Selective Catalytic Reduction (SCR) 
It is an emissions control technology that actively injects a liquid-reductant agent through a special catalyst in the exhaust stream. This form of emissions control is currently primarily used in the heavy trucking industry. The reductant source is called Diesel Exhaust Fluid. The DEF is an automotive-grade urea which sets of a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide which are the natural components of air. This technology is designed to allow NOx reduction reactions to take place in an oxidizing atmosphere. It is so called “selective” because it reduces the levels of NOx using ammonia as a reduction catalyst. SCR is a highly effective technology in reduction of NOx levels. It leads to a drop in 90% of this pollutant. The use of SCR in the past was primarily limited to marine vessels including cargo vessels, ferries and tugboats.
The main reason of its popularity and importance is that it is one of the most fuel efficient and cost reductive technologies for emissions control currently. As per the EPA standards, effective from January 2010, the particulate matter (PM) and nitrogen oxide (NOX) levels are near zero levels. The SCR can reduce NOx levels by 90% while simultaneously reducing HC and Co emissions by 50% - 90% and PM emissions by 30% - 50%. In the trucking industry, the use of SCR reports an increase in fuel economy by 3% - 5%.
Despite having an abundance of advantages to reduce emission and improve fuel economy, there are certain downsides. Primarily, the major concern is the replenishing of the DEF on a periodic basis. It has to be carried on an on-board tank. Depending on the vehicle, engine size and usage of the vehicle the size of the tank and rate of replenishment is determined. If designed well, the DEF can be used to ensure the individual driving the vehicle maintains his vehicle’s emissions standards. The tank can be designed such that the driver is warned of a low DEF level and as and when the DEF level reduces below a certain level, the vehicle doesn’t start and the DEF has to be replenished before the vehicle can be used.
Diesel Exhaust Fluid
It is a non-toxic fluid composed of purified water and automotive grade aqueous urea. It is available in a large range of dispensing and storage methods. DEF tanks range from 23 litres to 87 litres depending on the truck and its application. The tank fill opening is designed so as to accommodate a DEF fill nozzle to ensure only DEF is filled into the tank. A DEF nozzle is very different to a fuel nozzle and a fuel nozzle cannot fit into a DEF tank fill opening.
Diesel Particulate Filter 
It is device designed to remove diesel particulate matter or soot from the exhaust gases of a diesel vehicle. Soot particles are formed by the incomplete combustion if diesel fuel. The quality of fuel burned also has an influence on the formation of these particles. The sulphur content...