What do cars with petrol engines have in their exhaust system to reduce the amount of carbon monoxide unburned hydrocarbons and nitrogen oxides in their exhaust gases?

The catalytic converter reduces carbon monoxide, as well as unburned hydrocarbons.

The EGR system introduces exhaust gas to the intake, creating a lower combustion temperature, and in turn less oxides of nitrogen.

There are more advanced systems on newer cars, but these are the basics.

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Curtis

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