piston engines part 5 piston engine operations (fuel & ignition)
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PISTON ENGINES
Part 5
Piston Engine Operations(Fuel & Ignition)
The CarburettorRemember that the fuel is pumped to
the carburettor.
The carburettor is a fuel reservoir
and air delivery tube.
It controls the mixing of air and atomized fuel,
and the flow of the mixture to the engine,
therefore control of the fuel flow,
via the carburettor,
is control of the engine.
The Carburettor
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
Fuel is mostly supplied through the main jet nozzle,on the same level as fuel in the float chamber.
Air is sucked through the Venturi by air pressure,as the piston goes DOWN on an Induction Stroke.
This pressure reduction pulls fuelfrom the float chamber into the main jet.
FUEL FEED
AIR VENT
FUEL NOZZLE
FUEL LEVEL
VENTURI
LOWER AIR PRESSURE
FUEL FLOW
The Carburettor
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
FUEL FEED
AIR VENT
FUEL NOZZLE
FUEL LEVEL
VENTURI
LOWER AIR PRESSURE
FUEL FLOW
As fuel leaves the float chamber, air enters via the vent,
the float drops, opening a needle valve, which allows fuel to be replaced by the fuel pump.
Fuel entering the chamber forces air out of the vent, the float rises, closing the needle valve again.
NEEDLE VALVE
THIS ONLY HAPPENSWHEN ONE PISTON
IS ON THEINDUCTION STROKE
The throttle valve consists of a plate, mounted in a central spindle,
linked to the throttle lever (or foot pedal).
The plate can be any shapethat closely fits the tube in the carburettor.
The Throttle Valve
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
LINK TO THROTTLE
LEVER/PEDAL
VALVE SPINDLE
VALVE PLATE
An Open Throttle (Full Power) allows the Venturi to work at maximum efficiency.
A Low Throttle setting (Idle or Ticking Over)means the Venturi do not work very well,
with only sufficient Fuel/Air mixture to the engine.
The Throttle Valve
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
So an alternative, more effective,Venturi is needed.
OPEN THROTTLELOW THROTTLE
The Edge Gaps now become the Venturi for the Low Air Flow.
The Fuel Nozzle is re-designed,
with an extension to the throttle valve.
At Idle – Slow Running, the extension jet controls the fuel flow.
The Throttle Valve
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
OPEN THROTTLE
At Full Throttle the main Venturi
comes into operation.
LOW THROTTLE
Air Filters can cause restrictions to the Air Flow,with a slight vacuum before the carburettor.
This creates Pressure Differences between:
So the Float Chamber vent is closed off, and the Float Chamber is connected to
the Carburettor Air Intake.
Equalising pressureat Low Speed,and preventing too much Fuel mixing with the Air.
Balanced Air Pressure
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
ANDHere
VACUUM
Here
Efficient air/fuel mixing is essential.
So fuel needs to be ‘atomized’ (broken down into the smallest particles possible).
Fuel in liquid form doesn’t burn, it’s fuel vapour which burns,
so atomizing fuel creates a greater vapour surface.
A more efficient mix of fuel & air can be obtainedby fitting a
Diffuser Tubeonto the main jet.
Mixing Air and Fuel
FLOAT CHAMBERRESERVOIR
FLOAT
AIR/FUEL MIXTURE
FLOWS TO ENGINE
So let’s take a look here.
Mixing Air and FuelThe Diffuser Tube
Fuel level in theFloat Chamber
DIFFUSER TUBE
MAIN JET
Fuel feed fromthe Float Chamber
AIR INLET
Air mixes with Fuel
Increased flow of
mixed Air/Fuel
THROTTLEVALVE
IDLE SETTING the diffuser is full of fuel
but very little air,fuel supplied via idle jet
OPEN THROTTLEidle jet stops working,main Venturi comes
into operation
CRUISE SETTINGair bubbles through the diffuser holes,
improving atomization
HIGH-POWER SETTING
The throttle valve is fully open
and maximum airflowis passing throughthe main venturi.
The Pressure Capsule
Air at higher altitudes is ‘thinner’,causing lower pressure.
But the piston travels the same distance whatever the altitude it is working in.
Therefore at higher altitudes less air is taken into the engine.
Less airflow means a richer mixture, (more fuel than air),
which would exceed the ideal ratio of
15:1
The Pressure Capsule
A method of adjusting the fuel and air flow is requiredto maintain the idea 15:1 ratio.
Since air pressure is easily sensed, the device used is an Aneroid Capsule.
The aneroid capsule is a sealed chambermade from concertinaed thin metal.
The inside is a partial vacuum,
the outside senses the air pressure,
(termed as‘ambient’ pressure).
FLOAT
FLOAT CHAMBERRESERVOIR
ANEROIDCAPSULE
FLOAT
FLOAT CHAMBERRESERVOIR
The Pressure Capsule
At LOW Altitude – Higher Atmospheric Pressure,the capsule is compressed by the outside air pressure,
allowing the correct quantity of fuel through,for the weight of air passing into the engine.
FLOAT
FLOAT CHAMBERRESERVOIR
The Pressure Capsule
At HIGH Altitude – Lower Atmospheric Pressure,the capsule expands,
pushing the needle into the main jet,allowing a lower quantity of fuel
for the lower weight of air passing into the engine.
The result isthe air/fuel ratio is maintained at
15:1at all atmospheric
conditions.
Fuel Injection Systems
The injection system differs from the carburettor system in the way the fuel is delivered.
In carburettors, fuel is mixed with air before it enters the cylinders.
In injection systems, fuel is injected directly into the air,
either in the inlet manifold, or even directly into the cylinders.
Injection systems squirt high pressure fuel through a small nozzle, or jet, to atomise the fuel.
Fuel Injection SystemsHydro-Mechanical Injection System
Warm Up Regulator
Air Inletfrom air filter
Throttle Valve
Cold Start Valve
Auxiliary Air Device
Idle Speed Adjuster
Mixture Control Unit (Fuel Distribution Valve )
Air Sensor Plate
Fuel injector
The airflow is sensed by an ‘Air Sensor Plate’, which regulates the fuel flow to the engine in the
Mixture Control Unit/Fuel Distribution Valve.
In this system sufficient fuel is continuously injected into the inlet manifold.
Pump
Air Filter
ThrottleValve
Fuel Injection SystemsElectronic Injection System
Injection Control
Unit
EngineECU
Pressure Regulator
Common ‘Rail’
The electronic system is very much simpler, at least with the number of manufactured parts.Pressure is limited by the Pressure Regulator,
and the injectors fed from a common ‘Rail’ by the fuel pump.
The Injection Control Unit (ICU)controls the timing
of the injector valve opening,
in conjunction withthe Engine
Control Unit (ECU).
Fuel Injectors
Check of UnderstandingIn the carburettor
when is air sucked into the Ventri?
On the Induction down stroke
On the Compression up stroke
On the Compression down stroke
On the Induction up stroke
What shape are Throttle Valves?
Never Triangular
Any Shape
Always Square or Oblong
Always Circular
Check of Understanding
When does the Venturi work at maximum efficiency?
When the engine is at low power
When the engine is idle
When the engine is cruising
When the engine isat full power
Check of Understanding
What happens in a float chamber carburettor if an air filter causes restrictions to the air flow?
A vortex is formed
The pressure drops rapidly
The pressure rises rapidly
A vacuum is formed
Check of Understanding
Z
W
Y
X
Check of UnderstandingIn the diagram below,
which arrow points to the main jet?
At what throttle setting is a diffuser tubefull of fuel but with little air?
Ticking Over
Never
Cruising
Full Power
Check of Understanding
In a float chamber carburettor, what is the function of the device
marked with a ‘X’?
To prevent the fuel/air ratio from being too rich when the aircraft
climbs
To atomize the fuel
To supply extra fuel for acceleration
To prevent the fuel/air ratio from being too rich when the throttle
valve is opened
Check of Understanding
In a fuel injection system,which of the following does not apply?
A throttle valve is not needed
Fuel can be injected into the inlet manifold
Fuel can be injected into the cylinder
The fuel is not atomized
Check of Understanding
What is the normal fuel to air ratioin a piston engine?
15 : 1
1 : 5
5 : 1
1 : 15
Check of Understanding
PISTON ENGINES
End of Presentation
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