In a negative backpressure EGR system, when is the most exhaust gas recirculation achieved?

• A. At cruise, at normal operating temperature
• B. At idle
• C. At cold start
• D. At high RPM under load

Answer: (A)

In a negative backpressure EGR system, the amount of exhaust gas that can be recirculated depends on the pressure difference between the exhaust and intake paths and the engine’s operating conditions. When the engine is cruising at a steady, moderate load and is up to temperature, there’s a healthy exhaust flow and enough intake vacuum to pull the gas through the EGR valve smoothly. This combination yields the most effective and consistent EGR, helping to lower NOx without hurting performance. At cold start, the system is typically not active or limited because the engine isn’t warm enough for efficient EGR. At idle, exhaust flow is low, so even with high intake vacuum, the total EGR is small. At high RPM under load, manifold vacuum drops and throttle is wide open, making it harder for EGR to flow, so recirculation is reduced. So the peak EGR occurs during cruise at normal operating temperature.