7. External Exhaust Gas Recirculation

In diesel engines, NO_x (Oxides of Nitrogen) forms when the flame temperature in the combustion chamber exceeds 2000 K. Late injection of fuel into the combustion chamber decreases NO_x at the cost of fuel efficiency by 10–15%. Recirculating a part of the exhaust gas into the combustion chamber is termed as exhaust gas recirculation (EGR) which helps in reducing NO_x with a particulate trade off. Diesel oxidation catalyst and a particulate filter are, therefore, necessary to reduce the particulate. In this chapter, the complex interactions between the EGR, turbocharger, EGR cooler, air fuel ratio, combustion efficiency and pumping work are described. It is shown that EGR provides an effective means for reducing flame temperatures and NO_x emissions, particularly under lower air fuel ratio conditions. Combustion deterioration is predominant at higher load, low speed and lower boost conditions due to a substantial reduction of air fuel ratio with EGR.