Engine Fuel Metering Systems

What can happen if you have an excessively lean fuel/air mixture?

High cylinder head temp

Why is an aircraft equipped with a mixture control?

To be able to lean at altitude

If the volume of air passing through a carburetor venturi is reduced, what will happen to the pressure at the venturi throat?

Increase because velocity decreases

What is considered the best power mixture for the air/fuel ratio?

12:1

What does 11:1 refer to for air/fuel mixture ratio?

11 parts air to 1 part fuel

In a float carburetor, if the float sank to the bottom of the float bowl, what would happen to the fuel/air ratio?

the ratio would be too rich (enriches)

Which method is commonly used to adjust the level of a float in a float type carburetor?

Adding or subtracting shims under the needle seat

Where will the fuel level within the float chamber be of a properly adjusted carburetor?

Slightly lower than the discharge nozzle

What is least likely to occur during operation of an engine equipped with a direct cylinder fuel injection system?

less likely to back fire

What could cause an engine with a float carburetor to backfire or miss when the throttle is advanced?

Bad/INOP acceleration pump

On a carburetor without an automatic mixture control (AMC) as you ascend to altitude, the mixture will?

Enrich

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

Destroys surface tension and decrease fuel density

What is the fuel metering force of a conventional float-type carburetor in its normal operating range?

The pressure difference between discharge nozzle and the float chamber

If the main air bleed becomes clogged, what will happen to the fuel/air mixture as airflow increases through the carburetor?

Becomes richer

At what engine speed does the main metering jet actually function as a metering jet in a float-type carburetor?

Anything above idle

What would most likely cause a float carburetor to become flooded?

Needle valve not seating properly

Which system in a float carburetor compensates for the time lag in fuel supplied to the engine when the throttle is suddenly opened?

Accelerating system

How often should float carburetors be overhauled?

At engine overhaul

What controls the volume of the fuel/air mixture to the cylinders?

Throttle valve

What controls the ratio of the fuel/air mixture to the cylinders?

Mixture control

For idling speeds in a float carburetor, how is fuel discharged?

through the idle discharge nozzle after the throttle valve

What could cause an engine equipped with a float carburetor to run rich at full throttle?

Clogged main air bleed

During operation of an aircraft engine, the pressure drop in the carburetor venturi depends primarily upon what?

Velocity of the air through the venturi

What could cause an engine to run lean with high CHT at sea level or low altitude?

Automatic mixture control stuck in the extended position.

To determine if the float level is adjusted properly, a measurement is usually made from the top of the fuel in the float chamber to the what?

Parting surface of the carburetor

On a float carburetor, what is the purpose of the economizer valve?

Enrich the fuel air mixture at high power settings for cooling

What will happen to the fuel/air mixture if an aircraft engine with a carburetor is not compensated for altitude and temperature variations?

It will become richer as the altitude or temperature increases

What is the purpose of the air bleed openings used with continuous flow fuel injection nozzles?

Fuel vaporization, partially leans mixture

how is an engine with a pressure carburetor started?

Prime while mix control is at cutoff

What is the relationship between the accelerating pump and the enrichment valve in a pressure carburetor?

there Is no relationship

What adjustment is ordinarily used to make idle speed adjustments on a float type carburetor?

Throttle stop screw

What carburetor component actually limits the desired maximum airflow to the engine at full throttle?

Venturi

What type of engine driven fuel pump is used on most large aircraft reciprocating engines?

Positive displacement rotary vane type

In the TCM fuel injection system, what is the purpose of the engine driven fuel pump bypass valve?

To prevent a damaged or inop pump from blocking fuel flow to another pump in series

What is the function of the altitude compensating, or aneroid valve used with the TCM fuel injection system?

To prevent overly rich mixture during sudden acceleration

On a float carburetor, what is the purpose of the accelerator pump?

Prevents an overly lean mixture during a rapid acceleration

What is the major difference in fuel metering between the TCM and the RSA fuel injection system?

TCM uses just fuel to meter and the RSA uses both air and fuel to meter

What would most likely lead to detonation?

Too low of an octane level

Where is the engine fuel shutoff valve usually located?

Aft of the firewall

Why are fuel boost pumps operated?

provide a positive flow to the engine

During what period does the fuel pump bypass valve open and remain open?

When the boost pump pressure is higher than the fuel pumps pressure

A correct idling mixture will be indicated by

10-50 increase in RPM

What does the economizer system do in a float carburetor?

Increases fuel air mixture at high power

1.) The mixture at rated power is richer than the mixture at normal cruising range.

2.) The mixture at idle is richer than at rated power.

(Which statement is true)

Both true

How are discharge nozzles in a fuel injected reciprocating engine identified to indicate the flow range?

A letter stamped onto the hex

The mixture used at idle in air cooled recip engines is richer than the mixture at rated power

true

An aircraft engine continuous cylinder fuel injection system normally discharges fuel during which stroke(s)?

Continuously

Define continuous flow fuel injection....

Fuel is continuously supplied to the intake port

What could cause a single diaphragm accelerator pump to discharge fuel?

Increase in manifold pressure caused by throttle being opened

Where does the excess fuel from the fuel pump relief valve go?

Inlet of the pump

what causes pre-ignition ?

hot spots in the cylinder

what causes Backfire?

An excessively LEAN mixture

what causes after-fire?

An excessively RICH mixture

What are the three carburetor classifications

Up draft

Down draft

Side draft

Five systems found in most fuel metering systems

- Main metering system

- Idling system (richer than normal)

- Mixture control system

- Accelerator system

- Power enrichment/Economizer system (richer than normal)

Power enrichment system types

- Needle valve

- Air bleed

- Pressure operated

Limitations of a carburetor

float design does not respond well to sudden aircraft movement and unusual aircraft attitudes

What causes excessively rich fuel/air mixture?

Float is damaged or punctured and sinks in the float chamber

what causes the engine to run rich on a float type carb.

Fuel level in the bowl is set too high

what causes the engine to run to lean on a float type carb.

- Oversize main air bleed

- Fuel level in float set too low

- Float bowl vent blocked

Five components in the RSA fuel injection system

- Venturi housing

- Fuel metering unit

- Fuel regulator

- Flow divider

- Fuel nozzle

What type of engine utilizes a Aneroid valve ?

Turbocharged engines

Who can overhaul TCM fuel injection ?

The manufacture or a certified repair station