FAA Powerplant Written Practice Test

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

1. By a mechanical float

2. Using a diaphragm comparing Venturi and carb inlet air pressure

3. By electronic fuel injection

4. Through a manual lever adjustment

The correct answer is: Using a diaphragm comparing Venturi and carb inlet air pressure

In a pressure injection carburetor, the control of fuel to the discharge nozzle is achieved through a diaphragm mechanism that compares the pressure in the Venturi with the pressure in the carburetor inlet air. This design allows for more precise fuel delivery as the fuel flow can adjust dynamically to variations in engine demand and airflow. When the engine requires more power, the increase in airflow causes a drop in pressure in the Venturi section of the carburetor. The diaphragm senses this change and responds by opening a valve to allow more fuel to be injected into the airstream at the discharge nozzle. This interaction between the diaphragm and the differing pressures precisely regulates the fuel quantity to maintain optimal combustion performance. Other methods, such as mechanical floats or manual lever adjustments, would not provide the same level of responsiveness to engine conditions since they are more static or manually operated methods. Electronic fuel injection, while an advanced method of fuel management, is distinct from the operation of a pressure injection carburetor, which relies on the mechanical interplay of pressures and the diaphragm.