Which statement about EGR is true?

• A. EGR reduces HC emissions during engine warm-up.
• B. EGR reduces NOx emissions during engine operation.
• C. EGR increases CO2 emissions.
• D. EGR has no effect on emissions.

Answer: (B)

EGR works by cooling and diluting the combustion charge with inert exhaust gases, which lowers the peak flame temperature inside the cylinder. That temperature drop slows the chemical reactions that form nitrogen oxides, so NOx emissions decrease during normal engine operation. Because NOx is highly temperature-dependent, this cooling effect is the primary, most direct benefit of EGR, making the statement about reducing NOx the true one. EGR doesn’t inherently increase CO2 emissions; CO2 levels are tied to the amount of carbon burned and overall engine efficiency, and EGR’s dilution doesn't automatically raise CO2. Its impact on HC and CO can vary with operating conditions and may even worsen some emissions under certain circumstances, especially during warm-up when control strategies may limit EGR or catalytic efficiency. Overall, the key effect of EGR you’re testing here is its ability to reduce NOx during engine operation.