With a fixed throttle, and with increased mass airflow, what happens to EPR?

• A. EPR goes down
• B. EPR remains constant
• C. EPR goes up
• D. EPR fluctuates

Answer: (A)

EPR measures the ratio of the exhaust pressure to the compressor discharge pressure, and it tracks how much energy the turbine is producing to push the exhaust gas out. If you keep the throttle fixed, the fuel flow is constant. Now increase the mass airflow through the engine, so more air passes for the same amount of fuel. The flame has to heat a larger mass of air, so the overall exhaust gas temperature and energy per unit mass drop. The turbine can do less work to raise the exhaust pressure, so the exhaust pressure rise is smaller relative to the compressor discharge pressure. With the same compressor pressure but a lower turbine-driven exhaust pressure rise, the EPR decreases. So, with a fixed throttle and more mass airflow, EPR goes down.