ICE-19 — Exhaust Gas Recirculation (EGR)

ASSUMPTION — EGR rates vary by engine and operating point; a qualitative framework is given.

In this lesson you will learn:

• EGR’s effect on NOx
• Internal vs external EGR
• EGR trade-offs

1. Why Does NOx Form?

NOx forms when nitrogen and oxygen combine at high combustion temperature (ICE-25).

FACT — NOx formation rises steeply with temperature. EGR’s core idea is to lower combustion temperature.

2. How EGR Works

EGR feeds part of the exhaust back into the intake, diluting oxygen concentration and lowering combustion temperature.

FACT — Exhaust gas is largely inert (CO2, N2, H2O); adding it slows combustion and lowers peak temperature, reducing NOx.

3. Internal vs External EGR

• External EGR — gas is carried from exhaust to intake via a valve (and cooler).
• Internal EGR — residual gas is retained in the cylinder via valve overlap.

FACT — Cooled EGR keeps density while improving NOx reduction, so it is common in modern diesels.

4. Trade-offs

INTERPRETATION — Excessive EGR can reduce combustion efficiency/stability and increase particulates. Calibration manages the NOx target vs efficiency/smoke balance (ICE-41).

5. Summary

• EGR lowers combustion temperature to reduce NOx.
• External uses valve/cooler; internal uses overlap.
• Excessive EGR hurts efficiency and stability.

6. Sources

• Bosch, Emissions Control Technology
• Heywood, Internal Combustion Engine Fundamentals

Next Lesson

• ICE-20 — The Air Path: the whole air flow from intake to exhaust.