Exhaust Gas Recirculation

Effect of addition of diluents to the intake charge  for  lowering of  combustion temperatures and consequently reducing the formation of NO_x has been discussed in Module 2 .The heat capacity of the exhaust gas is higher than the air as it contains significant amount of tri-atomic gases CO₂ and water vapours.   Therefore, addition of exhaust gas to fresh intake charge has a higher effect in lowering the combustion temperatures compared to simple leaning of the charge..

EGR is defined as a mass percent of total intake flow:

(5.1)

where_i  is the total mass flow into the engine.

Typically, only about 5 to 10 % EGR rates are employed. At higher EGR rates, frequency of partial and complete misfire cycles increases resulting in unacceptably higher HC emissions and loss in fuel economy and power. EGR systems are made to operate mostly in the part-load range. These are deactivated at engine idle, because large amount of residual gas is already present in the cylinder. Many times the system is deactivated at full throttle conditions as the vehicle rarely operates under these conditions during city operation.
A schematic layout of EGR system is shown in Fig 5.8.  An EGR control valve is used to regulate flow of EGR depending upon engine operating conditions. The intake manifold pressure or exhaust back pressure may be used to control EGR rate as these parameters vary with engine load.   In the modern engines, EGR rate is controlled by the engine  electronic control unit. A pressure sensor in the exhaust or intake provides signal to the electronic control module of the engine, which in its turn regulates the operation of the EGR valve.
Electronically controlled EGR valves actuated by high-response stepper motor are being used on modern engines. Their fast response during transient operation makes it possible to reduce NO_x more than what is obtained by use of a mechanically controlled EGR valves.   Effectively a lower rate of EGR can be employed to obtain the same reduction in NO_x that results in a lower penalty on HC emissions