explain the mechanism of formation of COUPHC and NOx emissions ?

 

Mechanism of Formation of Emissions in IC Engines

In internal combustion engines (both SI and CI), incomplete and high-temperature combustion processes lead to the formation of harmful emissions. The major pollutants are:

• CO – Carbon Monoxide

• UHC – Unburned Hydrocarbons

• NOx – Oxides of Nitrogen

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1. Formation of CO (Carbon Monoxide):

Mechanism:

• CO is formed when there is incomplete combustion of fuel due to insufficient oxygen.

• In a rich mixture (more fuel than air), not all carbon gets oxidized to CO₂. Instead:

$$\text{C (from fuel)} + \frac{1}{2}O_2 \rightarrow CO$$

Causes:

• Rich air-fuel mixtures (especially in SI engines at full throttle)

• Poor mixing in CI engines

• Inadequate combustion time

• Low combustion temperatures

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2. Formation of UHC (Unburned Hydrocarbons):

Mechanism:

• Hydrocarbons (HCs) are part of fuel that fail to burn or partially burn.

• They escape the combustion process and appear in the exhaust.

Key Sources:

• Crevice volumes: Fuel-air mixture gets trapped in small gaps (e.g., piston ring grooves) and escapes combustion.

• Wall quenching: Flame extinguishes near the cooler cylinder walls before complete combustion.

• Incomplete combustion during cold starts or misfires.

• Fuel impingement: In CI engines, some fuel hits cylinder walls or piston crown and doesn't vaporize properly.

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3. Formation of NOx (Oxides of Nitrogen):

Mechanism:

• NOx (mostly NO and NO₂) is formed due to high-temperature oxidation of nitrogen in the intake air.

• Follows the Zeldovich Mechanism, which occurs significantly above 1800°C:

$$\text{N}_2 + O \rightarrow NO + N \\ \text{N} + O_2 \rightarrow NO + O$$

Conditions Favoring NOx Formation:

• High combustion temperatures

• High pressure and excess oxygen (lean mixtures)

• Long residence time at peak temperature

• Common in both SI (lean operation) and CI (diesel) engines

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Summary Table:

Pollutant	Cause	Engine Condition
CO	Incomplete oxidation of fuel	Rich mixtures, low oxygen
UHC	Unburned fuel	Flame quenching, misfire, cold start
NOx	High temperature oxidation of nitrogen	High combustion temp, lean mixtures

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Conclusion:

Understanding the mechanism of emission formation helps in designing control strategies like:

• Optimizing air-fuel ratios

• Using EGR (Exhaust Gas Recirculation)

• Catalytic converters

• Improving combustion chamber design

This leads to better engine performance with reduced environmental impact.