Engine Fuel & Fuel Metering Systems

Fuel System Requirements:

The engine fuel system must supply fuel to the engine’s fuel metering device under all conditions of ground and air operation; it needs to properly function with consistently changing altitudes and in any climate. The most common fuels for reciprocating engines are Jet A, AVGAS (Red), and 100LL (Blue).

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        Vapor Lock -

A vapor lock happens when fuel is overheated and vaporizes in the fuel lines. When vaporization happens, bubbles or vapor pockets are formed, causing obstruction of the fuel delivery to the engine.

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Basic Fuel System:

The basic parts of a fuel system include tanks, boost pumps, lines, selector valves, strainers, engine-driven pumps, and pressure gauges.

        Tanks -

Store flammable fuel and are exposed to various forces during flight, including vibration, aerodynamic forces, heat, cold, inertial loads, and lightning strikes.         

        Boost Pumps -

maintain fuel pressure in the lines leading to the engine-driven pump, preventing issues such as vapor lock at high altitudes and ensuring sufficient fuel flow under all flight conditions.

        Lines -

Transport fuel from the tanks to the engines efficiently and safely.

        Selector Valves -

Allow the pilot to choose which tank is feeding fuel to the engine.

        Fuel Strainers -

Used in aircraft fuel systems to remove dirt, water, and other foreign particles from the fuel.

        Engine-driven Pumps -

Deliver fuel directly to the engine by spraying it into the engine intake or cylinders, ensuring smooth engine operation.

        Pressure Gauges -

Measure the pressure of fuel as it flows from the storage/refueller tanks to aircraft.

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Carburetor Systems:

To provide for engine operation under various loads and at different engine speeds, each carburetor has six systems, main metering, idling, accelerating, mixture control, idle cutoff, and power enrichment or economizer.

        Main Metering System -

Supplies fuel to the engine at all speeds above idling.

        Idling System -

Supply fuel to the engine at low engine speeds.

        Accelerating System -

Supplies extra fuel during sudden increases in engine power.

        Mixture Control System -

Determines the ratio of fuel to air in the mixture.

        Idle Cutoff System -

Shuts off fuel to stop the engine.

        Power Enrichment / Economizer System -

Provides a richer air/fuel mixture when the engine is running at high power settings.

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Carburetor Types:

There are two different kinds of carburetor types used on aircrafts with reciprocating engines: The float-type, and the pressure-type.

        Float-Type Carburetors -

More common due to their simplicity and effectiveness with mixing air and fuel. However, the float-type carburetor does have disadvantages. For example, abrupt maneuvers and the necessity for low pressure discharging has negative effects on the float type carburetor. The main disadvantage, however, is its icing tendency.

        Pressure-Type Carburetors -

Pressure-type carburetors discharge fuel into the airstream at pressures well above the atmospheric pressure level. This leads to better vaporization and permits the discharge of fuel into the airstream on the engine side of the throttle valve.

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Carburetor Icing:

Float Type Carburetors:

Pressure Injection Carburetors: