Which statement about Pre- and Post-combustion emission control systems is true?

• A. Only Technician A is correct.
• B. Only Technician B is correct.
• C. Both Technicians A and B are correct.
• D. Neither Technician is correct.

Answer: (C)

Understanding how emission control works both before and after combustion helps explain why both statements can be true. Pre-combustion controls aim to limit pollutants before they’re formed by the engine. This includes keeping the air-fuel mixture precisely controlled for complete combustion, using high-quality fuel, proper ignition timing, and methods like Exhaust Gas Recirculation to lower combustion temperatures and reduce NOx formation. Post-combustion controls treat pollutants after combustion, right in the exhaust stream, with devices such as catalytic converters that oxidize carbon monoxide and hydrocarbons and reduce NOx, along with diesel oxidation catalysts, diesel particulate filters, and selective catalytic reduction systems for further NOx control. Because these approaches operate at different stages—one to prevent pollutant formation and the other to remove pollutants that are already formed—both perspectives are correct and reflect how modern emission control systems are designed to achieve lower emissions overall.