With the increasing demand for a cleaner environment and better air quality, a diesel engine designer is forced to meet stricter norms for exhaust emissions of an engine. Currently in India the prevailing emission norm is BS-III throughout the country & BS-IV in 13 selected cities, including metros & state capitals. Let us understand what makes the designer’s work challenging.
The various major pollutants a diesel engine emits through exhaust as a product of combustion are:-
Carbon monoxides (CO) Oxides of Nitrogen (NOx) Oxides of Sulphur (SOx)
Particulate matter (PM)
Particulate matter consists of soot & aerosols such as ash, various sulphates & silicates. Origin of emission products:
It is very difficult to reduce emission products all at a time as the requirements are conflicting:-
- CO: This forms when there is incomplete combustion & its formation reduces with higher combustion temperature in the cylinder.
- NOx: This increases with increase in combustion temperature in the cylinder.
- SOx: This is not dependent on combustion but depends on the sulphur content in diesel. This is controlled at refineries by reducing sulphur content.
- PM: This reduces with increase in combustion temperature in cylinder.
Thus the reduction of exhaust emission has conflicting requirements of controlling the temperature inside the cylinder. Two methods are popular to control emissions, namely:-
- Exhaust gas recirculation (EGR)
- Selective catalytic reduction (SCR)
EGR Method: In this method, limited quantity of exhaust gas is injected along with fresh intake air so that the peak combustion temperature is controlled. Sometimes the exhaust gas is cooled before mixing with the intake air to improve efficiency of the process. This in turn reduces the quantity of oxygen in the air & reduces the peak temperature inside cylinder thus reducing NOx. However, in this method the engine efficiency reduces. The schematic of the system is shown next.
SCR Method: In this method the engine is tuned for peak performance & the exhaust products are managed through after treatment at the exhaust pipe. A metered quantity of aqueous solution of urea is dosed in the exhaust system to convert NOx to by-products mainly water & N2. The schematic of the system is shown next.
PM: The particulates are controlled by using diesel particulate filters (DPF) in the exhaust system. The particulate matters are trapped in a filter in the exhaust pipe.
– Dr. Ajoy Das