OFR Driver and Aerial Operator Practice Exam

In a selective catalyst reductant system, Diesel Exhaust Fluid (DEF) is the substance injected to effectively reduce emissions, particularly nitrogen oxides (NOx). DEF is a solution made from urea and deionized water, which plays a crucial role in the Selective Catalytic Reduction (SCR) process. When DEF is introduced into the exhaust stream of diesel engines, it reacts with NOx under the influence of a catalyst, converting harmful emissions into nitrogen and water vapor, which are far less damaging to the environment. This process enhances compliance with environmental regulations and promotes cleaner air.

The other substances listed, such as compressed natural gas and liquefied petroleum gas, do not serve the same function as DEF in reducing NOx emissions. They are considered alternative fuels but do not engage in the SCR process used with DEF. Water vapor alone, while a natural component of exhaust, does not have the necessary chemical properties to facilitate the reduction of NOx in the manner that DEF does.