Summary of Aircraft Engine Fuel Systems
Aircraft Engine Fuel Systems: A Comprehensive Student Guide
Introduction
The aircraft fuel system supplies, meters, and delivers fuel from the tanks to the engine. This study material focuses on fuel pumps, fuel injection and metering components, turbine engine fuel controls, and important safety and operation practices. It excludes detailed material on carburetion, exhaust & mixture systems, and hydro‑pneumatic systems, which are covered elsewhere.
Definition: A fuel system is the set of components that store, filter, pressurize, meter, and deliver fuel to an aircraft engine in the proper proportion and state for combustion.
Fuel Pumps and Boost Pumps
Purpose and types
- Boost pumps are usually electric, often centrifugal, and have three main roles:
- Provide fuel pressure for starting.
- Pressurize fuel lines to prevent vapor lock.
- Transfer fuel between tanks when required.
- Engine-driven pumps supply fuel under pressure during engine operation and have a bypass or relief valve to protect the pump and system.
Definition: A boost pump is an electrically driven pump used to ensure adequate fuel pressure and prevent vapor lock during starting and low-pressure conditions.
Relief and bypass valves
- The pressure relief valve in an engine-driven pump returns excess fuel back to the pump inlet.
- A bypass valve allows fuel to flow around the engine-driven pump for starting or emergency operation if the pump fails.
Fuel Injection and Metering for Reciprocating Engines
Flow divider / manifold valve
- Two primary purposes of a manifold valve (flow divider):
- Provide a nearly constant discharge fuel pressure for smooth idling and distribution to injector nozzles.
- Provide a positive shutoff of fuel to the engine when the engine is shut down.
Mixture control (Teledyne-Continental system example)
- Full rich: All fuel is routed to the manifold valve and to the engine.
- Idle cutoff: All fuel is returned to the pump inlet; engine receives no fuel.
- Intermediate positions: Vary the proportion of fuel sent to the engine versus returned to the pump, changing the air–fuel ratio.
Definition: The mixture control varies the ratio of fuel delivered to the engine by routing fuel either to the injectors or returning it to the pump inlet.
Injector nozzle identification
- Injector nozzle flats are stamped with letters that indicate relative orifice size. The lower the letter (closer to A), the smaller the orifice flow.
Continuous-flow injection discharge
- Fuel is discharged through injector nozzles screwed into cylinder heads near the intake valves for each cylinder.
Pressure Carburetors and Related Components (limited coverage)
Note: Carburetion is covered elsewhere, but a few fuel-system-related items are included here when they interface with fuel pumps or ADI systems.
- Diaphragm soaking: Pressure-carburetor diaphragms must be soaked before flight to restore their flexibility to the calibrated condition.
- De-richment valve (with ADI): Automatically closes to lean the mixture when antidetonation injection (ADI, e.g., leaded fluid) is flowing; reopens when ADI stops to restore richer mixture.
- Starting position: On engines with a pressure carburetor, mixture control is placed in IDLE CUTOFF for start; the primer supplies fuel for cranking. Carburetor heat is placed in the COLD position for starting.
Fueling and Fuel Types for Turbine Engines
Common turbine fuels
- Jet A and Jet A-1: kerosene-based (similar to JP-5 in properties and flash/handling)
- Jet B: gasoline-based (similar to JP-4)
Definition: Jet A and Jet A-1 are kerosene-based turbine fuels commonly used in civil turbine aircraft; Jet B is a light distillate (gasoline-type) used in some cold-weather or specialty operations.
Additives and treatment
- Prist (biocide/antifreeze): Added to turbine fuel to (1) kill bacteria that can form oily scum in tanks and (2) lower the freezing point of entra
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Fuel System Overview
Klíčové pojmy: Boost pumps provide start pressure, prevent vapor lock, and transfer fuel, Engine-driven pump relief valves return excess fuel to pump inlet, Manifold/flow divider gives constant discharge pressure and positive shutoff, Teledyne-Continental mixture routes fuel to manifold at FULL RICH and returns to pump at IDLE CUTOFF, Injector nozzle letters indicate relative orifice size, Duplex nozzles provide effective spray over wide pressure range, Prist acts as biocide and antifreeze for turbine fuel water, Trim fuel control adjusts idle and maximum-thrust speeds, Filters may have relief valves to allow unfiltered flow if blocked, Vapor lock results from fuel boiling and is prevented by pressurizing and cooling fuel, Shutoff via mixture cut prevents accidental engine firing during propeller movement, After operation on turbine fuel, drain, flush, inspect compression and oil