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

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. The annual US usage of avgas was 186 million US gallons (700,000 m3) in 2008, and was approximately 0.14% of the motor gasoline consumption. From 1983 through 2008, US usage of avgas declined consistently by approximately 7.5 million US gallons (28,000 m3) each year.[6

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. 80/87 – Used in engines with low in engines with low compression ratio – phased out at this point/very limited availability. 100/130 – Mostly replaced by 100-LL 100LL – Most commonly used aviation gasoline, produced worldwide. 115/145 - Originally used as primary fuel for the largest, boost-supercharged radial engines needing this fuel's anti-detonation properties.[23] Now, limited batches are produced for special events such as unlimited air races. Many grades of avgas are identified by two numbers associated with its Motor Octane Number (MON).[7] The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the anti-knock index or "pump rating" given to automotive gasoline in the US. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.[8]

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. Supplemental Type Certificate for automotive gasoline. Automotive gasoline — known as mogas or autogas among aviators — that does not contain ethanol may be used in certified aircraft that have a Supplemental Type Certificate for automotive gasoline as well as in experimental aircraft and ultralight aircraft. Some oxygenates other than ethanol are approved. Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87 anti-knock index automotive gasoline. Examples include the popular Cessna 172 Skyhawk or Piper Cherokee with the 150 hp (110 kW) variant of the Lycoming O-320.[citation needed] Some aircraft engines were originally certified using a 91/96 avgas and have STCs available to run "premium" 91 anti-knock index (AKI) automotive gasoline. Examples include some Cherokees with the 160 hp (120 kW) Lycoming O-320 or 180 hp (130 kW) O-360, or the Cessna 152 with the O-235. The AKI rating of typical automotive fuel does not directly correspond to the 91/96 avgas used to certify engines. Sensitivity is roughly 8-10 points meaning that a 91 AKI fuel might have a MON of as low as 86. The extensive testing process required to obtain an STC for the engine/airframe combination helps ensure that for those eligible aircraft, 91 AKI fuel provides sufficient detonation margin under normal conditions.[citation needed]

Reciprocating Engine Fuels 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.

The Aircraft Float Carburetor Simple Venturi

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).

Fuel Metering Force HIGH LOW

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.

Air Bleed

Air Bleed

The Aircraft Float Carburetor Air Flow Limiter Throttle Butterfly Venturi size

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.

Back Suction Mixture Control

Variable Orifice Mixture Control

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 Carburetor Acceleration System Picks up fuel from bowl at idle and discharges it through the pump discharge when the throttle is opened.

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 Carburetor Float Carburetor Preflight Inspection No fuel leaking Sump all drain points

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 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

Airflow Sensing/Air Metering Force 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.