16.4 Piston engine fuel systems — carburettors, fuel injection and electronic engine control

Scope

The largest M16 sub-module (about 120 practice questions in this app). All at A level 1, B1/B3 level 2.

• 16.4.1 Carburettors: types, construction, operation; icing and heating.
• 16.4.2 Fuel injection systems: types, construction, operation.
• 16.4.3 Electronic engine control: engine control and fuel metering including FADEC; layout and components.

Purpose of fuel metering

Deliver fuel in the correct proportion to the mass of air entering the engine for every condition (start, idle, cruise, full power, acceleration, altitude), and atomise/vaporise it for even distribution to the cylinders.

Float-type carburettor

Principle: the venturi. Air speeds up in the venturi throat, its pressure falls (Bernoulli); the pressure difference between the float chamber (at near-ambient pressure) and the discharge nozzle in the throat draws fuel into the airstream. Fuel flow increases with airflow.

System	Function
Float and needle valve	Maintains a constant fuel level in the float chamber (slightly below the discharge nozzle outlet so fuel does not flow when the engine is stopped)
Main metering system	Main metering jet sizes fuel flow; discharge nozzle in the venturi; air bleed emulsifies fuel and reduces the tendency of the mixture to richen at higher airflow
Idle system	At closed throttle venturi suction is too weak, so fuel is drawn from idle discharge ports near the throttle valve edge, where suction is high; idle mixture adjustment screw
Accelerating system	Accelerating pump (piston or diaphragm linked to the throttle) squirts extra fuel when the throttle is opened quickly — prevents a momentary lean "hesitation"
Economiser (enrichment/power enrichment)	Adds extra fuel at high power settings for cooling and detonation suppression
Mixture control	Manual leaning to compensate for altitude (decreasing air density), and idle cut-off position to stop the engine by shutting off fuel (prevents dieseling/after-running)
Throttle valve (butterfly)	Controls volume of mixture → manifold pressure → power

Carburettor types: updraught, downdraught, side-draught. Float carburettors can be affected by negative g (fuel flow interruption) and are prone to icing.

Pressure-injection carburettor (older larger engines): fuel is metered by air pressure differentials acting on diaphragms (impact pressure and venturi suction) and injected under pressure downstream of the throttle — not affected by attitude, less icing.