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PPL AGK > Carburation and Injection > Flashcards

Carburation and Injection Flashcards

1
Q

a carburettor must be able to meter fuel for what? (8)

A
  • at all times above idle rpm
  • idling using a slow-running jet
  • engine acceleration using rapid throttle movement
  • power enrichment during high power operations such as during takeoff
  • idle cut-off when stopping the engine
  • mixture control at all altitudes and power settings
  • carb heating to stop ice formation
  • control of fuel during engine start
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2
Q

a simple float type carburettor consists of what? (4)

A
  • a venturi
  • throttle or butterfly valve
  • mainjet
  • float chamber with pivoted float which operates a needle valve
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3
Q

how does a carburettor work?

A
  • fuel in a float chamber is vented to atmospheric pressure.

then flows from the float chamber through the main metering jet into a venturi because of the difference in pressure.

fuel is atomised to form the air-fuel vapour (charge) that is sent to the cylinder to be ignited

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4
Q

what does the throttle butterfly do and how?

A

controls the pressure difference and the amount of fuel flowing.

as the butterfly is progressively closed, the speed of airflow is reduced and so pressure drop at venturi is reduced and less fuel flows from the float chamber.

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5
Q

what is atomisation and diffusion?

A
  • main carburettor jet allows fuel to be drawn into the air through the carb throat and also atomises the fuel by introducing air into the fuel flow exiting the jet.

a diffuser located at the nozzle helps mix the fuel and air together evenly.

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6
Q

how is atomisation and diffusion easily explained?

A

like sucking up water from a straw - water is drawn up in a steady stream but if there’s a hole in the side up of the straw you then suck up water and also some air mixing them.

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7
Q

why is an idling system used and what is it?

A

if the throttle butterfly closes completely, the engine will stop as no fuel or air can reach the cylinders.

idling system is incorporated where that when the throttle lever in the cockpit is fully closed, there’ll be a small amount of air, hence fuel, flowing through the carburettor throat

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8
Q

why is an acceleration system used? and how does it work?

A

Its used because the amount of fuel being drawn from the carburettor is dependent on the amount of air flowing through it, to accelerate the engine more quickly this system is used.

pushes more fuel into the carburettor throat via plunger, and when the throttle is open the plunger is pushed down squirting more fuel into the throat.

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9
Q

why is an enrichment system used and how does it work?

A

used to add extra fuel to the F-A mixture which helps cool the cylinders (useful on bigger engines) operating at higher power settings for long periods of time.

uses a secondary jet in the carb throat that is allowed to operate once the throttle is advanced beyond a certain point

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10
Q

why is a mixture control used?

A

to maintain the correct 15:1 ratio of air.

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11
Q

how does a mixture control work?

A

using the mixture control which uses a needle valve to let more or less fuel from the carburettor bowl into the nozzle.

moving the red mixture control raises or lowers the needle valve.

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12
Q

the idle cut off part of the mixture control system enables what?

A

engine to be stopped by cutting off the fuel supply at the carburettor.

ensures the cylinder contain insufficient fuel to burn and at the same time, the idle cut off system leaves the carburettor bowl full of fuel for the next start.

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13
Q

what is the construction of a simple mixture control system?

A

needle or jet restriction type which varies the flow of fuel through the main metering jet.

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14
Q

movement of the mixture control in the cockpit varies the position of the needle meaning what?

A

fully rich position, no restriction is placed on the flow of fuel to jets.

moving towards the lean position, needle is moved closer to its seat so reduces the amount of fuel which can flow to the jet.

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15
Q

what are the disadvantages of a float type carburettor?

A
  • susceptibility to manoeuvres including those which are less than 1g acting on the float and inverted flight which cause the float to stay closed.
  • icing
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16
Q

when the throttle lever is advanced or retracted throttle valve will what?

A

open or close like carburettor, but will only control the flow of air only not a fuel/air mixture.

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17
Q

the flow of fuel controlled by the throttle, that is mechanically linked to fuel control unit ensures that what?

A

that for a given throttle position, the correct proportion of fuel is metered into the fuel manifold value and hence to the fuel injectors.

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18
Q

as with carburettor, mixture controls what?

A

lean or rich. mixture by decreasing or increasing fuel flow into the fuel manifold valve.

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19
Q

what is the theoretical correct fuel-air ratio?

A

15 parts air to one part fuel by weight 15:1 known as stoichiometric mixture.

20
Q

what is the typical range of mixture strength?

A

9:1 - 17:1

21
Q

mixture are usually expressed with the fuel as unity therefore what is 9:1 and 17:1 recognised as?

A

9:1 has a lower air to fuel ratio and is considered rich, 17:1 the opposite.

22
Q

what is the correct mixture settings for different stages of flight?

A 
takeoff = full rich (unless operating in a high-density altitude
climb = full rich 
descent = full rich
23
Q

when can aircraft be leaned?

A

once at altitude and aircraft is at cruise speed.

24
Q

what is detonation?

A

the spontaneous combustion of the fuel-air mixture inside the cylinder after normal ignition by the spark plug

25
Q

what can cause detonation?

A

fuel-air mix being at too high a temp or pressure before it enters the cylinder which occurs if operating the engine at high power for long periods of time on a hot day and with a lean mix.

26
Q

what happens during detonation?

A

spark plug ignites the mixture and the remainder of the mixture ignites due to the high temperature and pressure in the cylinder rather than the burn spreading from the ignited portion near spark plugs.

27
Q

what is knocking?

A

named after the noise it makes.

and is caused by the shock waves that develop due to the very high pressures involved.

28
Q

what is pre-ignition?

A

ignition of the mixture before spark plugs have ignited it.

29
Q

what causes pre-ignition?

A

by prolonged detonation and by very hot carbon deposits that stick to the piston or walls of the cylinder.

30
Q

what can pre-ignition and detonation do?

A

cause damage to engine components.

deterioration of the bearing surfaces can result including piston and cylinder areas.

31
Q

what causes refrigeration icing? (3)

A

when fuel is vaporised in the carburettor throat, it causes the temperature in the throat to drop.

heat is taken from the air in the carburettor. the temperature of this air drops and if it reaches dew point ice will form.

process occurs between 4 - 25 degrees and when RH is more than 50%.

32
Q

what causes throttle icing?

A

forms mostly with a closed or partially closed throttle. as air flows faster through the venturi and the throttle butterfly, temp drops and reaching dew point freezes creating ice.

occurs up to 25 degrees and RH is generally greater than 50%

33
Q

what causes impact icing?

A

occurs when an aircraft is flying through visible moisture and OAT is below 0. aircraft becomes the condensation nuclei and as the cold, moist air impacts the surface of the plane, ice forms.

34
Q

what are the signs for icing issues? (5)

A
  • Engine RPM decrease
  • rough running engine
  • vibration
  • worse performance
  • engine failure
35
Q

when should you use carb heat?

A

if icing conditions exist or are experiencing symptoms.

36
Q

how is carb heat used? and how does it work?

A

selecting it to on/ hot.

sending warmer air into the carburettor to melt the ice or prevent it from forming. AS it melts RPM will rise and once back to normal you are out of icing conditions so it can be returned back to the original position.

37
Q

using carb heat for run-up checks what is expected and what should happen if the expected doesn’t occur?

A

turn carb heat on for RPM drop, and if there’s no change then it means carb heat isn’t working.

38
Q

what is the function of an inlet manifold?

A

inlet manifold runs form the carburettor to the inlet valves of the cylinder and is used to evenly distribute the mixture to each cylinder.

39
Q

what are the two types of fuel injection systems?

A

direct and indirect (continuous) flow.

40
Q

what is the process of direct flow fuel injection?

A

fuel is injected directly into the cylinder. flow is controlled by the fuel control unit which uses sensors in the engine to determine how much fuel is needed.

fuel is atomised through the nozzle of the injector

41
Q

what are the three main components of a direct fuel injection system and their uses?

A

fuel control unit - used for metering the correct amount of fuel based on throttle position and other info

injection pumps - draw fuel from the tank

injection nozzles - inject fuel into the system, the cylinder.

42
Q

what is the process of indirect (continuous) fuel injection?

A

fuel is not injected directly into the cylinders.

fuel goes to a manifold control valve which serves to equalise the fuel pressure. (called spiders)

from there the fuel goes to each cylinder via the injector nozzle.

43
Q

what does the continuous fuel flow injection system consist of? (4)

A

fuel control unit - which meters the correct amount of fuel needed

manifold valve - divides fuel up for each cylinder

injection pump - pump fuel form tanks to the manifold valve

injection nozzle - atomises and delivers the fuel to the inlet port. activated via a solenoid or is spring-loaded.

44
Q

what are the advantages of fuel injection? (6)

A
  • no refrigeration ice
  • more uniform delivery of fuel-air mix to cylinders
  • improved control of the fuel-air ratio
  • fewer maintenance problems
  • better throttle response
  • increased engine efficiency
45
Q

what are the disadvantages of fuel injection? (3)

A
  • vapour lock in fule lines making restarting a hot engine difficult
  • susceptible to fuel contamination from water or small debris in fuel lines
  • pilot needs a greater awareness of fuel distribution

PPL AGK (29 decks)

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