields as a solid fertilizer. The environmental grade urea, known as DEF, is liquid, comprised of 67.5% deionized water, and 32.5% high grade environmental urea (which is a mix of synthetic ammonia joined to carbon dioxide at a high heat).