INDUCTION AND FUEL METERING

1
INDUCTION AND FUEL METERING

• FEEDING THE HEAT ENGINE

2
AVIATION FUELS

• The production of aviation fuel is highly
  regulated.
• This combined with the fact that Avgas production
  accounts for only a fraction of a percent of
  global refinery output makes its price much
  higher than jet fuel or automotive gasoline.
• This high cost is driving research in alternative
  fuels for reciprocating aviation engines. (diesel)

3
FUEL PROPERTIES

• The fuel blends used in aviation are chosen
  because of the characteristics they exhibit.
• Heat Energy Content aviation gasoline is
  required to have a minimum of 18,700 Btu per
  pound.
• Vapor Pressure the amount of pressure required
  to hold the vapors in a liquid. Avgas must be
  vaporized in order to effectively mix it with
  oxygen in order to burn.
• Vapor pressure too low fuel will not vaporize
  properly and cause hard starting.
• Vapor pressure too high fuel will boil in the
  lines causing vapor lock.

4
FUEL PROPERTIES

• Critical Pressure and Temperature once reached a
  fuel will explode avgas must withstand the high
  pressure and temperature of the cylinder without
  detonating prematurely.
• Octane octane exhibits anti-detonation
  properties when blended with avgas it raises the
  critical pressure of the fuel.
• Heptane possesses detonation properties when
  blended with avgas it lowers the critical
  pressure of the fuel.
• Lead added to fuel for its ant-detonation
  qualities and valve lube functions.

5
FUEL GRADES

• It is important to consult the POH for any
  aircraft you operate to ensure the recommended
  grade of fuel is used.
• Aircraft engines are designed with specific fuel
  grades in mind. If an improper grade is used the
  engine may not operate efficiently or safely.
• Some aircraft engines are approved for a variety
  of grades.
• The source and quality of fuel must be assessed
  as well contaminated fuel could result in engine
  failure.

6
FILL ER UP WITH 100LL PLEASE
7
FUEL/AIR MIXTURE

• Stoichiometric mixture is the chemically correct
  mixture of fuel and air in which all the chemical
  elements are used. (15 pounds of air to 1 pound
  of fuel 151).
• This 151 mixture provides the maximum amount of
  heat energy released.
• However maximum power is produced at approx.
  121. (affected by engine induction and valve
  timing design)
• Because aircraft engines are built from the
  lightest possible materials they are highly
  susceptible to heat damage. For this reason
  temperature must be carefully monitored.

8
DETONATION

• The uncontrolled supersonic combustion of the
  fuel/air charge. Just after the charge is ignited
  by the plug the remaining unburned charge
  explodes. Almost instantaneous in nature, rather
  than the smooth progressive burning of normal
  combustion. The shockwave force can be
  destructive to engine components.
• Caused by excessively high combustion chamber
  temperatures (incorrect fuel, overheating,
  excessively lean mixture).
• Characterized by a knocking or pinging sound.
• Reduce power, enrichen mixture.

9
PRE-IGNITION

• Pre-ignition is the premature ignition of the
  fuel/air charge. May result in backfire as the
  charge ignites while the intake valve is still
  open.
• Caused by glowing carbon particles, or local hot
  spots within the combustion chamber.
• Characterized by backfire, can cause piston and
  cylinder damage.

10
EXHAUST GAS TEMPERATURE

• There is a direct relationship between the
  temperature of the exhaust gas and the mixture
  being burned.
• An EGT gauge allows us to monitor this
  temperature and set our mixture accordingly.
• Most aircraft have a recommended mixture setting
  which is a comprise between best economy and best
  power.

11
(No Transcript)
12
CYLINDER HEAD TEMPERATURE

• Cylinder head temperature is another important
  engine parameter to monitor.
• As each cylinder will create different operating
  temperatures, ground testing will reveal the hot
  cylinder to which the probe is applied.
• In some engines the hot cylinder will change
  depending on power setting.
• For this reason the use of multiple probe CHT
  gauges are used. The hot cylinder can always be
  monitored.

13
FUEL METERING

• The engine needs a method of vaporizing and
  mixing the proper amount of fuel with air. The
  pilot must be able to the control the volume of
  fuel/air mixture being burned, as well as the
  mixture ratio which affects economy and power.
• This is achieved through the use of fuel metering
  systems
• Float carburetion
• Pressure carburetion
• Fuel injection

14
FLOAT CARBURETOR

• Float carburetors are simple, reliable, and
  economic making them the perfect fit for entry
  level light aircraft.
• The float carburetors limitations are uneven
  mixture distribution and susceptibility to carb
  ice.
• The main function of the float carburetor is to
  sense the amount of air entering the engine
  (controlled by the pilot through throttle
  position) and meter the proper amount of fuel.
• It uses differential pressure between the venturi
  and the float chamber to accomplish this.

15
Decreased air pressure sucks fuel from the
discharge nozzle.
16
(No Transcript)
17

• At idle the engine sucks fuel through the idling
  jet.
• As the pilot opens the throttle valve by moving
  the throttle levers, more air is allowed to enter
  the engine, which in turn sucks fuel from the
  discharge nozzle.
• With the throttle fully open more air passes
  through the carb sucking more fuel.

18
In reality the higher air pressure inside the
float chamber pushes fuel to the lower air
pressure of the throttle body.
19
AIR BLEED

• The air bleed has three main functions
• Provide even mixture over a wide range of
  airflows.
• Improve fuel vaporization.
• Decrease fuel metering force needed, which
  decreases the amount of restriction required at
  the venturi. (less restriction to airflow)

20
MIXTURE CONTROL

• There are three types of mixture control
• Manual back suction- controls the amount of
  ambient air entering the float chamber.
• Manual variable orifice- controls size of the
  opening for fuel between float chamber and
  discharge nozzle.
• Automatic- bellows vary float chamber pressure in
  relation to atmospheric pressure.

21
AUTOMATIC MIXTURE CONTROL
22
ACCELERATION SYSTEM

• When the throttle is moved from idle to an open
  position there is a moment when the airflow is
  too low to provide enough fuel for acceleration.
• An accelerator pump can be incorporated to
  prevent momentary engine lag upon throttle
  application.
• When the throttle is positioned to idle the
  piston in the pump draws fuel into the pump
  cylinder, as the throttle is opened the fuel is
  discharged.

23
ECONOMIZER SYSTEMS

• Engines are designed to produce the maximum power
  available for their weight.
• At take off power they are unable to dissipate
  enough heat.
• An economizer enrichens the mixture during full
  throttle application in order to reduce heat.
  (the extra fuel absorbs this heat as it changes
  into vapor).

24
PRIMING

• The C-172 utilizes a piston type priming pump
  actuated manually by the pilot.
• The fuel is introduced directly into the
  combustion chamber.
• If the throttle is pumped fuel will enter the
  induction system through the accelerator pump
  within the carburetor.
• This can lead to engine fire.
• Aircraft equipped with fuel pumps accomplish
  priming through use of an electric boost pump.
  (B-95)

25
ADV./DISADV. FLOAT CARBS

• Advantages
• Economic
• Simple
• Less maintenance
• Disadvantages
• Formation of carb ice in throttle body.
• Gravity dependant (aircraft attitude disrupts
  fuel distribution).
• Imprecise fuel/air mixture (less fuel economy).
• Uneven fuel/air mixture to cylinders.

26
CARBURETOR ICE

• The term carb ice refers to the formation of ice
  within the throttle body of a carburetor due to
  cooling resulting from vaporization and
  expansion.
• Carb icing can occur under moist atmospheric
  conditions in temperatures ranging from approx.
  -13C to 38C even when no visible moisture is
  present.
• The formation of carb ice results in an airflow
  restriction within the carburetor and an
  associated power loss.

27
(No Transcript)
28
(No Transcript)
29
CARB HEAT
30
CARBURETOR ICE

• The formation of carb ice is recognized by a drop
  in rpm (fixed pitch) or MP (constant speed).
• Applying carb heat could result in a further drop
  in rpm or MP and possibly engine roughness as the
  ice is melted and ingested.
• The rpm or MP should then return to just below
  the original rpm or MP.
• It is important not to be scared out of using
  carb heat if rough running is initially
  experienced upon application.

31
CARBURETOR ICE

• How would carb ice be detected during runup?
• Upon application of carb heat
• A decrease in rpm.
• Followed by an increase in rpm as the ice melts.

32
CARBURETOR HEAT

• Carburetor heat increases the temperature of the
  induction air, which in turn reduces its density.
• This is shown by a decrease in power.
• The lean mixture required to run smoothly with
  carb heat on may result in detonation at high
  power settings.
• At normal cruise power the use of carb heat and
  recommended lean mixture will not create problems
  with detonation.

33
CARBURETOR HEAT

• When increasing power
• Increase throttle
• Carb heat - off
• When decreasing power
• Carb heat - on
• Decrease throttle

34
PRESSURE CARBURETOR

• Fuel is supplied under pressure via fuel pump.
• Venturi design is less drastic due to positive
  fuel pressure supply.
• This results in less carb ice susceptibility.
• Still utilizes an air pressure differential to
  meter fuel.
• Fuel is injected into throttle body downstream of
  the throttle valve. (helps prevent carb ice)

35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
ADV./DISADV. PRESSURE CARBS

• Advantages
• Not as susceptible to carb ice.
• Simple design.
• Not gravity dependant.
• More precise fuel/air mixture.
• Disadvantages
• More susceptible to carb ice than fuel injection.

39
FUEL INJECTION

• Fuel is pressure fed to the fuel nozzles which
  vaporize fuel into the air stream just outside of
  the cylinder (continuous flow).
• Direct injection systems inject fuel directly
  into the cylinder. (the air and fuel are mixed
  inside the combustion chamber).
• The fuel is metered one of two ways
• Venturi developed metering force (same as press.
  carb.)
• Engine rpm and throttle setting are sensed and
  the fuel pumps discharge a proportional amount of
  fuel.

40
(No Transcript)
41
FUEL INJECTION

• Airflow or rpmthrottle setting is measured and
  compared to fuel metering force by the fuel
  control unit (fuel regulator).
• Fuel pump supplies required fuel to flow divider
  through flexible tubing.
• Flow divider distributes fuel charge evenly to
  each fuel nozzle through stainless steel tubes.
• Fuel nozzles vaporize fuel into intake air flow
  just outside the cylinder (continuous flow) (or
  inside, with direct injection).

42
(No Transcript)
43
FUEL INJECTION

• Fuel nozzles incorporate bleed air holes to aid
  in fuel vaporization.
• Turbocharged engines must have another tube
  attached to the nozzle to supply turbocharged air
  to the bleed hole to prevent venting.
• If ambient pressure is below induction system
  pressure it could force fuel to vent out of the
  bleed hole.

44
Turbocharged engine nozzle
45
DIRECT INJECTION NOZZLE
46
VAPORLOCK

• Vaporlock can occur in fuel injected engines when
  attempting to restart a hot engine.
• The high temperatures of the recently shutdown
  engine cause the fuel in the injector lines to
  boil (vaporize).
• This vaporized (gaseous) fuel create resistance
  to the flow of fuel in the lines causing
  problematic starting.
• In order to start the engine liquid fuel must be
  pumped through the lines, purging the vapors.
• This is accomplished by selecting boost pump on
  for a period of time prior to start.
• This fills the lines to the pump and the pump
  itself with liquid fuel and a normal start may be
  accomplished.
• NOTE the hot start procedure is dependant on
  aircraft type. Consult the manual.

47
ADV./DISADV. FUEL INJECTION

• Advantages
• No carb ice
• Even air/fuel mixture distribution.
• Improved fuel economy.
• Better performance.
• Disadvantages
• More expensive.
• More maintenance required.

48
INDUCTION SYSTEMS

• The air used for mixture with fuel is obtained
  through the induction system.
• Normally aspirated engines simply take outside
  air and mix it with fuel. As an aircraft climbs
  and air density decreases limiting air for the
  fuel/air mixture, engine power will decrease.
• Induction air is always filtered to prevent
  contamination damage. If this filter were to
  become blocked we need a backup.
• An induction air blockage (debris or ice) will
  cause a loss of power (rpm or MP).

49
INDUCTION SYSTEMS

• The alternate air source on a carbureted engine
  will be the carb heat.
• When the carb heat is on the engine is supplied
  with unfiltered warm air.
• This warm, less dense air will create a lower
  engine output.

50
CARB HEAT
51
INDUCTION ALTERNATE AIR

• A fuel injected engine will have an alternate air
  system.
• Usually a spring loaded or magnetized alternate
  air door will automatically be sucked open if the
  main induction freezes over.
• The engine is then supplied with warm air from
  inside the cowling.
• This situation will be indicated to the pilot by
  a loss in MP.
• A pilot operated alternate air source should then
  be activated to supply the engine with more
  unfiltered warm air.

52
INDUCTION ALTERNATE AIR
53
INDUCTION ALTERNATE AIR
54
(No Transcript)
55
INERTIAL SEPARATORS

• Many turboprop applications incorporate inertial
  separators into the induction air system.
• A vane is deployed into the airflow which creates
  a sharp turn for the incoming air.
• Any heavier than air particles are unable to make
  the turn and vent overboard.
• These inertial separators guard against FOD,
  flame out, and ice particle damage.

56
INERTIAL SEPERATORS