Fuel Systems

Question 1

Characteristics of AVGAS.

Answer 1

Burns at around 18-24m/s
Good anti knock properties
85% carbon 15% hydrogen
SG of 0.72 at 15 degrees.

Question 2

AVGAS 100

Answer 2

Green
Octane rating 100

Question 3

AVGAS 100LL

Answer 3

Blue
Octane rating 100

Question 4

AVGAS 80

Answer 4

Red
Leaded fuel

Question 5

MOGAS

Answer 5

motor gasoline
Only for certain authorised aircraft (ultra lights)

Question 6

Pre flight fuel checks.

Answer 6

Clear and bright
Free of sediment
No entrained water
Max water content 30ppm

Question 7

Detonation - normal ignition BTDC

Answer 7

Shockwave from ignition triggers detonation elsewhere.

Question 8

Detonation - BTDC

Answer 8

Extremely rapid combustion from detonation source

Question 9

Detonation - TDC

Answer 9

Excessively high pressure at TDC cause mechanical shock.
Image in files, 19.

Question 10

Principal causes of detonation.

Answer 10

Poor anti Knock properties
Incorrect, excessively weak, mixture
Anything that might unduly raise temp or pressure
High engine temperature

Question 11

Pre ignition - BTDC before ignition

Answer 11

Hotspot triggers combustion

Question 12

Pre ignition - BTDC at ignition

Answer 12

Combustion is well advanced

Question 13

Pre ignition - TDC

Answer 13

Excessively high pressure at TDC causes mechanical shock.
Image in files, 20.

Question 14

Principal causes of pre ignition.

Answer 14

Hotspot within the cylinder, typically carbon deposits on a valve.
Ignition malfunction, damaged spark plug.
High cylinder temperatures.

Pre ignition can be more destructive than detonation.

Question 15

Mixture ratio range

Answer 15

Rich 8:1
Chemically correct 15:1
Lean 20:1

Question 16

Mixture ratios.

Answer 16

Ideal combustion takes place at 15:1.
This is not suitable for the whole flight.
Rich mixture 11:1 for takeoff
Lean mixture 17:1 for cruise.
Power cruise <15:1
Economy cruise >15:1

Question 17

Fuel delivery

Answer 17

Carburation
Fuel injection
Image in files, 21

Question 18

Carburation

Answer 18

Includes: Air inlet, main fuel jet, throttle valve, induction manifold, metered fuel delivered with air.
Induction manifold - distributes air to the engines cylinders.

Question 19

Fuel injection.

Answer 19

Includes: Air inlet, throttle valve, injector pump, metered fuel, injector, induction manifold.

Injector pump - pumps fuel into the cylinders.

Question 20

The basic float carburettor.

Answer 20

Fuel delivered under pressure from aircraft fuel tanks, goes through filter, goes through needle valve regulating fuel flow into engine. Enters the fuel tube, goes through the jet which controls how much fuel mixes with the air, enters Venturi, throttle butterfly controls the flow of mixture.

Image in files, 29

Question 21

Additional fuel metering

Answer 21

System is not sufficiently,
Take no account of air density, it would progressively get richer as altitude or engine speed increases.
Ram air effect in the air inlet would weaken the mixture with increased airspeed,

Question 22

The pressure balance duct.

Answer 22

Carburettor intake, pressure can rise above atmosphere at high forward speed.
Float chamber senses carburettor intake pressure.
Helps balance the pressure.

Question 23

The diffuser

Answer 23

Creates a fire mist of fuel allowing it to mix effectively with incoming air. Located above the jet.

Question 24

Slow running jet.

Answer 24

Tiny nozzle that controls the flow of fuel at low engine speeds.

Question 25

Power enrichment

Answer 25

Gives the engine a continuous boost of fuel.

Question 26

Accelerator pump

Answer 26

Gives the engine a quick boost if fuel.

Question 27

Inlet track

Answer 27

Fuel evaporated and pressure drop causes cooling.
Warm, moist air enters intake.
Maximum pressure drop when butterfly is closed.
Create spice build up.

Question 28

Induction and carburettor icing

Answer 28

Carburettor throat produces a drop in temperature because of the reduction in pressure and the evaporation of fuel.
-can drop by as much as 25 degrees.
At moderate or high levels of humidity, Ice will form in the throat.

Icing most likely at 18-25 degrees.

Question 29

First signs of carburettor/inlet icing.

Answer 29

Fixed pitch propeller - drop in rpm.
Variable pitch propeller - drop in manifold absolute pressure.