Lesson 7 fuels and fuel systems
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Lesson 7: Fuels And Fuel Systems. Fuels And Fuel Systems. Fuel: The energy source for the combustion process Combustion occurs when fuel comes into contact with oxygen, and the temperature of the mixture is raised to its kindling point.

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Fuels and fuel systems
Fuels And Fuel Systems

  • Fuel: The energy source for the combustion process

    • Combustion occurs when fuel comes into contact with oxygen, and the temperature of the mixture is raised to its kindling point.

    • The fuel and oxygen mix, and oxidation, or burning, occurs.

Air fuel ratio
Air : Fuel Ratio

  • Stoichiometric is a chemically correct mixture in which all of the chemical elements are used and none are left over. (15:1)

    • Fifteen pounds of air to one pound of gasoline.

    • 15:1 = 0.067

Air fuel ratio1
Air : Fuel Ratio

What air - fuel mixture would be used to produce the most power?

Air fuel ratio2
Air : Fuel Ratio

  • The design of the engine induction system and the valve timing requires a mixture that is slightly richer than chemically perfect in order to produce the maximum power.

  • This also runs cooler and prevents overheating and detonation under high engine loads.

  • Maximum power is normally considered to be produced with a mixture of approximately 12:1 or 0.083.

Exhaust gas temperature
Exhaust Gas Temperature

  • There is a direct relationship between the temperature of the exhaust gas and the mixture ratio being burned.

    • As mixture ratio is leaned, the EGT rises until peak temperature is reached, and then it drops off.

    • This peak EGT will always be reached with the same air : fuel ratio regardless of the power.

    • Used as a reference for adjusting the mixture.

Specific fuel consumption
Specific Fuel Consumption

  • The number of pounds of fuel burned per hour for each horsepower developed.

    Pounds of fuel burned per hour

    Brake horsepower produced

  • Used to rate or to compare the performance of aircraft engines.

  • Used rather than thermal efficiency.

Thermal efficiency
Thermal Efficiency

  • The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units.

Reciprocating engine fuels1
Reciprocating Engine Fuels

  • Composition

    • Aviation gasoline is a hydrocarbon fuel refined from crude oil.

    • Straight-run gasoline

    • All gasolines are blends of different hydrocarbons and additives.

    • Annual US usage of avgas was approximately 0.14% of motor gasoline consumption in 2008.

Reciprocating engine fuels2
Reciprocating Engine Fuels

Fuel Grades

(grade = octane)

  • Grade-80 RED

  • Grade-100 Green

  • Grade-100LL (Low Lead) Blue

  • Grade-115/145 Purple

    • The required grade of fuel must be placarded on the filler cap of the aircraft fuel tanks.

Reciprocating engine fuels3
Reciprocating Engine Fuels

  • Alternate Fuels

    • STC’s which permit the use of autogas or mogas in engines.

    • Lower price

    • No changes or adjustments to the engine are required

    • May be used interchangeably with avgas.

Reciprocating engine fuels4
Reciprocating Engine Fuels

  • Fuel Contamination

    • Solids

    • Water

    • Ice

    • Microorganisms


  • Water is one of the major sources of contamination.

    • At altitude the temperature is low enough to cause the water to condense out of the fuel and form free water.

    • The freed water can freeze and clog the fuel lines.

  • Water is slightly soluble in gasoline.

    • Fuel will hold more water in solution if it is warm than it will if it is cold.

Fuel metering systems1
Fuel Metering Systems

  • Principal Function is to sense the amount of air entering the engine at any moment and meter into that air an amount of fuel that will provide a uniform air : fuel ratio.

  • System will provide a uniform air : fuel ratio as the airflow varies.

The aircraft float carburetor
The Aircraft Float Carburetor

  • Airflow Sensing

    • The air measuring unit is the venturi.

    • Makes use of a basic law a physics:

      As the velocity of a gas or liquid increases, the pressure decreases.

The aircraft float carburetor2
The Aircraft Float Carburetor

  • Fuel Metering Force

    • Fuel from the aircraft’s tank is delivered to the float bowl of the carburetor.

    • The main fuel nozzle is located in the center of the venturi.

    • When air is flowing in the venturi a pressure differential between the venturi and the float chamber exist (Fuel Metering Force).

The aircraft float carburetor3
The Aircraft Float Carburetor

  • Air Bleed

    • Air bled into the main metering system decreases the fuel density and destroys surface tension.

    • This results in better vaporization and control of fuel discharge, especially at lower engine speeds.

The aircraft float carburetor4
The Aircraft Float Carburetor

  • Air Flow Limiter

    • Throttle Butterfly

    • Venturi size

The aircraft float carburetor5
The Aircraft Float Carburetor

  • Mixture Control System

    • Back Suction Mixture Control

      Varies the pressure in the float chamber between atmospheric and a pressure slightly below atmospheric.

    • Variable Orifice Mixture Control

      Changes the size of the opening between the float bowl and the discharge nozzle.

The aircraft float carburetor6
The Aircraft Float Carburetor

  • Mixture Control System (Idle System)

  • Pressure of the air at edge

    of the throttle valve and

    above the valve is low.

  • Fuel rises from the bowl

    due to the low pressure

    above the throttle valve.

The aircraft float carburetor7
The Aircraft Float Carburetor

  • Acceleration System

  • Picks up fuel from

    bowl at idle and

    discharges it through

    the pump discharge

    when the throttle is


The aircraft float carburetor8
The Aircraft Float Carburetor

  • Power Enrichment System

    • Removes some of the heat by enriching the fuel-air mixture at full throttle.

    • Some only provide full power enrichment when the throttle is all the way open.

    • When takeoff power is required, throttle should be opened fully.

The aircraft float carburetor9
The Aircraft Float Carburetor

  • Float Carburetor Preflight Inspection

    • No fuel leaking

    • Sump all drain points

The aircraft float carburetor10
The Aircraft Float Carburetor

  • Carburetor Icing And Heat Use

    • Carburetor ice means ice at any location in the induction system.

      • Impact ice

      • Fuel ice

      • Throttle ice

Carburetor ice
Carburetor Ice

  • Impact ice

    • Formed by the impingement of moisture-laded air at temperatures below freezing onto the elements of the induction system which are at temperatures below freezing.

    • Air scoop, heat valve, carburetor air screen, throttle valve and metering elements.

Carburetor ice1
Carburetor Ice

  • Fuel Ice

    • Forms when any air or fuel entrained moisture reaches a freezing temperature as a result of cooling of the mixture by fuel vaporization.

      • Cooler air holds less water vapor and the excess water is precipitated in the form of condensation.

      • Condensate freezes.

      • Can occur at ambient temperatures well above freezing.

Carburetor ice2
Carburetor Ice

  • Throttle ice

    • Formed at or near a partly closed throttle when water vapor in the induction air condenses and freezes due to the expansion cooling and lower pressure at the throttle.

    • Temperature drop normally does not exceed

      5° F.

  • How is carburetor ice formation prevented?

  • Advantages

    • Even fuel/air mixture distribution

    • More power

    • Less fuel

    • Less problems with carburetor ice

    Differences from float carburetors
    Differences from float carburetors

    • Fuel Injection: Deposits a continuous flow of fuel into the induction system near the intake valve just outside of the cylinder.

    • Carburetor: The correct amount of fuel is metered into the airflow.

    Two types
    Two Types

    • Bendix RSA

    • Teledyne-Continental

    Bendix fuel injection system
    Bendix Fuel Injection System

    • Uses a venturi and air diaphragm to develop a fuel metering force.

      • Impact tubes sense total pressure of air entering the engine. (Dynamic + Static)

      • Venturi senses its velocity.

      • Both combine to move the air diaphragm proportionally to the amount of air ingested into the engine.

    Fuel metering force
    Fuel Metering Force

    • Pressure drop across the orifice in the fuel injector nozzles.

    • Position of the ball valve in its seat.

    Idle system
    Idle System

    • Constant head spring pushes against the air diaphragm and forces the ball valve off its seat. (at low air flow)

    • As air flow increases the air diaphragm moves over.

    Idle rpm mixture control
    Idle RPM/Mixture Control

    • Limit the amount of air allowed to pass the throttle valve.

    • Limit the amount of fuel to flow to the discharge nozzles.

    Flow divider
    Flow Divider

    • At idle a spring holds the flow divider valve closed to oppose fuel flow until fuel pressure off-seats valve.

    • Creating down stream pressure for the fuel control.

    • Provides cut off of fuel at idle cut off.

    The teledyne continental fuel injection system
    The Teledyne-Continental Fuel Injection System

    • Meters fuel as a function of engine RPM.

    • No Venturi

    • Special engine driven pump produces the fuel metering pressure. (constant displacement pump)

    Mixture control
    Mixture control

    • Manual mixture control valve

    • Variable selector

    • Fuel is bypassed back to the tank.

    Throttle control
    Throttle control

    • Controls air valve and fuel valve.

    • Fuel valve is variable orifice

    Fuel manifold valve
    Fuel Manifold Valve

    • “Spider”

    • Similar to the flow divider of Bendix

    • Distributes fuel evenly

    • Provides positive shut off at idle cut-off position.

    Starting procedures bendix
    Starting Procedures (Bendix)

    • Mixture idle cut-off

    • Open throttle 1/8 inch

    • Master on

    • Boost pump on

    • Mixture full rich until indication of fuel flow

    • Return mixture to idle cut-off

    • Starter engage

    • At engine start move mixture to full rich

    Starting continental
    Starting (Continental)

    • Fuel on

    • Crack throttle 1/8 inch

    • Mixture full rich

    • Boost pump on high

    • Fuel flow indicated engage starter

    • Boost pump off

    Starting hot engine
    Starting HOT Engine

    • Mixture idle cut-off

    • Throttle open wide

    • Boost pump on high

    • Allow fuel to circulate 15-20 seconds

    • Boost pump off

    • Mixture full rich

    • Throttle 1/8

    • Engage starter

    • Continue normal start


    • Airflow Sensing/Air Metering Force

      • Float Carburetor: Venturi

      • Bendix: Impact Tubes and Venturi

      • Teledyne-Continental: N/A

    • Fuel Metering force

      • Float: Pressure Diff. between venturi and float chamber

      • Bendix: Balance between the air and fuel forces holds valve off its seat a stabilized amount for and given air flow.

      • Teledyne-Continental: Engine RPM

    • Mixture Control

      • Float: Back suction, Variable orifice, Needle valve at idle.

      • Bendix: Valve in the fuel control regulates the amount of fuel that can flow to main metering jet.

      • Teledyne-Continental: Variable Selector.