FAA Powerplant Written Practice Test

In a pressure injection carburetor, how is fuel to the discharge nozzle controlled?

1. By an electric pump

2. By a diaphragm that uses a comparison of Venturi air pressure and carb inlet air pressure

3. By a mechanical float system

4. By gravity feed from the fuel tank

The correct answer is: By a diaphragm that uses a comparison of Venturi air pressure and carb inlet air pressure

In a pressure injection carburetor, the fuel is delivered to the discharge nozzle through a diaphragm mechanism. This diaphragm operates by comparing the pressure created by the airflow in the Venturi section of the carburetor with the air pressure in the carburetor's inlet. The relative pressures influence the movement of the diaphragm, allowing it to either open or close a fuel passage, thereby controlling the amount of fuel that is injected into the airflow entering the engine. This approach ensures that the fuel delivery is closely matched to the actual air demand of the engine under various operating conditions, thus optimizing performance and ensuring efficient combustion. The diaphragm's sensitivity to pressure changes allows for smooth operation and a fine adjustment of the air-fuel mixture, which is critical for maintaining engine stability and power output.