Which NOx control technique lowers NOx formation by reducing combustion temperature through exhaust gas recirculation?

• A. Three-way catalytic converter
• B. Oxygenated fuels
• C. Exhaust Gas Recirculation
• D. Positive crankcase ventilation

Answer: (C)

Exhaust Gas Recirculation lowers NOx by cooling the combustion process. NOx forms mainly at high flame temperatures, so sending a portion of the exhaust back into the intake introduces inert gases (like nitrogen and CO2) that dilute the incoming charge and absorb heat. This slows down combustion and reduces the peak temperature, which suppresses the thermal formation of NOx.

That’s why this technique is the best match for lowering NOx through temperature reduction via recirculation. Other options operate differently: a three-way catalytic converter handles NOx after combustion when the engine runs near a specific air–fuel ratio, not by cooling the charge; oxygenated fuels change combustion chemistry but don’t rely on EGR to reduce peak temperature; and positive crankcase ventilation targets crankcase emissions rather than NOx formed in the combustion chamber.