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Lesson 7: Fuels And Fuel Systems

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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Lesson 7: Fuels And Fuel Systems

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  1. Lesson 7: Fuels And Fuel Systems

  2. 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.

  3. 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

  4. Air : Fuel Ratio What air - fuel mixture would be used to produce the most power?

  5. 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.

  6. 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.

  7. Exhaust Gas Temperature

  8. 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.

  9. 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.

  10. Reciprocating Engine Fuels

  11. 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.

  12. 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.

  13. 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.

  14. Reciprocating Engine Fuels • Fuel Contamination • Solids • Water • Ice • Microorganisms

  15. 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.

  16. Fuel Metering Systems

  17. 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.

  18. 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.

  19. The Aircraft Float Carburetor Simple Venturi

  20. 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).

  21. Fuel Metering Force HIGH LOW

  22. 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.

  23. Air Bleed

  24. Air Bleed

  25. The Aircraft Float Carburetor • Air Flow Limiter • Throttle Butterfly • Venturi size

  26. 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.

  27. Back Suction Mixture Control

  28. Variable Orifice Mixture Control

  29. 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.

  30. 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.

  31. 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.

  32. The Aircraft Float Carburetor • Float Carburetor Preflight Inspection • No fuel leaking • Sump all drain points

  33. 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

  34. 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.

  35. 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.

  36. 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?

  37. Fuel Injection Systems

  38. Advantages • Even fuel/air mixture distribution • More power • Less fuel • Less problems with carburetor ice

  39. 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.

  40. Two Types • Bendix RSA • Teledyne-Continental

  41. 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.

  42. Fuel Metering Force • Pressure drop across the orifice in the fuel injector nozzles. • Position of the ball valve in its seat.

  43. 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.

  44. 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.

  45. 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.

  46. 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)

  47. Mixture control • Manual mixture control valve • Variable selector • Fuel is bypassed back to the tank.

  48. Throttle control • Controls air valve and fuel valve. • Fuel valve is variable orifice

  49. Fuel Manifold Valve • “Spider” • Similar to the flow divider of Bendix • Distributes fuel evenly • Provides positive shut off at idle cut-off position.

  50. 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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