AMT Engine Fuel Metering Systems Questions and Answers 1 — Questions and Answers
Question 1: In a reciprocating engine's continuous-flow fuel injection system, what is the primary function of the air bleed incorporated into the fuel discharge nozzle?
- To provide positive fuel cutoff during engine shutdown.
- To filter the fuel just before it enters the intake port.
- To assist in atomizing the fuel and improve vaporization at lower power settings. (Correct answer)
- To meter the total quantity of fuel delivered to the cylinder.
Correct answer: To assist in atomizing the fuel and improve vaporization at lower power settings.
The air bleed in a continuous-flow fuel injection nozzle allows a small amount of air to be mixed with the fuel as it is discharged. This process helps to break up the solid stream of fuel into finer particles (atomization), which promotes better vaporization, especially at idle and other low power settings where fuel flow is minimal.
Question 2: A technician is troubleshooting a float-type carburetor that provides an excessively rich mixture as the aircraft climbs in altitude, even with no pilot input. Which component is most likely malfunctioning?
- The accelerator pump.
- The main metering jet.
- The throttle valve.
- The automatic mixture control (AMC). (Correct answer)
Correct answer: The automatic mixture control (AMC).
The automatic mixture control (AMC) is designed to compensate for changes in air density due to altitude. It uses a sealed bellows that expands as atmospheric pressure decreases, leaning the mixture accordingly. If the AMC fails, the carburetor will continue to meter fuel as if it were at a lower altitude, resulting in an increasingly rich mixture as the aircraft climbs into less dense air.
Question 3: Which of the following describes the fundamental operating principle of a pressure-type carburetor?
- Fuel is drawn into the venturi by a pressure differential created between the float chamber and the venturi throat.
- Fuel is delivered to a fuel metering unit under positive pressure, and the mixture ratio is controlled by the pressure of the metered fuel. (Correct answer)
- A sealed bellows expands and contracts with changes in air density to mechanically adjust a needle valve.
- Fuel is sprayed directly into the combustion chamber on the compression stroke, timed by the engine's rotation.
Correct answer: Fuel is delivered to a fuel metering unit under positive pressure, and the mixture ratio is controlled by the pressure of the metered fuel.
Unlike a float-type carburetor which relies on atmospheric pressure pushing fuel into a low-pressure venturi, a pressure-type carburetor meters fuel based on the principle of forcing fuel through jets under regulated pressure. The air metering force, created by the difference between ram air pressure and venturi pressure, acts on a diaphragm to control a fuel poppet valve, thereby regulating the amount of fuel delivered under pressure to the discharge nozzle.
Question 4: A pilot reports a gradual loss of RPM and engine roughness while flying on a warm, humid day at reduced power. Applying carburetor heat results in a further drop in RPM followed by a return to normal, smooth operation. What was the most likely cause of the initial problem?
- Vapor lock in the fuel lines.
- Carburetor icing. (Correct answer)
- Contamination in the main metering jet.
- A malfunctioning accelerator pump.
Correct answer: Carburetor icing.
The symptoms described are classic indicators of carburetor icing. The temperature drop from fuel vaporization and the venturi effect can cause moisture in the air to freeze, even on warm days. Applying carb heat introduces hot air, which melts the ice. The initial drop in RPM when heat is applied is due to the less dense hot air and the water from the melting ice being ingested by the engine. Once the ice is cleared, airflow is restored, and the engine runs smoothly again.
Question 5: In a Full Authority Digital Engine Control (FADEC) system, what is the relationship between the Electronic Engine Controller (EEC) and the Fuel Metering Unit (FMU)?
- The EEC is a hydromechanical backup that operates the FMU only if the primary system fails.
- The FMU senses engine parameters and sends mechanical signals to the EEC, which then trims the fuel flow.
- The EEC is a computer that receives sensor inputs and sends precise electrical signals to the FMU, which then mechanically adjusts fuel flow. (Correct answer)
- The FMU is the primary computer, and the EEC is a series of electronic sensors that provide data directly to the cockpit.
Correct answer: The EEC is a computer that receives sensor inputs and sends precise electrical signals to the FMU, which then mechanically adjusts fuel flow.
The FADEC system's 'brain' is the Electronic Engine Controller (EEC), a computer that processes data from various engine and aircraft sensors. Based on this data and pilot inputs, the EEC calculates the precise fuel flow required and sends electrical command signals to the Fuel Metering Unit (FMU). The FMU is essentially an electrically controlled hydromechanical device that physically regulates the amount of fuel delivered to the engine's fuel nozzles in response to the EEC's commands.
Question 6: What is a primary advantage of a fuel injection system over a float-type carburetor in a reciprocating engine?
- Reduced susceptibility to impact icing on the air filter.
- Simpler construction with fewer moving parts.
- Superior performance during aerobatic maneuvers and reduced risk of fuel vaporization icing. (Correct answer)
- Inherent ability to operate with lower octane fuels.
Correct answer: Superior performance during aerobatic maneuvers and reduced risk of fuel vaporization icing.
Fuel injection systems are less prone to icing because the fuel vaporization (and its significant cooling effect) occurs just before or inside the hot cylinder, not in a venturi upstream of the throttle valve. Additionally, because they do not rely on a float and gravity to manage fuel flow, they are much less susceptible to fuel starvation or flooding during abrupt changes in aircraft attitude, making them superior for aerobatic flight.
AMT Engine Fuel Metering Systems Questions and Answers 1
In a reciprocating engine's continuous-flow fuel injection system, what is the primary function of the air bleed incorporated into the fuel discharge nozzle?