Exhaust Gas Recirculation (EGR)

Exhaust Gas Recirculation (EGR) is an emission control system used to reduce NO_X emissions in particular. It redirects some amount of exhaust back to the combustion chamber of an engine to dilute the oxygen in the intake stroke and reduce the combustion temperature.

NO_X is produced at peak engine temperature and the emission is higher when the oxygen content in the intake is high (lean mixture). At high temperatures, nitrogen combines with oxygen to form various oxides of nitrogen (collectively known as NO_X). NO_X is responsible for the formation of smog in cities.

Design and Operation of EGR:

The exhaust from the engine is re-circulated only when a vacuum operated valve opens. The EGR valve is operated by the exhaust back pressure. At a particular back pressure range, EGR valve allows some amount of exhaust to be sent back to the combustion chamber.

The exhaust gas is an inert gas, hence it cannot be burned. It dilutes the fresh air in the inlet manifold, thus reducing the oxygen content. The inert gases also absorb the combustion chamber heat to reduce the peak temperature.

 The exhaust gases pass through an EGR cooler to reduce its temperature. Reducing the temperature of exhaust helps in reducing the peak temperature of the cylinder.

In an S.I. engine, EGR re-circulates 5 % to 15 % of the exhaust. Excess amount of recirculation can compromise engine efficiency. EGR is not used at engine idling speed to avoid unstable combustion. It is also not used at high loads as we require high power output to gain high speeds and EGR would just prevent high oxygen intake at high loads.

In a diesel engine, exhaust can be re-circulated as high as 50 % as diesel engine is not subjected to continuous flame front and knocking.

In the mid-1990s, pneumatic EGR valves became more popular in EGR system. It consists of an electromagnetic valve which is operated by vacuum. Vacuum is usually generated by the pressure difference between inlet manifold and exhaust manifold.