ICE-25 — Nitrogen Oxides (NOx)

ASSUMPTION — This lesson covers NOx formation conceptually; numeric formation rates are not given.

In this lesson you will learn:

• How NOx forms
• NO vs NO2
• Control methods

1. Formation Mechanism

NOx forms when nitrogen (N2) and oxygen (O2) react at high temperature.

FACT — NOx formation rises exponentially with temperature (thermal NOx). The higher the combustion temperature and the longer the gas dwells at high temperature, the more NOx forms.

2. NO vs NO2

FACT — Combustion first produces mostly NO (nitric oxide); NO later oxidizes to NO2 in the atmosphere. “NOx” means the sum of both; NO2 is more harmful to respiratory health.

3. Control Methods

• In-cylinder — EGR to lower temperature, injection/ignition timing.
• Aftertreatment — SCR (diesel) and TWC (gasoline).

FACT — Diesel NOx control is two-sided: EGR in-cylinder, SCR in the exhaust. Gasoline uses the TWC under stoichiometric operation.

4. Trade-offs

INTERPRETATION — NOx reduction usually conflicts with efficiency: lower combustion temperature means less NOx but possibly less efficiency. In diesels there is also a NOx-soot trade-off; calibration manages the balance (ICE-41).

5. Summary

• NOx forms from N2 + O2 at high temperature.
• NO forms first, then oxidizes to NO2.
• Controlled by EGR (in-cylinder) + SCR/TWC (aftertreatment).

6. Sources

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

Next Lesson

• ICE-26 — Lambda: air-fuel ratio control.