What does the acronym EGR stand for in diesel engines?

In diesel engines, EGR stands for Exhaust Gas Recirculation.

An EGR system routes a controlled portion of exhaust gas back into the engine’s intake. Because this returned gas contains relatively little oxygen and has already been heated, it dilutes the incoming air charge and lowers peak combustion temperatures. That helps reduce the formation of nitrogen oxides, or NOx, pollutants that increase sharply during hot, oxygen-rich combustion.

EGR became an important emissions-control technology as vehicle regulations tightened. In a modern diesel, the engine-control unit adjusts EGR flow according to conditions such as engine speed, load, coolant temperature, and intake-air requirements. Some systems use high-pressure EGR, low-pressure EGR, or a combination of both; an EGR cooler may lower the temperature of the recirculated gas before it reaches the intake.

The acronym is sometimes misread as “Emission Gas Recirculation” or “Exhaust Gas Regeneration,” but neither is the standard term. EGR is not a filter and does not remove pollutants directly. It changes combustion conditions, while components such as diesel particulate filters and selective catalytic reduction systems address soot and NOx through different mechanisms. Excessive soot or deposits can restrict EGR passages and cause rough running or warning lights.