Induction Systems

[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 7-7, 7-10]

(Last Update: 2020 Apr 24 0520Z)

Functions of Induction Systems

  1. brings in filtered air from the outside
    Outside air enters the induction system through an intake port on the front of the engine cowling. This port normally contains an air filter that inhibits the entry of dust and other foreign objects.
  2. mixes it (air) with fuel – carburetor system/fuel injection system
  3. delivers the fuel-air mixture to the cylinder where combustion occurs


Alternate source of air
  1. alternate air comes from inside the engine cowling – non-filtered air!
  2. Additionally, carburetor heat may be used as an alternate air source if the intake filter clogs

Since the filter may occasionally become clogged, an alternate source of air must be available. Usually, the alternate air comes from inside the engine cowling, where it bypasses a clogged air filter. Some alternate air sources function automatically, while others operate manually.


(2) Two types of induction systems are commonly used in small aircraft engines:

1. The carburetor system mixes the fuel and air in carburetor. Then this mixture through the intake manifold enters the combustion chambers.

2. The fuel injection system mixes the fuel and air in fuel-air control unit, which replaces the carburetor. Then this mixture through fuel manifold (fuel distributor) and individual fuel discharge nozzles enter the combustion chambers.

pressure-type carburetors ≠ fuel injection system


Carburetor Systems – Two categories:

  1. float-type carburetors
  2. pressure-type carburetors ( continuous-flow fuel injection system !)

  1. float-type carburetors
    • More common
    • x5 sub-systems : with 1) idling, 2)  accelerating, 3) mixture control,  4) idle cutoff, and 5) power enrichment systems
    • Mixture Control (= mixture needle) controls fuel to the discharge nozzle.
    • filtered air passing through a venturi ( a narrow throat in the carburetor) flows into the carburetor – this process causes a low-pressure area is created that forces the fuel to flow through a main fuel jet. The fuel-air mixture is then drawn through the intake manifold and then into the combustion chambers where it is ignited
    • Throttle in the flight deck – which control throttle valve in turn to regulate the flow of the fuel-air mixture to the combustion chambers
    • float-type carburetor’s distinct disadvantages x4: 
      1. fuel vaporization icing tendency
      2. mixture control skill is required
        [read more: # WHAT if fuel-air mixture is too rich/too lean?]
      3. do not function well during abrupt maneuvers
      4. incomplete vaporization and difficulty in discharging fuel into some types of supercharged systems
  2. pressure-type carburetors
    • discharges fuel into the airstream at a pressure well above atmospheric pressure
    • Advantages:
      • better vaporization
      • fuel vaporization icing is practically eliminated – fuel vaporization takes place after the air has passed through the throttle valve

Throttle : regulate the fuel-air mixture to the combustion chambers;

Mixture Control (= mixture needle) : controls fuel to the discharge nozzle

(3) Three Factors significantly Affecting Engine Performance (Carburetor-type Piston Engine)

Extended Reading –

continuous-flow fuel injection system

  • delivers fuel under pressure by a fuel pump.
  • x6 components : with engine-driven fuel pump, a fuel-air control unit, a fuel manifold (fuel distributor), discharge nozzles, an auxiliary fuel pump, and fuel pressure/flow indicators.
  • auxiliary fuel pump for engine starting and/or emergency use
  • fuel-air control unit, which replaces the carburetor, followed by that is the fuel manifold (fuel distributor), which distribute the fuel into the individual fuel discharge nozzles. The discharge nozzles, which are located in each cylinder head, inject the fuel-air mixture directly into each cylinder intake port.
  • advantages of using fuel injection:
    1. Reduction in fuel vaporization icing
    2. Better fuel flow
    3. Faster throttle response
    4. Precise control of mixture
    5. Better fuel distribution
    6. Easier cold weather starts
  • disadvantages of using fuel injection:
    1. Difficulty in starting a hot engine
    2. Vapor locks during ground operations on hot days
    3. Problems associated with restarting an engine that quits because of fuel starvation

Read More:

# Engine System

# Superchargers and Turbosuperchargers

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