Carburettor & Injection Systems

Carburetor Principles

• Function: Mixes fuel and air in the correct proportions before delivering it to the cylinders.
• Operation: Based on Bernoulli's Principle.
  • Air accelerates through a Venturi, causing a drop in static pressure and temperature.
  • This pressure drop draws fuel from the float chamber into the airstream.
• Diffuser: Maintains a constant mixture ratio across a wide range of engine speeds (prevents the mixture from becoming too rich at high airflow).
  • Uses a "pressure balance duct" to bleed air into the fuel nozzle, reducing suction at high power.
• Accelerator Pump: Prevents a temporary lean cut during rapid throttle advance.
  • Injects a shot of fuel into the venturi when the throttle is opened quickly, matching the rapid increase in airflow.

Induction Icing

• Susceptibility: Affects both carburetors and fuel injection systems (though carburetors are more prone due to fuel vaporization cooling).
• Carburetor Icing Factors:
  • Fuel Evaporation: Absorbs latent heat, cooling the air.
  • Venturi/Throttle Effect: Pressure drop causes temperature drop.
  • Result: Ice forms on the venturi walls and throttle valve, restricting airflow (drop in RPM/Manifold Pressure).
• Carburetor Heat:
  • Action: Supplies hot, unfiltered air to the intake.
  • Effect: Melts ice.
  • Side Effects:
    • Hot air is less dense $\rightarrow$ Power drops (~15%) and Mixture becomes Richer.
    • Combustion becomes less efficient (risk of detonation at high power).
  • Indication: Initial drop in RPM (due to hot air), followed by a rise in RPM (as ice melts and airflow is restored).
  • Usage: Use fully ON when icing is suspected. Avoid extended use at high power (takeoff/climb) to prevent detonation and power loss.

Priming & Starting

• Purpose: To inject extra fuel for starting (enrich the mixture), especially in cold conditions.
• Primer Pump: Forces atomized fuel directly into the cylinder intake ports (bypassing the carburetor).
• Throttle Pumping: Uses the accelerator pump to squirt fuel into the carburetor. Less effective and carries a higher risk of induction fire.
• Induction Fire on Start:
  • Procedure: Keep Cranking. This sucks the flames back into the engine.
  • If engine starts: Run at moderate RPM to extinguish.
  • If start fails: Shut down, mixture idle cut-off, fuel selector off, master off, evacuate, extinguish.

Fuel Injection

• Operation: Fuel is injected continuously into the inlet port (just before the valve).
• Advantages: No venturi restriction (more power), uniform mixture distribution, immune to refrigeration icing (though impact icing is still possible).
• Components: Engine-driven pump, fuel/air control unit, manifold valve, injector nozzles.