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Engine Fuel Systems
Carburettors and carburettor icing, fuel injection, mixture control and priming.
Engine fuel systems — summary notes
FreeMain ideas
- A float-type carburettor meters fuel using a venturi: the accelerating air drops the pressure and draws fuel from the float chamber; it adds a throttle, mixture control, idling system and accelerator pump for rapid opening.
- Carburettor icing forms when fuel vaporisation and the venturi pressure drop cool the mixture below freezing, icing the throttle/venturi — it can occur even in warm, humid air at reduced throttle, and is cleared/prevented with carburettor heat.
- Fuel injection meters fuel to each cylinder inlet port or directly, giving better cylinder-to-cylinder distribution, altitude compensation and freedom from carburettor icing; it needs an engine-driven pump plus an electric boost pump.
- Mixture control leans the fuel/air ratio for altitude and economy; a primer supplies extra fuel for starting; some modern (often Jet-A/diesel) piston engines use FADEC for automatic mixture and ignition control.
- Fuel is supplied by gravity and/or pumps through filters and selectors; water and contamination are drained and checked before flight.
- ⚠ Exam trap: carburettor icing can happen in WARM, humid air at low/cruise throttle because vaporisation and the venturi pressure drop cool the charge below 0 °C — apply carb heat; fuel-injected engines are not prone to carburettor (venturi) icing.
Solved examples
On a warm humid day at cruise power the RPM slowly drops and the engine runs rough on a carburetted aircraft. What is the likely cause and action?
Carburettor icing — vaporisation of fuel plus the venturi pressure drop has chilled the mixture below freezing and ice is restricting the venturi/throttle, even though the outside air is warm. The action is to apply full carburettor heat, which may briefly run rougher as the ice melts and passes through, then recovers.
Fuel systems concept map
FreeEngine Fuel Systems
Engine fuel systems quiz
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