During which driving condition is the Exhaust Gas Recirculation system active?

• A. Cruise
• B. Idle
• C. Acceleration
• D. Deceleration

Answer: (A)

EGR is used to lower NOx by recirculating a portion of exhaust back into the intake, which cools the combustion temperature. This approach is most effective when the engine is operating at steady, moderate load—such as cruising—where there’s enough operation to benefit from reduced peak temperatures but not so high demand that maximum power is needed. Opening the EGR during cruising reduces NOx without noticeably hurting performance. At acceleration, the engine needs more oxygen for power, so the EGR is typically kept mostly closed; at idle, there’s little NOx to manage, so EGR is generally not active. That’s why cruising is the condition where the EGR system is active.