why three way catalytic converter are employed in modern IC engine driven vehicles So the plate of pollution vs air fuel ratio and conversion efficiency vs air fuel ratio for all To major pollutants SI engines?

 

Why Three-Way Catalytic Converters (TWC) Are Used in Modern IC Engine Vehicles

A Three-Way Catalytic Converter (TWC) is employed in modern SI engine-driven vehicles primarily to simultaneously reduce all three major pollutants in the exhaust:

1. CO (Carbon Monoxide)

2. HC (Unburned Hydrocarbons)

3. NOx (Oxides of Nitrogen)

---

Functions of a TWC:

• Oxidation of CO → CO₂

  $$2CO + O₂ \rightarrow 2CO₂$$

• Oxidation of HC → CO₂ + H₂O

  $$CₓHᵧ + O₂ \rightarrow CO₂ + H₂O$$

• Reduction of NOx → N₂ + O₂

  $$2NO \rightarrow N₂ + O₂$$

---

Why It’s Called “Three-Way”:

It performs three reactions simultaneously:

• 2 oxidation reactions (for CO and HC)

• 1 reduction reaction (for NOx)

But for the TWC to work effectively, the air-fuel ratio (A/F ratio) must be kept very close to stoichiometric (≈ 14.7:1 for gasoline) using a closed-loop control system with oxygen sensors (lambda sensors).

---

Air-Fuel Ratio vs Pollutant Emissions Chart (Qualitative)

Below is a simplified conceptual chart of pollution vs. air-fuel ratio:

Emissions vs. Air-Fuel Ratio
|
|          NOx
|          /\
|         /  \
|        /    \
|       /      \
|------/--------\----- A/F Ratio (14.7)
|     /          \    \
|    /            \    \
|   /              \    \
|  /                \    \
| CO                  \    HC
|----------------------------->
     Rich             Lean

• CO and HC are high on rich side (A/F < 14.7).

• NOx peaks at stoichiometric or slightly lean conditions (A/F ~14.7–15.5).

• Best compromise point is at stoichiometric ratio, where the TWC can act on all pollutants effectively.

---

Conversion Efficiency vs Air-Fuel Ratio

This graph shows how effective the catalytic converter is at different A/F ratios:

Conversion Efficiency (%)
|
|        NOx
|       /‾‾‾\
|      /     \
|-----/       \----- A/F Ratio (14.7)
|    /         \    CO, HC
|   /           \   /
|  /             \ /
| /               /
|/               /
|---------------------->
   Rich       Stoichiometric      Lean

• Maximum conversion efficiency for all three pollutants occurs around stoichiometric (λ = 1).

• If the mixture is too rich: high CO and HC, poor NOx reduction.

• If the mixture is too lean: high NOx, poor CO and HC conversion.

---

Conclusion:

Modern IC engines use Three-Way Catalytic Converters because:

• They are most effective when A/F ratio is precisely controlled near stoichiometric.

• They significantly reduce CO, HC, and NOx emissions simultaneously.

• They are essential for meeting strict emission norms like Bharat Stage VI / Euro 6.

Therefore, engine control units (ECUs), oxygen sensors, and TWC together form a closed-loop emission control system in modern SI engine vehicles.