How does an EGR system reduce NOx emissions?

• A. By increasing exhaust gas temperature to reduce NOx formation
• B. Recirculates exhaust gas into the intake to lower combustion temperatures
• C. Uses a catalytic converter to convert NOx
• D. Reduces fuel delivery to the engine

Answer: (B)

EGR lowers NOx by recirculating a portion of the exhaust back into the intake. When exhaust gas is added to the incoming air-fuel mixture, it displaces some oxygen and has higher heat capacity. This makes the cylinder burn cooler and more slowly, reducing the peak flame temperature. Since NOx formation is highly sensitive to temperature, lower combustion temperatures mean far less NOx is formed.

This is the essence of how EGR works. Other approaches listed wouldn’t achieve the same effect: increasing exhaust gas temperature isn’t how EGR operates, catalytic converters reduce NOx after combustion rather than by diluting the mix, and simply reducing fuel delivery changes power output but not the cooling and diluting effect that EGR provides.