Introduction: Due to energy demand concerns, diesel engines have gained much attraction
recently compared to petrol engines because of their higher thermal efficiencies. However,
they emit a larger amount of NOx emissions into the atmosphere.
Objective: Nitrogen oxides are known as important ambient air pollutants that are responsible for
health problems, forest damage, and buildings corrosion. Therefore, using emissions control strategies
for diesel engines is required in order to have a cleaner environment. Urea-SCR (selective catalytic
reduction of NOx by urea) after-treatment system is considered as one of the most efficient
techniques available to reduce engine-out NOx emissions sufficiently.
Conclusion: This review article discusses all the methods suggested to diminish nitrogen oxides
emissions and then presents a comprehensive survey on developing urea-SCR unit -whether from
catalyst development aspect or from injection system modification point of view- in diesel engines
to meet strict emissions regulations.