Understanding Aircraft Carburetors and Exhaust Systems

4 0 7 . 0 3 Carburetors and Exhaust Systems

The Carburetor The carburetor has three important functions: • measure the correct quantity of fuel and vaporize this fuel, • mix it with air in the proper proportion, and • deliver the mixture to the cylinders.

Components of a Carburetor • Venturi • air is drawn into the venturi and because of its shape, the air is accelerated while the pressure is reduced • Nozzle • provides a passage for fuel from the float chamber to the venturi • reduced pressure draws fuel into the venturi where it is vaporized

Throttle Valve To Intake Manifold Float Chamber Components of a Carburetor • Throttle Valve • regulates the volume of fuel/air mixture • Intake Manifold • distributes the fuel/air mixture from the carburetor to the cylinders • Float Chamber • contains a consistent level of fuel in order to keep fuel supply steady

Components of a Carburetor • Needle Valve • opens and closes the fuel line and is controlled by the float • Vent • allows the pressure to be equalized with that of the changing outside air pressure • Idle Jet • used to keep the engine going when there is insufficient air flow to draw in fuel from the nozzle

Fuel/Air Mixture • The ratio of fuel to air is regulated by the pilot with the mixture control. • The throttle regulates the flow of air into the engine and creates turbulence at the butterfly valve to assist in the mixing of fuel and air. • The proportion of fuel to air is governed by weight and not by volume.

Fuel/Air Mixture • The chemically correct mixture is about 1:15, where it is one part fuel to fifteen parts air. • An engine will run hotter with a lean mixture because the lean mixture is slower burning, exposing the cylinder walls to high temperatures for a longer period of time. • An engine will run cooler with a richer mixture because the rich mixture burns more quickly.

Confirmation Stage NOTE: You must use the buttons in the Confirmation Stage

Aero Engines Carburetor and Exhaust Systems Let's try a few review questions on Aero Engines: Question #1 - How is the mixture of gasoline to air measured? Volume A B Weight C Mass D Area

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Mixture Control • As altitude increases, the density of air decreases. • Carburetors are calibrated for sea level operation. • Therefore, with altitude, the mixture would become over-rich, causing a waste of fuel and a loss of power. • Mixture control is fitted to adjust the amount of fuel being drawn from the nozzle. • Mixture control can be used to produce a rich or lean fuel/air mixture.

Rich Mixture • Besides lowering the combustion temperature, too rich a mixture will result in unburned wasted fuel. • It contributes to fouled spark plugs and combustion chamber deposits. • Can also cause rough engine operation, appreciable loss of power or engine failure. • Used for high power settings.

Lean Mixture • May cause rough engine operation, sudden “cutting out”, “popping back” or back-firing, detonation, overheating or appreciable loss of power. • Continual operation at too lean a mixture has also been responsible for engine failure. • Used for cruise power settings.

When to Lean the Engine • At cruise power, below approximately 75% of the rated RPM of the engine. • At any altitude above 3000 feet. • For take-off at high altitude airports. • After climbing to a higher altitude.

Why Lean the Engine Proper leaning of engine is both practical and economical. It results in: • economy of fuel, • a smoother running engine, • a more efficient engine, • extended range, • less spark plug fouling, • more desirable engine temperatures, and • cleaner combustion chambers.

Confirmation Stage NOTE: You must use the buttons in the Confirmation Stage

Aero Engines Carburetor and Exhaust Systems Let's try a few review questions on Aero Engines: Question #2 - When do you lean your engine? Take off at low altitude A Final approach B Climbing at high altitudes C Climbing at low altitudes D

Aero Engines Carburetor and Exhaust Systems Let's try a few review questions on Aero Engines: Question #3 - Why do you lean your engine? Extended range A Longer spark plug life B Cleaner engine C All the above D

Carburetor Icing • Forms under moist atmospheric conditions with air temperatures anywhere from approximately -5C to 30C. • Indicated by a loss of power (RPM drop). • Can cause complete engine failure.

Forms of Carburetor Ice There are three forms of carburetor ice: • fuel vaporization ice, • impact ice, and • throttle ice.

Prevention of Carb Icing • Carb icing does not occur in engines that have fuel injectors rather than a carburetor. • Carb heat uses air heated by the exhaust system which is pumped into the carburetor. • Results in initial drop in RPM. • If ice is present, its melting will give a short period of engine roughness.

Exhaust System • Basically a scavenging system. • Collects and disposes of the high temperature, noxious gases discharged by the engine. • Main function is to prevent the escape of these potentially destructive gases into the airframe and cabin. • Two types: • short stack system, and • collector system.

Short Stack Exhaust System • Used on non-turbocharged engines and on low powered engines. • Relatively simple: • a down stack from each cylinder, • an exhaust collector tube on each side of the engine, and • an exhaust ejector on each side of the cowling.

Collector Exhaust System • Used on most large engines and on all turbocharged engines. • Individual exhaust headers empty into a collector ring that collects the exhaust from all the cylinders. • One outlet from this rings routes the hot exhaust gas to the turbocharger. • An exhaust tailpipe carries the gases away.

Confirmation Stage NOTE: You must use the buttons in the Confirmation Stage

Aero Engines Carburetor and Exhaust Systems Let's try a few review questions on Aero Engines: Question #4 - At approximately what temperature range can carburetor ice form? -5°C to 30°C A B -30°C to -5°C C -30°C to 5°C D 5°C to 30°C

Aero Engines Carburetor and Exhaust Systems Let's try a few review questions on Aero Engines: Question #5 - Which is NOT a type of exhaust system? Short Stack A Collector B Venturi C Both B and C D

Aero Engines Carburetor and Exhaust Systems Congratulations!! You have now completed the Carburetor and Exhaust Systems lesson of the Aero Engines Module. Of course, this lesson is always available to you for future reference if required. You are now ready to move along to the next Aero Engines lesson you have not completed or to any other module you wish. You can advance to the Self Test Module if you feel ready to challenge the final exam. Good Luck!

Understanding Aircraft Carburetors and Exhaust Systems

Understanding Aircraft Carburetors and Exhaust Systems

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