List the factors responsible for formation of Nox during combustion. ?

 

🔥 Factors Responsible for the Formation of NOₓ (Oxides of Nitrogen) During Combustion

NOₓ emissions primarily include nitric oxide (NO) and nitrogen dioxide (NO₂), which are major pollutants formed during the high-temperature combustion of fuels, especially in IC engines.

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📌 Key Factors Contributing to NOₓ Formation:

1. 🌡️ High Combustion Temperature

• NOₓ formation increases exponentially above 2,000 K.

• At high temperatures, nitrogen (N₂) and oxygen (O₂) from air dissociate and react:

  $$N_2 + O_2 \rightarrow 2NO$$

2. ⏱️ Long Residence Time at High Temperatures

• The longer the gases remain at high temperature, the more NOₓ is produced.

• This is common in lean mixtures or engines under heavy load.

3. 💨 Excess Oxygen (Lean Mixtures)

• More oxygen availability supports the reaction between nitrogen and oxygen, forming more NOₓ.

• Lean burn engines are more prone to NOₓ formation.

4. 🔄 High Compression Ratio / Advanced Ignition Timing

• Leads to faster and hotter combustion, increasing peak temperatures and NOₓ production.

5. ⚙️ Engine Load and Speed

• Higher load = more fuel burned = higher temperatures → more NOₓ.

• Fast combustion at high speeds can also cause higher NOₓ formation.

6. 🔥 In-cylinder Turbulence

• Increased turbulence improves combustion efficiency, raising peak temperature and thus NOₓ levels.

7. 🛠️ Injection Pressure and Timing (in CI Engines)

• Early or high-pressure injection can raise cylinder temperature and pressure, increasing NOₓ.

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✅ Summary Table:

Factor	Effect on NOₓ Formation
High combustion temperature	Increases dissociation & NO formation
Long residence time at high temp	More time for NOₓ reactions to occur
Excess oxygen (lean mixtures)	Supports formation of thermal NO
Advanced ignition/injection timing	Increases peak pressure and temperature
High engine load	Increases fuel burned and heat release

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🧠 Conclusion:

NOₓ formation in engines is primarily driven by temperature, oxygen availability, and combustion timing. Engine design and control strategies (like EGR, retarded ignition timing, or lean-NOx traps) are used to minimize NOₓ emissions.