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. - PowerPoint PPT PresentationTRANSCRIPT
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Lesson 7: Fuels And Fuel Systems
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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.
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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
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Air : Fuel Ratio
What air - fuel mixture would be used to produce the most power?
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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.
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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.
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Exhaust Gas Temperature
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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.
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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.
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Reciprocating Engine Fuels
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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.
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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.
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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.
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Reciprocating Engine Fuels
• Fuel Contamination
• Solids
• Water
• Ice
• Microorganisms
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Water
• 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.
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Fuel Metering Systems
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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.
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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.
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The Aircraft Float Carburetor
Simple Venturi
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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).
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HIGH LOW
Fuel Metering Force
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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.
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Air Bleed
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Air Bleed
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The Aircraft Float Carburetor
• Air Flow Limiter
• Throttle Butterfly
• Venturi size
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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.
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Back Suction Mixture Control
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Variable Orifice Mixture Control
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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.
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The Aircraft Float Carburetor
• Acceleration System
• Picks up fuel from
bowl at idle and
discharges it through
the pump discharge
when the throttle is
opened.
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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.
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The Aircraft Float Carburetor
• Float Carburetor Preflight Inspection
• No fuel leaking
• Sump all drain points
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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
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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.
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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.
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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?
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Fuel Injection Systems
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Advantages
• Even fuel/air mixture distribution
• More power
• Less fuel
• Less problems with carburetor ice
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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.
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Two Types
• Bendix RSA
• Teledyne-Continental
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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.
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Fuel Metering Force
• Pressure drop across the orifice in the fuel injector nozzles.
• Position of the ball valve in its seat.
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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.
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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.
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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.
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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)
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Mixture control
• Manual mixture control valve
• Variable selector
• Fuel is bypassed back to the tank.
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Throttle control
• Controls air valve and fuel valve.
• Fuel valve is variable orifice
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Fuel Manifold Valve
• “Spider”
• Similar to the flow divider of Bendix
• Distributes fuel evenly
• Provides positive shut off at idle cut-off position.
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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
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Starting (Continental)
• Fuel on
• Crack throttle 1/8 inch
• Mixture full rich
• Boost pump on high
• Fuel flow indicated engage starter
• Boost pump off
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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
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Review
• Airflow Sensing/Air Metering Force– Float Carburetor: Venturi
– Bendix: Impact Tubes and Venturi
– Teledyne-Continental: N/A
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• 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
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• 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.
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