Vehicle emissions control What does a modern three-way catalytic converter do to the exhaust gases passing through it?

Introduction / Context:
The three-way catalytic converter (TWC) is the centerpiece of spark-ignition emission control. Knowing exactly which pollutants it targets improves understanding of diagnostics and regulations.

Given Data / Assumptions:

• Gasoline stoichiometric combustion with lambda control via oxygen sensor.
• Converter coated with precious metals (e.g., platinum, palladium, rhodium).
• Operating above light-off temperature.

Concept / Approach:
Three-way refers to simultaneous treatment of three pollutant groups: carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NOx). Oxidation reactions convert CO and HC to CO2 and H2O; reduction reactions convert NOx to N2 (with some N2O traces minimized by control strategies).

Step-by-Step Solution:
Engine control maintains lambda ~1 for optimal TWC efficiency.Oxidation: CO + O2 → CO2; HC + O2 → CO2 + H2O.Reduction: NOx → N2 + O2 (simplified overall).Thus, the TWC removes CO, HC, and NOx, not N2, H2O, or CO2.

Verification / Alternative check:
Tailpipe measurements with an analyzer show sharp drops in CO, HC, NOx when the converter is active and at proper air–fuel ratio.

Why Other Options Are Wrong:
Exhaust gas recirculation (EGR) recirculates gases; the TWC does not.

It does not specifically “burn fuel vapour only.”

N2, H2O, and CO2 are desired products, not pollutants to reduce.

Particulate filtration is for diesel/DGPF systems, not a standard TWC role.

Common Pitfalls:
Running rich or lean degrades TWC efficiency; misfires overheat and melt the substrate; delayed O2 sensor response causes excess NOx or HC slip.

Final Answer:
converts CO and HC to CO2 and H2O and reduces NOx to harmless gases