Under which condition is exhaust gas recirculation (EGR) required?

Exhaust Gas Recirculation (EGR) is a critical component in reducing nitrogen oxide (NOx) emissions in an engine, and it is typically utilized under conditions where the engine operates efficiently and generates higher combustion temperatures. In this context, the warm engine at part throttle scenario is ideal for EGR operation.

When the engine is warm and operating at part throttle, the combustion chamber temperature can become elevated, potentially leading to increased NOx production. By introducing a portion of the exhaust gas back into the intake manifold, the EGR system helps to lower these combustion temperatures, therefore reducing the formation of NOx emissions. The recirculated exhaust gases effectively dilute the air-fuel mixture, which promotes more complete combustion and lowers peak temperatures.

In contrast, a cold engine, whether at part throttle or idle, does not have the same combustion characteristics, and the EGR system is generally disabled to allow for optimal warm-up and performance. Additionally, during wide open throttle conditions, the engine operates at full load, where the need for EGR is limited since the focus is on maximum power output rather than emission control.