To open the diffuser bleed control valve, the engine must reach which speed?

• A. 7,000 rpm
• B. 7,600 rpm
• C. 7,500 rpm
• D. 7,750 rpm

Answer: (C)

The diffuser bleed valve is used to keep the airflow through the diffuser stable as the compressor load increases. As the engine speeds up and mass flow through the diffuser grows, the diffuser pressure can rise and push the compressor toward surge. Opening the valve lets some of that air bleed off into the bypass path, reducing the diffuser pressure and smoothing the flow into the compressor.

This bleed path is opened at a specific speed threshold, about 7,500 rpm, so the valve only acts when the compressor is approaching the part of its operating map where surge risk begins to rise. Below that speed, keeping the valve closed preserves maximum flow and efficiency; above it, bleeding helps maintain stability and prevent surge.

So, the engine must reach roughly 7,500 rpm for the diffuser bleed control valve to open.