Powerplant - AM.III.I.K1 - Fuel/air ratio and fuel metering, and carburetor theory and operation

Question 1

Why do aircraft fuel metering systems for reciprocating engines have mixture controls?

Answer 1

• air density decreases as aircraft ascends, causing mixture to become richer
• mixture control allows duel to be decreased to maintain a fuel/air mixture ration that produces the desired power

Question 2

What does an air/fuel mixture ration of 12:1 represent?

Answer 2

12 parts air and 1 part fuel

Question 3

What is the function of a float carburetor?

Answer 3

• float carburetors function is to measure airflow through the engine induction system and dispense the appropriate amount of gasoline unto the airflow for all engine operating parameters and conditions
• must also provide the fuel in a state that is as vaporised as possible by the time ignition occurs in the engine cylinders

Question 4

What two things are adjusted when setting the idling conditions on a float carburetor?

Answer 4

• idling RPM, by adjusting the throttle stop
• idling mixture by adjusting the idle needle valve

Question 5

What would likely cause a reciprocating engine equipped with a float carburetor to hesitate momentarily when the throttle is rapidly advanced from idle to full power?

Answer 5

a malfunctioning accelerator pump

Question 6

How does a pressure type carburetor operate?

Answer 6

• discharges fuel into the airstream at a pressure well above atmospheric
• this results in better vaporisation
• fuel discharged into on the engine side of the throttle valve
• thus, the danger of fuel vaporisation icing is practically eliminated
• Unaffected by rapid manuevres and rough air since its fuel chambers remain filled under all operating conditions