Engine Fuel Systems
Carburettors and carburettor icing, fuel injection, mixture control and priming.
Engine fuel systems, summary notes
- 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.
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
Engine Fuel Systems
Engine fuel systems quiz
Engine Fuel Systems, quiz
1. The purpose of the mixture control is to:
Adjust the fuel-to-air ratio as air density changes with altitudeChange engine RPM directly, the mixture lever acting as a fine adjustment on the throttleControl the supply of oil to the cylinder walls as the power setting is changed2. In the carburettor shown, the fuel is drawn from the float chamber into the airstream because the venturi:
Lowers the static pressure at the throatRaises the static pressure at the throatHeats the incoming air3. A float-type carburettor meters fuel using a:
Venturi (pressure drop draws fuel)MagnetoTurbine4. Carburettor icing can occur:
Even in warm, humid air at reduced throttleOnly below −10 °C outside airOnly in fuel-injected engines5. Carburettor icing is cleared/prevented by:
Applying carburettor heatLeaning the mixture fullyIncreasing RPM only6. Fuel injection differs from a carburettor in that it:
Is not prone to carburettor (venturi) icingAlways ices moreNeeds no fuel pump7. The mixture control is used to:
Lean the fuel/air ratio for altitude and economySet the ignition timingCool the oil8. The primer supplies:
Extra fuel for startingOil to the bearingsBoost pressure