Engine Fuel and Fuel Metering Systems - Carburetion

Flashcards covering aircraft reciprocating engine fuel/air mixtures, venturi principles, and float-type carburetor systems based on AVIA-1065 lecture materials.

What are the primary elemental components of atmospheric air by volume, and how do they behave during engine combustion?

Air consists of approximately $78\%$ nitrogen and $21\%$ oxygen. Nitrogen is inert and does not participate chemically in combustion, while oxygen combines with gasoline to burn.

How is engine power mechanically produced from the burning of gasoline and oxygen?

Burning gasoline and oxygen generates heat, which is absorbed by nitrogen and gaseous combustion byproducts, converting thermal energy into power through expansion.

Why does atmospheric air density decrease as an aircraft climbs to higher altitudes?

Air density decreases because there is less atmospheric air pushing down from above and gravitational attraction is weaker farther from Earth's center.

Why must an engine's fuel/air mixture be leaned as the aircraft climbs to higher altitudes?

Air density decreases with altitude, meaning there is less air weight and less oxygen in a given volume; the fuel flow must be reduced (leaned) to maintain the correct fuel-to-air proportion.

What are the extreme rich and lean air-to-fuel mixture limits within which gasoline will burn in an engine cylinder?

Air and gasoline mixtures as rich as 8:1 and as lean as 16:1 will burn in an engine cylinder; beyond these limits, lean or rich blowout can occur.

At what air-to-fuel ratio does an internal combustion aircraft engine develop maximum power?

An engine develops maximum power at an air-to-fuel ratio of approximately 12:1 by weight (a fuel/air ratio of $0.083$).

What is a stoichiometric fuel/air mixture, and what is its specific proportion?

A stoichiometric mixture is the chemically correct ratio where all fuel and oxygen are completely consumed, consisting of $0.067\,\text{lb}$ of fuel to $1\,\text{lb}$ of air (an air/fuel ratio of 15:1).

What is specific fuel consumption (SFC), and how is it calculated?

SFC is a measure of the economical use of fuel defined as the fuel weight burned per hour per horsepower: $\text{SFC} = \frac{\text{lbs fuel/hour}}{\text{horsepower}}$.

What is the typical fuel/air ratio range for best economy cruise power on aircraft with manual leaning?

The best economy range is between $0.060$ and $0.065$ fuel/air ratios (representing an air-to-fuel ratio range of 16:1 to 15:1).

How do rich and lean fuel/air mixtures provide internal cylinder cooling?

Internal cylinder cooling is achieved by unused evaporating fuel when fuel/air ratios are richer than $0.067$, and by excess unburned air when ratios are leaner than $0.067$.

Why is an engine operated on a rich mixture at full power even though it has high fuel consumption?

A rich mixture is required at near full power to prevent engine overheating and detonation.

What condition causes engine backfire in the induction system?

Backfire is caused by the extremely slow burning of an excessively lean mixture; if combustion continues until the intake valve opens, the flame ignites incoming fresh mixture into the induction system.

What is engine detonation?

Detonation is an uncontrolled, explosive burn of the fuel charge caused by hot spots or high cylinder temperatures from an excessively lean mixture.

How does fluid velocity affect pressure according to the Venturi principle demonstrated here?

As the velocity of air increases through the narrow throat of the venturi, its static pressure decreases proportionally.

How do updraft, downdraft, and sidedraft carburetors differ in orientation?

Downdraft carburetors route incoming air downward through the venturi barrel; updraft carburetors route air upward; sidedraft carburetors direct air horizontally into the induction system.

What is the function and position of the throttle valve in a carburetor?

Located between the venturi and the engine cylinders, the throttle valve controls airflow volume into the engine, directly regulating power output.

Why is the fuel level inside a float chamber maintained slightly below the discharge nozzle outlet?

It is maintained slightly below the nozzle outlet to prevent fuel from overflowing or dripping into the barrel when the engine is shut down.

What differential pressure causes fuel to discharge from the nozzle into the venturi airstream?

The pressure difference between higher (atmospheric) pressure inside the vented float chamber and the low static pressure created at the venturi throat.

What is the purpose of the main air bleed in a float carburetor?

The air bleed introduces air into the fuel being drawn through the discharge nozzle, reducing fuel density and promoting complete fuel atomization and vaporization.

What effect does installing a smaller or larger carburetor metering jet have on the fuel/air mixture?

A larger metering jet increases fuel flow, creating a richer mixture; a smaller metering jet restricts fuel flow, creating a leaner mixture.

What are the six distinct systems built into an aircraft carburetor (mnemonic MIAMIP)?

1. Main metering, 2. Idling, 3. Accelerating, 4. Mixture control, 5. Idle cutoff, and 6. Power enrichment (or economizer).

Why cannot the main metering system supply fuel during engine idling?

At idle, the throttle is almost closed, so air velocity through the venturi is too low to create enough pressure drop to draw fuel from the main discharge nozzle.

Which carburetor system is depicted delivering fuel directly at the edge of the closed throttle valve?

The idling system, which uses manifold depression behind the closed throttle to discharge an idle fuel-air emulsion via the idling jet.

What is the function of the accelerating system in an aircraft carburetor?

It temporarily injects an extra spray of fuel when the throttle is opened rapidly to prevent a momentary lean stumble caused by fuel inertia lag behind rapid airflow increase.

What are the two common mechanical types of manual cockpit mixture controls in float carburetors?

The needle-type mixture control and the back-suction mixture control.

How does a needle-type mixture control alter the fuel-air ratio?

It raises or lowers a tapered needle valve in the passage leading to the main discharge nozzle, opening wide for full rich and restricting fuel flow for lean settings.

How does a back-suction mixture control adjust fuel metering in a float carburetor?

It routes low venturi or manifold pressure into the sealed float chamber, reducing the pressure differential across the metering jet and lowering fuel discharge.

Why are aircraft reciprocating engines stopped using the idle cutoff system rather than the ignition switch?

Stopping fuel flow completely drains combustible vapor from cylinders and manifolds, preventing dieseling, post-ignition kickback, and accidental starting if the propeller is moved by hand.

Why is the power enrichment system also known as an economizer?

It allows the engine to operate on an economical lean mixture across all cruising speeds, opening extra fuel flow only when high power is demanded.

How does air density at an altitude of $18{,}000\,\text{ft}$ compare to air density at sea level?

Air at $18{,}000\,\text{ft}$ is only half as dense as sea-level air, meaning an engine cylinder contains only half as much oxygen by weight per stroke.