UNIT 4 Review for Carburetor Syst
UNIT 4 Review for Carburetor Syst AMS 271
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This 15 page Study Guide was uploaded by Jose Manuel on Saturday March 26, 2016. The Study Guide belongs to AMS 271 at Embry-Riddle Aeronautical University - Daytona Beach taught by Mr. Lazy in Spring 2016. Since its upload, it has received 12 views. For similar materials see Aircraft Reciprocating PowerPlant & Systems in Applied Mathematics at Embry-Riddle Aeronautical University - Daytona Beach.
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Date Created: 03/26/16
Carburetor exam review #4 Fuel Filters & Strainers • Installed in the lowest point in the fuel system. • Located between the fuel tank and the fuel metering device. • Located between the fuel tank and the engine-driven fuel pump. • Water and sediment collects and is drained from the fuel strainer bowl. Fuel Strainers Service fuel strainers periodically. • Typically 25 to 50 hours time in service. • Remove and clean strainer bowl. • Remove and clean or replace filter. Pumps Pump Feed Fuel Systems • Fuel boost pump • Engine driven pump Hand-Operated Pumps • Wobble pump • Like a hand-operated boost pump • Handle located in the cockpit • Emergency pump • Transfer fuel • Larger aircraft • Starting fuel-injected engines • These pumps were found on early Beechcraft Bonanzas Fuel Boost Pump • Provides a positive fuel pressure to the engine driven pump. • Minimizes vapor lock. • Can be an emergency fuel pump. • Used for engine starting. Engine Driven Fuel Pump • Provides a positive fuel flow to the fuel metering device. • Positive displacement vane type pump • Diaphragm type pump Fuel Pumps • Diaphragm-type • Lycoming 4 & 6 cylinder engines • High pressure for fuel injected engine • About 40 PSI • Low pressure for carbureted engine 2 • About 10 PSI Usually not repaired or overhauled by a mechanic • No parts or literature available • Overhauled by certificated repair station or manufacturer Atmospheric Conditions • Air Density is the mass or weight per unit volume of air (pounds per cubic foot) • Density decreases with an increase of: • Altitude • Air temperature • Water content • If air density decreases, a fuel air mixture will become richer - more fuel than desired. A mixture control in the carburetor or fuel injections system compensates for this Relative Humidity Amount of water vapor in the air as compared to what the air can hold at a given temperature. 3 Composition of the Atmospheric • 78% nitrogen • 21% oxygen • 1% argon, CO , other gases 2 • Standard day conditions: • 29.92” Hg • 59º F (15º C) FUEL / AIR MIXTURES Mixture Control Knobs • Mixture control full forward • Mixture is full rich • Moving the mixture control aft • Progressively leans the mixture • Mixture control full aft • Idle cut of - engine shut down 4 Power Setting Best power: Lean & Rich Lean Power The leanest fuel/air ratio that can be used and have the engine develop its maximum RPM or manifold pressure for a given throttle position. Any further leaning will cause a rapid drop of in engine power. Rich Best Power: The richest fuel/air mixture that can be used and have the engine develop its maximum RPM or manifold pressure for a given throttle position. Best Economy close to 16:1 ratio The mixture ratio that develops the greatest amount of power for the least amount of fuel flow. 5 Obtained by leaning the mixture until obtaining the highest EGT and RPM while the throttle is in a set position. May cause excessive engine temperatures if used for extended periods of time To Obtain Best economy ratio: Lean mixture from full rich until obtaininghighest engine EGT To Obtain Best Power Ratio From the best economy ratio setting, slightly enrichen the mixture to obtain an approximate 25º F temperature drop from the peak EGT MA4-SPA Carburetor Type / Design Type: Updraft Carburetor 6 • Induction air flows upward Design: Marvel Schebler MA4- SPA MA: Marbel Schelber 4: Standard SAE #4 Throttle Mounting Flange A: Altitude Mixture Control P: Accelerator Pump S: Oversize Bore Systems • Main metering system • Idle system 7 • Accelerating system • Mixture control system • Power enrichment/economizer system Main Metering Systems Controls the fuel flow at all engine speeds above idle Functions Supply the correct amount of fuel to the engine at all speeds above idle Proportion fuel/air mixture Decrease pressure at the main discharge nozzle Control airflow at full throttle 8 Meters fuel Main metering jet Main discharge nozzle Controls airflow Venturi(s) Throttle valve Idle System At idle, airflow through the carburetor venturi is very low due to the position of the throttle valve. 9 a separate idle system is incorporated into the carburetor and only operates when the throttle valve is in the idle or nearly closed position. Idle system - provides fuel at idle Idle fuel delivery passages Idle mixture adjusting screw Idle speed adjusting screw Fly holes Accelerating system Rapid opening of the throttle causes the mixture to become momentarily too lean. 10 Causes the engine to misfire An accelerating system provides extra fuel to minimize this temporary problem. & prevent leaning of the mixture during rapid acceleration Plunger Check valves Accelerator pump discharge tube Mixture control system allow varying the fuel/air mixture to compensate for changes in altitude and temperature. Manual AMC Manual Mixture Control 11 • As altitude increases atmosphere becomes less dense. • o compensate, aircraft carburetors have a manual mixture control •Pilot can lean or enrichen the mixture to suit operating conditions. • Prevents mixture from becoming too rich. • Economize fuel. AMC Automatic Mixture Control Controlled by a bellows. 12 Air pressure decreases as altitude increases. Bellows expands limiting fuel flow. Economizer System Air cooled engine cylinder head temperatures (CHT) can rise to unacceptable levels during high power settings. Take of Climb Additional fuel is fed into the induction system to cool the cylinder heads Also known as a power enrichment system Varies the amount of air pressure on the fuel in the fuel chamber Pressures varies as to the position of the throttle valve to the economizer port located in the top portion of the main venturi 13 FLY HOLES Provide a smooth transition between idle and higher engine speeds They are holes drilled into the throttle bore below the idle fuel delivery hole 14 15
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