Emission Control Systems

Operation of the Catalytic Converter

The effect of changing air/fuel ratio on the levels of NO x (solid green), CO (black) and HC (dotted green) produced in the engine. The diagram also shows qualitatively how the engine power output changes with the A/F ratio. A general relationship between levels of CO, HC and NO x released from the engine and the A/F ratio is shown in the graph. At A/F ratios somewhat above stoichiometric (14.7:1) – that is, when the engine is operating under fuel-lean , net oxidising conditions – low levels of HC and CO are produced in the engine, and there is a peak in NO x concentration. At higher A/F values, NO x falls, but the hydrocarbon concentration increases as the engine begins to misfire.