Carburetion and Mixture Control

13.1 The Carburetor

• The carburetor's job is to mix fuel and air in the correct ratio for combustion under all
• perating conditions39. The most common type on light aircraft is the
• float-chamber carburetor, which is simple but prone to icing40.
•  Venturi Principle: The carburetor uses a venturi—a narrow throat in the air intake passage.
• As air flows through the venturi, its velocity increases and its static pressure drops

• (Bernoulli's Principle)414141. This low pressure is used to draw fuel from the float chamber
• into the airstream42.
•  Mixture Correction: As engine speed increases, a simple carburetor tends to deliver an
• verly rich mixture. To correct this, a
• diffuser well or an air bleed diffuser is used to introduce air into the fuel discharge tube,
• preventing the mixture from becoming too rich as airflow increases43434343.
•  Accelerator Pump: When the throttle is opened quickly, there can be a temporary
• weakening of the mixture, causing the engine to hesitate (a 'flat spot')44. An accelerator pump,
• linked to the throttle, injects a small amount of extra fuel into the venturi to prevent this45.

13.2 Mixture Control

• As an aircraft climbs, air density decreases. A carburetor, being a volumetric device, will not
• automatically reduce the fuel flow to match, causing the mixture to become progressively
• richer with altitude46. A manual mixture control is required to correct this.
•  Practical Mixture Ratios: While the chemically correct (stoichiometric) ratio is about 15:1
• (air to fuel), this does not give the best results in an engine47. Piston engines are often run
• slightly richer than this for stable operation48.
•  Leaning the Mixture: The pilot uses the mixture control to lean the fuel/air ratio. For
• economy cruise, a lean mixture (e.g., 17.4:1) can be used, which reduces Specific Fuel
• Consumption (SFC)49. However, excessively lean mixtures burn slower and hotter, which can
• lead to overheating and 'popping back' into the induction manifold50.
•  Full Rich for Takeoff: For takeoff, a very rich mixture (e.g., 10:1) is used. The excess,
• unburnt fuel has a cooling effect inside the cylinder, which helps prevent detonation at high
• power settings51.