Aircraft Engine Review: Carburetors and Fuel Systems

Aircraft Engine Review & Turbine Engine Cooling

Fuel Quantity Indicator

• Not gravity fed, electronic type (capacitance type).

• Capacitance:

  • Two parallel plates with a dielectric flowing in the middle.

  • Capacitor stores electrons.

  • Attracts negative electron charges.

  • The amount of electron charges absorbed determines the density differential between air pressures and liquid.

  • Tells the amount of electrons of the air and the amount of electrons on the liquid (oil).

Fuel Oil Cooler

• Receives hot liquid heat exchange.

• Allows bypassing of congealed oil.

• Test for flow control valve, temperature regulating valve, bypass valve.

Pressure Type Carburetor

• Has all six systems in one sub-assembly.

Four Sub-Assemblies:

1. Fuel control

2. Automatic mixture control

3. Regulating unit

4. The whole body

Differences Between Carb Types:

• Pressure type:

  • Discharges fuel through calibrated nozzles.

  • Venturi creates a pressure differential (between chamber A and chamber B).

Regulating Unit

• Two elements flowing: Air and Fuel.

• Air from venturi:

  • Tunnel valve controls airflow.

  • Increased flow decreases pressure; decreased flow increases pressure (Venturi principle).

  • Air passes through tubes: one to chamber A (inlet pressure), one through idle cutoff to a venturi.

  • Venturi changes air pressure; higher pressure pulls in from Chamber B.

  • Differential pressure between Chamber A and B is divided by a diaphragm, held by springs (small and large).

  • This is the poppet valve assembly.

Fuel Supply

• Fuel supplied by a pump (positive displacement type).

• Fuel passes through a fuel strainer to the fuel control unit.

• Vapor vent:

  • If it loses buoyancy, increased fuel returns to the fuel tank.

  • Risk of pressure type carburetor running out of fuel.

• Poppet valve controls fuel flow inside Chamber D (unmetered fuel).

• Diaphragm separates fuel and air.

• Unmetered fuel in chamber D passes through the main metering jet.

• Pressure differentials between chambers A and B determine fuel flow.

  • Higher pressure in B pushes more fuel from D through the main metering jet to Chamber C.

  • Lower pressure in B lessens fuel flow.

• Fuel pressure in Chamber C must be constant, aided by the fuel control subassembly.

• Fuel control uses high air pressure/boosted air pressure.

• Fuel control includes:

  • Automatic Rich

  • Power enrichment/economizer.

  • Accelerating pump.

    • Air pressure differential sets it.

    • Uses pressure above the throttle.

    • Works upon sudden movement of the throttle valve.

    • Pushes fuel out.

Automatic Mixture Control (AMC)

• Contains metallic bellows (sealed inside).

• Sealed at 28 inches of mercury.

• Senses differential pressures (internal fuel/air pressures, external air pressure changes).

• Adjusts fuel discharge for altitude changes.

• Uses boost venturi, pushing a diaphragm with a needle valve assembly.

• Fuel is sprayed and emulsified (air from emulsification reduces liquid density for faster flow).

Icing

• Pressure type carburetor eliminates throttle ice.

• Still susceptible to fuel evap and impact ice, thus requiring a carburetor heat system.