The U.S. Environmental Protection Agency (EPA) sets standards for the emissions of nitrogen oxides (NOx) for Medium and Heavy-Duty vehicles. The EU and Canada also have their own limits. For the regulations, reference EUR-Lex and the Canadian Justice Laws website. Manufacturers for these types of engines examined a number of technologies to meet these requirements. Selective Catalytic Reduction (SCR) was seen as a means of meeting the continually improving environmental standards.
SCR converts nitrogen oxide compounds (generically referred to as NOx) with the use of a catalyst into nitrogen (N2); which is rather harmless as it composes about 78 percent of the atmosphere.
What Is DEF Fluid Made of?
Diesel Exhaust Fluid (DEF) which is composed of 32.5 percent urea and 67.5 percent deionized water. For those who like trivia, urea is considered to be the first organic compound to be synthesized from inorganic chemicals.
Unlike the safe and fairly inert N2, NOx causes a whole host of issues such as health problems when inhaled, it can turn into nitric acid, both create and destroy ozone and cause a number of other issues. So, less NOx in the atmosphere, particularly around the areas where we live and work, is a very good thing. DEF is used with diesel engines to achieve this effect.
Petrol/gasoline engines run cooler, so they tend not to produce as much NOx as their diesel counterparts. The result of the lower operating temperature for petrol/gasoline engines is less pollution from NOx compounds and therefore they do not need to use SCR to reduce the NOx in the exhaust.
With these advantages over ammonia it is understandable why it is the material of choice. As always, there is room for improvement, and we can look forward to seeing this replaced by even better technologies.