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Aircraft Fuel System and Float Type Carburetor Notes

Aircraft Fuel System and Float Type Carburetor

Fuel Flow Initiation

  • A specific button must be pressed to allow fuel to flow in the fuel lines.
  • Fuel flows into the fuel lines and then to the carburetor.

Fuel Inlet Screen

  • The fuel passes through a fuel inlet screen.
  • In a float-type carburetor, the screen is typically paper, reusable, and cleanable.

Float Chamber and Needle Valve

  • Fuel passes through a needle valve seat assembly into the float chamber.
  • The fuel level causes the float to rise.
  • Float level is measured from the top of the float to the parting surface.
  • Float height can be adjusted (if the carburetor allows) by adding or removing shims.

Air Pressure in Float Chamber

  • The float chamber contains air at a pressure of 14.7 PSI (or 29.9 centimeters of mercury).
  • The air pressure pushes the fuel down through the main metering passageway.
  • Fuel fills the accelerating fuel chamber and then the main discharge nozzle.

Fuel Levels

  • The fuel level in the main discharge nozzle is higher than in the float chamber.

Six Systems of Float Type Carburetor

  1. Idling System
  2. Idle Cutoff
  3. Main Metering
  4. Mixture Control
  5. Accelerating System
  6. Economizer/Power Enrichment

Engine Start

  • Mixture control is set to full rich.
  • The idling system works in conjunction with the mixture control during startup.

Pressure at Venturi During Idle

  • There is minimal usable pressure at the venturi during idle.
  • The throttle valve is mostly closed, creating usable pressure at the edge of the throttle valve.
  • This causes the idling system to provide a full rich fuel-air mixture.

Idling System Details

  • Air enters through the idle air bleed and is injected out to the idle jet.

Transition to Taxi

  • The throttle valve is opened to a half-open position.
  • Pressures at the throttle valve edge are reduced.

Main Metering System

  • Usable pressures shift to the main venturi and the air pressure inside the float chamber.
  • The main metering system takes over when the speed is above idle.
  • Fuel is vacuumed out at the throat of the venturi where the lowest pressure exists.
  • Large fuel droplets (not vapors) are sucked out.

Flight and Mixture Control

  • Takeoff: Full throttle, full rich mixture.
  • Ascending: Full rich mixture.
  • Cruising: Rich mixture.

Float Rupture During Flight

  • If the float ruptures, it causes an excessively rich mixture.

Ice Formation

  • Ice formation (impact ice or fuel evaporation ice) can occur.
  • If ice clogs the system during full power, the mixture becomes excessively rich.
  • If ice clogs the system when the throttle is half open, the mixture leans out.

Main Air Bleed Blockage at Full Power

  • If the main air bleed gets clogged during full power (throttle valve fully open), the mixture goes from rich to excessively rich.

Main Air Bleed Blockage During Cruising

  • If the main air bleed gets clogged during cruising (throttle valve half open), the mixture leans out.
  • Throttle valve position affects airflow and fuel metering.

Consequences of Blockage

  • Fully open throttle with blockage: airflow is high, but metering is lost, resulting in an excessively rich mixture.
  • Half-open throttle with blockage: insufficient airflow to vacuum fuel, leading to a lean mixture.

Emulsification and Fuel Delivery

  • The system emulsifies fuel and pushes it out.
  • If this is blocked, fuel delivery is compromised.

Accelerating System

  • Rapid throttle opening activates the accelerating system.
  • The accelerating pump provides a temporary full rich mixture to prevent power lag.
  • Manifold pressure and RPM spike up temporarily.

Economizer System

  • The economizer needle valve opens to provide a gradual, constant enrichment of the fuel-air mixture.
  • This maintains power output and provides internal cooling during valve overlap.

Landing and Idle

  • After landing and taxiing to a parking spot, the engine is set to idle.
  • If the engine stops without the mixture control being set to idle cutoff, it indicates an issue.

Potential Issues at Idle

  • Blocked idle jet or idle air bleed can cause the engine to stop.
  • A blocked idle air bleed leads to a lean mixture and engine shutdown.

Idle Cutoff System

  • The idle cutoff system works with back suction to vent out air pressures.
  • This reduces the air pressure pushing fuel, cutting off fuel flow.

Indicators

  • Monitor manifold pressure and RPM.