FAA Powerplant Written Practice Test

From which source does sufficient pressure to feed fuel into the idle circuit primarily come when idling?

1. Battery

2. Fuel tank

3. From the engine

4. Fuel pump

The correct answer is: From the engine

The correct choice is that the sufficient pressure to feed fuel into the idle circuit primarily comes from the engine. During engine operation, particularly at idle, the engine generates a vacuum or suction that helps draw fuel into the idle system from the fuel sources. This process is essential because the idle circuit must provide the correct fuel-air mixture to maintain a stable engine operation at low RPMs. While fuel pumps play a crucial role in supplying fuel under various operating conditions, most of the pressure specifically for idling is managed through the engine's design and its components that utilize the pressure differentials created during combustion. This ensures that the idle circuit receives the necessary amount of fuel without the additional pressures that may be generated by pumps or external sources.