DIESEL EXHAUST GAS AFTERTREATMENT

The combustion process and hence the exhaust gas composition and its thermodynamic state in diesel engines differ from SI engines. The main differences are;

• The overall air-fuel ratio in the diesel engines varies from about 19:1 to 75:1 resulting in large variations in the exhaust gas composition with excess oxygen always present in the exhaust gases.
• Due to heterogeneous combustion in diesel engines, a large concentration of particulate matter is present in the exhaust gases.
• The exhaust gas temperature varies usually from 150 to 350º C. The gas temperatures at the exit turbocharger are further lower compared to temperatures at the exhaust port  due to expansion in the turbine 

      In the European heavy duty engine cycle the exhaust temperatures vary from 200 - 400º C although in the US transient cycle the gas temperatures may reach up to 600 C. On the other hand in the driving cycle for light duty vehicles the gas temperatures vary in the range of 150 350 C only. Until the year 2000, the diesel vehicle emission standards in the US and Europe were largely met by use of improved injection system, engine combustion improvements, EGR and turbocharging.  The three-way catalytic converters are unable to function in diesel engines as a high amount of excess oxygen is always present in the exhaust gases.  Hence, the nature of exhaust treatment in diesel engines is considerably different than for the stoichiometric SI engines. In the light duty diesel vehicle segment, diesel oxidation catalysts have found application for the Euro 2 and 3 vehicles. For the later standards such as Euro 4 and 5, advanced forms of exhaust aftertreatment like diesel particulate filters and lean de-NOx catalysts are being employed.

Exhaust aftertreatment in diesel engines may be grouped in two broad categories;

• Diesel catalytic exhaust aftertreatment and
• Diesel particulate filters (DPF)