Lesson 5 Fuel Systems

Question 1

AVGAS has a specific gravity of…

Answer 1

0.72kg/l

Question 2

How fast does AVGAS burn?

Answer 2

18-24m/s (60-80ft/s)

Question 3

Why is fuel measured in mass?

Answer 3

Volume has a different density depending on conditions ie heat

Question 4

AVGAS has good anti knock properties and resists detonation. What component determines this?

Answer 4

Octane level

Question 5

Liquid in fuel lines can form a pocket of vapour which restricts the flow of fuel. What is this known as?

Answer 5

Vapour Lock

Question 6

What is AVGAS made up of?

Answer 6

85% Carbon
15% Hydrogen

Question 7

Pre flight you must take a fuel sample to check that is is free of contaminants. It must be:

Answer 7

Clear and bright
Free of sediment
No entrained water

(Max water content 30ppm)

Question 8

Pre ignition can cause early combustion. What are the 3 principal causes?

Answer 8

Hotspot within cylinder (usually caused by debris)

Ignition malfunction ie damaged spark plug

High cylinder temperatures

The primary cause is an excessively weak mixture

Question 9

What is the ratio of a RICH mixture?

Answer 9

8:1

Question 10

What is the ratio of a stoichiometrically chemically correct mixture?

Answer 10

15:1

Question 11

What is the ratio of a LEAN mixture?

Answer 11

20:1

17:1 can be used in cruise for efficiency

Question 12

What ratio should your TAKEOFF mixture be?

Answer 12

11 or 12:1

Power demand is very high in this phase, the excess fuel provides cooling to to cylinders

Question 13

What mixture ratio gives you peak EGT (Exhaust Gas Temperature)?

Answer 13

15:1

Question 14

What kind of mixture can lead to high cylinder head temperatures?

Answer 14

LEAN

The flame rate is slower due to less fuel in the charge so the flame is in contact with the cylinder head for longer.

Question 15

What is the equation for calculating Total Pressure?

Answer 15

TOTAL PRESSURE = DYNAMIC PRESSURE + STATIC PRESSURE

PT = PD + PS

Question 16

When you are travelling faster and more air goes into the inlets resulting in an increase of static pressure this is known as what?

Answer 16

Ram effect

Question 17

Why do we fit a pressure balance duct?

Answer 17

To counteract Ram effect and stop the mixture getting too lean

Question 18

Why is a carburettor fitted with a diffuser?

Answer 18

To mitigate the risk of high power/altitude which gives a too rich mixture

Question 19

What are the four principal causes of detonation?

Answer 19

Fuel with poor anti knock properties

Excessively weak mixture

Anything that might raise temp or pressure eg low RPM with high manifold absolute pressure

High engine temperature

Question 20

Pre ignition worsens with ___ RPM whereas detonation reduces as RPM ____

Answer 20

increased, increase

Question 21

At what mixture ratio does the highest combustion temperature occur?

Answer 21

15:1

Question 22

What is the purpose of the pressure balance duct in a carburettor?

Answer 22

To ensure that the correct mixture ratio is maintained across a range of airspeeds and flight conditions

Question 23

What purpose does the diffuser have in a carburettor?

Answer 23

It maintains the correct mixture ratio across a wide range of engine speeds and aids atomisation to help produce a good emulsion of mixture and fuel

Question 24

If you double the air flow through the engine the total pressure increases and static pressure decreases by how much?

Answer 24

x 4

Question 25

At very small throttle openings the pressure drop in the Venturi is negligible resulting in an insufficient flow of fuel from the main jet. What is introduced to counteract this?

Answer 25

Slow running jet. Placed right next to the butterfly to take advantage of strong pressure drop. Provides a rich mixture at idle.

Question 26

The power enrichment system in a carburettor is used when?

Answer 26

At high power settings to provide a rich mixture to cool the cylinders. Also known as economiser system because the needle is closed at all but high power settings, despite being less economic

Question 27

The accelerator pump is used in what conditions?

Answer 27

Emergency. The accelerator pump is used when the throttle is opened rapidly and injects fuel directly into the Venturi to counteract the increase in airflow and the delay in fuel delivery which could cut out the engine (known as LEAN CUT)