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.

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

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 Ratio

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

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

• 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.

Exhaust Gas Temperature

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

• 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 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.


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.


• 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.


• Fuel Contamination

• Solids

• Water

• Ice

• Microorganisms

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.

Fuel Metering Systems

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

• 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.


Simple Venturi


• 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).

HIGH LOW

Fuel Metering Force


• 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.

Air Bleed


• Air Flow Limiter

• Throttle Butterfly

• Venturi size


• 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.

Back Suction Mixture Control

Variable Orifice Mixture Control


• 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.


• Acceleration System

• Picks up fuel from

bowl at idle and

discharges it through

the pump discharge

when the throttle is

opened.


• 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.


• Float Carburetor Preflight Inspection

• No fuel leaking

• Sump all drain points


• Carburetor Icing And Heat Use

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

– Impact ice

– Fuel ice

– Throttle 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 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 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?

Fuel Injection Systems

Advantages

• Even fuel/air mixture distribution

• More power

• Less fuel

• Less problems with carburetor ice

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

• Bendix RSA

• Teledyne-Continental

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

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

• Position of the ball valve in its seat.

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

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

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

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

• Meters fuel as a function of engine RPM.

• No Venturi

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

Mixture control

• Manual mixture control valve

• Variable selector

• Fuel is bypassed back to the tank.

Throttle control

• Controls air valve and fuel valve.

• Fuel valve is variable orifice

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)

• 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)

• 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

• 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

Review

• 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.

Lesson 7: Fuels And Fuel Systems

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