Piston engines use either a carburettor or fuel injection to form the air/fuel mixture.
Float-type carburettor
Classic system (e.g. Cessna 152, older C172):
- Fuel is atomised by a venturi using the pressure drop.
- Float maintains a constant fuel level in the float chamber.
- Main jet meters fuel into the induction air.
Pros: Simple, robust, low maintenance. Con: Susceptible to carburettor icing (see §2.5).
Fuel injection
Modern system (e.g. C172S with Lycoming IO-360, DA40, Aquila):
- Mechanical injection (Bendix RSA, Continental TCM) — servo measures airflow, manifold distributes fuel via individual injectors per cylinder in the intake port.
- Direct cylinder injection (rare in PPL trainers) delivers fuel during the intake stroke.
Pros:
- No carburettor icing (no venturi pressure drop + evaporation)
- Better cylinder-to-cylinder mixture distribution
- Higher specific power
Cons:
- Vapour lock possible (see §3.4)
- Hot starts can be tricky (residual heat boils fuel in the lines — AFM procedure)
- Induction icing at filters or manifold in visible moisture still possible → alternate air / carb heat function provided